Continuous measurement of alveolar CO2 tension during the hyperventilation test in routine electroencephalography

Abstract

1. A method is described for automatic and continuous measurement of the alveolar carbon dioxide during the hyperventilation test in routine electroencephalography. It depends upon the specific absorption of infrared by CO2, as measured by an infra-red analyzer (K. Luft) which transforms changes in absorption into deflections of a membrane-condenser. The CO2 concentration is recorded through the EEG amplifier on the ink-writer tape, and it appears as an alternating voltage of 7 cycles per second, the amplitude of each cycle representing the instantaneous CO2 concentration in the analyzer. 2. Using this method, 98 healthy flying cadets were tested during a routine three-minute hyperventilation. There was a wide distribution of hypocapnic levels reached, covering the range of 10–28 mm. of Hg. The median of CO2 tensions after one minute was 22.5 mm; after two minutes it was 20.5 mm., and after three minutes, 18.5 mm. Hypocapnia persisted in each case for more than two minutes after “normal breathing” was demanded. 3. The first appearance of slow wave activity in the EEG occurred over a wide range of alveolar CO2 tensions, Whereas a number of cases went as low as 10–15 mm. without exhibiting slow waves, others showed a response as high as 25–28 mm. Hg when hyperglycemia was not assured. With a selected group of five subjects, it was demonstrated that raising the blood sugar above 130 mgm. per cent necessitated a lower CO2 tension for the elicitation of a given slow wave response, as compared to the fasting state. — The progress through the different degrees of slow wave build-up was accompanied by only slight changes in alveolar CO2 tension. In half of the cases, the transition from 5–8 per sec. activity to 2–4 per sec. activity occurred within a range of ± 1 mm. 4. The disappearance of slow wave activity after termination of hyperventilation generally occurred at a higher CO2 level than that of its appearance during hyperventilation. In the mean, 2–4 per sec. waves disappeared at a tension 2.3 mm. higher, and 5–8 per sec. waves required an increment of 3.2 mm. Persistence of slow waves beyond two minutes post-hyperventilation indicated that there was either a facile EEG response at a relatively high CO2 level or an inadequate recuperation from a low CO2 level.