The mesenteric hemodynamic response to circulatory shock: an overview.

The mesenteric hemodynamic response to circulatory shock is characteristic and profound; this vasoconstrictive response disproportionately affects both the mesenteric organs and the organism as a whole. Vasoconstriction of post-capillary mesenteric venules and veins, mediated largely by the alpha-adrenergic receptors of the sympathetic nervous system, can effect an "autotransfusion" of up to 30% of the total circulating blood volume, supporting cardiac filling pressures ("preload"), and thereby sustaining cardiac output at virtually no cost in nutrient flow to the mesenteric organs. Under conditions of decreased cardiac output caused by cardiogenic or hypovolemic shock, selective vasoconstriction of the afferent mesenteric arterioles serves to sustain total systemic vascular resistance ("afterload"), thereby maintaining systemic arterial pressure and sustaining the perfusion of non-mesenteric organs at the expense of mesenteric organ perfusion (Cannon's "flight or fight" response). This markedly disproportionate response of the mesenteric resistance vessels is largely independent of the sympathetic nervous system and variably related to vasopressin, but mediated primarily by the renin-angiotensin axis. The extreme of this response can lead to gastric stress erosions, nonocclusive mesenteric ischemia, ischemic colitis, ischemic hepatitis, ischemic cholecystitis, and/or ischemic pancreatitis. Septic shock can produce decreased or increased mesenteric perfusion, but is characterized by an increased oxygen consumption that exceeds the capacity of mesenteric oxygen delivery, resulting in net ischemia and consequent tissue injury. Mesenteric organ injury from ischemia/reperfusion due to any form of shock can lead to a triggering of systemic inflammatory response syndrome, and ultimately to multiple organ dysfunction syndrome. The mesenteric vasculature is therefore a major target and a primary determinant of the systemic response to circulatory shock.