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• DR. SUMAN KHAWAS
• DNB TRAINEE, 1ST YEAR
• BOKARO GENERAL HOSPITAL
- Mechanical ventilation (MV) is used to assist or replace
- It is implemented with special devices that can support
ventilatory function and improve oxygenation through the
application of high-oxygen-content gas and positive pressure.
• The primary indication for initiation of MV is respiratory failure, of
which there are two basic types:
• (1) hypoxemic, which is present when arterial O2 saturation (Sao2)
<90% occurs despite an increased inspired O2 fraction and usually
results from ventilation-perfusion mismatch or shunt; and
• (2) hypercarbic, which is characterized by elevated arterial carbon
dioxide partial pressure (PCO2) values (usually >50 mmHg) resulting
from conditions that decrease minute ventilation or increase
physiologic dead space such that alveolar ventilation is inadequate
to meet metabolic demands.
-Acute respiratory failure with hypoxemia (acute respiratory distress syndrome,
heart failure with pulmonary edema, pneumonia,
complications of surgery and trauma),
which accounts for ~65% of all ventilated cases.
-Hypercarbic ventilatory failure—e.g., due to coma(15%), exacerbations of chronic
obstructive pulmonary disease (COPD; 13%),
and neuromuscular diseases (5%)
-Used to reduce cerebral blood flow in patients with increased intracranial pressure .
-Used frequently in conjunction with endotracheal intubation for airway protection to
prevent aspiration of gastric contents in otherwise unstable patients during gastric
lavage for suspected drug overdose or during gastrointestinal endoscopy.
-In critically ill patients, intubation and MV may be indicated before the performance
of essential diagnostic or therapeutic studies if it appears that respiratory failure
may occur during those maneuvers.
Contraindications for Noninvasive
TYPES OF MECHANICAL VENTILATION
• noninvasive ventilation (NIV)
• invasive (or conventional mechanical)
INVASIVE MECHANICAL VENTILATION
• Conventional MV is implemented once a cuffed tube is inserted into the
trachea to allow conditioned gas (warmed, oxygenated, and humidified) to
be delivered to the airways and lungs at pressures above atmospheric
• avoid brain-damaging hypoxia
• administration of mild sedation may facilitate the procedure. Opiates and
benzodiazepines are good choices (S/E ON HEMODYNAMICS: DEPRESSED
CARDIAC FUNCTION AND LOW SVR)
• Morphine can promote histamine release from tissue mast cells and may
worsen bronchospasm in patients with asthma :
• fentanyl, sufentanil, and alfentanil are acceptable alternatives
• Ketamine increases systemic arterial pressure and cause hallucinatory
• shorter-acting agents—etomidate and propofol—have been used for both
induction and maintenance; have fewer adverse hemodynamic effects; but
are more expensive.
PRINCIPLES OF MV
• Once the patient has been intubated, the basic
goals of MV are to optimize oxygenation while
avoiding ventilator-induced lung injury due to
overstretch and collapse/re-recruitment.
• K.a. “protective ventilatory strategy”
• To prevent: high airway pressures and volumes
and overstretching of the lung as well as
collapse/re-recruitment which leads to poor
clinical outcomes (barotrauma and volume
MODES OF VENTILATION
• Mode refers to the manner in which ventilator breaths are triggered,
cycled, and limited .
• Trigger, either an inspiratory effort or a time-based signal, defines
what the ventilator senses to initiate an assisted breath.
• Cycle refers to the factors that determine the end of inspiration. For
example, in volume-cycled ventilation, inspiration ends when a specific
tidal volume is delivered. Other types of cycling include pressure
cycling and time cycling.
• Limiting factors are operator-specified values, such as airway
pressure, that are monitored by transducers internal to the ventilator
circuit throughout the respiratory cycle ; if the specified values are
exceeded, inspiratory flow is terminated, and the ventilator circuit is
vented to atmospheric pressure or the specified pressure at the end of
expiration (positive end-expiratory pressure, or PEEP)
Assist-Control Mechanical Ventilation
• ACMV is the most widely used mode of ventilation
• an inspiratory cycle is initiated either by the patient’s inspiratory effort
or, if none is detected within a specified time window, by a timer signal
within the ventilator.
