Mechanical ventilation: breath types and modes
Tidal breaths delivered during mechanical ventilation are produced by flow of gas that is either interactive with a patient’s intrinsic effort or entirely controlled by the ventilator. The stages of the mechanical breath that determine the breath type are: 1) trigger (initiates the breath), 2) target (controls gas delivery during the breath), and 3) cycling (transition from inspiration to expiration). When a patient initiates a breath, the patient’s inspiratory effort causes a drop in the ventilator circuit pressure (or a decrement in circuit flow), resulting in breath delivery (i.e. assisted breath). The amount of gas delivered during a breath is governed by 3 parameters that can be set on the ventilator: pressure (cm H20), flow (L/min) and volume (mL).
The breath type describes the variables that are controlled during breath delivery. A volume-cycled breath delivers a breath at a controlled flow rate that is either constant or decelerating and cycles after a preset tidal volume is delivered (i.e. volume assist control breath). A time-cycled breath applies a constant pressure for a set amount of time, producing variable flow and tidal volume dependent upon a patient’s airway resistance and lung compliance (i.e. pressure assist control breath). Finally, a flow-cycled breath similarly applies a constant pressure, but cycles when flow degrades to a preset fraction of peak flow (typically 25 percent), rather than after a preset inspiratory time (i.e. pressure support breath). The various modes are determined by the combination of breath types that are delivered. One concept to remember is that both volume and pressure cannot be controlled at the same time during breath delivery. In flow-targeted breaths (e.g. volume assist control breath), changes in compliance, resistance or patient effort will change the circuit pressure. In contrast, during a pressure-targeted breath, changes in lung compliance, airway resistance or patient effort will alter the flow rate and resultant tidal volume.
Mechanical ventilators continuously measure the pressure, flow and volume of each breath and display the values over time. These ventilator graphics allow a clinician to visually assess each variable (pressure, flow and volume) that governs breath delivery. As a result, the mode of mechanical ventilation can be determined from visual inspection of ventilator graphics by discerning which parameters, pressure, flow or volume, are held constant (i.e. independent variables) and which parameters vary with lung compliance and resistance (dependent variables). Assess the ventilator graphics below and see if you can “name that mode.”
Name that mode:
A. Pressure assist control
Pressure assist control (PAC) mode is patient or time-triggered, pressure targeted (control variable) and time cycled. As you can see in the graphic display, both the applied pressure and inspiratory time are held constant, and therefore set by the clinician. If the compliance abruptly changes (e.g. development of acute pulmonary edema), the tidal volume would decrease and thus minute ventilation is not guaranteed.
B. Volume assist control
Volume assist control (VAC) mode is patient or time-triggered, flow targeted (control variable) and volume cycled. In VAC, the peak inspiratory flow rate and inspiratory flow pattern (square wave or decelerating) are set by the clinician. An advantage of VAC is that minute ventilation is guaranteed; however, increases in airway resistance or worsening lung and chest wall compliance can increase peak inspiratory and plateau pressures, unless a preset limit is exceeded.
C. Pressure regulated volume control
Pressure regulated volume control (PRVC) is a dual control mode that allows the practitioner to select a “target” tidal volume and inspiratory time. The ventilator automatically adjusts the inspiratory pressure to target the preset volume. During breath #2 above, the lung compliance improves, causing the tidal volume to increase. Subsequently, the ventilator reduces the applied pressure to bring the tidal volume back to the set target in breath #4. So, is PRVC a volume control or pressure control mode? In an upcoming post, Dr. Phil Dellinger will answer this question and bring clarity to a topic that confused me during my fellowship training!