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.