PHYSIOLOGIC CONSEQUENCES OF CONGENITAL HEART DISEASE 



425 



of operation; and construction for ease of sterilization 

 and removal of air bubbles entrapped in the hydraulic 

 system (266). Strain-gauge manometers of the un- 

 bonded type, adapted for recording of blood pressure, 

 more nearly approach fulfillment of these require- 

 ments than do other manometers now available from 

 the stocks of commercial suppliers. A recent review 

 of strain-gauge manometers and their application to 

 recording of intravascular and intracardiac pressures 

 has appeared (235). 



The most serious difficulty in the problem of at- 

 taining high-fidelity recordings of intracardiac and 

 great-vessel pressures by means of catheter-mano- 

 meter systems is the avoidance of artifacts generated 

 by the motions of the catheter caused by the heart- 

 beat. Such artifacts cannot be eliminated when 

 conventional catheter-manometer systems are used 

 (270). They can be minimized, however, by the use 

 of miniature manometers mounted at the catheter 

 tip (94, 227). Such a miniature manometer has also 

 been used with success as an intracardiac microphone 

 for intracardiac phonocardiography (226). 



Detnminatioii of Blood Gases 



MANOMETRic METHODS. \'an Slyke & Neill (250) pub- 

 lished a gasometric technique for determination of 

 the oxygen and carbon dioxide content of blood, 

 which later was modified by Sendroy and associates 

 (218) and by Roughton and co-workers (201). This 

 technique or modification of it is commonly used for 

 determination of percentage saturation of blood with 

 oxygen. This requires that both the oxygen content 

 and the oxygen capacity (in volumes per cent) of 

 the blood Ije meastired so that, after appropriate 

 corrections for the oxygen in physical solution, the 

 percentage of hemoglobin saturated with oxygen 

 can be estimated. 



The principal advantage of this method is its 

 accuracy, for with good techniques oxygen content 

 can be determined within ±0. i volume per cent. 

 This method, however, is time consuming, and it is 

 usually true that the values from such analyses are 

 not available until after the cardiac-catheterization 

 procedure has been completed. Another disadvantage 

 is that the number of samples of blood that can be 

 obtained and analyzed from various sites in the heart 

 and great vessels is severely limited. 



POLAROGRAPHic OXYGEN ELECTRODE. An inert iiictal 

 such as platinum, gold, or mercury negatively charged 

 in an electrolyte solution will gi%e up electrons to 



dissolved oxygen gas, reducing it to H2O2 or OH~. 

 The current measured pa.ssing into solution from the 

 electrode is directly related to the availability of 

 oxygen at the metal surface. Bare platinum and 

 dropping mercury have been widely used in the 60 

 years since the technique was first described, but for 

 blood oxygen tension (PO2) the proteins interfered 

 with analysis. 



Stow and associates (233) developed a membrane- 

 covered electrode for measuring blood carbon dioxide, 

 and in 1956 Clark (59) introduced the use of the 

 membrane-covered electrode for the measurement of 

 blood oxygen tension. The membrane, which is a 

 suitable plastic permeable to oxygen but impermeable 

 to protein molecules, protects the electrode from the 

 "poisoning effect" caused by most biologic fluids. 

 The electrode is usually incorporated in an airtight 

 cuvette which can be temperature controlled and 

 in which the blood in contact with the membrane 

 interface can be stirred. Various techniques have been 

 developed to accomplish this (157, 219, 228). 



The polarographic electrode has been used to 

 measure the oxygen content of whole blood (180), 

 the principle being to inject a measured amount of 

 blood into a larger volume of solution which frees 

 the oxygen from the hemoglobin, releasing it into 

 .solution where the rise in p02 is proportional to the 

 o.xygen content of the original blood. For this pur- 

 pose, ferricyanide and carbon monoxide have been 

 used. The accuracy available is principally dependent 

 on calibration of the electrode. 



SPECTROPHOTOMETRIC METHODS. It is possible tO 



determine the oxygen saturation of blood by means 

 of widely used spectrophotometric methods. The 

 physical basis of these methods rests on the difference 

 in ab.sorption, by oxygenated and reduced hemo- 

 globin, of red light at a wavelength of about 640 

 m^u, and on their similarity in absorption of infrared 

 light at a wavelength of about 800 m/i. These measure- 

 ments are usually made on light transmitted through 

 blood, although reflected light also has been success- 

 fully used. 



A number of instruments and procedures are 

 available for photometric determination of the oxy- 

 gen saturation of blood. 



Hemoreflector. This instrument and its use have been 

 described in detail by Zijlstra (287). The intensity of 

 light (600-680 mju) reflected from a 0.5 ml-sample of 

 whole blood diluted to i ml is measured. The instru- 

 ment consists of a glass-bottom cuvette for the blood 

 sample that is held in a revolvable turret housed in 



