Effects of focal cooling in the ventrolateral medulla on chemoresponsiveness in dogs

Studies in cats and dogs have shown that the ventrolateral region of the medulla participates significantly in the shaping of the respiratory rhythm. The purpose of this study was to examine the effects of unilateral focal cooling (15-20 degrees C) in the ventrolateral medullary region on respiratory responses to hypercapnia and hypoxia in dogs. A cryoprobe was used to cool selected locations in the ventrolateral medulla in 9 anesthetized and vagotomized dogs. Diaphragmatic electromyogram (EMG) was measured with implanted electrodes. The animals were ventilated artificially at a constant rate with 100% O2 and the inspired gas was switched to 7% CO2 in O2 or 10% O2 in N2 to determine the response to hypercapnia or hypoxia. The sites cooled ranged 4.0-8.0 mm rostral to obex, 3.0-5.5 mm lateral to midline, and within 1.5 mm deep from the ventral surface of the medulla. Unilateral focal cooling in this region significantly decreased the responses of both the amplitude and the rate of rise of diaphragmatic EMG to hypercapnia and hypoxia. These results support the hypothesis that neural structures in the ventrolateral medulla are important in the respiratory responses to hypoxia and hypercapnia as well as for the setting of respiratory drive and timing.