286 CIRCULATION 



The maximal mechanical efficiency was obtained by moderate 

 increase of both pressure and output and reached 26 per cent. 



It has been suggested that a term Energy Index should be used 

 to denote the total force developed by the heart per minute. If 

 the systolic blood pressure be 120 mm. Hg and the diastolic 80 mm., 

 then the force of each beat would be 120 -f 80 =200 mm. Hg. This 

 figure multiplied by the number of beats indicates the total force 

 per minute. From an examination of about 26,500 recruits for 

 the American army the normal index was found to be about 

 20,000 mm. Hg per minute. This Systolic-Diastolic-Rate index 

 designates the amount of effort which the heart is putting forth. 

 A high S.D.R. index indicates increased cardiac effort due 

 either to inability of the heart to carry on its normal work at a 

 normal rate or to the presence of some resistance to flow in the 

 circulation. 



For example, adrenalin causes an increased blood pressure by con- 

 stricting the peripheral vessels. A man who had S.P. = 108, D.P. = 70, 

 P.R. = 84, giving an index of 19,852, had a small injection of adrenalin. 

 His S.D.R. index then rose to 23,520, made up from S.P. = 140, D.P. = 70, 

 P.R. = 112. 



The diagrammatic section of the heart (Fig. 58) demonstrates 

 that the walls of the left ventricle are much thicker than those 

 of the right. The mean of a large number of determinations 

 furnishes the ratio of 6-8 : 1. This may be interpreted as indi- 

 cating that the left ventricle develops about seven times as much 

 pressure as the right ventricle. Proof confirmatory of this 

 deduction is obtained by determining the hydrostatic pressures 

 necessary completely and symmetrically to fill these two chambers. 

 The right ventricle is dilated by a seventh of the pressure employed 

 in equally dilating the other ventricle. 



A dog weighing 10 kilos with an average aortic pressure of 100 mm. Hg, 

 and an output of 2,000 c.c. of blood per minute, develops pressure in 

 right and left ventricles of 25 and 150 mm. Hg respectively a ratio of 

 25 : 150 = 1 :6. 



The pressure developed in a distended hollow elastic vessel 

 depends on (i) the elasticity of the walls, (ii) the degree of dis- 

 tension and (iii) inversely, the radius of curvature of the walls. 

 The volume output from both ventricles is the same and their 

 radii of curvature are similar. There remains only a marked 

 difference in elasticity. As both are formed from the same 

 material, alteration in elasticity must be brought about by 

 alteration in wall thickness. 



