[Deterioration of oxygenation with mechanical ventilation, the lung time constant and cardiac output]

The influence of the lung time constant (tc) and preset volume on a ventilator-cardiac output ratio (Vv/Q) on the decrease in the oxygenation of blood (PaO2/FiO2) due possibly to mechanical controlled ventilation, was examined with special reference to the time of passive expiration in 18 patients. The degree of deterioration of oxygenation appeared to depend mainly on the time following FRC, during which the blood continued to perfuse the lungs until the next active insufflation by the ventilator. The characteristics of the negative exponential function indicated that the lungs theoretically were passively evacuated by 98% at tc x 4 (= TC) sec. The value for the expiratory time-TC/TC was 3. 7 +/- 1.3 before reduction of this expiratory time. Following reduction, 11 subjects showed improved oxygenation while no change was noted in the other 7. In the former Vv/Q was 1.19 +/- 0.21 and in the latter 0.94 +/- 0.03 before reduction, showing significantly smaller values in the latter (p less than 0.01). Within each of the four [(expiratory time-TC)/TC]/Vv/Q zones between 1.0 and 5.0 (1.0-2.0, 2.0-3.0, 3.0-4.0, 4.0-5.0), Vv/Q of those who showed no change in oxygenation was always less than Vv/Q of those who showed improved oxygenation (p less than 0.01). Thus, it is possible that a relatively prolonged exhalation time compared to the lung time constant, under the conditions of cardiac output dominance in Vv/Q, could be one of the causes of deterioration of oxygenation which occurs frequently during mechanically controlled ventilation.