![]() Prone position improves V A /Q matching by more evenly distributing V A and Q. However, at high levels, it increases dead space ventilation and hypercarbia. Ventilation/perfusion ratios less than 1 are responsible for hypoxemia in asthmatics, but no correlation exists between V A /Q mismatch and the degree of small airway obstruction.ĭuring mechanical ventilation, positive end-expiratory pressure decreases the proportion of shunt in PARDS. The primary V A /Q abnormality in pediatric acute respiratory distress syndrome (PARDS) is intrapulmonary shunt. Perfusion is affected more by gravitational forces than ventilation but is of secondary importance in Q and V A. Gravity increases pulmonary blood flow and ventilation to the base of the lung. The fractal design of the lung and gravitational effects on the flow of air and blood affect V A /Q matching. However, at 15 and 20 cm H2O PEEP's high VA/Q and dead space ventilation with CO2 retention again developed in all but the most severely affected (shunt greater than 40%) dogs.Gas exchange is optimized when pulmonary perfusion (Q) and alveolar ventilation (V A ) are tightly matched. In dogs with pulmonary edema, PEEP's of 5 and 10 cm H2O resulted in dramatic reductions in shunt, virtual obliteration of low VA/Q regions, and market improvement in Pao2. PEEP greater than or equal to 10 cm H2O resulted in a reduction in Pao2 and an increase in PaCO2. In these animals increasing PEEP caused progressive depression of cardiac output, associated with an increase in ventilation to both high VA/Q and unperfused regions. Normal dogs had little or no shunt, and no areas of low (less than 0.1) or high VA/Q (greater than 10.0) at zero end-expiratory pressure (intermittent positive-pressure breathing). Tidal volume and frequency were held constant at all times with mechanical ventilation during intravenous pentobarbital and gallamine anesthesia. The effects of positive end-expiratory pressure (PEEP) at 5, 10, 15, and 20 cm H2O on the distribution of ventilation-perfusion (VA/Q) ratios was determined in four normal dogs and in ten with oleic acid-induced acute hemorrhagic pulmonary edema. ![]()
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