Role of ventilation in maintaining cardiac output under positive-pressure breathing.

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Abstract
Simultaneous measurements of ventilation (VE) and cardiac output (Q) were obtained in normal subjects breathing either at ambient pressure or against a positive pressure (PPB) of 5, 10, or 25 cm H2O. When this pressure is increased, Q tends to decrease. This may however be masked by concomitant changes in VE. When the latter is constant at 10 liters. min-1, Q decreases by approximately 0. 2 liter. min-1 for each additional cm H2O of PPB. Therefore, at this ventilatory level, a PPB of 30-35 cm H2O would be associated with a Q value of zero. At any level of PPB an increase in VE results in an increase in Q. This increase is 0. 3 liter. min-1 per liter. min-1 change in VE when frequency of breathing is constant, but only 0. 15 when frequency is increased, which my indicate that tidal volume per se may be involved.

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