Shock is the clinical syndrome that results from inadequate tissue perfusion. Irrespective of cause, the hypoperfusion-induced imbalance between the delivery of and requirements for oxygen and substrate leads to cellular dysfunction. The cellular injury created by the inadequate delivery of oxygen and substrates also induces the production and release of damage-associated molecular patterns (DAMPs or "danger signals") and inflammatory mediators that further compromise perfusion through functional and structural changes within the microvasculature. This leads to a vicious cycle in which impaired perfusion is responsible for cellular injury that causes maldistribution of blood flow, further compromising cellular perfusion; the latter ultimately causes multiple organ failure (MOF) and, if the process is not interrupted, leads to death. The clinical manifestations of shock are also the result, in part, of autonomic neuroendocrine responses to hypoperfusion as well as the breakdown in organ function induced by severe cellular dysfunction (Fig. 270-1).
Shock-induced vicious cycle.
When very severe and/or persistent, inadequate oxygen delivery leads to irreversible cell injury; only rapid restoration of oxygen delivery can reverse the progression of the shock state. The fundamental approach to management, therefore, is to recognize overt and impending shock in a timely fashion and to intervene emergently to restore perfusion. This often requires the expansion or reexpansion of intravascular blood volume. Control of any inciting pathologic process (e.g., continued hemorrhage, impairment of cardiac function, or infection), must occur simultaneously.
Clinical shock is usually accompanied by hypotension (i.e., a mean arterial pressure (MAP) <60 mmHg in previously normotensive persons). Multiple classification schemes have been developed in an attempt to synthesize the seemingly dissimilar processes leading to shock. Strict adherence to a classification scheme may be difficult from a clinical standpoint because of the frequent combination of two or more causes of shock in any individual patient, but the classification shown in Table 270-1 provides a useful reference point from which to discuss and further delineate the underlying processes.
Table 270–1. Classification of Shock |Favorite Table|Download (.pdf)
Table 270–1. Classification of Shock
Normally when cardiac output falls, systemic vascular resistance rises to maintain a level of systemic pressure that is adequate for perfusion of the heart and brain at the expense of other tissues such as muscle, skin, and especially the gastrointestinal (GI) tract. Systemic vascular resistance is determined primarily by the luminal diameter of arterioles. The metabolic rates of the heart and brain are high, and their stores of energy substrate are low. These organs are critically dependent on a continuous supply of oxygen and nutrients, and neither tolerates severe ischemia for more than brief periods (minutes). Autoregulation (i.e., the maintenance of blood flow over a wide range of perfusion pressures), is critical in sustaining cerebral and coronary perfusion despite significant hypotension. However, when MAP drops to ≤60 mmHg, blood flow to these organs falls, and their function deteriorates.
Arteriolar vascular smooth muscle has both α- and β-adrenergic receptors. The α1 receptors mediate vasoconstriction, while the β2 receptors mediate vasodilation. Efferent sympathetic fibers ...