RELATIONSHIP OF DIAPHRAGMATIC CONTRACTILITY TO DIAPHRAGMATIC BLOOD FLOW IN A NEWBORN MODEL

Abstract

We determined the relationship of diaphragmatic contraction rate to diaphragmatic blood flow (Qdi), metabolism, and contractility in 6 newborn lambs. The diaphragm was paced for 15 min. at slow (SC, 20/min) and fast (FC, 100/min) contraction rates each followed by a 30 min. recovery period in open-chested, mechanically ventilated lambs. Qdi was measured with radiolabeled microspheres. Transdiaphragmatic pressure (Pdi) was measured with the abdomen tightly casted to preserve length and geometry of the diaphragm. Diaphragmatic fatigue was defined as a reduction in Pdi at stimulation frequencies of 10-100 Hz. With SC there was a mild reduction in Pdi at all stimulation frequencies. Pdi was profoundly reduced at all stimulation frequencies during FC with only partial recovery. During SC there was a significant increase compared to control In Qdi (273%), diaphragmatic O2 delivery (ODdi 273%), O2 consumption (VO2di, 593%), and fractional O2 extraction (Edi, 188%). During FC there was a further increase in Qdi (332%), ODdi (331%), and VO2di (729%). These increases were also significantly different from SC (P<0.05). Edi was increased compared to control (188%) but similar to SC. Lactate gradient (lac v-a) remained unchanged. We conclude that diaphragmatic fatigue at fast contraction rates is unrelated to a limitation in aerobic metabolism, as the muscle is able to increase ODdi and VO2di while lac v-a remains unchanged.