Oxygenation and Oxidation 



an inert system, rather than the manometry per se, are responsible for 

 the errors of this method 16 . For example, if met- or ferrihaemoglobin, 

 which does not bind O a , is present in the original blood, it may revert 

 to Hb and thence to Hb0 2 in the time consumed before and during 

 equilibration for 2 capacity 26 ' 27 . Such a reversion of methaemoglobin 

 will result in obtaining a value for the denominator too high in respect 

 to the numerator in the above equation, and the percentage saturation 

 will be underestimated. 



„ _ (0 2 content — dissolved Oo) X 100 



2. Percentage saturation = - — — — — ; — : 



Total pigment 



This is the percentage saturation determined by a combination of 

 gasometry for 2 content and ' colorimetry ' or photometry for total 

 haemoglobin pigments. The combined method 28 has advantages over 

 the purely gasometric, in that the interpretation of the total pigment 

 value is unambiguous and the determination may be performed 

 promptly and simultaneously with that of the 2 content. 2 capacity 

 as a measure of total blood pigment is strictly valid only if no other 

 haemin derivatives besides Hb and Hb0 2 are present in the sample. 

 However, this method suffers from the uncertainty inherent in the use 

 of two independent and different techniques (gasometry and photo- 

 metry) whose exactness of alignment with each other may be question- 

 able. Interestingly enough, the presence of methaemoglobin in the 

 original blood may still result in an underestimation of percentage 

 saturation by the combined method, just as in the first procedure. In 

 the present case the numerator will be too low for the denominator. 



• TTUA [Hb0 2 x 100] 



3. Percentage saturation, or Percentage Hb0 2 = ^ — -, — : V 



[Total pigment] 



This is the percentage saturation defined by the direct spectrophoto- 

 metric analytical procedure of Drabkin and Schmidt 10 . It is the only 

 method in which the percentage saturation is unequivocally equal to 

 what it is regarded to represent, the percentage of oxyhaemoglobin, 

 since the fraction of each component (HbO a and Hb) in the sample is 

 determined directly. For the most accurate results the total pigment 

 concentration as cyanmethaemoglobin or ferrihaemoglobin cyanide, 

 MHbCN, is independently measured spectrophotometrically 29 . We 

 prefer this procedure, though it may be side-stepped in the method. If 

 the total pigment concentration in blood samples from normal dogs 

 or from normal young men (non-smokers) is determined spectro- 

 photometrically either as Hb0 2 (fully oxygenated), Hb (after deoxygen- 

 ation with Na 2 S 2 4 ), or MHbCN (after conversion with ferricyanide 

 and cyanide), the results agree within 0-5 per cent, the analytical error. 

 This has led to the conclusion 10 that the concentration of methaemo- 



39 



