End-tidal gas composition is not correlated with episodic breathing in hibernating ground squirrels

Abstract

During hibernation, the breathing pattern of the golden-mantled ground squirrel (Spermophilus lateralis) consists of short episodes of breathing separated by pauses ranging in length from less than 1 min to greater than 30 min. Computerized tomography scans of two hibernating animals during apnea indicated that the glottis was closed and that apneic oxygenation could not occur. Analysis of end-tidal gas composition, indicative of arterial blood gas composition, revealed no clear thresholds in gas composition for initiating or terminating episodes of breathing. Over the course of a breathing episode, however, O2 extraction fell exponentially, while CO2 excretion fell in a linear fashion. The breathing episode terminated at the point where O2 extraction asymptoted, suggesting that the length of the episode was just sufficient to repay the O2 debt that accumulated during the preceding period of apnea. The oxidative cost of the events associated with breathing episodes in these animals was calculated to be approximately 90% of the total metabolic rate during hibernation. This suggests that metabolic rate is not constant during hibernation but varies in a cyclic fashion associated with the breathing pattern. It further suggests that although the levels of blood gases play a key role in establishing the total level of ventilation, the cyclic variations in their composition, associated with periods of apnea and eupnea, do not by themselves initiate or terminate breathing episodes.