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Microcirculatory responses in cat sartorius muscle to hemorrhagic hypotension
I P Torres Filho1, M A Boegehold, E Bouskela
1Department of Physiology, University of Arizona College of Medicine, Tucson 85724.
Insights
Hemorrhagic hypotension caused increased vascular resistance in cat muscles. Despite widespread capillary stoppage, smaller arterioles dilated, potentially offsetting resistance increases.
Area of Science:
- Physiology
- Microcirculation Research
- Hemodynamics
Background:
- Hemorrhagic hypotension leads to increased vascular resistance.
- The precise microcirculatory changes driving this response are not fully understood.
Purpose of the Study:
- To investigate microcirculatory alterations during hemorrhagic hypotension.
- To elucidate the mechanisms behind the rise in vascular resistance.
Main Methods:
- Studied microcirculatory vessel diameter and red blood cell velocity in cat sartorius muscles.
- Maintained hemorrhagic hypotension at 60 mmHg for 4 hours.
Main Results:
- Vascular resistance increased by ~70%; volume flow and red blood cell velocity decreased.
- Capillary blood flow ceased in ~60% of capillaries.
- Arterioles >45 microns constricted, while arterioles <45 microns dilated, with smaller vessels showing greater changes.
Conclusions:
- Arteriolar dilation, particularly in smaller vessels, may counteract increased vascular resistance.
- Constriction of larger arteries and capillary blockage by formed elements could offset this dilatory effect.
Abstract:
The purpose of this study was to examine changes in the microcirculation that might explain the rise in vascular resistance during hemorrhagic hypotension. Diameter and red blood cell velocity of microcirculatory vessels in exteriorized cat sartorius muscles were studied during 4 h of hemorrhagic hypotension at 60 mmHg. During hypotension, vascular resistance of the muscles rose approximately 70% while calculated volume flow in arterioles and venules fell to about the same degree. Average red blood cell velocity for all capillaries showed a comparable decline. Red blood cell flow stopped in approximately 60% of capillaries, but the extent of stoppage varied greatly among capillary fields. Arterioles larger than 45 microns constricted 9-29%, with the largest arterioles showing the greatest constriction. Arterioles smaller than 45 microns dilated 34-56%, with the smallest arterioles showing the greatest dilation. Venular diameter did not change with hemorrhage. The predominance of arteriolar dilation, especially in the later stages of hypotension, should lead to a fall in vascular resistance of the muscle. This effect may be offset by constriction of arteries outside the microcirculatory field observed and blockage of capillaries or venules by formed elements.