I. GENERAL PRINCIPLES. Indications for mechanical ventilation (MV) include hypoxic and hypercapnic respiratory failure. Intrinsic lung disease can result in hypoxemia and/or pump failure manifested by hypercapnia and hypoxemia. Pure hypercapnic respiratory failure can result from central nervous system (CNS) depression, respiratory muscle fatigue or weakness due to peripheral nervous system disease or an intrinsic muscle disorder, chest wall mechanical defects, and mediators of diseases that affect respiratory muscles (e.g., sepsis). Positive-pressure MV is currently the predominant means of providing ventilatory support, as opposed to negative-pressure ventilation. MV may be invasive (delivered through an endotracheal tube [ETT] or tracheostomy tube) or noninvasive positive-pressure ventilation (NIPPV) (delivered to the patient through a full-face or nasal mask).

1. Volume-cycled MV: delivers a guaranteed preset volume (Vt) with each breath that is specified by the operator. Peak inspiratory pressures (PIPs) generated by the ventilator are variable with each breath, depending on airway resistance or compliance. A “pop-off” pressure is assigned to prevent excessive peak pressures that abort the breath when that pressure limit is reached. The time that it takes to deliver the Vt (inspiratory time or Ti) is also controlled by the operator because it is dependent on the volume, inspiratory flow rate ([V with dot above]i), and waveform characteristics (square or decelerating waveforms), which are all specified by the operator.

a. Assist control (AC): All breaths are assisted. The patient initiates a breath and a set inspiratory flow and Vt are delivered with each breath. However, if the patient’s intrinsic rate falls below the preset basal rate, then all the breaths delivered are control breaths, spaced at regular time intervals. AC is also a time-triggered mode that delivers a preset volume if the patient does not initiate any spontaneous breaths. During the control and assisted breaths, the Vt and inspiratory flow and characteristics are exactly the same with each breath.

i. Advantages include that a guaranteed Vt will be delivered, and when patients are in synchrony with the ventilator, this mode allows for minimal patient effort and rest for fatigued respiratory muscles.

ii. Disadvantages include the following: The potential for induced respiratory alkalosis in patients with high respiratory drive (e.g.,
liver failure), patient asynchrony and respiratory muscle fatigue can occur, and I:E ratio can vary because the variable respiratory rate (RR) can alter the expiratory phase.

b. Synchronized intermittent mandatory ventilation (SIMV): SIMV can deliver three kinds of breaths—spontaneous, assisted, and mandatory breath. If no breaths are initiated within a period of time, a mandatory breath will be delivered. If the machine senses that the patient has taken a spontaneous breath just before the mandatory breath, the machine will recycle and then wait for the next spontaneous breath and assist it.

i. Advantages include an insured minimum [V with dot above]_E and backup rates for patients with apneas.

ii. Disadvantages include the following: The least beneficial mode for weaning, I:E ratio cannot be fully controlled given the variability in RR and presence of spontaneous breaths, and it does not provide the same degree of respiratory muscle rest as AC mode.

2. Pressure-limited MV delivers a flow until a preset pressure limit that is set by the operator is reached. PIP is therefore always the same as the sum of the preset pressure limit for each breath and the positive end-expiratory pressure (PEEP) value. Vt is variable with each breath, according to airway resistance and compliance.

a. Pressure support (PS): Every breath is an assisted breath. Each breath is triggered by the patient’s respiratory effort. The patient determines the inspiratory flow rate and shape of the waveform as well as the RR. When a preset pressure limit is reached, inspiratory flow slows to <0.5 L/min and the machine cycles off.

i. Advantages include the following: better patient synchrony, it limits PIP, and probably is as effective as spontaneous breathing trial weaning.

ii. Disadvantages include the following: Apnea backup breaths are infrequent and less responsive than the backup from AC, inadequate volumes could be delivered if the ETT is blocked, and decreased lung compliance can cause preset pressure limit to stop inspiratory flow before an adequate Vt is delivered; asynchrony can occur due to high inspiratory pressure settings, low respiratory drive, and airflow obstruction with dynamic hyperinflation and with air leaks.

b. Pressure control (PC): PC is similar to AC in that control breaths are delivered at a preset time interval but with a preset pressure limit rather than a preset volume. RR and time to maximal pressure limit are both operator set, and spontaneous breaths can be interspersed between the mandatory breaths.

i. Advantages include the following: It limits PIP and plateau pressure (Pplat) to minimize barotrauma and it can control or extend Ti for inverse ratio ventilation (IRV) to increase mean airway pressure (MAP) and augment oxygenation.

ii. Disadvantages include the following: It cannot ensure minimal [V with dot above]_E with airway obstruction or poor compliance, sedation with
or without paralysis is necessary for IRV (I:E > 2:1), extended inspiratory time with circuit leaks, and exaggeration of inspiratory time can limit time for passive exhalation and contribute to autoPEEP.

c. Bilevel: a form of pressure-support ventilation that allows for unrestricted spontaneous breathing that switches between a high and low airway pressure based on an adjustable time sequence. Cycling between the two pressure settings can be synchronized with the patient’s spontaneous breathing to maximize the I-pressure during inspiration and the E-pressure during expiration.

i. Potential advantage is that it is theoretically more comfortable to the patient, resulting in less agitation and less need for sedation.

ii. Potential disadvantages include that it is not well studied and safety has not been well established.

d. Airway pressure release ventilation (APRV): an extreme form of bilevel ventilation, maintaining a long period of high pressure followed by a very short period of low pressure (the “release”). This results in an inverse I:E ratio of 8 to 9:1. It is a time-triggered, pressure-limited, time-cycled mode that also allows for the patient to have spontaneous breathing.

i. Potential advantages include that it may improve oxygenation in patients with severe ARDS, decreases the frequency of opening and closing of alveoli, limits the amount of alveolar stretching factors thought to promote lung injury, and decreases airway pressures.

ii.

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