Invasive Mechanical Ventilation

Abstract

Invasive mechanical ventilation is a potentially lifesaving intervention for acutely ill patients. The goal of this review is to provide a concise, clinically focused overview of basic invasive mechanical ventilation for the many clinicians who care for mechanically ventilated patients. Attention is given to how common ventilator modes differ in delivering a mechanical breath, evaluation of respiratory system mechanics, how to approach acute changes in airway pressure, and the diagnosis of auto-positive end-expiratory pressure. Waveform interpretation is emphasized throughout the review.

Fig. 1.

(A) Pressure, volume, and time waveforms in assist-control volume control (AC-VC) mode using a decelerating flow strategy. Note the second breath is patient triggered as evidenced by a negative deflection of the pressure waveform. The flow rate set on the ventilator represents the peak flow rate (solid black arrow), which linearly decelerates to near zero with each breath. Inspiration ends after a preset tidal volume is achieved. Peak airway pressure is not set by the clinician but rather a consequence of respiratory system mechanics, tidal volume, inspiratory flow rate and pattern, and patient effort. Note that during the second and third breaths, airway pressure decreases while flow rate and tidal volume remain unchanged reflecting a favorable change in respiratory system mechanics. Expiratory flow (arrowhead) is not controlled by the ventilator but is instead dependent on patient effort and respiratory system mechanics. (B) Pressure, volume, and time waveforms in the assist-control pressure control (AC-PC) mode. Note that the second breath is patient triggered as evidenced by a negative deflection of the pressure waveform. With each breath, a preset inspiratory pressure above positive end-expiratory pressure is delivered for a set inspiratory time, after which the breath is cycled off. Tidal volume is determined by the patient’s respiratory system mechanics, inspiratory effort, inspiratory pressure, and inspiratory time. Note that during the second and third breaths, tidal volume increases despite an unchanged inspiratory pressure and inspiratory time, reflecting a favorable change in respiratory system mechanics or increased patient effort. (C) Pressure, volume, and time waveforms in pressure-regulated volume control (PRVC) mode. During a test breath (gray box), the ventilator estimates the inspiratory pressure and flow rate needed to achieve a preset goal tidal volume in a preset inspiratory time. Inspiratory pressure is continually adjusted to correct discrepancies between the goal and delivered tidal volume. Note during the fifth breath the delivered tidal volume exceeds the goal tidal volume (solid arrow). Inspiratory pressure is decreased by the machine on subsequent breaths to bring the delivered tidal volume closer to the goal tidal volume.

Fig. 2.

(A) Pressure, volume, and time waveforms in pressure support (PS) ventilation. All breaths are patient triggered as evidenced by the negative deflections in the pressure waveform (gray circles). With each breath, a set inspiratory pressure is delivered above positive end-expiratory pressure. Per the equation of motion, constant pressure waveforms require decelerating inspiratory flow, and when this inspiratory flow falls to a preset percentage of peak inspiratory flow (solid arrow), the machine cycles off. Tidal volume will vary based on respiratory system mechanics and patient effort. (B) Pressure, volume, and time waveforms in SIMV-VC with pressure support. Flow-targeted volume-cycled mandatory breaths are given at a rate set by the clinician (gray box). These breaths are “synchronized” to patient effort (and thereby assisted) if a patient attempts to trigger a breath near the time of set breath delivery (gray circle). Between synchronized and mandatory breaths, the patient may take spontaneous breaths which are generally supported with pressure support (middle two breaths).

Fig. 3.

(A) Pressure and flow waveforms for constant (left) and decelerating (right) inspiratory flow patterns. Note the “shark fin” appearance of the pressure waveform with constant inspiratory flow caused by the steady increase in distending pressure (shaded area) coupled with a constant resistive pressure (unshaded area). With decelerating inspiratory flow, a square pressure waveform is observed as decreasing flow causes a decline in resistive pressure during breath delivery. (B) Pressure and flow waveforms during an inspiratory hold maneuver. During inspiration, airway pressure rises from positive end-expiratory pressure (PEEP) to peak inspiratory pressure. Inspiratory flow is then stopped, eliminating resistive pressure and causing airway pressure to fall to a plateau pressure (P_plt). The difference between peak inspiration pressure and P_plt represents airway resistive pressure. The difference between P_plt and PEEP is determined by tidal volume and respiratory system compliance. (C) Diagnostic approach to elevated peak pressures on a mechanical ventilator. ETT, endotracheal tube.

Fig. 4.

Pressure and flow waveforms in a patient with autopositive end-expiratory pressure. Note that expiratory flow does not return to zero before the start of the next breath (arrows).