Organ perfusion during controlled hypotension.

Relatively short periods of unwanted hypotension, such as those associated with "shock" states, may be followed by irreversible organ damage. In contrast, controlled hypotension is rarely followed by organ damage, although minor reversible changes in function may occur (Rollason and Hough, 1960; Enderby, 1961; Linacre, 1961). The usual explanation for this paradox is that in the former situation hypotension is accompanied by severe decreases in organ blood-flow, with associated acid-base and metabolic changes, whereas with deliberate hypotension organ blood-flow is well maintained and the other changes do not occur. Clearly, even under controlled conditions, there must be some arterial pressure at which organ flow is no longer adequate. Indeed, the question is commonly asked, "What reduction in arterial pressure is safe?" In fact, what is really required is the answer to the question, "Is the flow adequate?" When considering individual organs, measurement of blood-flow is difficult, and in the main the techniques are only practical under somewhat artificial conditions, or are confined to animal studies. The practical anaesthetist, therefore, has relied on measuring systemic arterial pressure, and after accepting some value as "normal" has assumed that the individual pressures and therefore perfusion of the various organs in the body is adequate. It does not necessarily follow that the perfusion of an individual organ is reflected in the systemic arterial pressure even if one assumes, perhaps incorrectly, that the pressure there is indeed related to flow.