Basics of mechanical ventilation

Basics of Mechanical Ventilation

Origins of mechanical ventilation
•Negative-pressure ventilators
(“iron lungs”)
• Non-invasive ventilation first
used in Boston Children’s
Hospital in 1928
• Used extensively during polio
outbreaks in 1940s – 1950s
•Positive-pressure ventilators
• Invasive ventilation first used at
Massachusetts General Hospital
in 1955
• Now the modern standard of
mechanical ventilation
The era of intensive care medicine began with positive-pressure ventilation
The iron lung created negative pressure in abdomen
as well as the chest, decreasing cardiac output.
Iron lung polio ward at Rancho Los Amigos Hospital
in 1953.

Outline
•Theory
• Ventilation vs. Oxygenation
• Pressure Cycling vs. Volume Cycling
•Modes
•Ventilator Settings
•Indications to intubate
•Indications to extubate
•Management algorithim

Principles (1): Ventilation
The goal of ventilation is to facilitate CO2 release and maintain normal PaCO2
•Minute ventilation (VE)
• Total amount of gas exhaled/min.
• VE = (RR) x (TV)
• VE comprised of 2 factors
• VA = alveolar ventilation
• VD = dead space ventilation
• VD/VT = 0.33
• VE regulated by brain stem,
responding to pH and PaCO2
•Ventilation in context of ICU
• Increased CO2 production
• fever, sepsis, injury, overfeeding
• Increased VD
• atelectasis, lung injury, ARDS,
pulmonary embolism
• Adjustments: RR and TV
V/Q Matching. Zone 1 demonstrates dead-space ventilation
(ventilation without perfusion). Zone 2 demonstrates normal
perfusion. Zone 3 demonstrates shunting (perfusion without
ventilation).

Principles (2): Oxygenation
The primary goal of oxygenation is to maximize O2 delivery to blood (PaO2)
•Alveolar-arterial O2 gradient
(PAO2 – PaO2)
• Equilibrium between oxygen in
blood and oxygen in alveoli
• A-a gradient measures efficiency
of oxygenation
• PaO2 partially depends on
ventilation but more on V/Q
matching
•Oxygenation in context of ICU
• V/Q mismatching
• Patient position (supine)
• Airway pressure, pulmonary
parenchymal disease, small-
airway disease
• Adjustments: FiO2 and PEEP
V/Q Matching. Zone 1 demonstrates dead-space ventilation
(ventilation without perfusion). Zone 2 demonstrates normal
perfusion. Zone 3 demonstrates shunting (perfusion without
ventilation).

Non- invasive ventilation
 Advantages:
1.allows speech
2.ideal for patients with nocturnal hypoventilation
3.complications of intubation –avoided
 Disadvantages:
1. patient must be NPO
2. lack of airway protection
3. patient should be alert with normal respiratory drive
4. cannot fully sedate patients
5. slower correction of blood gas abnormalities
6. upper airway must be intact
7. cannot provide full 100% ventilatory support
8. apparatus uncomfortable for patients

Patients likely to benefit from non-invasive
ventilation:
1. respiratory failure likely to be reversible within
24-48 hrs
 acute reversible cardiogenic pulmonary oedema
 acute exacerbation of COPD
 post-op respiratory failure
 laryngeal oedema
2.Chronic use in progressive respiratory failure
neuromuscular disease with respiratory muscle
weakness
lung transplant candidiates awaiting doners
3. Acute respiratory failure in patients not willing
intubation
 terminally ill patients & AIDS patients

Types of non-invasive ventilation: CPAP &
BiPAP
1.CPAP:
Patient continuously receives a set air pressure, during both
inspiration and expiration.
Patient has full control over respiratory rate, inspiratory time , and
depth of inspiration.
Best suited for:
 patients with obstructive sleep apnea
 cardiac patients requiring ventilatory assistance but not
requiring immediate intubation
Advantages of increase in pressure at end- expiration:
 increase in FRC
 decrease afterload to the left ventricle
 increased patency of alveoli ( esp those atelectatic due to
pulmonary edema or poor inspiratory effort)
Mode of delivery: nasal vs face mask. Pressure 5-20cm of H2O.

2.BiPAP
This provides a set inspiratory pressure and a
different set expiratory pressure
Patient has full control over the respiratory rate,
inspiratory time and depth of inspiration
The unit senses the beginning and end of
inspiration so that the delivery pressure can be
changed from the expiratory pressure to the
inspiratory pressure and vice versa.
In simple way
BiPAP=CPAP+Pressure support during
inspiration

BiPAP is indicated only when
CPAP alone is insufficient to improve dyspnea or
hypoxemia in patients with
 respiratory muscle fatigue
 underlying COPD
 high pCO2.
Initial setting:
IPAP: 8cm H2O
EPAP: 4cm H2O
O2 @ 2-5 L/min

Invasive Ventilation:
Pressure ventilation vs. volume ventilation
Pressure-cycled modes deliver a fixed pressure at variable volume (neonates)
Volume-cycled modes deliver a fixed volume at variable pressure (adults)
•Pressure-cycled modes
• Pressure Support Ventilation (PSV)
• Pressure Control Ventilation (PCV)
• CPAP
• BiPAP
•Volume-cycled modes
• Control
• Assist
• Assist/Control
• Intermittent Mandatory Ventilation
(IMV)
• Synchronous Intermittent
Mandatory Ventilation (SIMV)
Volume-cycled modes have the inherent risk of
volutrauma.

Pressure Support Ventilation (PSV)
Patient determines RR, VE, inspiratory time – a purely spontaneous mode
• Parameters
• Triggered by pt’s own breath
• Limited by pressure
• Affects inspiration only
• Uses
• Complement volume-cycled
modes (i.e., SIMV)
• Does not augment TV but
overcomes resistance created
by ventilator tubing
• PSV alone
• Used alone for recovering
intubated pts who are not
quite ready for extubation
• Augments inflation volumes
during spontaneous breaths
• BiPAP (CPAP plus PS)
PSV is most often used together with other volume-cycled modes.
PSV provides sufficient pressure to overcome the resistance of the ventilator
tubing, and acts during inspiration only.

Advantages of PSV
 Avoids patient-ventilator asynchrony
 Patient comfortable-having full control over their
ventilatory pattern and minute ventilation
 Avoids breath stacking and auto PEEP
Disadvantages
 Spontaneous mode –can’t be used in heavily
sedated, paralyzed or comatose patients
 Respiratory muscle fatigue can develop if
pressure support is set too low

Pressure Control Ventilation (PCV)
Ventilator determines inspiratory time – no patient participation
•Parameters
•Triggered by time
•Limited by pressure
•Affects inspiration only
•Best reserved for patients with ARDS
•Disadvantages
•No guaranteed tidal volume thus no
guaranteed VE
•Requires frequent adjustments to
maintain adequate VE
•Pt with noncompliant lungs may require
alterations in inspiratory times to achieve
adequate TV
•Air trapping can be a problem
•Heavy sedation required

CPAP and BiPAP
CPAP is essentially constant PEEP; BiPAP is CPAP plus PS
•Parameters
 CPAP – PEEP set at 5-10 cm H2O
 BiPAP – CPAP with Pressure Support (5-20 cm H2O)
 Shown to reduce need for intubation and mortality in
COPD pts
Indications
 When medical therapy fails (tachypnea, hypoxemia,
respiratory acidosis)
 Use in conjunction with bronchodilators, steroids,
oral/parenteral steroids, antibiotics to prevent/delay
intubation
 Weaning protocols
 Obstructive Sleep Apnea

