Pressure-control ventilation in managing acute respiratory distress syndrome

adjusts inspiratory gas flow to maintain Paw as a square wave. Titrating inspiratory flow rate (V̇I) to maintain Paw as a square wave results in a variable, decreasing ramp flow waveform (Figure 1A). Peak V̇ occurs as inspiration commences, so that the flow-resistive properties of the patientventilator system can be utilised to create a ‘pressure-step’ that approaches the peak Paw target (Paw = ̇ V x resistance). Because the initial V̇ may be very high (>100 L/min when a traditional-sized VT is used),8 lung volume and alveolar pressure rise rapidly in consequence. To maintain Paw as square-wave, V̇ then must ‘taper off’ at a pace that is determined by the respiratory system compliance (Crs). A decreasing ramp flow pattern can also be created during volume control ventilation (VCV). The difference is that both the peak V̇ and the rate at which flow tapers-off are fixed and independent of patient effort, Crs or respiratory system resistance (Rrs). The search for mechanical ventilation strategies that enhance lung-protection in patients with acute respiratory distress syndrome (ARDS) and acute lung injury (ALI) has focused interest on pressure-regulated modes that prevent high airway pressure (Paw), which is believed to be injurious to the lungs. Pressure-control ventilation (PCV) is the most common pressure-regulated mode, wherein the ventilator actively monitors and adjusts the gas flow to maintain Paw as a square wave during inspiration. This results in a characteristic decreasing ramp flow pattern that may result in improved pulmonary mechanics, gas exchange and patient work of breathing. A literature review of 17 prospective clinical trials of PCV and other pressure-regulated modes in patients with ARDS/ALI has produced mixed results. There is some evidence that pressure-regulated modes may effect modest improvements in respiratory system compliance, oxygenation, dead-space fraction and patient work of breathing. However, nearly all studies were done with a traditional-sized tidal volume which may have influenced the effects of the decreasing ramp flow pattern. Therefore, the extension of these results to the era of low tidal volume ventilation remains uncertain. In the only large, multicentred, randomised, controlled trial of patients with ARDS/ALI, PCV was not shown to have improved morbidity or mortality compared to volume control ventilation.