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The study to be reported was undertaken with the purpose of thr.owing light on the mechanism of vasomotor collapse in man. During a previous investigation (1), it was noted that small doses of sodium nitrite, which in normal subjects would produce no symptoms and slight if any circulatory changes in the prone position, would in the upright position lead to progressive vasomotor collapse, often terminating in syncope. Return to the prone position was followed by immediate recovery without ill effects. By varying the dose of sodium nitrite and the angle of tilting the body, the duration as well as the degree of the circulatory collapse could be regulated. This method, then, offered an opportunity of recording observations on vasomotor collapse under well-controlled experimental conditions. This seemed particularly desirable in view of the fact that the study of vasomotor collapse, as it occurs in various diseases, is difficult because it develops unexpectedly in seriously ill patients and often terminates fatally. METHODOur primary aim was to observe simultaneously several aspects of the peripheral circulation before, during and after recovery from vasomotor collapse. The heart rate was counted by arterial palpation, by auscultation over the precordium, or from the pulse waves on the plethysmographic tracings. The arterial blood pressure was determined in the upper arm at heart level by the usual auscultatory method, using a mercury manometer. The arterial blood pressure and the pulse volume were registered by sphygmometric oscillometers.The venous pressure was measured in the foot by the indirect method of Krogh, Turner and Landis (2). In some experiments the pressure in the femoral vein was determined by direct venous puncture, after the method of Moritz and von Tabora (3). All measurements are given in relation to the right auricle (second costal interspace). The skin temperature of the hands and feet was taken by means of a thermocouple.1 This investigation was aided in part by a grant from the Josiah Macy, Jr., Foundation.The blood flow through the hands was measured by the plethysmographic method of Hewlett and Van Zwaluwenburg (4) as modified by Freeman (5). The average of S to 10 determinations was taken for each flow. The plethysmographs were arranged on adjustable stands which allowed free movement up or down as the subject was tilted. Usually the blood flow was determined through one hand at a temperature of 320 C., and through the other at 450 C. At longer, followed by a return to the prone position; (2) the effect of oral administration of 0.12 to 0.18 grams (2 or 3 grains) of sodium nitrite in the horizontal position for one hour; (3) the effect of the same amount of sodium nitrite followed in 10 to 20 minutes by elevation to the upright position. While in the upright position the subjects were urged to remain motionless. At the height of the vasomotor collapse, which was usually 73

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