Effects of Inspiratory Flow Rate Alterations on Gas Exchange During Mechanical Ventilation in Normal Lungs

The influence of inspiratory flow rate (TI), without changing respiratory frequency, tidal volume, and FIo2, was investigated in 11 normal lungs in patients undergoing mechanical ventilation because of central respiratory failure due to stable coma. The patients were anesthetized and paralyzed. They first received a conventional ventilation (TI = 25 percent, pause = 10 percent) and then, were submitted to four different TI values, randomly administered without any end-inspiratory pause (EIP) (TI = 20 percent; TI = 33 percent; TI = 50 percent; TI = 67 percent). In the middle and at the end of the procedure, a return to basal conditions was introduced. At each ventilator setting, the following were obtained: respiratory flow (Pneumotachograph Fleish No. 2), airway pressure, FRC changes (inductive plethysmography), arterial and mixed venous blood gases, hemodynamic data, and V˙A/Q˙ ratios distribution using multiple inert gases technique. EIP suppression provides a significant increase in V˙A/Q˙ mismatch (until TI = 50 percent) and in shunt effect (between 3 and 9 percent of cardiac output [Q˙T]. The absence of simultaneous PaO2 change is due to increasing P V˙o2 linked to a higher Q˙T. The shorter the Ti, the higher the PaCO2 connected with a relative alveolar hypoventilation. However, increasing TI without EIP significantly decreases ventilation distribution inequalities. This improvement is concomitant with a rise in FRC (FRC67-FRC20 = 0.340 ± 0.450, p