Controlled Hyperventilation-Reply

In Reply.  —Levy cites two references to demonstrate that controlled HV only briefly reduces ICP and risks subsequent rebound hypertension increasing mortality from head trauma. These conclusions overstate or misinterpret the cited literature.Raichle et al¹ studied normal dogs and humans undergoing controlled or voluntary HV, respectively, and demonstrated a transient fall in CBF followed by elevated CBF above baseline values. These investigators, however, lowered PaCO₂ below 20 mm Hg. We emphasized in our review that such severe hypocapnia produces EEG abnormalities and cerebral lactic acidosis, and that hypocapnia in head trauma should be maintained between 25 to 30 mm Hg. Indeed, the cited reference demonstrated a fivefold increase in systemic lactate production during peak HV. Therefore, the transient fall in CBF and subsequent rebound may have occurred because increased CSF lactate overcomes the cerebral vasoconstrictive effects of hypercapnia, which is based on raising extracellular pH. Additionally, transposing