Influence of changes in arterial carbon dioxide tension on the electroencephalogram and posterior tibial nerve somatosensory cortical evoked potentials during alfentanil/nitrous oxide anesthesia

The effects of variation of arterial CO2 tension (Pa(CO₂)) on the electroencephalogram (EEG) and posterior tibial nerve somatosensory cortical evoked potentials (PTN-SCEP) during opioid/N₂O anesthesia have not been well documented. We studied the effects of hypocapnia (Pa(CO₂) ~ 23 mm Hg) and hypercapnia (Pa(CO₂) ~ 50 mmHg) during steady-state alfentanil/N₂O anesthesia in 16 patients. EEG and PTN-SCEP were recorded continuously, while Pa(CO₂) was altered in 15-min intervals by varying the inspired CO₂ concentration. Hypocapnia caused significant increases in power in the delta, theta, and beta bands (P < 0.01), with the greatest increase observed in the alpha band. Relative power increased in the alpha band but remained unchanged in the delta, theta, and beta bands. Median frequency and 95% spectral edge frequency were unaltered during hypocapnia. In contrast, hypercapnia caused a significant decrease of power in the alpha and beta bands, whereas delta and theta power remained unchanged. This was reflected in a significant decrease of the 95% spectral edge frequency, from 8.9 (6.7-11.6) to 7.0 (5.6-8.6) Hz. All EEG parameters returned to normal upon restoration of normocapnia. There was a significant negative correlation between power in the alpha band and end-tidal CO₂ in all patients (r = -0.47 to -0.89). PTN-SCEP latencies and amplitudes were not significantly different from control values during hypocapnia and hypercapnia. It is concluded that variations in Pa(CO₂) within the limits 20-50 mmHg produce substantial changes in the EEG power spectrum, especially in the alpha band (8-12 Hz), but do not alter PTN-SCEP.