Volume cycled modes
•Control Mode
• Pt receives a set number of
breaths and cannot breathe
between ventilator breaths
• Similar to Pressure Control
•Assist Mode
• Pt initiates all breaths, but
ventilator cycles in at initiation
to give a preset tidal volume
• Pt controls rate but always
receives a full machine breath
Ventilator delivers a fixed volume

Assist/Control Mode
 Assist mode unless pt’s
respiratory rate falls below
preset value
 Ventilator then switches to
control mode and provides full
Vt at a minimum preset rate
Advantage
 provides near complete resting
of ventilatory muscles
 can be used in awake, sedated
or paralyzed patients.
Disadvantages
• Rapidly breathing pts can
overventilate and induce severe
respiratory alkalosis and
hyperinflation (auto-PEEP)

IMV and SIMV
Volume-cycled modes typically augmented with Pressure Support
•IMV
• Pt receives a set number of
ventilator breaths
• Different from Control: pt can
initiate own (spontaneous) breaths
• Different from Assist: spontaneous
breaths are not supported by
machine with fixed TV
• Ventilator always delivers breath,
even if pt exhaling

SIMV
 Most commonly used mode
 Ventilator provides set tidal volume at a preset rate
 If pt has respiratory drive, the mandatory breaths are synchronized
with the pt’s inspiratory effort
 When a ventilator breath is programmed to occur, the ventilator
waits a predetermined trigger period; any patient initiated breath
during this trigger period results in a programmed ventilator
delivered breath

SIMV contd.
 The patient can take additional breaths but Vt of these extra
breaths is dependent on the patient’s inspiratory effort
Advantages of SIMV
 Theoritically results in improved blood return to the right ventricle
owing to intermittent negative pressure (spontaneous) breaths
 Patient comfort due to the control over their ventilatory pattern
and minute ventilation
Disadvantage
 Can result in chronic respiratory fatigue if set rate is too low;
characterized by
high respiratory rate
rising pCO2
 Air trapping can occur

Vent settings to improve <oxygenation>
•FIO2
• Simplest maneuver to quickly increase PaO2
• Long-term toxicity at >60%
• Free radical damage
•Inadequate oxygenation despite 100% FiO2
usually due to pulmonary shunting
• Collapse – Atelectasis
• Pus-filled alveoli – Pneumonia
• Water/Protein – ARDS
• Water – CHF
• Blood - Hemorrhage
PEEP and FiO2 are adjusted in tandem

Vent settings to improve <oxygenation>
•PEEP
• Increases FRC
• Prevents progressive atelectasis and
intrapulmonary shunting
• Prevents repetitive opening/closing (injury)
• Recruits collapsed alveoli and improves
V/Q matching
• Resolves intrapulmonary shunting
• Improves compliance
• Enables maintenance of adequate PaO2
at a safe FiO2 level
• Disadvantages
• Increases intrathoracic pressure (may
require pulmonary a. catheter)
• May lead to ARDS
• Rupture: PTX, pulmonary edema
PEEP and FiO2 are adjusted in tandem
Oxygen delivery (DO2), not PaO2, should be
used to assess optimal PEEP.

Vent settings to improve <ventilation>
•Respiratory rate
• Max RR at 35 breaths/min
• Efficiency of ventilation decreases
with increasing RR
• Decreased time for alveolar emptying
•TV
• Goal of 10 ml/kg
• Risk of volutrauma
•Other means to decrease PaCO2
• Reduce muscular activity/seizures
• Minimizing exogenous carb load
• Controlling hypermetabolic states
•Permissive hypercapnea
• Preferable to dangerously high RR
and TV, as long as pH > 7.15
RR and TV are adjusted to maintain VE and PaCO2
•I:E ratio (IRV)
• Increasing inspiration time will
increase TV, but may lead to
auto-PEEP
•PIP
• Elevated PIP suggests need for
switch from volume-cycled to
pressure-cycled mode
• Maintained at <45cm H2O to
minimize barotrauma
•Plateau pressures
• Pressure measured at the end
of inspiratory phase
• Maintained at <30-35cm H2O to
minimize barotrauma

Alternative Modes
• I:E inverse ratio ventilation (IRV)
• ARDS and severe hypoxemia
• Prolonged inspiratory time (3:1) leads to
better gas distribution with lower PIP
• Elevated pressure improves alveolar
recruitment
• No statistical advantage over PEEP, and
does not prevent repetitive collapse and
reinflation
• Prone positioning
• Addresses dependent atelectasis
• Improved recruitment and FRC, relief of
diaphragmatic pressure from abdominal
viscera, improved drainage of secretions
• Logistically difficult
• No mortality benefit demonstrated
• ECHMO
• High-Frequency Oscillatory
Ventilation (HFOV)
• High-frequency, low amplitude
ventilation superimposed over
elevated Paw
• Avoids repetitive alveolar open and
closing that occur with low airway
pressures
• Avoids overdistension that occurs
at high airway pressures
• Well tolerated, consistent
improvements in oxygenation, but
unclear mortality benefits
• Disadvantages
• Potential hemodynamic compromise
• Pneumothorax
• Neuromuscular blocking agents

Airway Pressure Release (APR)
 Ventilator supplies a low level of CPAP alternating with a relatively
high level of CPAP
 At intermittent times the higher CPAP level is reduced for one or
more breaths, permitting rapid exhausting of the gas in the
expiratory reserve volume and resulting in a higher tidal volume
Advantage:
Venous return preserved
Barotrauma and overdistension is minimized
Permits spontaneous breaths
Disadvantage: requires high CPAP, suited only for post op & mildly
diseased lungs

Treatment of respiratory failure
•Prevention
• Incentive spirometry
• Mobilization
• Humidified air
• Pain control
• Turn, cough, deep breathe
•Treatment
• Medications
• B-2 Agonist
• Theophylline
• Steroids
• CPAP, BiPAP
• Intubation
The critical period before the patient needs to be intubated

Indications for intubation
•Criteria
• Clinical deterioration
• Tachypnea: RR >35
• Hypoxia: pO2<60mm Hg
• Hypercarbia: pCO2 > 55mm Hg
• Minute ventilation<10 L/min
• Tidal volume <5-10 ml/kg
• Negative inspiratory force <
25cm H2O (how strong the pt
can suck in)
•Initial vent settings
• FiO2 = 50%
• PEEP = 5cm H2O
• RR = 12 – 15 breaths/min
• VT = 10 – 12 ml/kg
• COPD = 10 ml/kg (prevent
overinflation)
• ARDS = 8 ml/kg (prevent volutrauma)
• Permissive hypercapnea
• Pressure Support = 10cm H2O
How the values trend should significantly impact clinical decisions

Indications for extubation
•Clinical parameters
• Resolution/Stabilization of
disease process
• Hemodynamically stable
• Intact cough/gag reflex
• Spontaneous respirations
• Acceptable vent settings
• FiO2< 50%, PEEP < 8, PaO2
> 75, pH > 7.25
•General approaches
• SIMV Weaning
• Pressure Support Ventilation
(PSV) Weaning
• Spontaneous breathing trials
and use of T-piece
• Demonstrated to be superior
No weaning parameter completely accurate when used alone
Numerical
Parameters
Normal
Range
Weaning
Threshold
P/F > 400 > 200
Tidal volume 5 - 7 ml/kg 5 ml/kg
Respiratory rate 14 - 18 breaths/min < 40 breaths/min
Vital capacity 65 - 75 ml/kg 10 ml/kg
Minute volume 5 - 7 L/min < 10 L/min
Greater Predictive
Value
Normal
Range
Weaning
Threshold
NIF (Negative
Inspiratory Force)
> - 90 cm H2O > - 25 cm H2O
RSBI (Rapid
Shallow Breathing
Index) (RR/TV)
< 50 < 100
Marino P, The ICU Book (2/e). 1998.

