Institute for Clinical and Translational Science

Purpose

During anesthesia, maneuvers which cause the least disturbance of cerebral oxygenation with the greatest decrease in intracranial pressure would be most beneficial to patients with intracranial hypertension. Both head-up tilt (HUT) and hyperventilation are used to decrease brain bulk, and both may be associated with decreases in cerebral oxygenation. In this observational study, our null hypothesis was that the impact of HUT and hyperventilation on cerebral tissue oxygen saturation (SctO₂) and cerebral blood volume (CBV) are comparable.

Surgical patients without neurological disease were anesthetized with propofol-remifentanil. Before the start of surgery, frequency-domain near-infrared spectroscopy was used to measure SctO₂ and CBV at the supine position, at the 30° head-up and head-down positions, as well as during hypoventilation and hyperventilation.

Thirty-three patients were studied. Both HUT and hyperventilation induced small decreases in SctO₂ [3.5 (2.6)%; P <0.001 and 3.0 (1.8)%; P <0.001, respectively] and in CBV [0.05 (0.07) mL·100 g⁻¹; P <0.001 and 0.06 (0.05) mL·100 g⁻¹; P <0.001, respectively]. There were no differences between HUT to 30° and hyperventilation to an end-tidal carbon dioxide (ETCO₂) of 25 mmHg (from 45 mmHg) in both SctO₂ (P = 0.3) and CBV (P = 0.4).

The small but statistically significant decreases in both SctO₂ and CBV caused by HUT and hyperventilation are comparable. There was no correlation between the decreases in SctO₂ and CBV and the decreases in blood pressure and cardiac output during head-up and head-down tilts. However, the decreases in both SctO₂ and CBV correlate with the decreases in ETCO₂ during ventilation adjustment.