Continuous pressure breathing is a technique developed to increase the alveolar oxygen tension by breathing oxygen at higher than ambient pressure. In other words the pressure within the alveoli of the lungs is higher than that outside the chest wall. It is found that as much as 16 mm. Hg of such extra pressure can be tolerated for periods of 30 to 60 minutes by the average healthy man. However, as the extra pressure is increased above this figure, evidence of a deleterious effect upon the circulation becomes increasingly apparent. At pressures of 25 to 40 mm. Hg circulatory collapse and fainting become frequent. Often in cases in which this does not ensue there may develop sensations of impending collapse with pallor and sweating, necessitating termination of the test. Although the blood pressure remains above the control level the pulse is small and difficult to feel and the cardiac output is diminished (1). When using these higher pressures not only is much blood pooled in the veins of the arms and legs but also there is a progressive hemoconcentration due to the passage of fluid from the blood stream into the tissue spaces under .the influence of the raised intravascular pressure (2). A considerable fraction of the total blood volume is thus more or less sequestrated from the general circulation under these conditions and when collapse ensues it is in large part due to decreased effective blood volume. The actual total volume of blood thus isolated cannot be readily assessed and there is need of a reliable but simple test which could be used to assay the impairment of circulation induced by pressure breathing.With this aim in view we studied the blood pressure changes occurring during continuous pressure breathing. This was determined in the usual manner on the arm. Such an approach proved unsatisfactory since a fall in the pressure was usually only observed when the subject was on the verge of syncope. Otherwise the blood pressure was the same as the control value or, as was usual, it was somewhat elevated. We then decided to investigate the value of a determination of the changes in kidney function produced by pressure breathing. METHODThe specific function of the kidney studied was its capacity to excrete urea. This information is obtained by determining the rate of urea excretion during any period and dividing this by the blood urea concentration at the time, thus obtaining a value essentially the same as the kidney clearance. Urea was chosen because of the accuracy with which it can be determined and the constancy of its clearance in a given individual under standard conditions. In addition it has been shown to be responsive to physiological changes in the circulatory system which take place in such conditions as severe exercise (3) and hemorrhage (4). The test was carried out as follows: The subject went without breakfast in the morning and drank a liter of water on rising and 500 ml.

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