428 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



temic, but may be a region such as the forearm, 

 or an organ such as the kidnc-v. Although there is 

 considerable debate regarding the exact anatomic 

 location and piiysiologic basis of \ascular resistance, 

 the concept appears to have practical value. It has 

 been recognized in the field of nonbiologic hydraulics 

 that resistance of multiple-pipe systems can be calcu- 

 lated by application of the principles of Ohm's law. 

 Circulatory resistances, by analogy, are calculated in 

 a similar manner. Thus, the simplest formula for 

 calculating resistance is: 



resistance = 



mean pressure gradient 

 blood flow 



In recent vears many workers in this field have 

 made use of Aperia's formula so as to give the result 

 in absolute units. Resistance then is expressed as: 



Pressure differences (dynes/cm^) 

 Flow (ml/sec) 



dynes sec cm ' 



R = 



(Pi - P«)-i332 



where R = resistance in dynes sec cm~°. Pi — Pj = 

 the pressure loss across the resistance circuit in 

 millimeters of mercury, 1332 is the factor for con- 

 version to dynes, and Q, = blood flow in milliliters 

 per second through this circuit. 



In practice, the exit (venous) pressure for the 

 systemic circuit is so small in relation to the entering 

 pressure that it is often disregarded and the mean 

 systemic arterial pressure is used in place of the 

 pressure loss. In the pulmonary vascular bed this is 

 not true and the actual pressure loss across the 

 pulmonary vascular bed should be measured. 



Just exactly what is measured by "resistance" 

 remains problematic. According to Poiseuille's law, 

 which is strictly applicable only to steady (non- 

 pulsatile) flow of a Newtonian fluid through rigid 

 tubes, resistance varies directly with the vessel length 

 and blood viscosity, and inversely with the cross- 

 sectional area. Other factors being equal and un- 

 changed in the same patient, changes in resistance 

 are presumed to reflect changes in cross-sectional 

 vascular area. A decrease in resistance could be due 

 to increased vascular distention of a passive nature 

 resulting from an increase in transmural pressure or 

 due to true vasodilatation. The exact mechanism and 

 significance of a measured decrease in vascular 



resistance are frequently difficult to interpret with 

 certainty. 



CALCULATION OF VALVE AREAS. Taylor and co-workers 

 applied hydraulic formulas to the study of the 

 relationship of the flow through a patent ductus 

 arteriosus to the size and the pressure gradient across 

 the ductus (248). 



Gorlin & Gorlin (118) have made use of similar 

 hydraulic principles in calculating the valve areas 

 from available hemodynamic data. The formula, an 

 adaptation of the standard equation for hydrokinetic 

 orifices (76), is fully discussed in Chapter 20. 



bulicator-Dilution Curves 



An indicator-dilution curve is a plot of concen- 

 tration of a substance at a given site in the circulation 

 against time following its injection at another site in 

 the blood stream. 



Measurement of cardiac output from such dilution 

 curves was first advocated by Stewart (230) in 1897. 

 The use of an indicator-dilution method for measure- 

 ment of blood volume was popularized by the work 

 of Keith and co-workers (148) and others during 

 World War I, and in more recent years the use of 

 indicator-dilution techniques for measurement of 

 blood flow has been established on a firm basis by 

 the work of Hamilton and colleagues (127); these 

 methods have also been applied to the diagnosis and 

 in\'estigation of various forms of cardio\'ascular 

 disease (181, 242). 



The diagnostic as well as the in\estigative value of 

 these techniques has gradually received progressively 

 widespread recognition (25, 39, 74, 140, 175). The 

 rate of this progression has, however, been greatly 

 accelerated in the last 2 or 3 years with more general 

 availability of suitable instrumentation and, particu- 

 larly, the introduction of foreign gases for use as 

 indicators in these techniques. The first of these was 

 nitrous oxide {177), which was rapidly followed by 

 the use of radioactive gases, krypton 85 (165, 207), 

 I"' ethyl iodide (lo, 54), and more recently, hydrogen 

 (60) and hydrogen plus ascorbic acid (61, 62). 

 Thermal dilution (100) and external scanning 

 techniques (135, 189) have also been applied to the 

 diagnosis of congenital heart disease. 



The use of foreign gases as indicators in these 

 dilution techniques has contributed greatly to the 

 ease of application of the techniques by opening up a 

 bloodless method of what in fact amounts to a) 



