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 from rostral and intermediate areas of the ventral medullary surface in four intact, drug-free cars during acute hypovolemia. Scattered light images, collected during 660 and 560 nm illumination to measure cellular activity and hemodynamic aspects, respectively, were digitized at 50 frames/s during baseline, and during withdrawal of 20-30% blood volume. Hypovolemia elicited a profound hypotension and eventual bradycardia. In all cats, a modest increase in ventral medullary surface reflectance (activity decline) accompanied initial blood loss; as hypovolemia continued, and blood pressure declined, reflectance switched to a decline (activity increase), with the lowest reflectance occurring at maximal blood loss. Hypovolemia elicited multiple transient physiologic behaviors, including tachycardia, tachypnea, intermittent isolated and sustained bursts of enhanced inspiratory efforts, and extensor activation of the somatic musculature. The phasic physiological behaviors during hypovolemia were accompanied by partial recovery of medullary surface reflectance and blood pressure towards baseline values; however, reflectance continued to decrease as blood pressure progressively fell after these recovery efforts. Patterns of reflectance were not uniform over areas examined; isolated regions of enhanced or diminished reflectance appeared upon the overall images, Optical signals indicating hemodynamic changes followed the neural activity patterns, but not precisely. Regions within the ventral surface are responsive to hypovolemia, and to transient behaviors associated with momentary restoration of blood pressure; these ventral surface areas may assume essential roles in the systemic response to hypovolemic induced shock. (C) 1999 IBRO. Published by Elsevier Science Ltd.