EFFECT OF ACUTE HYPERCAPNIA ON DIAPHRAGMATIC AND LIMB MUSCLE CONTRACTILITY

The purpose of this study was to determine whether acute hypercapnia d epresses 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 cer vical 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 durin g transcutaneous supramaximal ulnar nerve stimulation and arterialized venous blood gas determinations were obtained before, during, and aft er CO2 breathing. Twitch Pdi was not significantly different from base line at any time after CO2 breathing. Similarly, twitch Pdi did not si gnificantly change from the beginning to the end of the period of CO2 breathing. In contrast, adductor pollicis twitch force was significant ly less than baseline 2 min after CO2 breathing (p < 0.01). Furthermor e, during CO2 breathing, adductor pollicis twitch force fell significa ntly, reaching statistical significance at 8 min, decreasing still fur ther by 13 min, and then plateauing. During the final 2 min of CO2 bre athing, adductor pollicis twitch force averaged 86.5 +/- 2.7% of the b aseline value (p < 0.0007). During CO2 breathing, Pco(2) rose to 60.1 +/- 0.5 mm Hg, whereas pH fell to 7.27 +/- 0.004 units. Breathing 8% C O2 elicited a moderate hyperpnea, and minute ventilation during the fi nal minute of CO2 breathing averaged 54.74 +/- 4.90 L/min. To ensure t hat this hyperpnea did not augment diaphragmatic activity enough to po tentiate the twitch, five subjects voluntarily mimicked their CO2 hype rpnea on a separate occasion. Twitch Pdi was not significantly differe nt 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 mil dly depresses limb muscle contractility, and (2) acute moderate hyperc apnia did not produce significant changes in diaphragmatic contractili ty.