Ventrolateral medullary surface blood flow determined by hydrogen clearance

The H2 clearance technique was used to determine the blood flow of the postulated respiratory chemosensitive areas near the ventrolateral surface of the medulla. In 12 pentobarbital sodium-anesthetized cats, flow (mean +/- SD) was measured from 25-micron Teflon-coated platinum wire electrodes implanted to a depth of 0.3-0.7 mm. Flow (in ml X min-1 X 100 g-1, n = 35) was 52.8 +/- 28.5 in hypocapnia [arterial CO2 partial pressure (PaCO2) = 21.8 +/- 1.6 Torr], 57.8 +/- 27.5 in normocapnia (PaCO2 = 31.9 +/- 2.2 Torr), and 75.0 +/- 31.7 in hypercapnia (PaCO2 = 44.5 +/- 3.0 Torr). Flow determined from 15 electrodes in adjacent pyramidal tracts (white matter) was less at all levels of CO2; 22.9 +/- 12.3 in hypocapnia, 29.1 +/- 15.9 in normocapnia, and 33.9 +/- 13.9 in hypercapnia. In hypoxia [arterial O2 partial pressure (PaO2) = 39.9 +/- 6.3 Torr] ventrolateral surface flow rose to 87.9 +/- 47.6, and adjacent white matter flow was 35.8 +/- 15.6. These results indicate that flow in the postulated central chemoreceptor areas exceeds that of white matter and is sensitive to variations in PaCO2 and PaO2.