Spontaneous Breathing Trials
•Settings
• PEEP = 5, PS = 0 – 5, FiO2 < 40%
• Breathe independently for 30 –
120 min
• ABG obtained at end of SBT
•Failed SBT Criteria
• RR > 35 for >5 min
• SaO2 <90% for >30 sec
• HR > 140
• Systolic BP > 180 or < 90mm Hg
• Sustained increased work of
breathing
• Cardiac dysrhythmia
• pH < 7.32
SBTs do not guarantee that airway is stable or pt can self-clear secretions
Causes of Failed
SBTs
Treatments
Anxiety/Agitation Benzodiazepines or haldol
Infection Diagnosis and tx
Electrolyte abnormalities
(K+, PO4-)
Correction
Pulmonary edema, cardiac
ischemia
Diuretics and nitrates
Deconditioning,
malnutrition
Aggressive nutrition
Neuromuscular disease Bronchopulmonary hygiene,
early consideration of trach
Increased intra-abdominal
pressure
Semirecumbent positioning,
NGT
Hypothyroidism Thyroid replacement
Excessive auto-PEEP
(COPD, asthma)
Bronchodilator therapy
Sena et al, ACS Surgery: Principles and
Practice (2005).

T-piece trial
Advantage
 Simple
 Provides strenuous exercise to the ventilatory muscles
followed by periods of full ventilatory muscle rest;
theoretically the best way to condition the inspiratory
muscles

Continued ventilation after successful SBT
•Commonly cited factors
• Altered mental status and inability to
protect airway
• Potentially difficult reintubation
• Unstable injury to cervical spine
• Likelihood of return trips to OR
• Need for frequent suctioning
Inherent risks of intubation balanced against continued need for intubation

Need for tracheostomy
•Advantages
• Issue of airway stability can be
separated from issue of readiness
for extubation
• May quicken decision to extubate
• Decreased work of breathing
• Avoid continued vocal cord injury
• Improved bronchopulmonary
hygiene
• Improved pt communication
•Disadvantages
• Long term risk of tracheal stenosis
• Procedure-related complication
rate (4% - 36%)
Prolonged intubation may injure airway and cause airway edema
1 - Vocal cords. 2 - Thyroid cartilage. 3 - Cricoid
cartilage. 4 - Tracheal cartilage. 5 - Balloon cuff.

Ventilator management algorithim
Initial intubation
• FiO2 = 50%
• PEEP = 5
• RR = 12 – 15
• VT = 8 – 10 ml/kg
SaO2 < 90% SaO2 > 90%
SaO2 > 90%
• Adjust RR to maintain PaCO2 = 40
• Reduce FiO2 < 50% as tolerated
• Reduce PEEP < 8 as tolerated
• Assess criteria for SBT daily
SaO2 < 90%
• Increase FiO2 (keep SaO2>90%)
• Increase PEEP to max 20
• Identify possible acute lung injury
• Identify respiratory failure causes
Acute lung injury
No injury
Fail SBT
Acute lung injury
• Low TV (lung-protective) settings
• Reduce TV to 6 ml/kg
• Increase RR up to 35 to keep
pH > 7.2, PaCO2 < 50
• Adjust PEEP to keep FiO2 < 60%
SaO2 < 90% SaO2 > 90%
SaO2 < 90%
• Dx/Tx associated conditions
(PTX, hemothorax, hydrothorax)
• Consider adjunct measures
(prone positioning, HFOV, IRV)
SaO2 > 90%
• Continue lung-protective
ventilation until:
• PaO2/FiO2 > 300
• Criteria met for SBT
Persistently fail SBT
• Consider tracheostomy
• Resume daily SBTs with CPAP or
tracheostomy collar
Pass SBT
Airway stable
Extubate
Intubated > 2 wks
• Consider PSV wean (gradual
reduction of pressure support)
• Consider gradual increases in SBT
duration until endurance improves
Prolonged ventilator
dependence
Pass SBT
Pass SBT
Airway stable
Modified from Sena et al, ACS Surgery:
Principles and Practice (2005).

 Afterload
 afterload = wall tension (T) during contraction
where Ptm= transmural pressure, R=radius and
H=wall thickness
 transmural pressure=intraventricular pressure-pleural
pressure
 pleural pressure increased by positive pressure
 therefore transmural pressure and afterload must be
decreased by positive pressure ventilation

Advanced Mechanical
Ventilation

Advanced Mechanical Ventilation Outline
Consequences of Elevated Alveolar Pressure
Implications of Heterogeneous Lung Unit Inflation
Adjuncts to Ventilation
• Prone Positioning
• Recruitment Maneuvers
Difficult Management Problems
• Acute Lung Injury
• Severe Airflow Obstruction
• An Approach to Withdrawing Ventilator Support
Advanced Modes for Implementing Ventilation
Case Scenarios

Consequences of Elevated Alveolar Pressure--1
Mechanical ventilation expands the lungs and chest wall by pressurizing the
airway during inflation. The stretched lungs and chest wall develop recoil
tension that drives expiration.
Positive pressure developed in the pleural space may have adverse effects on
venous return, cardiac output and dead space creation.
Stretching the lung refreshes the alveolar gas, but excessive stretch subjects the
tissue to tensile stresses which may exceed the structural tolerance limits of this
delicate membrane.
Disrupted alveolar membranes allow gas to seep into the interstitial
compartment, where it collects, and migrates toward regions with lower tissue
pressures.
Interstitial, mediastinal, and subcutaneous emphysema are frequently the
consequences. Less commonly, pneumoperitoneum, pneumothorax, and
tension cysts may form.
Rarely, a communication between the high pressure gas pocket and the
pulmonary veins generates systemic gas emboli.

Partitioning of Alveolar Pressure is a Function of Lung
and Chest Wall Compliances
Lungs are smaller and
pleural pressures are
higher when the chest
wall is stiff.

Hemodynamic Effects of Lung Inflation
Lung inflation by positive pressure causes
• Increased pleural pressure and impeded venous return
• Increased pulmonary vascular resistance
• Compression of the inferior vena cava
• Retardation of heart rate increases
These effects are much less obvious in the presence of
• Adequate circulating volume
• Adequate vascular tone
• Spontaneous breathing efforts
• Preserved adrenergic responsiveness

Hemodynamic Effects of Lung Inflation
With Low Lung Compliance, High Levels of
PEEP are Generally Well Tolerated.

Effect of lung expansion on pulmonary vasculature. Capillaries
that
are embedded in the alveolar walls undergo compression even as
interstitial vessels dilate. The net result is usually an increase in
pulmonary vascular resistance, unless recruitment of collapsed units
occurs.

Conflicting Actions of Higher Airway Pressure
Lung Unit Recruitment and Maintenance of Aerated Volume
• Gas exchange
 Improved Oxygenation
 Distribution of Ventilation
• Lung protection
 Parenchymal Damage
 Airway trauma
Increased Lung Distention
• Impaired Hemodynamics
• Increased Dead Space
• Potential to Increase Tissue Stress
…Only If Plateau Pressure Rises

Diseased Lungs Do
Not Fully Collapse,
Despite Tension Pneumothorax
…and
They cannot always
be fully “opened”
Dimensions of a fully
Collapsed Normal Lung

Tidally Phasic Systemic Gas Embolism
End-Expiration End-Inspiration

Consequences of Elevated Alveolar Pressure--2
In recent years there has been intense interest in another and
perhaps common consequence of excessive inflation pressure--
Ventilator-Induced Lung Injury (VILI).
VILI appears to develop either as a result of structural breakdown
of the tissue by mechanical forces or by mechano-signaling of
inflammation due to repeated application of excessive tensile
forces.
Although still controversial, it is generally agreed that damage can
result from overstretching of lung units that are already open or
from shearing forces generated at the junction of open and
collapsed tissue.
Tidally recurrent opening and closure of small airways under high
pressure is thought to be important in the generation of VILI.
Prevention of VILI is among the highest priorities of the ICU
clinician caring for the ventilated patient and is the purpose
motivating adoption of “Lung Protective” ventilation strategies.
Translocation of inflammatory products, bacteria, and even gas
may contribute to remote damage in systemic organs and help
explain why lung protective strategies are associated with lower
risk for morbidity and death.

