Critical Care Medicine

Answer 1 PIP, 33 cm H2O; Pp, 33 cm H2O; VT, 200 mL; RR, 15 breaths/min. PC-IRV is performed by setting an inspiratory pressure, inspiratory time with the inspiration/expiration ratio greater than 1:1 (normal, &#$139; 1 [eg, 1:3]), and RR. Understanding mechanical ventilator support modes is made easier by analyzing 3 variables: trigger (what initiates the breath), target (what controls gas delivery during the breath), and cycle (what terminates the breath). In pressure-control mode, the target is pressure (the set inspiratory pressure) and the cycle is time (inspiration ends after a set inspiratory time passes). VT varies depending on the set inspiratory pressure, inspiratory time, airway resistance (patient and ventilator tubing), and compliance of the lung and chest wall. Inspiratory pressure is rapidly achieved with high initial gas flows, and full inspiratory pressure is reached early in the inspiratory cycle. The selected inspiratory pressure is held throughout the inspiratory cycle and then followed by a rapid deceleration in flow. When lung compliance decreases (mucous plugging, pulmonary edema, or pneumothorax in this case), VT goes down, but the selected inspiratory pressure is maintained throughout the cycle. Because this patient is paralyzed, the RR cannot increase. In volume-control mode, the target is flow (the set IFR), and the cycle is volume (inspiration ends after the set VT is achieved). When lung compliance decreases, airway pressure increases, maintaining the set VT and flow rates. PC-IRV mode may be used in ARDS when severe hypoxemia and ventilation/perfusion mismatch is present. The advantages of PC-IRV mode are alveolar recruitment and auto-PEEP, which may improve oxygenation, compliance, and functional residual capacity (FRC). Disadvantages of using PC-IRV include the need for deep sedation and neuromuscular blockade, risk for barotrauma, and development of hypotension that severely limit its use in hemodynamically unstable patients.1,2 There are no randomized prospective studies demonstrating improved survival with PC-IRV in ARDS. REFERENCES 1. Chatburn RL. Classification for mechanical ventilators. Respir Care 1992;37:1009-25. 2. Armstrong BW Jr, MacIntyre NR. Pressure-controlled, inverse ratio ventilation that avoids air trapping in the adult respiratory distress syndrome. Crit Care Med 1995;23:279-85. Click here to return to the questions

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