Cardiovascular changes during varied thermal stress

The effect of thermal stress on healthy mongrel dogs was studied at ambient temperatures of 40, 45 and 50 degrees C. At 40 degrees C there was a linear increase in cardiac output, heart rate and oxygen consumption and cardiovascular failure did not occur even after 6 hours of exposure. At 45 degrees C the circulation was hyperdynamic up to 60 min but at 90 min there was a steep fall in oxygen consumption, cardiac output, stroke volume and mean arterial pressure whereas heart rate increased significantly. At 50 degrees C the hyperdynamic changes reached peak values in 30 min; but by 60 min, there was a collapse of cardio-vascular function leading to rapid decline and ultimate death. It seems that the point at which there is a sudden fall in oxygen consumption, cardiac output, stroke volume and mean arterial pressure could be taken as an index of significant heat injury.