426 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION 1 



the same cabinet as the source of light, photocells 

 and control circuits, and a separate sensitive galva- 

 nometer for indication of the output of the photocell 

 assemljly. 



Beckman DU spectrophotometer with special cuvettes. 

 This widely used spectrophotometer allows accurate 

 measurement of the optical density of samples of 

 blood to relatively monociiromatic light of selected 

 wavelengths. For these procedures the blood is placed 

 in special cuvettes and usually hemolyzed. Nicholson 

 and colleagues (182) have described a cuvette in 

 which the thickness of the blood film is 2.4 mm; in 

 another cuvette, described by Nahas (179), the 

 thickness of the blood film is o.i mm. 



An excellent discussion of the techniques and theory 

 of spectroscopic determination of the oxygen solution 

 of blood is given by Drabkin {84). 



Oximetry. An oximeter is a photoelectric photometer 

 for measurement of the fraction of hemoglobin in 

 blood that is in the oxygenated state. One type, the 

 ear oximeter, is designed to measure the oxygen satu- 

 ration of blood circulating in a particular tissue, 

 usually the pinna, of an intact animal or human 

 being. A second type, the cuvette oximeter, is de- 

 signed to measure the oxygen saturation of blood 

 outside the body during or soon after withdrawal 

 from various sites in the vascular system. 



Oximeters are classified on the basis of whether 

 they are relative- or absolute-reading instruments. 

 The former usually measures the light transmitted 

 or reflected by l:)lood at only one spectral region 

 (about 640 mti) and is usually adjusted during opera- 

 tion to indicate a known value for the oxygen satura- 

 tion of the blood being analyzed. This type measures 

 only changes in oxygen saturation that may occur 

 after this initial adjustment and cannot make an 

 independent (absolute) measurement of the actual 

 saturation. An absolute-reading oximeter measures 

 light transmitted or reflected by blood at two wave- 

 lengths; an initial adjustment to indicate a known 

 value of oxygen saturation is not required, so that the 

 instrument can make an independent (absolute) 

 measurement of the oxygen saturation of the l)lood 

 being analyzed. 



The cuvette oximeter has the advantage that 

 whole blood can be drawn directly from arterial or 

 venous sampling sites through tlie cuvette for analysis. 

 Use of the single-scale recording assembly also allows 

 nearh- instantaneous determination of the blood 

 oxygen saturation. Determinations on flowing i)lood 

 are of primary importance for cardiac catheteriza- 

 tion, studies of pulmonary function, monitoring or 



study of extracorporeal pump-oxygenator assemblies, 

 and any application that requires dynamic measure- 

 ments. Also multiple samples may be obtained, since 

 the blood can be collected in a sterile syringe and 

 reinfused into the patient's blood stream after the 

 analysis has been made. 



It has been shown that the accuracy of cuvette 

 oximetry in determination of the oxygen saturation 

 of whole blood is comparable to that of a spectro- 

 photometer used on hemolyzed blood, which is 

 limited to in vitro applications (271). The applica- 

 tion in diagnostic cardiac catheterization of a cuvette 

 oximeter utilizing the reflection principle has recently 

 been described (36). 



The application of gas chromatography to the 

 determination of blood gases has been perfected in 

 recent years and offers distinct advantages in sim- 

 plicity and speed of anahses for many applications 

 (166, 196). 



Calculation of Blood Flows and Shunts 



Development of applications of the direct Fick 

 principle for determination of blood flow using the 

 technique of intracardiac catheterization has sim- 

 plified determination of cardiac output in man. This 

 method is the one generally used today. The equation 

 for calculation of cardiac output is as follows: 



Qb 



Vo, 



Cao. — Cv,i, 



where Q,b is the cardiac output in liters per minute, 

 V02 is oxygen consumption in milliliters per min- 

 ute, Cao., is the oxygen content of the blood 

 leaving the lungs, and Cvq.. is the oxygen content 

 of mixed venous blood returning to the lungs. In 

 order to obtain a representative sample of mixed 

 venous blood, the venous blood u,sually is withdrawn 

 from the pulmonary artery. 



This equation can also be used to determine sys- 

 temic (Qs) and pulmonary (Q,p) blood flows when 

 the blood flow to one of these systems is greater as a 

 result of shunted blood. The equations would then be: 



Qp = 



Vo, 



Cpvd. — Cpao. 



and 



Qs = 



Vo, 



Csaoj — Cvo2 



Cpvo., and Cpaon are the oxygen contents of pul- 

 monary-vein and pulmonary-artery blood samples. 



