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Since the early work of Blumgart, Yens and Weiss (1 to 4) many methods have been recommended for measuring the circulation time in various segments of the circulation. In general, these methods have consisted of injecting a substance into an antecubital vein and noting its arrival either at the lung, as with the ether (5) and paraldehyde (6) tests, or at some point along the systemic circulation such as the tongue, brain or carotid body. The fact that the arm to right heart circulation time varies so widely (2, 4) even in the normal individual constitutes a disadvantage to any test which involves the injection of a substance into the antecubital veins. Moreover, subtraction of the arm to lung time (ether, paraldehyde methods) from arm to tongue or carotid body time can give only an indirect value for the measure of the pulmonary to systemic capillary time. Another disadvantage common to many of the methods described in the literature is that the end-point is either subjective or at best not very sharp. Furthermore the end-points with some of the methods are quite unpleasant and at times alarming to the patient. The CO2 inhalation method described by Bornstein (7) in 1912 and improved by Gubner et al. (8) obviates intravenous injection and is a direct measure of pulmonary to systemic capillary time, but the fact that 2 breaths of 50 per cent CO2 are necessary raises the question as to the correct starting time for the measurement. Furthermore, the end-point which consists of stimulation of respiration and flushing of the face is not always sharp. If concentrations other than 50 per cent are used, different values for the circulation time are obtained. In the course of employing the Millikan (9) oximeter to study the degree of arterial unsaturation during breath holding, the idea presented it-

AN OBJECTIVE METHOD FOR DETERMINING CIRCULATION TIME FROM PULMONARY TO SYSTEMIC CAPILLARIES BY THE USE OF THE OXIMETER