Cerebral effects of hypocapnia plus nitroglycerin-induced hypotension in dogs

This study examined the effect of hypocapnia (PaCO2 20 mm Hg) on cerebral metabolism and the electroencephalogram (EEG) findings in 12 dogs during nitroglycerin (NTG)-induced hypotension. Previous studies suggest that NTG is a more potent cerebral vasodilator than sodium nitroprusside or trimethaphan. It was speculated that combining hypocapnia with NTG-induced hypotension would cause less disturbance of cerebral metabolism and the EEG than the disturbances previously reported when hypocapnia was combined with hypotension induced by sodium nitroprusside or trimethaphan. All 12 dogs were examined at 1) normocapnia with normotension; 2) hypocapnia with normotension; and 3) hypocapnia combined with NTG-induced hypotension to mean arterial blood pressure (MABP) levels of 60, 50, and 40 mm Hg. In six dogs the cerebral metabolic rate of oxygen was determined, and the EEG was evaluated using compressed spectral analysis. Brain tissue metabolites were calculated in the other six dogs. During normotension, hypocapnia caused no deterioration of cerebral metabolism or of the EEG. Hypocapnia combined with NTG-induced hypotension caused a decrease of the power of the alpha and beta 2 spectra of the EEG at MABP's of 60 mm Hg or less. At an MABP of 40 mm Hg, brain tissue phosphocreatine and the cerebral energy charge decreased, while the brain tissue lactate:pyruvate ratio increased. Thirty minutes after restoration of normocapnia with normotension, cerebral metabolites returned to initial values, but the power of the EEG alpha and beta 2 spectra was decreased compared to baseline values. The cerebral metabolic disturbances and EEG alterations seen here with hypocapnia plus NTG-induced hypotension were similar to those previously reported with hypocapnia plus sodium nitroprusside-induced hypotension, and less than those previously reported with hypocapnia plus trimethaphan-induced hypotension. For hyperventilated patients, administration of NTG may be a better hypotensive treatment than trimethaphan, but similar in effect to sodium nitroprusside.