VENTILATION-PERFUSION IMBALANCE AFTER HEAD TRAUMA

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Abstract
To investigate the role of ventilation-perfusion (.ovrhdot.VA/.ovrhdot.Q) imbalance in the hypoxemia observed after head injury, 5 male subjects (17-26 yr of age) with isolated head trauma and subsequent hypoxemia were studied. Disturbances of ventilation and perfusion were assessed using the steady-state elimination of 6 inert gases of different solubilities. Paired studies were conducted during mechanical ventilation with a volume-cycled ventilator and during spontaneous ventilation. Distributions recovered from studies of spontaneous ventilation show a mode of ventilation and perfusion near a .ovrhdot.VA/.ovrhdot.Q of 1.0. Of the cardiac output 41% was distributed to a 2nd population of lung units with low .ovrhdot.VA/.ovrhdot.Q (< 0.1) and shunt. During mechanical ventilation, perfusion to these regions of low .ovrhdot.VA/.ovrhdot.Q decreased to 21% of the cardiac output, whereas shunt fraction was unchanged. This was associated with a marked broadening of the .ovrhdot.VA/.ovrhdot.Q mode near 1.0, relative to the studies during spontaneous ventilation. Mean functional residual capacity during mechanical ventilation was different from that during spontaneous ventilation. Head injury possibly can lead to hypoxemia through a failure of .ovrhdot.VA/.ovrhdot.Q regulatory mechanisms.

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