Recognized Mechanisms of Airspace Injury
“Stretch”
“Shear”
Airway Trauma

Mechanisms of Ventilator-Induced Lung Injury
(VILI)
High airway pressures may injure the lungs by repeated overstretching of
open alveoli, by exposing delicate terminal airways to high pressure, or by
generating shearing forces that tear fragile tissues.
These latter shearing forces tend to occur as small lung units open and
close with each tidal cycle and are amplified when the unit opens only
after high pressures are reached.
To avoid VILI, end-inspiratory lung pressure (“plateau”) should be kept
from rising too high, and when high plateau pressures are required,
sufficient PEEP should be applied to keep unstable lung units from
opening and closing with each tidal cycle.
Independent of opening and closure, tissue strain is dramatically
amplified at the junctions of open and closed lung units when high
alveolar pressures are reached.
Extremely high tissue strains may rip the alveolar gas-blood interface.
Repeated application of more moderate strains incite inflammation.
Such factors as breath frequency, micro-vascular pressure, temperature,
and body position modify VILI expression.

End-Expiration
Tidal Forces
(Transpulmonary and
Microvascular
Pressures)
Extreme Stress/Strain Moderate Stress/Strain
Mechano signaling via
integrins, cytoskeleton, ion channels
inflammatory cascade
Cellular Infiltration and Inflammation
Rupture Signaling
Marini / Gattinoni CCM 2004
Pathways to VILI

Microvascular Fracture in ARDS
A Portal for
Gas &
Bacteria?
Hotchkiss et al
Crit Care Med
2002;
1 

The Problem of Heterogeneity
The heterogeneous nature of regional mechanical properties presents major
difficulty for the clinician, who must apply only a single pressure or flow
profile to the airway opening.
Heterogeneity means that some lung units may be overstretched while others
remain airless at the same measured airway pressure.
Finding just the right balance of tidal volume and PEEP to keep the lung as
open as possible without generating excessive regional tissue stresses is a
major goal of modern practice.
Prone positioning tends to reduce the regional gradients of pleural and trans-
pulmonary pressure.

Spectrum of Regional Opening Pressures
(Supine Position)
Superimposed
Pressure Inflated 0
Alveolar Collapse
(Reabsorption) 20-60 cmH2O
Small Airway
Collapse
10-20 cmH2O
Consolidation 
(from Gattinoni)
Lung Units at Risk for Tidal
Opening & Closure
=
Opening
Pressure

Different lung regions may be overstretched or underinflated, even
as measures of total lung mechanics appear within normal limits.
Alveolar Pressure
LungVolume
UPPER LUNG
TOTAL LUNG
LOWER LUNG

Recruitment Parallels Volume As A
Function of Airway Pressure
Recruitment and
Inflation (%)
Frequency
Distribution of
Opening
Pressures (%)
Airway Pressure (cmH2O)

%
Opening and Closing Pressures in ARDS
50
Paw [cmH2O]
0 5 10 15 20 25 30 35 40 45 50
0
10
20
30
40
Opening
pressure
Closing
pressure
From Crotti et al
AJRCCM 2001.
High pressures may be needed to open some lung units, but once open,
many units stay open at lower pressure.

Dependent to Non-dependent
Progression of Injury

Histopathology of VILI
Belperio et al, J Clin Invest Dec 2002; 110(11):1703-1716

Links Between VILI and MSOF
Biotrauma and Mediator
De-compartmentalization
Slutsky, Chest 116(1):9S-16S

Airway Orientation in Supine Position

Prone Positioning Evens The Distribution of
Pleural & Transpulmonary Pressures

Prone Positioning Relieves Lung Compression
by the Heart
Supine
Prone

Proning May Benefit the Most Seriously Ill
ARDS Subset
SAPS II
MortalityRate
> 49 40- 49 31- 40 0 - 31
0.0
0.1
0.2
0.3
0.4
0.5 Supine
* p<0.05 vs
Supine
Prone
*
Quartiles of SAPS II

Proning Helped Most in High VT Subgroup At
Risk For VILI
VT/Kg
< 8.2 8.2- 9.7 9.7- 12 > 12
0.0
0.1
0.2
0.3
0.4
0.5
SupineMortalityRate
Quartiles of VT /Predicted body
weight
* p<0.05 vs
Supine
Prone
*

How Much Collapse Is Dangerous
Depends on the Plateau
R = 100%
20
60
100
Pressure [cmH2O]
20 40 60
TotalLungCapacity[%]
R = 22%
R = 81%
R = 93%
0
0
R = 0%
R = 59%
From Pelosi et al
AJRCCM 2001
Some
potentially
recruitable
units open only
at high
pressure
More Extensive
Collapse But
Lower PPLAT
Less Extensive
Collapse But
Greater PPLAT

Recruiting Maneuvers in ARDS
The purpose of a recruiting maneuver is to open
collapsed lung tissue so it can remain open during tidal
ventilation with lower pressures and PEEP, thereby
improving gas exchange and helping to eliminate high
stress interfaces.
Although applying high pressure is fundamental to
recruitment, sustaining high pressure is also important.
Methods of performing a recruiting maneuver include
single sustained inflations and ventilation with high
PEEP .

Theoretical Effect of Sustained Inflation on Tidal Cycling
Rimensberger ICM 2000
VOLUME
(% TLC)
Benefit from a recruiting
maneuver is usually
transient if PEEP remains
unchanged afterward.

Three Types of Recruitment Maneuvers
S-C Lim, et al
Crit Care Med 2004

How is the Injured Lung Best Recruited?
Prone positioning
Adequate PEEP
Adequate tidal volume (and/or intermittent ‘sighs’?)
Recruiting maneuvers
Minimize edema (?)
Lowest acceptable FiO2 (?)
Spontaneous breathing efforts (?)

Severe Airflow Obstruction
A major objective of ventilating patients with severe airflow
obstruction is to relieve the work of breathing and to minimize
auto-PEEP.
Reducing minute ventilation requirements will help impressively
in reducing gas trapping.
When auto-PEEP is present, it has important consequences for
hemodynamics, triggering effort and work of breathing.
In patients whose expiratory flows are flow limited during tidal
breathing, offsetting auto-PEEP with external PEEP may even
the gas distribution and reduce breathing effort.

Auto-PEEP Adds To the Breathing Workload
The pressure-volume areas
correspond to the inspiratory
mechanical workloads of auto-PEEP (AP)
flow resistance and tidal elastance.

Gas Trapping in Severe Airflow Obstruction
Disadvantages the respiratory muscles and increases
the work of tidal breathing
Often causes hemodynamic compromise, especially
during passive inflation
Raises plateau and mean airway pressures,
predisposing to barotrauma
Varies with body position and from site to site within the
lung

Volume Losses in Recumbent Positions
Note that COPD patients lose
much less lung volume than
normals do, due to gas
trapping and need to keep the
lungs more inflated to minimize
the severity of obstruction.
Orthopnea may result.

