538 HANDBOOK OF PHYSIOLOGY ^^ CIRCULATION I 



QUANTITIES 



INCREASED 

 CONTROL CONTRaCTILITY 



RATES 



INCREASED 

 CONTROL CONTflaCTIUTY 



FIG. 2 Increased ventricular "contractility" is characterized 

 in terms of the variables indicated in figure i . Changes in 

 quantities (i.e., pressure, flow/stroke and diameter or work/ 

 stroke) are not altered as much as the rates (i.e., rate of changes 

 of pressure, rate of flovi', rate of change of diameter, and rate of 

 performing work, which is power), during increased contrac- 

 tility. 



leading to changes in cardiac function rather closely 

 simulating those of exercise. Included in the latter 

 category are administration of epinephrine accom- 

 panied by tachycardia, administrationof isoproterenol, 

 and electrical stimulation of selected sites in the 

 diencephalon (36, 47). The typical ventricular res- 

 ponses under these conditions involve at least si.x 

 major components: /) more rapid rise in ventricular 

 pressure during isovolumetric contraction, 2) more 

 rapid fall in ventricular pressure during isovolumetric 

 rela.xation, j) faster myocardial shortening and more 

 rapid ventricular ejection, 4) more complete systolic 

 ejection, 5) elevated peak systolic tension due in part 

 to faster systolic ejection, and 6) increased maximum 

 possible isometric contraction tension. The evidence 

 for the occurrence of the last component is the observa- 

 tion that, when the aorta is clamped, sympathetic 

 stimulation of the exposed heart drives ventricular 

 pressure to levels as high as 500 mm Hg during 

 ventricular contractions which are almost isometric 

 (i, 24). 



Contractility, a Generic or a Specific Term 



In physiological circumstances, the factors defining 

 contractility tend to vary together even though they 

 may involve different mechanisms. For example, a 

 faster-rising contractile tension may reflect more 



nearly siinultaneous excitation of the various bundles 

 of myocardium. This mechanism would be a function 

 of the conduction system. The shorter duration of 

 systole indicates a more rapid repolarization of the 

 individual muscle membranes, a function of the 

 transmembrane ion exchanges. The faster myocardial 

 shortening to produce more rapid ejection must 

 involve a change in the myocardial contractile 

 elements. The more rapid isovolumetric relaxation 

 might involve either more synchronous repolariza- 

 tion or an altered rate of relaxation of the contractile 

 elements. A more complete systolic ejection suggests 

 either greater contractile tension or less loss of tension 

 due to shortening owing to unknown changes at the 

 cellular or molecular level. To lump all these factors 

 under a single term such as '"increased contractility" 

 may obscure the issues and serve as a semantic 

 obstacle to an understanding of the various 

 mechanisms. Contractility must be treated as a term 

 covering many different factors, so that we may 

 define and understand each aspect individually. We 

 must not close our minds to the possibility that 

 mechanisms which are completely unsuspected at 

 present play important roles in the control of the 

 heart. 



THE NATURE OF SPONTANEOUS 

 CARDIOVASCULAR .ADJUSTMENTS 



The experimental techniques illustrated in figure i 

 have been used in various combinations for continuous 

 analysis of left \entricular performance in healthy 

 active dogs, fully recovered from the surgical installa- 

 tion of the recording devices. Although it is unwise 

 to extrapolate from dogs to man, such studies may 

 depict circulatory response patterns that indicate 

 what measurements should be explored in man. By 

 comparing the continuous analysis with the available 

 information on human reactions, one should gain 

 .some insight into the extent to which these reactions 

 correspond to the patterns in dogs. 



The most prominent feature of cardiovascular 

 function, in dogs, under '"normal" conditions is the 

 unremitting fluctuation. The heart rate, ventricular 

 and arterial pressures, ventricular dimensions, and 

 flow all \ary continuously. Many deviations in the 

 records are obviously associated with such things as 

 moving all or part of the body and altered respiratory 

 activity a; well as with eating, exercise, and startle 

 reactions (fig. 3). In contrast, prominent cardiovas- 

 cular adaptations wiiich cannot be attributed to any 



