This presentation deals with the basic physics of human ventillation. I have made an effort to clarify most of the venti lingo , so as to make way for further discussions on ventilator use. Hope it turns out to be helpful for you. Thank you.

1. Presented By : Dr. Sonali Paradhi Mhatre

2. Physiology Of Respiration
 Spontaneous Breathing
The driving force needed to
overcome the elastic airflow resistive
and inertial properties of the
respiratory system is a result of the
Intrapleural pressure changes
generated by the respiratory
muscles.

3. Physics of Spontaneous Respiration :

4. Pressure mechanism during Spontaneous
Respiration :

6. Pressure
mechanism
during
Spontaneous
Respiration :

7. Positive
Pressure
Ventilation
Negative
Pressure
ventilation
Types of Ventilation

8. Negative Pressure Ventilation
 A negative pressure
ventilator, often referred to
colloquially as an iron lung,
is a form of medical
ventilator that enables a
person to breathe when
normal muscle control has
been lost or the work of
breathing exceeds the
person's ability.
 The negative form of
pressure ventilation has
been almost entirely
superseded by positive
pressure ventilation

9. Positive Pressure ventilation
 Inflating the lungs by
inserting Positive
pressure on the airways.
 Alveoli forced to expand
 Air entry in alveoli.
 Thus, Inspiration.

10. Factors influencing the airway
mechanics
Physical Factors
Properties of gas
molecules, flow,
airway diameter,
airway elastic
properties, airway
compliance
Physiological
Factors
Tone of
tracheobronchial
smooth muscles.
Pathological
factors
Airway lumen plugging,
mucosal edema, airway
wall compressions,
tracheo/
bronchomalacia.

12. Tidal Volume : Volume of air inspired / expired in each
breath.
Minute Ventillation : Volume of air inspired / expired during
each minute.
MV = Vt * Breath Rate

13. Anatomic dead space :
volume of gas in the
conducting airways.
Alveolar dead space:
Volume of gas in poorly
ventillated alveoli.
Physiologic dead space
= anatomic D/S + alveolar
D/S

14.  Inspiratory Reserve Volume : Maximal volume of gas that can inspired
from the peak of tidal volume.
 Expiratory Reserve Volume : Maximal volume of gas that can be
expired after a normal tidal expiration.
 Residual volume : Volume of gas that remains in the lung.

15.  Total Lung capacity : Amount of gas in the respiratory system after
maximal inspiration.
TLC = Vt + IRV + ERV + RV
 Vital capacity : Maximal amount of gas that can be expelled from the
lungs after maximal inspiration.
VC = IRV + Vt + ERV

16.  Functional Residual capacity : Volume of air in the lungs when the
respiratory system is at rest (i.e.) Volume of gas in lung at the end of
expiration and before the next inspiration.
FRC = ERV + RV
 FRC is the volume of gas above which a normal tidal volume oscillates.

17. Normal Lung volumes in neonate :
 Tidal volume = 5 – 8 ml/kg
 Dead space volume = 2 – 2.5 ml/kg
 Alveolar space volume = 60 – 320 ml/kg
 Minute ventillation = 200 – 480 ml/kg/ min
Residual volumes and capacity :
 Residual volume = 10 – 15ml/kg
 Functional residual capacity = 25 – 30ml/kg
 Total lung capacity = 50 – 90 ml/kg
 Vital capacity = 35 – 80 ml/kg

19.  Compliance describes the
elasticity or distensibility of
the lungs or respiratory
system (lungs+ chest wall).
 It is also described as the
change in volume per unit
change in pressure.
 Normal lung compliance in a
newborn =
3 - 5 ml/cmH2O/kg.
 In RDS, lung compliance =
0.1 – 1 ml/cmH2O/kg

20.  Resistance describes the ability
of the gas conducting parts of the
lungs or respiratory system (lung+
chest wall) to resists airflow.
 Normal lung resistance =
25 – 50 cm/H2O/L/sec.
 Resistance of lungs are not
markedly changed in RDS or
acute pulmonary disorders , but
can be increased to 100
cm/H2O/L/sec or more by small
sized ET tube use or a nasal
blockage.

21.  The Tc of the respiratory
system is the measure of
the time necessary for the
alveolar pressure to reach
63% of the change in
airway pressure.
 A duration of Inspiration
equal to 3-5 Tc is needed
for a relatively complete
inspiration.
Tc = Raw × C

22.  Thus, Ins and exp duration should be around 240 – 400 ms. (0.24 –
0.4s).
 If the Inspiratory time is short ( <3 – 5 Tc ): Decreased tidal volume
delivery and mean airway pressure.
 If the Expiration time is short (<3 – 5 Tc) :
Gas trapping occurs causing decreased tidal volume, hypercapnia .

24.  Mean airway pressure typically
refers to the mean pressure applied
during positive-pressure mechanical
ventilation.
 The average pressure exerted on the
airway and lungs from the beginning
of inspiration until the beginning of the
next inspiration
 Mean airway pressure correlates
with alveolar ventilation, arterial
oxygenation, hemodynamic
performance, and barotrauma.
MAP = K (PIP – PEEP) (TI / TI+TE) + PEEP

26.  PIP is the pressure gradient between the onset and end of
inspiration.
 This determines the the tidal volume and the minute
ventilation.
 Increase in PIP will increase MAP, PaO2 and cause
increased CO2 elimination.
 S/E : Increased risk of barotrauma , volutrauma and BPD.
It is thus important to adjust PIP as per lung compliance &
thus ventillate with optimum tidal volumes.