PEEP in Airflow Obstruction
Effects Depend on Type and Severity of Airflow
Obstruction
Generally Helpful if PEEP  Original Auto-PEEP
Potential Benefits
• Decreased Work of Breathing
• Increased VT During PSV or PCV
• (?) Improved Distribution of Ventilation
• (?) Decreased Dyspnea

Inhalation Lung Scans in the Lateral Decubitus Position for a
Normal Subject and COPD Patient
Normal
COPD
No PEEP
PEEP10
Addition of 10 cm H2O PEEP
re-opens dependent airways
in COPD

PEEP
Flow Limitation “Waterfall”

Adding PEEP that approximates auto-PEEP may reduce the
difference in pressure between alveolus (Palv) and airway
opening, thereby lowering the negative pleural (Pes) pressure
needed to begin inspiration and trigger ventilation.

Adding PEEP Lessens the Heterogeneity of End-expiratory
Alveolar Pressures and Even the Distribution of Subsequent
Inspiratory Flow.

PEEP may offset
(COPD) or add to
auto-PEEP
(Asthma),
depending on
flow limitation.
Note that adding
8 cmH2O PEEP
to 10 cmH2O of
intrinsic PEEP
may either
reduce effort
(Pes, solid
arrow) or cause
further hyper-
inflation (dashed
arrow).
Ranieri et al, Clinics in Chest Medicine 1996; 17(3):379-94
ASTHM
A
COP
D

Conventional Modes of Ventilatory Support
The traditional modes of mechanical ventilation—Flow-regulated volume Assist Control
(“Volume Control”, AMV, AC)) or Pressure-Targeted Assist Control (“Pressure Control”),
Synchronized Intermittent Mandatory Ventilation (SIMV)—with flow or pressure
targeted mandatory cycles), Continuously Positive Airway Pressure (CPAP) and
Pressure Support can be used to manage virtually any patient when accompanied by
adequate sedation and settings well adjusted for the patient’s needs. Their properties
are discussed in the “Basic Mechanical Ventilation” unit of this series.

Positive Airway Pressure Can Be Either Pressure or
Flow Controlled—But Not Both Simultaneously
Dependent
Variable
Dependent
Variable
Set Variable
Set Variable

Decelerating flow profile is an option in flow
controlled ventilation but a dependent variable in
pressure control.
Decelerating Flow Pressure Control
Peak pressure is a function of flow;
plateau pressure is not

Patient-Ventilator Interactions
Coordination of the patient’s needs for flow, power, and cycle timing
with outputs of the machine determine how well the ventilator
simulates an auxiliary muscle under the patient’s control.
Flow controlled, volume cycled ventilator modes offer almost
unlimited power but specify the cycle timing and flow profile.
Traditional pressure targeted modes (Pressure Control (PCV) and
Pressure support (PSV)) provide no greater power amplitude than
that set by the clinician but do not limit flow. PCV has a cycle time
fixed by the clinician, and in that sense is as inflexible as VCV.
Pressure Support (PSV) allows the patient to determine both flow
and, within certain limits, cycle length as well. Because flow is
determined by respiratory mechanics as well as by effort,
adjustments may be needed to the inspiratory flow offswitch criterion
of PSV so as to coordinate with the patient’s needs.
Modification of the rate at which the pressure target is reached at the
beginning of inspiration (the ramp or ‘attack’ rate) may be needed to
help ensure comfort

Pressure Support ‘off-switch’ is a set flow value or a set % of
peak inspiratory flow. The patient with airflow obstruction may
need
to put on the brake with muscular effort to slow flow quickly
enough
to satisfy his intrinsic neural timing.

Tapered
inspiratory
‘attack’ rate and a
higher
percentage of
peak flow off
switch criterion
are often more
appropriate in
airflow
obstruction than
are the default
values in PSV.
Airflow Obstruction

Although early flows
are adequate, mid-
cycle efforts may not
be matched by
Decelerating Flow
Control (VCV).
Pressure Controlled
breaths (PCV) do not
restrict flow.
Since the flow
demands of severely
obstructed patients
may be nearly
unchanging in severe
airflow obstruction ,
decelerating VCV
may not be the best
choice.

Interactions Between Pressure Controlled
Ventilation and Lung Mechanics
The gradient of pressure that determines tidal volume is the difference
between end-inspiratory (Plateau) and end-expiratory alveolar pressure
(total PEEP).
In PCV, tidal volume can be reduced if the inspiratory time or the
expiratory time is too brief to allow the airway and alveolar pressures to
equilibrate.
The development of auto-PEEP reduces the gradient for inspiratory flow
and curtails the potential tidal volume available with any given set
inspiratory airway pressure.
These conditions are most likely to arise in the setting of severe airflow
obstruction. Therefore, modifications of the breathing frequency and
inspiratory cycle period (‘duty cycle’) may powerfully impact ventilation
efficiency.

Alveolar
Pressure
Airway
Pressure
Residual Flows

Deformed Airway Pressure Waveforms
Paw Reflects Effort and Dys-Synchrony During
Constant Flow Ventilation

High Pressure Alarm
Variable Tidal Volume or Pressure Limit Alarm During Pressure Contro

What to Do When the Patent and Ventilator are
“Out of Synch”?
Frequent pressure alarms during Volume Assist Control or Tidal Volume alarms
during Pressure Control both mean that the patient is not receiving the desired
tidal volume.
If not explained by an important underlying change in the circuit properties (e.g.,
disconnection or plug) or in the impedance of the respiratory system, this often
arises from a timing “collision” between the patient and ventilator’s cycling
rhythms.
To quickly regain smooth control without deep sedation, the patient must be
given back some degree of control over cycle timing.
This timing control is conferred by introducing flow off-switched cycles in the
form of high level pressure support. Pressure Support alone, or SIMV at a
relatively low mandated rate together with PSV, are the logical choices to provide
adequate power, achieve flow synchrony and yield cycle timing to the patient.
Pressure Controlled SIMV, where the inspiratory time of the PCV cycle is set to
be a bit shorter than the observed PSV cycle, is a good strategy for this purpose.

Advanced Interactive Modes of
Mechanical Ventilation
Airway pressure release/BiPAP/Bi-Level
Combination or “Dual control” modes
Proportional assist ventilation
Adaptive support ventilation
Automatic ET tube compensation

Combination “Dual Control” Modes
Combination or “dual control” modes combine features of pressure
and volume targeting to accomplish ventilatory objectives which
might remain unmet by either used independently.
Combination modes are pressure targeted
 Partial support is generally provided by pressure support
 Full support is provided by Pressure Control

Combination “Dual Control” Modes
Volume Assured Pressure Support
(Pressure Augmentation)
Volume Support
(Variable Pressure Support)
Pressure Regulated Volume Control
(Variable Pressure Control, or Autoflow)
Airway Pressure Release
(Bi-Level, Bi-PAP)

Pressure Regulated Volume Control
Characteristics
Guaranteed tidal volume using “pressure control”
waveform
Pressure target is adjusted to least value that satisfies
the targeted tidal volume minimum
Settings:
Minimum VE
Minimum Frequency
Inspiratory Time per Cycle

Compliance Changes During Pressure
Controlled Ventilation

PRVC Automatically Adjusts To
Compliance Changes

Several modes allow the physician to allow for variability in patient efforts while
achieving a targeted goal. Volume support monitors minute ventilation and tidal
volume , changing the level of pressure support to achieve a volume target. Volume
assured pressure support allows the patient to breathe with pressure support,
supplementing the breath with constant flow when needed to achieve the targeted
tidal volume within an allocated time. Proportional assist (see later) varies pressure
output in direct relation to patient effort.

