Pressure-flow relationships in an in vitro model of compartment syndrome. Shrier, Ian, and Sheldon Magder. Herzl Family Practice Centre and Lady Davis Institute, Sir Mortimer B. Davis Jewish General Hospital, Montreal, Qc.; Critical Care Division, Royal Victoria Hospital, Montreal, Qc.; and the Meakins -Christie Laboratories, McGill University, Montreal, Qc.
APStracts 2:0106A, 1995.
Compartment Syndrome is a condition in which an increase in intramuscular pressure (Pim) decreases blood flow to skeletal muscle. According to the Starling resistor (i.e. vascular waterfall) model of blood flow, the decrease in flow could occur through an increase in arterial resistance (Rart), or an increase in the critical closing pressure (Pcrit). An increase in venous resistance (Rven) might also decrease flow, but only when the downstream pressure becomes greater than Pcrit. To determine which explains the decrease in flow, we produced the equivalent of an increase in intramuscular pressure by putting the canine gastrocnemius muscle in an airtight box and then pump-perfused it. We measured Pcrit, Rart, arterial compliance (Cart), small venular pressure (Pel, measured by the double occlusion technique) and Rven. Box pressure (Pbox) was adjusted to produce 100% (Pbox=0), 90%, 75%, and 50% of control flow. An increase in Pbox limited flow mainly through an increase in Pcrit (75-85%), with only small changes in Rart (15-25%), and no change in Cart. Increases in Pbox also produced a vascular waterfall in the venous circulation. Increases in Pbox increased Pel but the transmural pressure always remained less than control. Finally, small increases in Pbox did not change Pel despite a 10% reduction in flow. This provides further evidence that a vascular waterfall exists at the arteriolar level in the skeletal muscle circulation. We conclude that increases in the pressure surrounding a muscle limit blood flow mostly through increases in Pcrit, although increases in Rart also play a small role. Transmural pressure across the small venules (and presumably the capillaries) decreases with increases in Pim which contradicts the currently held belief that Compartment Syndrome is due to swelling which causes ischemia which causes more swelling and more ischemia. 

Received 9 May 1994; accepted in final form 28 November 1995.
APS Manuscript Number A440-4.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 21 March 1995.