DAVID L. DRABKIN 



such values for per cent Hb0 2 corresponded respectively to 65 and 

 80 mm of Hg for the arterial p0 2 , whereas the p0 2 of the alveolar air 

 was known to be of the order of 100 mm. Herein lay the origin of the 

 concept, accepted and regarded as authoritative for many years, that 

 an appreciable difference (Ap0 2 ) existed between the tensions of 

 oxygen in alveolar air and arterial blood. This Ap0 2 was accorded 

 the status of a ' physiological ' phenomenon and was explained by the 

 hypothesis that ' oxygen equilibrium is not attained until after passage 

 through the lung capillaries ' 21 . 



During the late war I had the pleasure of participating with Roughton 

 in a memorable session of our Philadelphia Physiological Society 

 (meeting of 16 May 1944) at which three independent investigations, 

 with mutually concordant results, were simultaneously reported. The 

 validity of the measurements, upon which the Ap0 2 was based, was 

 challenged, and the existence of the sacred cow (or at least its size) 

 was questioned. Roughton and his colleagues 16 had analyzed the 

 sources of error in the indirect gasometric determination of the oxygen 

 saturation of blood, and concluded that the results were about 2 per 

 cent too low. J. H. Comroe Jr. and R. D. Dripps Jr. 22 had found by 

 direct measurement (with an adaptation of the Scholander micro gas 

 analyzer 23 ) that the/?0 2 of arterial human blood was of the order of 

 97 mm of Hg, or only slightly lower than that of alveolar air. Of both 

 historic and present interest is the fact that the magnitude of the newly 

 reported arterial p0 2 agreed with two early and, at that time, excep- 

 tionally high values published by Barcroft and his team 24 ' 25 . Finally, 

 D. L. Drabkin and C. F. Schmidt 10 had succeeded in measuring 

 directly by means of spectrophotometry the percentage of oxyhaemo- 

 globin in the arterial blood of dog and man. In both species the mean 

 values for this quantity were 98-5 per cent, or 3 to 3-5 per cent higher 

 than that deduced from the indirect gasometric measurements. 



The simplest way to suggest the potential errors to which older 

 determinations of percentage saturation are subject appears to be to 

 define this quantity in terms of the analytical procedures used to 

 obtain it. 



, „ . (0 2 content - dissolved O a ) X 100 



1 . Percentage saturation — ,^ : T . — : — , ^ . — 



(0 2 capacity — dissolved 2 ) 



This is the percentage saturation obtained by the indirect gasometric 

 technique. The analysis for 2 content is usually performed promptly 

 upon the fresh blood sample, collected anaerobically. The analysis for 

 2 capacity is carried out upon an aliquot, equilibrated with air or an 

 oxygen mixture to attain full saturation. This consumes time, and the 

 changes which may occur in the processing of the blood, which is not 



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