THE CONTROL OF THE FUNCTION OF THE HEART 



515 



of the extent to which vagal stimulation can alter 

 atrial contractility. This was done in hearts both with 

 and without heart block. Figure 21 (upper) shows 

 the changes induced in the amplitude of the atrial 

 "a" wave and the consequent changes in left ventricu- 

 lar diastolic pressure before, during, and after distal 

 vagal nerve stimulation in a heart block preparation. 

 During stimulation indicated by signal at bottom, 

 atrial pressure was higher and left ventricular pressure 

 lower in late diastole in spite of a longer diastole. 

 This response is also observed in the dog without 

 heart block and in which the heart rate is either main- 

 tained constant or allowed to decrease during vagal 

 stimulation as in figure 21 (lower). The control trac- 

 ing is at the left, and the arrows in the third beat 

 indicate the atrial "a" wave and the consequent 

 increase in left \entricular end diastolic pressure and 

 myocardial segment length. The tracing at the right 

 is during stimulation of the efferent vagus nerve. In 

 addition to the bradycardia produced in the un- 

 paced heart, the atrial quieting also diminished the 

 end diastolic augmentation of both the ventricular 

 pressure and myocardial segment length. 



E. Effect of Autonomic Nerve Stimulation on the 

 Atrial Transport Function (Relation between 

 Mean LA Pressure and LI ED Pressure) 



The atrium performs much like a booster pump, 

 the function of which is to augment the transfer of 

 fluid from the feed line into the primary or power- 

 generating element of the system. There are two main 

 consequences when the action of a booster pump is 

 increased or decreased. The first is that more or less 

 fluid will be forwarded into the main pump, pro- 

 \ided that the latter does not have a stop which 

 limits input. This effect was demonstrated in sections 

 II B and 11 D. The second effect is that for a given 

 input into the main pump, the pressure in the feed 

 line behind the booster pump will be lower when the 

 actixity of the booster pump is increased and higher 

 when booster pump activitv is diminished. 



The pressure in the atrium not only provides for 

 the progress of ijlood into the ventricle but is also the 

 central pressure head which must be exceeded bv liie 

 pres.sure of the blood in the veins feeding into it. 

 From the point of \iew of integrating the heart with 

 the total organism it is, therefore, pertinent to inquire 

 whether a change in the acti\ity of the atrium modifies 

 the mean pressure in it for any given end diastolic 

 pressure, stroke work, and cardiac output (67a). 

 Figure 22 shows the plot of mean left atrial pressure 



FIG. 22. LVED = left ventricular end diastolic pressure; 

 LA (mean) = mean left atrial pressure. 



against left ventricular end diastolic pressure before 

 and during autonomic nerve stimulation. The con- 

 tinuous line joining solid dots shows the relation 

 during a control run in which mean left atrial pres- 

 sure, left ventricular end diastolic pressure, and stroke 

 \olume were intermittently increased by the infusion 

 of blood. The continuous line joining open circles 

 shows the effect of left stellate ganglion stimulation on 

 this relation. The dashed line joining .solid dots 

 shows the control curve in another experiment and 

 the dashed line joining the open circles shows the 

 effect of efferent vagal nerve stimulation on this 

 relation. During cardiac sympathetic nerve stimula- 

 tion tlie mean left atrial pressure is lower for any 

 given left ventricular end diastolic pressure than in 

 the control run. Conversely, during efferent vagal 

 nerve stimulation the mean left atrial pressure is 

 higher for any given left ventricular end diastolic 

 pressure than in the control run. Changes in atrial 

 contractility contribute substantially to these altered 

 relations. At any given heart rate during cardiac 

 s\rnpathetic nerve stimulation the atrium contracts 

 more vigorously and empties more completely. This 

 decreases residual atrial volume and places the 

 atrium on a lower portion of its pressure-volume 

 curve, so that for any given AV it requires a smaller 

 AP. Conversely, during efferent vagal nerve stimula- 

 tion atrial contractility is decreased and emptying is 

 less complete. Therefore, residual atrial volume is 

 increased, and the atrium is placed on a higher por- 

 tion of its pressure volume cur\e, thus requiring a 

 large AP for any AV. Further, the shorter the time 

 required for atrial ssstole, the longer the period during 

 which the atrium will be relaxed and therefore more 



