Assist/control (A/C) ventilation, otherwise known as continuous mandatory ventilation (CMV), is a mode that delivers a preset volume or pressure at a specified rate, but allows the patient to trigger an assisted breath at any time (Figure 32-1). The A/C ventilation can be pressure or volume controlled. The machine is set to “sense” the patient’s negative inspiratory effort. It is therefore triggered to deliver the preset tidal volume or inspiratory pressure. All delivered breaths, whether mandatory or patient triggered, will be delivered by the ventilator according to the set parameters (volume or pressure). The fraction of inspired oxygen concentration (FiO2) and positive end-expiratory pressure (PEEP) are also set by the operator and remain the same for every breath delivered (whether mandatory or patient triggered).
Assist control ventilation. (Reproduced with permission from Longnecker DE, Brown DL, Newman MF, Zapol WM, eds. Anesthesiology, 2nd ed. New York: McGraw-Hill Medical; 2012.)
The A/C rate is the minimum number of full ventilator breaths the patient will receive. The actual respiratory rate is equal to the A/C rate plus any patient-triggered breaths per minute. If volume control is used, the delivered tidal volume will be constant but the pressure may change with each breath. If pressure control is chosen, the pressure of each delivered breath will be constant but the tidal volume may change.
INTERMITTENT MANDATORY VENTILATION
Intermittent mandatory ventilation (IMV) is a volume control mode that will deliver a preset volume at a preset rate. As with A/C mode, the operator must set FiO2 and PEEP. In contrast to A/C, if the patient takes a breath on their own, the machine will not provide any additional support. In a straight IMV mode, if the patient’s breath is “out of sync” with the machine, a set breath could be delivered while the patient is attempting to take a breath. For this reason, synchronized IMV (SIMV) was developed. In this mode, the ventilator senses the patient’s attempts at spontaneous breathing but will not deliver a set breath at the same time. The advantage of SIMV is that the patient may not want or even require the full preset tidal volume with any given spontaneous breath. However, the patient must expend significant energy to take a breath through a full ventilator circuit. To overcome the higher work of breath, clinicians often combine pressure support ventilation (PSV) with SIMV ventilation (Figure 32-2). The combination of SIMV with PSV has proved to be an excellent ventilator weaning mode.
Intermittent mandatory ventilation. (Reproduced with permission from Longnecker DE, Brown DL, Newman MF, Zapol WM, eds. Anesthesiology, 2nd ed. New York: McGraw-Hill Medical; 2012.)