MJ Mador, T Wendel and TJ Kufel
The Division of Pulmonary and Critical Care Medicine, State University of New York at Buffalo, Veterans Administration Medical Center 14215, USA.

The purpose of this study was to determine whether acute hypercapnia depresses limb muscle and diaphragmatic contractility. Twelve subjects breathed 8% CO2 for 20 min on two separate occasions. On one occasion, twitch transdiaphragmatic pressure (Pdi) was obtained during both cervical magnetic and transcutaneous stimulation before and 2, 10, and 30 min after CO2 breathing. In addition, twitch Pdi was obtained during CO2 breathing in five subjects using cervical magnetic stimulation. On the other occasion, adductor pollicis twitch force was measured during transcutaneous supramaximal ulnar nerve stimulation and arterialized venous blood gas determinations were obtained before, during, and after CO2 breathing. Twitch Pdi was not significantly different from baseline at any time after CO2 breathing. Similarly, twitch Pdi did not significantly change from the beginning to the end of the period of CO2 breathing. In contrast, adductor pollicis twitch force was significantly less than baseline 2 min after CO2 breathing (p < 0.01). Furthermore, during CO2 breathing, adductor pollicis twitch force fell significantly, reaching statistical significance at 8 min, decreasing still further by 13 min, and then plateauing. During the final 2 min of CO2 breathing, adductor pollicis twitch force averaged 86.5 +/- 2.7% of the baseline value (p < 0.0007). During CO2 breathing, Pco2 rose to 60.1 +/- 0.5 mm Hg, whereas pH fell to 7.27 +/- 0.004 units. Breathing 8% CO2 elicited a moderate hyperpnea, and minute ventilation during the final minute of CO2 breathing averaged 54.74 +/- 4.90 L/min. To ensure that this hyperpnea did not augment diaphragmatic activity enough to potentiate the twitch, five subjects voluntarily mimicked their CO2 hyperpnea on a separate occasion. Twitch Pdi was not significantly different from baseline at any time after voluntary mimicking. To ensure that changes in diaphragmatic contractility were not missed by our twitch measurements, Pdi was measured during bilateral transcutaneous phrenic nerve stimulation at 10 Hz in four subjects. Again, Pdi during 10 Hz stimulation was not significantly different from baseline at any time after CO2 breathing. In conclusion, (1) acute moderate hypercapnia mildly depresses limb muscle contractility, and (2) acute moderate hypercapnia did not produce significant changes in diaphragmatic contractility.

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