Effect of inspiratory flow rate on gas exchange during mechanical ventilation.

We studied the effect of changes in inspiratory flow rate (VI) on gas exchange and respiratory system mechanics during mechanical ventilation. Seven patients with respiratory failure caused by severe chronic airway obstruction (CAO group) and 6 patients with other causes of respiratory failure (ORF group) were studied at 3 VI settings, 40 L/min, 70 L/min, and 100 L/min. In the CAO group, as VI increased from 40 L/min to 100 L/min, arterial oxygen tension increased 18%, venous admixture decreased 23.2%, dead space to tidal volume ratio decreased 23.7%, and tidal compliance of the respiratory system increased 31.2%. In the ORF group, as VI increased, there was little change in most parameters measured. The changes in the CAO group may be explained by redistribution of ventilation to low ventilation-perfusion ratio units, which empty more completely during the longer expiratory times allowed at high VI. Using a simple two-compartment lung model we determined that an increase in expiratory time from 4.2 to 6.1 s (the increase observed in the CAO group as VI increased from 40 L/min to 100 L/min) will result in redistribution of ventilation to long expiratory time constant respiratory units and produce changes in lung mechanics and gas exchange similar to those observed experimentally. We concluded that mechanical ventilation at high VI in patients with CAO is associated with improvement in gas exchange and more even distribution of ventilation, which appear to be due to the increased time available for alveolar emptying.