Method for estimation of the perfusion defect in shock

Abstract

A method for estimating the distribution of systemic blood flow and oxygen transport is described and preliminary data obtained in anesthetized dogs before and during acute hypoxia under control, hypovolemic and terminal conditions. We assume there is a capillary blood flow made up of: nutritional blood flow, which is defined as the minimum flow needed to transport the consumed oxygen, and the reserve blood flow, the flow required for the reserve oxygen transport. The latter is estimated from the change in oxygen transport produced by hypoxia. The nonnutritional (shunt) flow is the difference between cardiac output and the capillary flow. With hypovolemia, the calculated nutritional flow decreased, but expressed as a percentage of cardiac output, it increased from mean control values of 25 to over 50%. At the same time, the reserve flow decreased from control values of 55% to zero, while nonnutritional flow and as a percent of cardiac output increased significantly. The data suggest that this method may provide a useful index of the systemic perfusion defect of shock. Poor tissue perfusion as well as low cardiac output, stagnant anoxia, decreased effective circulating blood volume and other rather ill defined factors have been implicated as the causes of shock [5, 6, 7, 141. Low blood flow has been documented in hemorrhagic shock, cardiogenic shock, and in the terminal stages of other categories of shock. However, traumatic shock, septic shock and most other clinical shock syndromes not associated with hypovolemia or central pump failure have normal or high cardiac output [7]. Moreover, in the early period of most clinical shock states there is an increased cardiac drive of central origin from increased autonomic neural activity; this stimulates cardiac output unless limited by central pump failure or hypovolemia [12]. Clearly, then, explanation of the physiologic defect of shock must be sought in other than low blood flow. Following the initial studies of Berggren [l], Riley and Coumand [lo] developed a method for estimating pulmonary capillary blood flow and blood flow through shunts, “pulmonary venous admixture.” The latter could be derived, if the “ideal” alveolar PO, were measurable; the composition of the “ideal” alveolar air and of the “ideal” blood leaving the alveolar capillary was defined as the gas composition that would occur if the alveolar air were homogenous throughout all parts of the lung and if perfect equilibrium of the gases were reached between blood and gas phases [lo]. Also implied is that the patient is in a steady state, that the inspired air entering the alveoli and the venous blood entering the alveolar capillary are constant throughout the lung and finally that this could be demonstrated by direct sampling and analysis [lo]. In the Riley method [lo], end-capillary oxygen tension (Pcoz) which is assumed equal to alveolar oxygen tension (PAo,) is estimated by giving the subject 100% O2 by a closed system for 20 min to wash out all alveolar Nz; alveolar gas then consists only of water vapor, CO, and 0,. The partial pressure of water may be assumed to be 47 torr, when alveolar gas is completely