Physiological and ventral medullary surface activity during hypovolemia

The objective was to determine ventral medullary surface responses to blood loss sufficient to induce shock. We examined changes in scattered light fr om rostral and intermediate areas of the ventral medullary surface in four intact, drug-free cars during acute hypovolemia. Scattered light images, co llected during 660 and 560 nm illumination to measure cellular activity and hemodynamic aspects, respectively, were digitized at 50 frames/s during ba seline, and during withdrawal of 20-30% blood volume. Hypovolemia elicited a profound hypotension and eventual bradycardia. In all cats, a modest incr ease in ventral medullary surface reflectance (activity decline) accompanie d initial blood loss; as hypovolemia continued, and blood pressure declined , reflectance switched to a decline (activity increase), with the lowest re flectance occurring at maximal blood loss. Hypovolemia elicited multiple tr ansient physiologic behaviors, including tachycardia, tachypnea, intermitte nt isolated and sustained bursts of enhanced inspiratory efforts, and exten sor activation of the somatic musculature. The phasic physiological behavio rs during hypovolemia were accompanied by partial recovery of medullary sur face reflectance and blood pressure towards baseline values; however, refle ctance continued to decrease as blood pressure progressively fell after the se recovery efforts. Patterns of reflectance were not uniform over areas ex amined; isolated regions of enhanced or diminished reflectance appeared upo n the overall images, Optical signals indicating hemodynamic changes follow ed the neural activity patterns, but not precisely. Regions within the ventral surface are responsive to hypovolemia, and to tr ansient behaviors associated with momentary restoration of blood pressure; these ventral surface areas may assume essential roles in the systemic resp onse to hypovolemic induced shock. (C) 1999 IBRO. Published by Elsevier Sci ence Ltd.