ROUTINE cardiovascular monitoring during surgery and in the early postoperative period provides no direct information concerning cardiac output and peripheral resistance. Only blood pressure and pulse rate are regularly measured. Since pressure is a function of flow and resistance, we ordinarily rely on a pressure drop to tell if flow is significantly decreased. The concept is basic to care of the injured. It works usually but pressure alone is not a sensitive indicator of flow since it may be maintained by an increase of resistance when flow decreases.
P = R × [unk]
where P = pressure, R = resistance, and Q̇ = flow. Direct measurement of flow, general and regional, would clearly be valuable. Lacking such measurements practically, however, we might be helped by an indicator of peripheral resistance. Then, theoretically at least, with both pressure and resistance, we could predict flow.
[unk] = P/R
The defects in this simplification are many, but nonetheless,