This study tested the hypothesis that hypocapnia superimposed upon hypotension produces a further reduction in cerebral blood flow velocity (CBFV). In 12 newborn piglets, CBFV was measured continuously by Doppler ultrasound through an artificial fontanelle. Hypotension was induced by removing 30 ml/kg of blood. Increasing the ventilator rate reduced the average arterial carbon dioxide tension from 5.5 to 2.0 kPa. When mean arterial pressure (MAP) was held steady at 45 mm Hg or above, hypocapnia produced a substantial drop in CBFV but, in all the piglets with MAP below 38 mm Hg, hypocapnia failed to change CBFV by 10%. Hypocapnia produced an increase in lactate in sagittal sinus blood but cerebral venous hypoxanthine concentrations were not affected by hypocapnia. Hyperventilation (without haemorrhage) produced a significant drop in MAP, preventable by infusing colloid. Hypocapnia itself does not further reduce CBFV in the hypotensive piglet. However, the pressure effect of hyperventilation may significantly impair the cerebral circulation.
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