Describe adaptive pressure control.
Compare approaches to dual-control modes.
Compare volume support, AutoMode, SmartCare, and average volume-assured pressure support (AVAPS).
Compare the control of airway pressure during proportional-assist ventilation (PAV), tube compensation, and neurally adjusted ventilatory assist (NAVA).
Explain the function of airway pressure-release ventilation.
Explain breath delivery with adaptive support ventilation and Intellivent.
Discuss the rationale for mandatory minute ventilation.
With each generation of ventilators, new modes and variations on previous modes become available. Numerous ventilator modes from a variety of manufacturers exist now. The purpose of this chapter is to describe the technical and clinical aspects of advanced modes of ventilation that have recently become available. Although promoted by manufacturers, the clinical value of many of these modes remains unproven. Use of these modes is often based on their availability and clinician bias, rather than evidence that they are superior to traditional modes.
Adaptive Pressure Control
Adaptive pressure control is negative feedback closed-loop pressure-controlled ventilation (PCV). Tidal volume is a feedback control for breath-by-breath adjustment of pressure control. 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. With adaptive pressure control, the ventilator increases or decreases pressure on a breath-by-breath basis to deliver the desired VT.
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 minimally variable minute volume (Figure 8-1). 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. If breaths exceed set tidal volume in the presence of strong inspiratory efforts by the patient, the ventilator may excessively reduce the level of support, leading to asynchrony. On some ventilators, a low-pressure limit and a high-pressure limit can be set. If the pressure level increases in an attempt to maintain tidal volume in a patient with airflow obstruction, there might be an increase in air trapping.
(A) The effect of tidal volume increases, such as a result of an increase in compliance or an increase in patient effort. (B) The effect of tidal volume decreases, such as a result of a decrease in compliance or a decrease in patient effort. (Reproduced with permission from Branson RD, Johannigman JA. The role of ventilator graphics when setting dual-control modes. Respir Care. 2005;50(2):187-201.)