658 
PHYSIOLOGY 
RESULTS 
In the first series of experiments, when the 
cardiac nerves were intact, isorhythmic disso- 
ciation was evident in every animal when the 
ventricles were paced at a frequency close to the 
spontaneous S-A nodal rate. The changes in the 
P-St (and P-R) intervals during isorhythmic 
dissociation were very characteristic and re- 
sembled those observed in patients with com- 
plete heart block and ventricular pacing (Fig- 
ure 1) . In the experimental animals, the P wave 
wandered rhythmically back and forth across 
the pacing artifact (St) and subsequent QRS 
complex. In the tracing recorded at slow paper 
speed shown in Figure 4, near the left border of 
200- 
ART. 
PRESS. 
mm HG 
0- 
500 
i 
P-St 
MSEC 
250- 
0 - 
A 
Figure 4. — Record from an animal in the first series of 
experiments (circulation intact, except for complete 
heart block), showing the changes in arterial blood 
pressure and P-St interval when the ventricles were 
paced at a constant frequency of 150 per min. A 
wide bore cannula in the left subclavian artery was 
connected by rubber tubing to an elevated blood res- 
ervoir. This tubing was clamped to the left of the 
arrow, and undamped to the right of the arrow in 
order to minimize the cyclic blood pressure fluctua- 
tions. The sharp vertical deflections downward of the 
P-St tracing indicate that the P had moved from just 
in front of, to just behind, the St, whereas the verti- 
cal upward deflections denote the crossing of the P 
and St in the opposite direction. Isorhythmic disso- 
ciation is denoted by the rhythmic crossing of the P 
back and forth across St (to the left of the arrow) ; 
loss of synchronization is denoted by the saw-tooth 
appearance of the P-St tracing (to the right of the 
arrow). Time scale-vertical deflections denote 10 sec 
intervals. 
the record the P wave was first located just 
after the St, as denoted by the P-St interval of 
between 350 and 400 msec. The constant St-St 
interval was 400 msec; i.e., the ventricle was 
being paced at a fixed frequency of 2.5 
stimuli/sec, or 150 stimuli/min. 
With such a large P-St interval (i.e., with the 
P occurring just after the St, and hence after 
ventricular activation), the blood pressure was 
about 100 mm. Hg (left border of Figure 4). 
When the P moved across the St, as indicated 
by the first sharp vertical deflection in the bot- 
tom tracing, the blood pressure began to rise. 
As the P-St increased over the next 5 sec to a 
maximum interval of 80 msec, the blood pres- 
sure rose to a maximum value of 170 mm. Hg. 
Conversely, as the P began to move back again 
toward St, the diminution in the P-St interval 
was accompanied by a fall in blood pressure. As 
the P moved across St, the P-St tracing was de- 
flected sharply upward again. It may be noted 
in the figure that the blood pressure continued 
to decline as the P moved behind St (denoted by 
the P-St intervals > 250 msec) . 
In Figure 4 to the left of the arrow, the P 
wave oscillated rhythmically about the St (and 
QRS), with a period of about 20 sec (i.e., about 
3 times/min). An increase in the P-St interval 
denoted that the S-A nodal discharge frequency 
temporarily exceeded the fixed ventricular pac- 
ing frequency, whereas a decrease in the P-St 
interval signified the opposite condition. The 
rhythmic oscillation of the P about the St indi- 
cated that the mean frequency of the atrial pace- 
maker was equal to the fixed frequency of the 
artificial ventricular pacemaker; i.e., iso- 
rhythmic A-V dissociation prevailed. 
At the arrow in Figure 4, the clamp was re- 
moved from the tubing which connected a can- 
nula in the left subclavian artery with an ele- 
vated blood reservoir. This resulted in a marked 
attenuation of the rhythmic oscillations of the 
arterial blood pressure. Concomitantly, the 
rhythmic oscillation of the P wave about the St 
ceased; i.e., the P-St interval first increased 
progressively for about 10 sec. until it reached a 
maximum of about 300 msec, and then it de- 
creased continuously thereafter. Near the right 
border of the figure, the five vertical upward de- 
flections of the P-St tracing, with slower step- 
