Effect of acute carbon monoxide exposure on cardiopulmonary function of the awake rat

Unanesthetized male Sprague-Dawley rats were exposed for 20 min to 0 (control), 2000, or 4000 ppm carbon monoxide, and cardiopulmonary responses were evaluated. Venous blood samples were taken prior to exposure and at 2, 4, 7, 11, 15, and 20 min during exposure. Responses for CO-exposed animals were compared at 20, 30, 40, 50, and 60% carboxyhemoglobin (COHb), and these values were compared to control values at similar times during exposure. Fifty percent COHb was attained within 4 to 8.5 min at 4000 ppm and 11 to 14 min at 2000 ppm. For both CO exposure groups, mean arterial pressure first decreased at 40% COHb and fell to 69% of baseline at 60% COHb. Tidal volume and breathing frequency increases resulted in minute volume elevations of 52 (2000 ppm) and 77% (4000 ppm) at 60% COHb. Minute volume was elevated at 50 and 60% COHb at 4000 ppm, but was not elevated until 60% COHb for 2000-ppm exposures. The earlier ventilatory stimulation at 4000 ppm may have been a by-product of increased agitation observed in rats exposed to the higher concentration.