Determinations of Physiological Dead-Space and Alveolo-Arterial Oxygen Gradients by the Indirect Methods of Riley and Enghoff: I. Studies on the Experimental Error of the Method and on the Variations Observed in Normal Individuals

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Abstract
Alveolar-arterial oxygen gradient[long dash](A-a)O2i[long dash]and physiological deadspace[long dash]VDi[long dash]determined by the indirect methods of Riley and Enghoff, do not always give results identical with a "true" mean (A-a)O2 gradient and a theoretically conceived "true" physiological deadspace i.e. anatomical + alveolar deadspace. Increase of "true" mean (A-a)O2 gradients and true physiological deadspace caused by uneven distribution of ventilation perfusion is underestimated when the indirect methods are used for the determinations. This underestimation, however, appears for clinical purposes to be insignificant compared to the methodical errors and the variations observed in normal individuals. The methodical errors of (A-a)O2i and VDi determinations are prone to be great. In our hands the (A-a)O2i gradients are, judged by duplicate tests,, measured, within + 6.6 mm Hg if a single test is used, and within [plus or minus] 4.8 mm Hg if the mean of two tests is used. The corresponding ranges for deadspace measurements were [plus or minus] 78 cc and [plus or minus] 54 cc and for the VDi x 100/VE ratio [plus or minus] 10.6 and [plus or minus] 7.6. If the 95.5% confidence interval is used for estimation of the normal ranges, the upper normal limits for the (A-a)O2i gradient and the VDi x 100/VE ratio should be 19 mm Hg and 46, respectively, in subjects without cardiopulmonary diseases. The values obtained in all our 60 "normal" subjects are within these ranges. In our material VDi does not exceed 220 ml in females when the tidal volume is less than 600 ml and 250 ml in males when the tidal volume is less than 800 ml. The magnitude of VDi seems to be dependent on tidal volume, frequency and mode of breathing. Similar correlations have not been demonstrated for the (A-a)O2i gradient.