27.  PEEP determines the lung volume during expiration phase,
improves the V/Q mismatch and prevents alveolar collapse.
 It also contributes to the pressure gradient and thus affects tidal
volume and minute ventilation.
 Increased PEEP increases FRC, improves oxygenation, increases
MAP.
 S/E : PEEP increase reduces the pressure gradient during
inspiration…thus tidal volume decreases and CO2 elimination
decreased.
 It can cause over distension of the lungs. High PEEP can cause
decreased cardiac output & O2 transport. Increases risk for Air
leak syndromes.

28.  Fraction of inspired oxygen (FiO2) is the fraction or percentage
of oxygen in the space being measured.
 Room air = 21% (0.21)
 Oxygen-enriched air has a higher FiO2 than 0.21, up to 1.00,
which means 100% oxygen. FiO2 is typically maintained below 0.5
even with mechanical ventilation, to avoid Oxygen toxicity.
 Changes in FiO2 changes the alveolar oxygen pressure (i.e.)
oxygenation.
 Very high FiO2 can cause hyperoxic lung damage.
 Recommendations : Target SPO2 = 90 – 95 % in preterms.

29.  Rate determines the minute ventilation and thus affects CO2
elimination.
 Ventillation at high rates (>60/min) may help with synchronization
of ventillator and spontaneous patient breaths.
 Infants with smaller and less compliant lungs tend to breath faster.
 High rates are associated with insufficient inspiratory time/
expiratory time (i.e.) low TV/ air trapping can occur respectively.

30.  The effects of changes in inspiratory and expiratory times on gas
exchange are influenced strongly by the relations of these times to
the inspiratory and expiratory time constants, respectively.
 An inspiratory time 3-5 times longer than the time constant of the
respiratory system allows relatively complete inspiration. A long
inspiratory time increases the risk of pneumothorax. Shortening
inspiratory time is advantageous during weaning.
 Patients with chronic lung disease may have a prolonged time
constant. In these patients, a longer inspiratory time (closer to 0.8
second) may result in improved tidal volume and better carbon dioxide
elimination.

31.  The major effect of an increase in the I:E ratio is an
increased MAP and thus improved oxygenation.
 Physiologic ratio is 1:1 to 1:1.5
 For minimum FiO2 and PEEP requirement, reversed I:E ratio
are recommended.
 Prolonged expiratory rates (I:E = 1:2 to 1:3) are preferred
during MAS and weaning.

32.  Flow is the rate of volume delivery .
 Suboptimal flow can cause Rheotrauma.
 Inadequate flow can cause air hunger, asynchrony & increased
work of breathing.
 Excess flow may contribute to turbulence, hyperinflation, inefficient
gas exchange & Inadvernt PEEP.
 Minimum flow of atleast 2 times of minute ventilation volume is
recommended. Usually started at flow of 4 – 8 L/min.

33. Ventillator
Control variables
Pressure control
Volume control
Flow control
Phase Variables
Trigger
Limit
Cycle
Baseline pressure
(PEEP)

34. Pressure
control
• Controls the
airway
pressure more
than the body
surface
pressure.
• The ventilator
maintains the
same pressure
waveform, at
the mouth
regardless of
changes in
lung
characteristics.
Volume
controller
• Ventilator volume
delivery and
volume waveform
remain constant
and are not
affected by
changes in lung
characteristics.
• Volume is
measured.
Flow controller
• Venti controls the
tidal volume but
does not directly
measure it.
• Ventilator volume
delivery and
volume waveform
remain constant
and are not
affected by
changes in lung
characteristics.
Time controller
• This type controls
the timing of the
venti cycle but
not the pressure/
volume.
• Pressure, volume,
and flow curves
can change as
lung
characteristics
change. Time
remains constant.
• Most high
frequency
ventillators are
time controlled.

35. Ventilatory Phases
Inspiration
Starts
(TRIGGER
POINT)
Maximum
Inspiration
(LIMIT)
Inspiration
ends &
Expiration
starts
(CYCLE)
Expiration
ends
(BASELINE
PRESSURE)

36. Trigger
 This is the variable which is used by the ventilator and used to trigger inspiration.
 Inspiration begins when the variables reach the preset value point.
Pressure
trigger
Flow trigger
Time
trigger
1. Time : Ventillator gives a breath at the
predefined time.
Used in Intermittent mandatory ventilation
2. Pressure : When inspiratory effort is
detected as a change in the end expiratory
pressures, venti is triggered to start
inspiration.
3. Flow trigger : Involves less patient effort
and usually used in infant ventilators.

37. Other parameters
Limit :
 During inspiration, the pressure
,volume, flow increases.
 Limits the restriction value, but
does not limit duration.
 Many neonatal ventilators are
PRESSURE LIMITED.
Cycle :
Inspiration is terminated after a
preset tidal volume has been
delivered by the ventilator.
Many ventillators, including
HFOV are time cycled.
Flow cycles can also be used.