Airway Pressure Release and Bi-Level
Airway Pressure
Over the past 20 years, modes of ventilation that move the airway pressure
baseline around which spontaneous breaths occur between two levels have been
employed in a variety of clinical settings.
Although the machine’s contribution to ventilation may resemble inverse ratio
ventilation, patient breathing cycles occur through an open (as opposed to
closed) circuit, so that airway pressure never exceeds that value desired by the
clinician.
Transpulmonary pressure—the pressure across the lung—can approach
dangerous levels, however, and mean airway pressure is relatively high.

Inverse Ratio Airway Pressure Release
(APRV), and Bi-Level (Bi-PAP)

Bi-Pap &Airway Pressure Release
Characteristics
Allow spontaneous breaths superimposed on a set
number of “pressure controlled” ventilator cycles
Reduce peak airway pressures
“Open” circuit / enhanced synchrony between patient
effort and machine response
Settings:
Pinsp and Pexp (Phigh and Plow)
Thigh and Tlow

Unlike PCV, BiPAP Allows Spontaneous
Breathing During Both Phases of Machine’s
Cycle

Bi-Level Ventilation
With Pressure Support

Modes That Vary Their Output to Maintain
Appropriate Physiology
Proportional Assist Ventilation
(Proportional Pressure Support)
- Support pressure parallels patient effort
Adaptive Support Ventilation
- Adjusts Pinsp and PC-SIMV rate to meet
“optimum” breathing pattern target
Neurally Adjusted Ventilatory Assist
- Ventilator output is keyed to neural signal

Proportional Assist Ventilation (PAV)
Proportional assist ventilation attempts to regulate the pressure output of the
ventilator moment by moment in accord with the patient’s demands for flow
and volume.
Thus, when the patient wants more, (s)he gets more help; when less, (s)he
gets less. The timing and power synchrony are therefore nearly optimal—at
least in concept.
To regulate pressure, PAV uses the monitored flow and a calculated
assessment of the mechanical properties of the respiratory system as inputs
into the equation of motion of the respiratory system.
More than with any other currently available mode, PAV acts as an auxiliary
muscle whose strength is regulated by the proportionality constant
determined by the clinician.
At present, PAV cannot account for auto-PEEP, and there is a small chance
for inappropriate support to be applied.

Proportional Assist Amplifies Muscular Effort
Muscular effort (Pmus) and
airway pressure assistance
(Paw) are better matched for
Proportional Assist (PAV)
than for Pressure Support
(PSV).

Goals of Adaptive Support Ventilation
Reduce Dead-space Ventilation
Avoid Auto-PEEP
Discourage Rapid Shallow Breathing
Mirror Changing Patient Activity

Adaptive Lung Ventilation
Synchronized Intermittent Pressure Control
Rate
Inspiratory Pressure
PSV cycling characteristics
Physician Settings
Ideal body weight
% Minute Volume
Maximal Allowed Pressure

Neural Control of Ventilatory Assist
(NAVA)
Although still in its development phase, the possibility of
controlling ventilator output by sensing the neural traffic flowing
to the diaphragm promises to further enhance synchrony.
NAVA senses the desired assist using an array of esophageal
EMG electrodes positioned to detect the diaphragm’s
contraction signal.
The reliability of neurally-controlled ventilator assistance needs
to be determined before its deployment in the clinical setting.

Neural Control of Ventilatory Assist (NAVA)
Neuro-VentilatoryCoupling
Central Nervous System

Phrenic Nerve

Diaphragm Excitation

Diaphragm Contraction

Chest Wall and Lung
Expansion

Airway Pressure, Flow and
Volume
New
Technology
Ideal
Technology
Current
Technology
Ventilator
Unit

Electrode Array in Neurally Adjusted
Ventilatory Assist (NAVA)
Sinderby et al, Nature Medicine; 5(12):1433-1436

NAVA Provides Flexible Response to Effort
Volume
PAW
DGM
EMG
Sinderby et al, Nature Medicine; 5(12):1433-
1436

Automatic Tube Compensation
The endotracheal tube offers resistance to ventilation both on
inspiration and on expiration.
A low level of pressure support can help overcome this pressure
cost, but its effect varies with flow rate.
Automatic tube compensation (ATC) adjusts its pressure output in
accordance with flow, theoretically giving an appropriate amount
of pressure support as needed as the cycle proceeds and flow
demands vary within and between subsequent breaths.
Some variants of ATC drop airway pressure in the early portion of
expiration to help speed expiration.
Supplemental pressure support can be provided to assist in tidal
breath delivery.

External and Tracheal Pressures Differ
Because of Tube Resistance
ATC offsets a fraction of tube
resistance

Valve Control Maintains Tracheal Pressure
During ATC
Pressure
Support
Pressure
Support
ATC ATC
Fabry et al, ICM 1997;23:545-552

ATC Adjusts Inspiratory Pressure to Need
Postop
Critically
Ill

Discontinuation of Mechanical Ventilation
To discontinue mechanical ventilation requires:
 Patient preparation
 Assessment of readiness
 For independent breathing
 For extubation
 A brief trial of minimally assisted breathing
 An assessment of probable upper airway patency after extubation
 Either abrupt or gradual withdrawal of positive pressure,
depending on the patient’s readiness

Preparation: Factors Affecting
Ventilatory Demand

The frequency to tidal volume ratio (or rapid shallow breathing index, RSBI) is a
simple and useful integrative indicator of the balance between power supply and
power demand. A rapid shallow breathing index < 100 generally indicates adequate
power reserve. In this instance, the RSBI indicated that spontaneous breathing
without pressure support was not tolerable, likely due in part to the development of
gas trapping.
Even when the mechanical requirements of the respiratory system can be met by
adequate ventilation reserve, congestive heart failure, arrhythmia or ischemia
may cause failure of spontaneous breathing.

Integrative Indices Predicting Success

Measured Indices Must Be Combined With
Clinical Observations

Three Methods for Gradually
Withdrawing Ventilator Support
Although the majority of patients do not require gradual withdrawal of ventilation,
those that do tend to do better with graded pressure supported weaning than
with abrupt transitions from Assist/Control to CPAP or with SIMV used with only
minimal pressure support.

Extubation Criteria
Ability to protect upper airway
 Effective cough
 Alertness
Improving clinical condition
Adequate lumen of trachea and larynx
 “Leak test” during airway pressurization with the
cuff deflated

Drager Savina
Modes and Waveforms

Terms
 Vt
 Set volume
 ∆P
 PIP
 Vt dependent upon C/Raw
 f
 Set
 Spontaneous
 MV = Vt x f
 PEEP
 Ti
 Set
 Patient regulated

April 17 Advanced Core
Effects of PIP and PEEP on Vt

Mean Airway Pressure
(Paw, MAP)

Reminders
 Ventilation relates to CO2
 Managed by f and Vt (MV)
 Oxygenation
 Function of MAP and FiO2

 Preset volume is delivered to patient
 Inspiration ends once volume is delivered
 Volume constant, pressure variable
 Ensures desired amount of air is delivered to
lungs regardless of lung condition
 May generate undesirable(high)
airway pressures
Volume –Targeted Ventilation

 Preset inspiratory pressure is delivered to patient
 Pressure constant, volume variable
 Clinician determines ventilating pressures
 Volumes may increase or decrease in response
to changing lung conditions
Pressure-Targeted Ventilation

Breath Types
 Two basic breath types
 Mandatory breaths – delivered according to set
ventilator parameters
 Demand breaths (spontaneous) – triggered by the
patient

Phase Variables
A Trigger: START
Patient (Assisted)-
flow or pressure
Machine (Time controlled)
B Limit: TARGET
Volume
Pressure
C Cycle: STOP
Time
Flow
AB
B
B
A
C
With permission: Rob Diblasi, RRT-NPS, Clinical Specialist, Viasys

