Continuous infusion indicator dilution measurement of limb blood flow and vascular response to magnesium sulfate in normotensive and hypertensive men

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Abstract
A constant infusion, indicator dilution technique for blood flow measurements in the forearm and hand of man was tested and validated in vitro and in vivo. This technique employs jet injection to improve mixing of indicator with arterial blood. The mixing characteristics of the jet injection system were studied in vitro in tubing simulating the brachial artery of man. In addition, actual blood flows in the isolated pump-perfused forelimbs of five dogs were compared with constant infusion, indicator dilution calculated flows. Measurements were also made of mixing and of blood flow in the forearm and hand of man. The technique was used to compare forearm and hand vascular responses with constant intrabrachial arterial infusions of magnesium sulfate in 13 normotensive and 13 essential hypertensive men. In vitro and in vivo the jet injection system significantly improved mixing of indicator with blood, as compared with mixing produced by standard infusion techniques, without causing hemolysis. In 30 measurements in isolated, perfused dog forelimbs the correlation coefficient between actual and calculated blood flow was 0.992. Resting limb vascular resistance in the hypertensive group was significantly higher than in the normotensive group. Limb vascular resistance in all 26 men decreased in response to intrabrachial-arterial infusion of 0.25% magnesium sulfate (8 ml/min). Rate of infusion of Mg++ was 0.162 mEq/min. There was a significant positive linear correlation between level of initial limb vascular resistance and magnitude of response to magnesium sulfate. Vascular response data adjusted for this source of variation were similar in hypertensives and normotensives. The data suggest that this constant infusion, indicator dilution technique allows accurate calculation of total limb blood flow in man, provided that anomalous bifurcation of the brachial artery is not present. The data also suggest that the jet injection system improves mixing of substances with arterial blood. Thus, use of this system should especially aid reliability of studies of limb vascular responses to vasoactive agents infused into the brachial artery.