Effects of mild hypothermia on the cerebral microvascular tone
Acute blood loss is associated with deterioration of blood circulation, including microcirculation. Clinical and experimental studies are focused on the search for the possibility of neutralizing the consequences of such impairment. The use of hypothermia is considered not only as a method to improve survival, but also as a method to improve cerebral microcirculation in hemorrhage. The study was aimed to assess the state of the rats’ cerebral arteries in cases of mild hypothermic exposure after acute moderate blood loss. The study involving anesthetized Wistar rats was performed by vital microscopy. We assessed the responses of pial arteries (initial diameter 10–40 µm) in animals cooled to the rectal temperature of 34 °С under conditions of hemodynamic stability and when simulating blood loss (20% of total blood loss) in normothermic animals and animals with mild hypothermia. The findings showed that 3.5 h of exposure were associated with vasoconstriction in animals of all studied groups. Hypothermic state of the body was associated with initial decrease in the diameter by 9% of the baseline (24.9 ± 0.9 µm to 22.7 ± 0.7 µm; p < 0.05) followed by restoration of the diameter after 2 h (to 25.7 ± 1.7; p > 0.05). Blood loss was associated with the decrease in the diameter of cerebral blood vessels by 20–25% within the first hour (23.4 ± 0.7 µm to 17.6 ± 1.1 µm; p < 0.001) and no subsequent restoration (the diameter was 16.7 ± 0.8 µm after 3.5 h of monitoring). When using hypothermia, vasoconstriction following blood loss was 8–10% in the first 45 min of monitoring (22.6 ± 1.3 µm to 20.3 ± 1.2 µm; p < 0.05), then the constriction decrease was observed (the diameter was 21.4 ± 1.4 µm after 3.5 h of monitoring). It was concluded that the use of mild hypothermia resulted in the reduced vasoconstrictor effect of moderate blood loss on the pial microvessels.
Keywords: hypothermia, blood loss, cerebral vessels