M. N. LEVY 
659 
wise downward deflections indicate that there is 
a fairly constant difference between the sponta- 
neous frequency of the S-A node and the fixed 
ventricular pacing frequency; i.e., the A-V dis- 
sociation is no longer isorhythmic. 
A similar phenomenon in the experiments of 
the second series (arterial system not intact) is 
shown in Figure 5. To the left of arrow 1 and to 
the right of arrow 2, the tubing connecting the 
cannula in the descending thoracic aorta with 
the servo-controlled pump was clamped. Hence, 
no attempt was being made to regulate the ar- 
terial blood pressure. The ventricular pacing 
frequency had been adjusted to a value near the 
spontaneous S-A nodal frequency in this dog 
with complete A-V block. It is evident that iso- 
rhythmic dissociation prevailed in the left and 
right thirds of the figure, as evinced by the 
rhythmic oscillations of the P wave about St. 
This is manifested in the lower tracing of Fig- 
ure 5 by the characteristic variations in the F- 
St interval. The changes in the P-St interval 
were accompanied by blood pressure oscillations 
similar to those noted in the previous figure. 
Between the arrows (Figure 5), the clamp 
was removed and the servo-controlled pump 
was used to hold the arterial blood pressure 
constant at a value approximately halfway be- 
tween the peaks and troughs of the oscillating 
blood pressure in the left and right thirds of the 
figure. It is evident that when the blood pres- 
sure variations were severely attenuated, the P 
no longer roamed back and forth across St, but 
it progressively swept through successive car- 
diac cycles. This is manifested by the saw-tooth 
200- 
ART. 
PRESS. 100- 
fMM.HG) 
0- 
500- 
P-St 
(MSEC) "0- 
0-^ 
MM 
Figure 5. — Record from a typical experiment in the 
second series, in which the arterial system was not 
intact. Between arrows 1 and 2, the arterial blood 
pressure was held relatively constant by means of a 
servo-controlled pump. Remainder of legend as in 
preceding figure. 
appearance of the P-St tracing in the middle 
third of Figure 5 (between the arrows) . 
When the cardiac nerves were intact, it was 
possible to produce isorhythmic A-V dissocia- 
tion in all animals with complete heart block in 
this study. When either the vagi or stellate gan- 
glia were sectioned, it was still possible to elicit 
synchronization, although the range of ventric- 
ular pacing frequencies which would still 
achieve isorhythmicity was diminished. In 
slightly over half the animals, when both the 
vagi and stellate ganglia were sectioned, it was 
no longer possible to elicit isorhythmicity. How- 
ever, whenever the blood pressure oscillations 
were severely attenuated, it was never possible 
to produce synchronization. This was true no 
matter what the state of the cardiac innerva- 
tion. 
In the second series of experiments in which 
the blood pressure could be regulated by means 
of a servo-controlled pump, the importance of 
the blood pressure oscillations was further elu- 
cidated. The blood pressure could be varied as a 
function of the P-St interval under the control 
of an analog computer. It was shown in a pre- 
ceding study*' that when the cardiac nerves 
were intact, if blood pressure was altered suffi- 
ciently per unit change in the P-St interval, 
isorhythmicity could be achieved. It was postu- 
lated that the blood pressure oscillations evoked 
A-V synchronization via the baroreceptor re- 
flexes, at least in part. 
In order to test this hypothesis, the blood 
pressure of the dogs in the second series was 
held constant and the vagi were transected in 
the neck. Stimulating electrodes were attached 
to the distal segments of these nerves and the 
frequency of stimulation was varied as a func- 
tion of the P-St interval, as described 
previously.^ The results of a representative ex- 
periment are shown in Figure 6. To the left of 
arrow 1 and to the right of arrow 2, the vagal 
stimulation period (reciprocal of the fre- 
quency) was made to decrease as the P-St inter- 
val increased from 0 to some optimal value, as 
established by the computer. The changes in the 
vagal stimulation period which accompany the 
varying P-St intervals are evident in the lower 
tracing of the figure. When the P wave moves 
behind St (as manifested by the high values of 
