E. DONG, G. C. M. WIEDERHOLD 
1095 
(a) 
P R 
P R 
(b) 
P R 
r 
1 
0 500 msec 
Figure 5. — EKG records of the effect of multiple stimuli. 
cycle is increased. Increasing the number of 
stimuli in the stimulation changes the configura- 
tion of the total response curve as given in the 
second curve, v^here the number of stimuli in 
the stimulation is 3. There is still a time period 
near 200 msec before the end of the cycle in 
which this degree of inhibition results in no 
prolongation of the contained heart beat. These 
data show that the cardiac cycle is not tempo- 
rally uniform in its response to vagal stimula- 
tion. 
Now, the question arises : Is there a relation- 
ship between the first observation concerning 
frequency locking and the second observation of 
phase sensitivity? 
If we examined the phase relationship be- 
tween the vagal stimulation and the P wave 
onset in the first experiment we carried out, you 
will note that as the frequency of the vagal 
stimulation increases during the period of fre- 
quency locking, the time relationship between 
the stimulation and the P wave varies. The 
phase is shifting in a direction predicted by the 
phase sensitivity plots ; as the stimulation fre- 
quency increased, the stimuli are placed in a 
phase of a cardiac cycle in which that same 
level of stimulation will result in less inhibi- 
tion: thus, an increased heart rate rather than 
a decreased heart rate is achieved. 
Now, what is the implication of these obser- 
vations for the heart rate control system? In 
cardiac transplantation the donor heart con- 
taining the atrium and ventricle are excised 
and implanted in the new host, thus simulating 
an artificial heart. In our system of transplant- 
ing the heart, the atria are left behind. The 
donor ventricles beat independently of the re- 
cipient atria each having their own pacemaker. 
A typical intra-atrial recording demonstrating 
the electrograms in a human transplant is 
shown in Figure 7. The two components, donor 
and recipient, were separated out by computer 
methods. The two rates and the phase relation- 
ship where then plotted on the incremental 
plotter (Figure 8). 
i200n 
1000- 
800- 
600- 
400- 
zio" 
-i6o 
p- 
p-p 
Interval 
(msed 
oP CP 
o 
o o 
o 8 
□ □ 
No. of Stim. 
A I 
□ 3 
o 5 
A 
0 
Stimulus 
iSo 
Interval 
260 3A0 
(msec) 
»3o sJx 
Figure 6. — Effect of timing on heart period. 
