The oxygen saturation of arterial blood in man can be measured continuously in situ by means of bichromatic photoelectric colorimetry of the intact fully flushed ear. The accuracy of the device as determined by gas analysis of arterial blood samples is from 3 to 8 percent. The entire optical and photoelectric system, comprising a miniature lamp bulb, two color filters, and two selenium barrier‐layer photo‐cells, weighs 30 grams, and slips over the shell of the ear. One of the color filters transmits a wave‐length band which is equally absorbed by oxy‐ and reduced hemoglobin, thus providing a means of measuring the amount of total hemoglobin in the optical path, independent of its degree of oxygen saturation. The other color is very differently absorbed by the two hemoglobin forms. Several direct reading forms of the instrument are discussed.
REFERENCES
1.
K.
Matthes
, Arch. Exp. Path. and Pharm.
179
, 698
(1935
) recommends the use of histamine introduced through the skin by electrophoresis as a vaso‐dilator for the purposes of such measurements. We have tried this method and also xylol applied to the skin, but neither of them give better results—and both are more irritating—than the application of gentle heat.2.
3.
Pappenheimer
, J. Physiol.
99
, 184
(1941
), has recently made a very careful study of the same relationship, and has found it to be accurately valid above 50 percent saturation. Matthes (reference 1) assumed that the straight line relationship applied to measurements on the intact human ear, but his paper contains too few experimental details to indicate whether he verified it or not. All three of these workers were concerned with only a single color, red. None of them utilized a second wave‐length to obtain independent information about the amount of total hemoglobin present, and all of them required taking samples of blood for calibration.A 2‐color method for determining oxygen saturation in very dilute hemoglobin solutions has been used by
G. A.
Millikan
, J. Physiol.
79
, 152
(1933
),who also adapted it for use in intact tissues,
Proc. Roy. Soc.
B123
, 218
(1937
).4.
It has been shown by independent research of the Coleman Electric Company that the most effective crossover wave‐length of this filter‐ear‐photo‐cell combination is not in the green, as the author had supposed, but in the near infra‐red. The blood thickness of the ear is sufficient to block the green light almost completely. This discovery does not affect the principle of operation, as the “green” color is defined by its property of being equally absorbed by the two pigments; it was, however, important in explaining a serious weakness in the early standardization procedure, and in indicating a means of remedying the difficulty.
5.
6.
The Barach‐Eckman injector and oxygen mask, J. Aviation Med. (March, 1941), p. 3, is very well adapted to this use. Instead of attaching the injector to an oxygen tank, as is done in oxygen therapy, it is supplied from a nitrogen tank. The calibrated settings on the injector are recalculated and give a convenient series of oxygen percentages from 21 to 0, instead of 21 to 100.
7.
This relation does not appear to be in agreement with theory, but it is not possible to form a definite conclusion on this point because of the uncertainty as to the value of the scattered light correction. Our rather unsatisfactory values for this correction certainly yield a relation very different from the theoretical one.
8.
These adjustments were originally made with rheostats in the lamp circuit until it was shown independently by Smaller (reference 9) and Coleman that adjustment in the galvanometer circuit is more convenient, compact, and reliable, as well as less expensive.
9.
B. Smaller, U.S. War Department report. EXP‐M‐54‐653‐102 (May 4, 1942) and EXP‐M‐49‐696‐16 (Aug. 28, 1942).
10.
11.
A one‐color instrument, operating in the red wavelength region, has in fact been used for this purpose by
R. D.
McClure
and F. W.
Hartman
, Ann. Surg.
112
, 791
(1940
)and a 2‐color device designed for clinical work has been described by
J. R.
Squire
, Clinical Science
4
, 331
(1940
).
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© 1942 American Institute of Physics.
1942
American Institute of Physics
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