AN ANIMAL MODEL OF 
ISORHYTHMIC ATRIOVENTRICULAR DISSOCIATION 
M. N. Levy* 
In patients with atrioventricular dissociation, the 
atria and ventricles are under the control of presumably 
independent pacemakers. Yet, several lines of evidence 
indicate that there is a distinct interaction between 
these pacemakers. The strongest evidence is that the 
separate pacemakers may discharge at the same fre- 
quency; such an arrhythmia is termed isorhythmic 
dissociation. By means of a series of experiments on 
anesthetized dogs, it was possible to determine the mecha- 
nism responsible for the synchronization of these seem- 
ingly independent pacemakers. Complete heart block 
was produced acutely by injecting 95% ethanol directly 
into the A-V conduction tissue. An electrode catheter 
was inserted into the right ventricle, and the ventricle 
was paced at a constant rate close to the prevailing fre- 
quency of discharge of the S-A node. It was found that 
the S-A node then assumed the mean frequency of the 
artificial pacing rate of the ventricles, a condition which 
simulates clinical isorhythmic dissociation. The S-A 
nodal frequency was observed to oscillate rhythmically 
about the constant frequency of ventricular pacing. 
Hence, the P waves of the electrocardiogram were 
found to pass periodically from slightly in front of to 
slightly behind the QRS complexes. When the P waves 
preceded the QRS complexes, atrial contraction was 
able to contribute to ventricular filling, and the arterial 
blood pressure began to rise. Presumably, this stimu- 
lated the arterial baroreceptors, which reflexly deceler- 
ated the S-A node. This caused the atrial contractions 
to occur after the beginning of ventricular systole, and 
therefore arterial blood pressure began to drop. This in- 
itiated a reflex acceleration of the S-A node, the P 
waves again moved in front of the QRS complexes, and 
the cycle was repeated continuously. Interference with 
the baroreceptor reflexes by preventing the blood pres- 
sure fluctuations or by sectioning the efferent nerves to 
the heart was found to terminate atrioventricular syn- 
chronization. Hence, the baroreceptor reflex appears to 
be responsible for synchronization of atria and ventri- 
cles in this arrhythmia. 
INTRODUCTION** 
In patients with atrioventricular dissociation, 
the atria and ventricles are controlled by inde- 
* Department of Investigative Medicine, Mt. Sinai Hospital, Cleve- 
land, Ohio. 
** This work was supported by Grant HE-10951 from the U.S. 
Public Health Service. 
pendent pacemakers. Hov\^ever, it has been rec- 
ognized for many years that some sort of inter- 
action exists betw^een these two presumably 
independent pacemakers.^ Most commonly, this 
interaction is manifested by a phenomenon 
which has been termed "accrochage" (from ac- 
crocher, to hook together) . It consists of a tran- 
sient but recurrent tendency for the frequency 
of discharge of one of the pacemakers to ap- 
proach the frequency of the other, such that the 
two pacemakers discharge temporarily at ap- 
proximately the same frequency.^ Hence, the 
atria and ventricles appear to be locked to- 
gether briefly. 
This tendency toward synchronization of two 
presumably independent pacemakers may be 
more persistent, with the result that the atrial 
and ventricular pacemakers may discharge at 
the same mean frequency, sometimes for pro- 
tracted periods of time. Such a condition is 
termed "isorhythmic A-V dissociation." Despite 
the same mean frequency, there is indisputable 
evidence that no direct communication need 
exist between the two pacemakers. The best il- 
lustration is afforded by a patient with complete 
heart block, in whom an artificial pacemaker 
has been installed. When the ventricles are 
paced at a constant frequency which is close to 
the spontaneous discharge rate of the S-A node, 
the atrial pacemaker will assume the same 
mean frequency as that of the artificial ventric- 
ular pacemaker.^ Small changes in the fre- 
quency of ventricular pacing will indjuce equal 
changes in the spontaneous rate of the natural 
atrial pacemaker. Confidence that this syn- 
chronization of the natural with the artificial 
pacemaker is not ascribable to a spurious 
spread of the artificial pacemaker current to the 
atria is furnished by the observation that the P 
wave of the electrocardiogram oscillates slowly 
back and forth across the QRS complex. Hence, 
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