Adaptive pressure control is closed-loop PCV. Tidal volume is a feedback control for breath-by-breath adjustment of pressure control (Figure 8-2). All breaths are patient or ventilator-triggered, pressure-controlled, and time-cycled. This mode is available on most current ICU ventilators and has various names, dependent on the manufacturer, such as AutoFlow, pressure-regulated volume control (PRCV), volume control + (VC+), adaptive pressure ventilation, volume-targeted pressure control, and pressure-controlled volume guarantee. The ventilator delivers a test breath and calculates system compliance. A number of breaths are delivered to test the pressure control necessary to achieve the desired tidal volume based on the compliance calculation. The ventilator then increases or decreases the pressure on a breath-by-breath basis to deliver the desired VT.
Control logic for adaptive pressure control mode.
Perhaps the most important advantage of this mode is the ability of the ventilator to change inspiratory flow to meet patient's demand while maintaining a constant minute volume (Figure 8-3). An important disadvantage of this mode is that the tidal volume remains constant and peak alveolar pressure increases as the lungs become less compliant (eg, acute respiratory distress syndrome [ARDS]), which could result in alveolar overdistention and acute lung injury. With this mode, breaths can exceed set tidal volume in the presence of strong inspiratory efforts by the patient. When this occurs, the ventilator may excessively reduce the level of support, leading to asynchrony. On some ventilators a low-pressure limit as well as a high-pressure limit can be set.
(A) The effect of a tidal volume increases, such as an increase in compliance or an increase in patient effort. (B) The effect of a tidal volume decreases, such as a decrease in compliance or a decrease in compliance. (Reproduced with permission from Branson RD, Johannigman JA. The role of ventilator graphics when setting dual-control modes. Respir Care. 2005; Feb; 50(2):187-201.)
Volume support (VS) is closed-loop control of pressure support ventilation (PSV). Tidal volume is used as feedback control to adjust the pressure support level. All breaths are patient-triggered, pressure-limited, and flow-cycled (Figure 8-4). A test breath with a low-pressure is applied. The delivered tidal volume (exiting the ventilator) is measured and compliance is calculated. A number of breaths are then delivered to test the calculated pressure to deliver the set tidal volume. The ventilator then attempts to maintain a constant delivered tidal volume on a breath-to-breath manner. Since VS is a variation on PSV, the breath is flow-cycled.
Control logic for volume support mode.
There are several potential issues with this mode. Auto-positive end-expiratory pressure (auto-PEEP) may occur if the pressure level increases in an attempt to maintain tidal volume in a patient with airflow obstruction. In the patient with a high ventilatory demand, ventilator support will decrease, which could be the opposite of the desired response. This mode of ventilation is available on most current generation ICU ventilators.
AutoMode allows the ventilator to switch between mandatory and spontaneous breathing modes. If the patient is apneic, the ventilator will provide VCV, PCV, or PRVC. If the patient triggers a breath, the ventilator switches from VCV to VS, from PCV to PSV, or from PRVC to VS. If the patient becomes apneic, the ventilator reverts to VCV, PCV, or PRVC.
Average Volume-Assured Pressure Support
Average volume-assured pressure support (AVAPS) is a form of adaptive pressure control available on some ventilators for noninvasive ventilation. It maintains a VT equal to or greater than the target VT by automatically controlling the minimum and maximum inspiratory positive airway pressure (IPAP) setting. AVAPS averages VT over time and gradually changes the IPAP over several minutes to achieve the target VT. If the patient's effort decreases, IPAP is increased to maintain the target tidal volume. On the other hand, if the patient's effort increases, IPAP is reduced. As with other types of adaptive pressure control, there is a concern that the ventilator will inappropriately decrease support if respiratory drive increases.