The use of pressure-controlled ventilation (PCV) increased substantially after 1995, when intensivists became increasingly aware of ventilator-induced lung injury (VILI) and the risks of high inspiratory pressures. Familiarity with the concept of permissive hypercapnia contributed to this change, helping physicians to overcome the old and stringent mindset on arterial blood gases.1–4 Tidal volume or minute ventilation requirements were increasingly regarded as secondary goals during mechanical ventilation and the apparent security provided by PCV, keeping airway pressures under strict limits, gained broader acceptance. Recent surveys in intensive care units demonstrate that PCV is now used in up to 25% of ventilated patients, usually in the most severe cases5,6 (including pediatric patients7). Studies, describing the implications of PCV on the cardiovascular system, work of breathing, regional mechanics, risks of VILI, and recruitment maneuvers are now available. These studies increase physician comfort in moving away from volume-controlled ventilation.
In contrast to the increasing acceptance of PCV, the use of inverse-ratio ventilation (IRV)—that is, the prolongation of inspiratory time to the point of inverting the conventional inspiratory-to-expiratory (I:E) ratio—is now very rare. This decline was caused by recent progress in our understanding on the pathophysiology of lung collapse, and demonstration of unnecessary hemodynamic compromise. The original benefits in terms of oxygenation ascribed to IRV,8–14 and the apparent reduction of peak inspiratory alveolar pressures11,12 are now supplanted by more consistent effects of recruiting maneuvers followed by optimization of positive end-expiratory pressure (PEEP),15–20 especially during controlled mechanical ventilation. It is only in the more specific context of airway pressure release ventilation (APRV) that the use of IRV has still survived, although surrounded by great skepticism and controversy.21,22
In this chapter, we do not revitalize IRV. We believe there are always safer, more predictable, and more efficient ventilatory solutions to be implemented at the bedside. IRV was extensively discussed in the second edition of this book. At the end of the present chapter, we present a few aspects of IRV that still offer great insights about the pathophysiology of acute respiratory failure.
Ventilators regulate the pressure profile applied to the airways or the pattern of flow delivery. Somewhat imprecisely, flow-controlled ventilation has been designated “volume-controlled” ventilation (VCV). We avoid this convention because the criterion by which the ventilator ceases to pressurize the airway (initiates deflation) may be a specified value of delivered volume, pressure, elapsed time, or flow. The variable, however, actively controlled by the ventilator during “volume-controlled” breaths is in reality inspiratory flow. Therefore, the modes of ventilation currently used in medical practice should be classified as pressure-controlled or flow-controlled and as time-cycled, volume-cycled, flow-cycled, or pressure-cycled. Pressure-support ventilation (PSV) is an example of a pressure-controlled, flow-cycled mode, whereas PCV is an example of pressure-controlled, time-cycled mode.
In flow-controlled modes, the waveform theoretically can be of any desired contour; in practice, ...