Continuous Mandatory Ventilation
CMV
 Controlled Ventilation
 Can be volume oriented or pressure oriented
 Intended for patients without spontaneous breathing
 Inspiration is initiated by timing device.
 Machine controlled breath
Set:
 Vt (tidal volume) or PIP
 f (frequency/rate) (Vt x f = MV)
 Ti (inspiratory time)
 PEEP

Press
ure
Flow
Volume
(L/min)
(cm H2O)
(ml)
Time (sec)
Time-Cycled
Set PC level
From CD “Esssentials of Ventilator Graphics”. ©2000 RespiMedu. With permission
Control Mode
(Pressure-Targeted)

Pressure Assist Control
(Controlled)

CMVAssist
 Synchronized mandatory breaths with
spontaneous breathing
 Breaths triggered by patient unless rate falls
below set level
 Actual rate may be > set f (due to patient
triggering vent
 Each breaths pressure of volume is pre set

Assist-Control Mode
(Pressure-Targeted Ventilation)
Pressure
Flow
Volume
(L/min)
(cm H2O)
(ml)
Set PC level
Time (sec)
Time-Cycled
Patient Triggered, Pressure Limited, Time Cycled Ventilation

Assist-Control Mode
(Volume-Targeted Ventilation)
Time (sec)
Flow
(L/m)
Pressure
(cm H2O)
Volume
(mL)
Preset VT
Volume Cycling
Patient triggered, Flow limited, Volume cycled Ventilation

Pressure Assist Control
(Assisted)
Paw
Paw
Paw

SIMV
 Synchronized Intermittent
Mandatory Ventilation
 Fixed pattern and rate
 Synchronized with patient’s
spontaneous breathing
 Patient’s may breathe between
ventilator breaths, but breaths
are not supported
 Used for weaning
 Apnea back up rate

SIMV
Spontaneous Breaths
Flow
(L/m)
Pressure
(cm H2O)
Volume
(mL)

Pressure Support
 Decelerating, variable inspiratory flow (varies
depending upon patient needs)
 No set rate. Patient initiates all breaths
 Flow triggered
 Set pressure above PEEP (Delta P)
 Pressure limited
 Patient initiates exhalation
 Flow cycled
 Set PEEP
 All breaths supported
 Apnea back up available
 Commonly used as a “weaning mode”

Pressure Support (PS)
 Augmented pressure for
insufficient spontaneous breathing
 Triggered by flow
Or
 Inspired vol > 25 ml
 Terminated by insp flow of 0
during phase I
Or
 Insp flow phase II <25% peak insp
flow of previous breath
Or
 4 sec

Pressure Support Ventilation
Paw CPAP
Paw PS

Flow Synchronized
Ventilation
 Aka “flow cycle”- allows patient to determine
their own I- time by terminating the breath
once a certain percentage of the peak
inspiratory flow is met
 May improve preload and eliminate V/Q
mismatching
 Improves patient/ventilator dsy-synchrony
 May improve oxygenation and ventilation in
spontaneously breathing patients

Synchronized IMV with PS
(SIMV/PS)
 Constant inspiratory pressure
 Decelerating, variable inspiratory flow rate
 SIMV breaths
 Set minimum rate (Mandatory breaths):
 Synchronized with patients spontaneous efforts
 Set pressure above PEEP (Delta P)
 Time cycled or Flow cycled
 PS breaths
 Spontaneous efforts are supported with lower ∆P
 Flow cycled with Set Max Ti
 Set PEEP (same for SIMV and PS breaths)
 Used for “acute” phase or as a “weaning” mode
 Weaning commonly consists of weaning IMV rate and then
IMV ∆P

SIMV + PS
 PS adds to SIMV
 Provides augmentation of unsupported
spontaneous breath
 Each breath is a pt triggered breath
 Vol. delivered during mechanical breaths is
preset
 Vol delivered by PS breath is dependent on
 Level of PS
 Pt’s lung compliance
 Airway resistance
 Pt’s inspiratory effort
 Flow-time curve
 flow pattern of spontaneous breaths is
decelerating instead of rounded-shape as in
unsupported spontaneous breaths
 Pressure-time curve
 indicates the level of set pressure support
 PS level will be maintained throughout
inspiration. Essentials of Ventilator Graphics ©2000 RespiMedu
SIMV+PS
SIMV+PS
Time (sec)Time (sec)
FlowFlow
PressurePressure
VolumeVolume
(L/min)(L/min)
(cm H(cm H22O)O)
(ml)(ml)
Set PSlevelSet PSlevel

Press
ure
Flow
Volume
(L/min)
(cm H2O)
(ml)
SIMV + PS
(Pressure-Targeted Ventilation)
PS Breath
Set PS level
Set PC level
Time (sec)
Time-Cycled
Flow-Cycled

Paw
Paw
SIMV/PS
Mandatory Breaths
SIMV: Mandatory (patient or machine init
Spontaneous breaths

SIMV/PS
Spontaneous Breaths

 Preset pressure is maintained in the airway
 Patient must breathe spontaneously - no mechanical
breaths delivered
 “breathing at an elevated baseline”
 Increases lung volumes, improves oxygenation.
Continuous Positive Airway Pressure
(CPAP)

CPAP
Flow
Pressure
Volume
(L/min)
(cm H2O)
(ml)
CPAP level
Time (sec)

PCV+ (BIPAP)
 A pressure controlled /time cycled
mode
 Patient can breath spontaneously any
time
 So, a time cycled alternation between
two CPAP levels
 Time cycled change of pressure
produces controlled vent similar to
PCV
 Non-invasive ventilation
 Set IPAP to obtain level of pressure
support
 Improve ventilation
 Set EPAP to obtain level of CPAP
 Improve oxygenation

Consequences of
Asynchrony
 “Fighting” the ventilator
 Inconsistent Vt delivery
 Increased work of breathing
 Inefficient gas exchange
 Increased caloric expenditure
 Increased oxygen demand
 Barotrauma
 Disturbances in cerebral perfusion

Trigger Sensitivity- Inappropriate
Flow Trigger
e
Sensitivity level
Time
Flow
Time
Patient Effort

Trigger Sensitivity- Appropriate Flow
Trigger

Air leak -
Related to ETT or Circuit
Volume
Pressure
Flow
Volume
Time

Air leak-
Related to ETT or Circuit
Volume Flow
Volume
Time

Autocycling- Secondary to Leak

Resolved Autocycle- Flow
Trigger Increased

Excessive Inspiratory
Time
 Created when Inspiratory time exceeds the time
constants of the lung or when active exhalation occurs
 May increase WOB and “Fighting” of the ventilator
 May increase intra-thoracic pressure compromising
cardiovascular status
 May result in an insufficient expiratory time and gas
trapping
 May cause hypercarbia

Excessive Inspiratory Time
Inspiratory Time= .5 secs

Excessive Inspiratory Time

Insufficient Expiratory Time
 Expiratory flow is unable to return to
baseline prior to the initiation of the next
mechanical breath
 Incomplete exhalation causes:
 gas trapping
 dynamic hyper-expansion
 development of intrinsic PEEP
 Can be fixed by decreasing I-time

Gas Trapping with Inappropriate
Inspiratory Time
Inspiratory Time 1.2 s
Inspiratory Time 0.8 s

Air Trapping
Inspiration
Expiration
Normal
Patient
Time (sec)
Flow(L/min)
Air Trapping
Auto-PEEP
}

Before and After
Flow Cycle Added
Before
After

Increased Expiratory
Resistance
 Prolonged expiratory flow indicates an
obstruction to exhalation
 Possible causes include:
 Obstruction of a large airway
 Bronchospasm
 Secretions
 Bias flow too high

