Contribution of the spontaneous crossed-phrenic phenomenon to inspiratory tidal volume in spontaneously breathing rats

Dougherty BJ, Lee KZ, Lane MA, Reier PJ, Fuller DD. Contribution of the spontaneous crossed-phrenic phenomenon to inspiratory tidal volume in spontaneously breathing rats. J Appl Physiol 112: 96-105, 2012. First published October 27, 2011; doi:10.1152/japplphysiol.00690.2011.-Spinal cord hemisection at C2 (C2HS) severs bulbospinal inputs to ipsilateral phrenic motoneurons causing transient hemidiaphragm paralysis. The spontaneous crossed-phrenic phenomenon (sCPP) describes the spontaneous recovery of ipsilateral phrenic bursting following C2HS. We reasoned that the immediate (next breath) changes in tidal volume (V-T) induced by ipsilateral phrenicotomy during spontaneous breathing would provide a quantitative measure of the contribution of the sCPP to postinjury V-T. Using this approach, we tested the hypothesis that the sCPP makes more substantial contributions to V-T when respiratory drive is increased. Pneumotachography was used to measure V-T in anesthetized, spontaneously breathing adult male rats at intervals following C2HS. A progressive increase in V-T (ml/breath) occurred over an 8 wk period following C2HS during both poikilocapnic baseline breathing and hypercapnic respiratory challenge (7% inspired CO2). The sCPP did not impact baseline breathing at 1-3 days postinjury since V-T was unchanged after ipsilateral phrenicotomy. However, by 2 wk post-C2HS, baseline phrenicotomy caused a 16 +/- 2% decline in V-T; a comparable 16 +/- 4% decline occurred at 8 wk. Contrary to our hypothesis, the phrenicotomy-induced declines in V-T (%) during hypercapnic respiratory stimulation did not differ from the baseline response at any postinjury time point (all P > 0.11). We conclude that by 2 wk post-C2HS the sCPP makes a meaningful contribution to V-T that is similar across different levels of respiratory drive.