Motor-evoked potential changes during hypoxic hypoxia.

Motor-evoked potentials (MEPs) from forearm muscles were recorded in response to single-shock electrical stimulation of motor cortex of rats (n = 15) under pentobarbital anesthesia and controlled room air ventilation. In addition, electroencephalograms (EEGs) were recorded for all animals. Following baseline MEP recording in room air (21% O2), animals were subjected to graded hypoxia of either 15.75%, 10.5%, or 5.25% oxygen for 10 minutes, then followed by room air ventilation for 15 minutes. The mean baseline latency, amplitude, and duration of the evoked muscle response were 4.3 +/- 0.4 mseconds, 556 +/- 476 microV, and 9.6 +/- 2.3 mseconds, respectively. At moderate hypoxia (15.75%), the latency was 4.2 +/- 0.5 mseconds and the amplitude and the duration were 530 +/- 356 microV (n = 14), and 9.5 +/- 2.2 mseconds, (n = 14). These values did not deviate significantly from baseline (p > 0.56). Only one animal lost MEPs at the 15.75% hypoxia level. At 10.5% hypoxia, 27% of animals (n = 4) lost MEP within minutes. In the remaining animals (n = 11), there was a trend toward a prolongation of latency and a decrease of both amplitude and duration. All animals lost MEPs under extreme hypoxia (5.25%) within 2 minutes. No change was seen in the EEG recording until the level of extreme hypoxia was reached. The loss of MEPs at this level of hypoxia was concurrent with the loss of EEGs. We conclude that hypoxia effects MEPs in experimental animals.