Increased Expiratory Resistance
Normal Resistance
Increased Resistance

Response to Bronchodilator
Before
Time (sec)
Flow(L/min)
PEFR
After
Long TE
Higher PEFR Shorter TE

Airway Obstruction- Secretions

Airway Obstruction- Secretions
BEFORE SXN AFTER SXN

Insufficient PSV
Negative
hysteresis

Rise Time- Slow

Rise Time- Fast

Lung Compliance Changes and
the P-V Loop
Volume (mL)
Preset PIP
VTlevels
Paw (cm H2O)
COMPLIANCE
Increased
Normal
Decreased
PressureTargetedVentilation

Flow-Volume Loop
Volume (ml)
PEFR
FRC
Inspiration
Expiration
PIFR
VT

Airway Secretions/
Water in the Circuit
Inspiration
Expiration
Volume (ml)
Flow
(L/min)
Normal
Abnormal

Air Trapping
Inspiration
Expiration
Volume (ml)
Flow
(L/min)
Does not return
to baseline
Normal
Abnormal

Air Leak
Inspiration
Expiration
Volume (ml)
Flow
(L/min)
Air Leak in mL
Normal
Abnormal

References
 Sunil K Sinha, Steven M Donn Weaning from assisted ventilation: art or science?
Arch Dis Child Fetal neonatal Ed 2000;83:F64-F70
 Steven M Donn, Sunil K Sinha Newer modes of mechanical ventilation for the
neonate. Pediatrics 2001, 13:99-103
 Martin Keszler Pressure support ventilation and other approaches to overcome
imposed work of breathing. Neo Reviews Vol. 7 No. 5 May 2006
 March C. Mammel Editorial: New modes of neonatal ventilation: Let there be
light. Journal of Perinatology (2005) 25, 624-625
 Amal Jubran Critical illness and mechanical ventilation: Effects on the diaphragm.
Respiratory Care Sept. 2006 Vol 51 No 9
 Senaida C. Reyes, Nelson Claure, Markus Tauscher, Carmen D’Ugard, Silvia
Vanbuskirk, Eduardo Bancalari Randomized, controlled trial comparing
synchronized intermittent mandatory ventilation and synchronized intermittent
mandatory ventilation plus pressure support in preterm infants. Pediatrics
2006:118:1409-1417
 Waldo Osorio, Nelson Claure, Carmen D’Ugard, Lamlesh Athavale, Eduardo
Bancalari Effects of pressure support during and acute reduction of synchronized
intermittent mandatory ventilation in preterm infants. Journal of Perinatology
2005: 25:412-416

References
 Yoshitsugu Yamada MD and Hong-Lin Du MD Effects of Different
Pressure Support Termination on Patient-Ventilator Synchrony. Respir
Care 1998;43(12):1048-1057
 Jan Klimiek, Colin Morley, Rosalind lau, Peter Davis Does measuring
respiratory function improve neonatal ventilation? Journal of Paediatrics
and Child Health 42 (2006) 140-142
 Steven M Donn, Sunil Sinha Invasive and Noninvasive neonatal
mechanical ventilation. Respiratory Care April 2003 Vol 48 No 4
 Martin Keszler Volume targeted ventilation Neo Reviews Vol. 7 No. 5
May 2006
 Irfan U Cheema, Jagjit S.Ahluwalia Feasibility of tidal volume guided
ventilation in newborn infants: a randomized, crossover trial using the
volume guarantee modality. Pediatrics 2001; 107;1323-1328
 Viasys Clinical Monograph Series, Avea Modes
 Viasys Clinical Monograph Series, Ventilator Management of neonates
and infants using respiratory mechanics
 Viasys Clinical Monograph Series, New approaches to mechanical
ventilatory support

Auto Flow
 Settings menu
 Insp flow
automatically
adjusted to changes
in C, R and to
patient demands
 Always set high Paw
and Vt alarms

Notes on Setting
Pressure A/C
 Monitor the I:E ratio to ensure it doesn’t inverse
 If it does, ensure Trigger and Ti are appropriately set
 If your “adjustable Ti” is not sufficient to provide a good I:E
ratio, notify the MD. He/she may “order” a Flow Cycle
 Monitor Vte to ensure the Ti is long enough to provide adequate
Vt

Pressure Control
Ventilation
 Constant inspiratory pressure
 Decelerating, variable inspiratory flow rate
 Set pressure above PEEP ((Delta P) for ALL breaths
 Set minimum rate (Mandatory breaths):
 Time cycled (Set Ti) or Flow cycled
 Patients spontaneous breaths (Demand breaths):
 Time cycled (Set Ti) or Flow cycled
 At SAME settings as Mandatory Breaths
 Set PEEP (Same for both Mandatory and Demand breaths)
 All breaths (set + demand) look exactly the same

Pressure Control
 Variable flow capability based on patient demand
 Reduced patient inspiratory muscle workload
 Adjustable inspiratory time (Time or Flow)
 Uniform Vt delivery
 Commonly used for “acute” phase of illness
 Rate commonly set below infants “spontaneous” rate
 Beware I:E ratio and risk of breath stacking, inadvertent PEEP if
Ti not set appropriately
 Weaning from this mode is generally accomplished by lowering
pressure above PEEP (Delta P)

SIMV + PS + PEEP
 In this graph, flow-time and
volume-time curves are similar to
the SIMV + PS
 Pressure-time waveform shows
elevated baseline of pressure.
 This indicates the level of
applied PEEP

Notes on Setting
SIMV/PS
 SIMV
 Inspiratory Rise Time to match PS Rise Time
 SIMV Ti: Flow Term (if used) to match PS Flow Term
 PS
 Rise Time and Flow Term to match SIMV setting
 Max Ti to match SIMV set Ti
 Monitor Vte to ensure Ti is not set too short
 Monitor flow/pressure waveforms to ensure Ti is not too
long

Pressure Control Ventilation
C = VT / PC
Flow
Pressure
Volume
Cl
Cl
Set PC level
Time (sec)
(L/min)
(cm H2O)
(ml)

Controlled Mode
Volume Targeted
Flow
Pressure
Volume
Time (sec)
(L/min)
(cm H2O)
(ml)

Controlled Mode (Pressure-
Targeted Ventilation)
Press
ure
Flow
Volume
(L/min)
(cm H2O)
(ml)
Time (sec)
Time-Cycled
Set PC level
Time Triggered, Pressure Limited, Time Cycled Ventilation

Autocycling?
Things To Try!
 Ensure there is NO leak in circuit
 Perform EST (from Set-Up menu)
 Ensure flow sensor is not wet
 Increase flow trigger sensitivity
 Increase Bias flow 1.5 ml > Trigger Sensitivity
 Set Pressure Trigger
 Remove flow trigger (From patient wye & vent)
 Notify MD of % leak – May choose to ∆ ETT

Flow Cycle

Begin Inspiration
Begin Expiration
Paw(cmH2O)
Time (sec)
Distending
(Alveolar)
Pressure Expiration
Inflation Hold
(seconds)
Begin Expiration
Paw(cmH2O)
Time (sec)
Begin Inspiration
PIP
Pplateau
(Palveolar
Transairway Pressure (PTA)
} Exhalation Valve Opens
Expiration
PIP

CPAP + PSV
Set PS level
CPAP level
Time (sec)
Flow
(L/m)
Pressure
(cm H2O)
Volume
(mL)
Flow Cycling

PIP vs Pplat
Normal High Raw
High Flow
Low Compliance
Time (sec)
Paw(cmH2O)
PIP
PPlat
PIP
PIP PIP
PPlat
PPlat
PPlat
Interpretation of Ventilator Graphics v.1 ©2000 RespiMedu

Basics of mechanical ventilation

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