Mechanical ventilation is the process by which the fraction of inspired oxygen (FIO )2at
21% (room air) or greater is moved into and out of the lungs by a mechanical ventilator.
Indications for mechanical ventilation include (1) apnea or impending inability to
breathe, (2) acute respiratory failure generally defined as pH 7.25 with a PaCO ≥50
mm Hg, (3) severe hypoxia, and (4) respiratory muscle fatigue.
Types of Mechanical Ventilation
Negative pressure ventilation involves the use of chambers that encase the chest or
body and surround it with intermittent subatmospheric or negative pressure.
o Negative pressure ventilation is delivered as noninvasive ventilation and an
artificial airway is not required.
o Negative pressure ventilators are not used extensively for acutely ill patients.
However, some research has demonstrated positive outcomes with the use of
negative pressure ventilation in acute exacerbations of chronic respiratory failure.
Positive pressure ventilation (PPV), used primarily with acutely ill patients, pushes air
into the lungs under positive pressure during inspiration. Expiration occurs passively as
in normal expiration. Modes of PPV are categorized into two groups:
o Volume ventilation involves a predetermined tidal volume (V ) that is delivered
with each inspiration, while the amount of pressure needed to deliver the breath
varies based on the compliance and resistance factors of the patient-ventilator
o Pressure ventilation involves a predetermined peak inspiratory pressure while
the VTdelivered to the patient varies based on the selected pressure and the
compliance and resistance factors of the patient-ventilator system.
Careful attention must be given to the VTto prevent unplanned
hyperventilation or hypoventilation.
Settings of Mechanical Ventilators
Mechanical ventilator settings regulate the rate, depth, and other characteristics of
ventilation and are based on the patient’s status (e.g., ABGs, body weight, level of
consciousness, muscle strength). The ventilator is tuned as finely as possible to match the
patient’s ventilatory pattern.
Modes of volume ventilation:
o Ventilator mode is based on how much WOB the patient ought to or can perform and
is determined by the patient’s ventilatory status, respiratory drive, and ABGs.
o Ventilator modes are controlled or assisted.
With controlled ventilatory support, the ventilator does all of the WOB.
With assisted ventilatory support, the ventilator and the patient share the
o Controlled mandatory ventilation (CMV) delivers breaths that are delivered at a set
rate per minute and a set VT, which are independent of the patient’s ventilatory efforts.
Patients perform no WOB and cannot adjust respirations to meet changing
o Assist-control ventilation (ACV) delivers a preset V at T preset frequency, and
when the patient initiates a spontaneous breath, the preset VTis delivered.
The patient can breathe faster than the preset rate but not slower.
This mode allows the patient some control over ventilation while providing
some assistance and is used in patients with a variety of conditions (e.g.,
Guillain-Barré syndrome, pulmonary edema, acute respiratory failure).
Patients require vigilant assessment and monitoring of ventilatory status,
including respiratory rate, ABGs, SpO ,2and SvO /S2vO . 2
If it is too difficult for the patient to initiate a breath, the WOB is
increased and the patient may tire and or develop ventilator
asynchrony (i.e., the patient “fights” the ventilator).
o Synchronized intermittent mandatory ventilation (SIMV) delivers a preset V at a T
preset frequency in synchrony with the patient’s spontaneous breathing.
Between ventilator-delivered breaths, the patient is able to breathe
The patient receives the preset FIO2concentration during the spontaneous
breaths but self-regulates the rate and volume of those breaths.
Potential benefits of SIMV include improved patient-ventilator synchrony,
lower mean airway pressure, and prevention of muscle atrophy as the patient
takes on more of the WOB.
Modes of pressure ventilation:
o With pressure support ventilation (PSV), positive pressure is applied to the airway
only during inspiration and is used in conjunction with the patient’s spontaneous
The patient must be able to initiate a breath in this modality.
A preset level of positive airway pressure is selected so that the gas flow rate
is greater than the patient’s inspiratory flow rate.
Advantages to PSV include increased patient comfort, decreased WOB,
decreased oxygen consumption, and increased endurance conditioning.
o Pressure-controlled/ inverse ratio ventilation (PC-IRV) combines pressure-limited
ventilation with an inverse ratio of inspiration (I) to expiration (E). Normal I/E is 1:2.
With IRV, the I/E ratio begins at 1:1