• operator-specified tidal volume
• ACMV is commonly used for initiation of MV because it ensures a
backup minute ventilation in the absence of an intact respiratory drive
and allows for synchronization of the ventilator cycle with the patient’s
• PROBLEMS: Respiratory alkalemia , myoclonus or seizures
• Dynamic hyperinflation leading to increased intrathoracic pressures
(so-called auto-PEEP) if the patients respiratory mechanics are such
• inadequate time is available for complete exhalation between
• Auto-PEEP can limit venous return, decrease cardiac output, and
increase airway pressures, predisposing to barotrauma.
Intermittent Mandatory Ventilation
• the operator sets the number of mandatory breaths
of fixed volume to be delivered by the ventilator;
between those breaths, the patient can breathe
• In synchronized mode (SIMV):
• mandatory breaths are delivered in synchrony with the
patient’s inspiratory efforts at a frequency determined
by the operator.
• If the patient fails to initiate a breath, the ventilator
delivers a fixed-tidal-volume breath and resets the
internal timer for the next inspiratory cycle.
• SIMV differs from ACMV in that only a preset number
of breaths are ventilator-assisted.
• allows patients with an intact respiratory drive to
exercise inspiratory muscles between assisted breaths.
• useful for both supporting and weaning intubated
• may be difficult to use in patients with tachypnea AS
they may attempt to exhale during the ventilator-
programmed inspiratory cycle.
• Then, the airway pressure may exceed the inspiratory
pressure limit, and the ventilator-assisted breath will
be aborted, and minute volume may drop below that
programmed by the operator.
• Its patient-triggered, flow-cycled, and pressure-limited .
• It provides graded assistance and differs from the other two modes in that the
operator sets the pressure level (rather than the volume) to augment every
spontaneous respiratory effort.
• level of pressure is adjusted by observing the patient’s respiratory frequency.
• During PSV, the inspiration is terminated when inspiratory airflow falls below a
• With PSV, patients receive ventilator assistance only when the ventilator detects an
inspiratory effort .
• PSV is often used in combination with SIMV to ensure volume-cycled backup for
patients whose respiratory drive is depressed.
• PSV is well tolerated by most patients who are being weaned from MV;.
• Its time-triggered, time-cycled, and pressure-limited
• A specified pressure is imposed at the airway opening
• tidal volume and inspiratory flow rate are dependent,
rather than independent
• preferred mode of ventilation for patients in whom it is
desirable to regulate peak airway pressures, such as
- preexisting barotrauma,
- post– thoracic surgery patients, in whom the shear
forces across a fresh suture line should be limited
INVERSE-RATIO VENTILATION (IRV)
• Its a variant of PCV that incorporates the use
of a prolonged inspiratory time with the
appropriate shortening of the expiratory time.
• IRV has been used in patients with severe
hypoxemic respiratory failure.
• This approach increases mean distending
pressures without increasing peak airway
• Several nonconventional strategies have been evaluated for their ability to
improve gas exchange and survival rates in severe hypoxemic respiratory
• These strategies include:
• High-frequency oscillatory ventilation (HFOV),
• Airway pressure release ventilation (APRV),
• Partial liquid ventilation (PLV) using perfluorocarbons and the
administration of nitric oxide gas delivered through the airways.
• “Salvage” techniques like:
Extracorporeal membrane oxygenation (ECMO)
• ECMO to be considered in patients with severe respiratory failure refractory
to conventional therapy.
• Others include:
• Proportional assist ventilation (PAV)[p,v,t and resp. resistance and
• Neurally adjusted ventilatory-assist ventilation (NAV) [neural activation of
• PAV/NAV : new modes to enhance patient ventilator synchrony.
PROTECTIVE VENTILATORY STRATEGY
• Set a target tidal volume close to 6 mL/kg of
ideal body weight.
• Prevent plateau pressure (static pressure in the
airway at the end of inspiration) exceeding 30 cm
• Use the lowest possible fraction of inspired
oxygen (Fio2) to keep the Sao2 at ≥90%.
• Adjust the PEEP to maintain alveolar patency
while preventing overdistention and
• The SBT involves an integrated patient assessment during
spontaneous breathing with little or no ventilatory support.
• The SBT is usually implemented with a T-piece using 1–5 cmH2O
CPAP with 5–7 cmH2O or PSV from the ventilator to offset
resistance from the endotracheal tube.
• Once it is determined that the patient can breathe spontaneously, a
decision must be made about the removal of the artificial airway,
which should be undertaken only when it is concluded that the
patient has the ability to protect the airway, is able to cough and
clear secretions, and is alert enough to follow commands.