Transvascular fluid exchange with application to the brain. An experimental study in cat skeletal muscle and brain

University dissertation from Department of Anaesthesia and Intensive Care, University Hospital Lund

Abstract: This study evaluated transcapillary fluid exchange in the cat brain and in a cat skeletal muscle enclosed in a plethysmograph. A method for determination of capillary permeability (capillary filtration coefficient and reflection coefficient) and effects of transcapillary hydrostatic pressure changes were studied We found that capillary fluid permeability in a tissue can be determined by the capillary filtration coefficient, even when arterial pressure and vascular tone are changed. By evaluating relative variations in the reflection coefficient for albumin rather than absolute values, common error factors were reduced. The reflection coefficient for albumin decreased following surgical trauma, with a resultant decrease in plasma albumin, whereas the influence on fluid permeability was insignificant. Dural puncture reduces intracranial pressure due to a hydrostatic pressure gradient generated by the distal opening of the spinal fluid column, especially in the upright position. This may cause an increased transvascular pressure, dilation of the cerebral outflow veins and - when the blood-brain barrier is disrupted - increased fluid filtration to the interstitial tissue, causing oedema and even brain stem herniation. We found that in our muscle model and in the cat brain with depressed autoregulation and disrupted blood-brain barrier (induced by intrathecal endotoxin), the arterial pressure and the tissue pressure changed in parallel. Variations in venous pressure and in the surrounding fluid volume did not influence the tissue pressure. We conclude that following a traumatic head injury, or during a cerebral infection, elevation of the arterial pressure will increase transcapillary filtration and aggravate the interstitial brain oedema, whereas a decreased arterial pressure will decrease the oedema.

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