656 
PHYSIOLOGY 
the atrial frequency is, at times, slightly greater 
than the ventricular frequency, and, at other 
times, slightly less than the ventricular fre- 
quency, although the mean frequencies are the 
same. With surreptitious spread of the artificial 
pacemaker current, the relationship of the P 
wave to the QRS complex would probably re- 
main constant. 
An example of persistent isorhythmic A-V 
dissociation which occurred spontaneously in a 
44-year old man with chronic myocarditis is il- 
lustrated by the cyclic changes in P-R interval 
which are plotted in Figure 1. In his electrocar- 
diogram (see Figure 5, reference 3), the P 
wave was usually buried in the QRS complex, or 
only the very beginning of the P wave was evi- 
dent just before the onset of the QRS. How- 
ever, the P wave periodically moved out in front 
of the QRS, remained there briefly, and then re- 
turned to be obscured by the QRS, as shown in 
Figure 1. During those periods in which the P 
■wave moved to the left relative to the QRS (i.e., 
those periods of increasing P-R interval), the 
atrial activation frequency exceeded the ventric- 
ular activation frequency, whereas the con- 
verse was true when the P wave wandered back 
toward the QRS. However, on the average, the 
P and QRS frequencies were equal; i.e., iso- 
rhythmicity prevailed. 
In this patient, there were changes in the ar- 
terial blood pressure which accompanied the pe- 
SEC. 
.15 - 
TIME, SEC. 
Figure 1. — The rhythmic changes in P-R interval 
which were observed in a 44 year old man with 
chronic myocarditis, in whom isorhythmic A-V dis- 
sociation appeared spontaneously. 
Figure 2. — The changes in arterial blood pressure 
(upper tracing), in mm. Hg, which accompanied the 
changes in the P-R interval of the electrocardiogram 
(lower tracing) in the same patient as in Figure 1. 
riodic variations in the P-P interval (Figure 
2). These variations of the blood pressure pro- 
vide a clue to the mechanism producing the syn- 
chronization. It is evident from the figure that 
when the P wave preceded the QRS complex 
(left half of the tracing), the arterial blood 
pressure was significantly greater than when 
the P wave was obscured by the QRS (right 
half of the record) . 
In order to elucidate the mechanism produc- 
ing isorhythmic A-V dissociation, the clinical 
condition was simulated by means of an animal 
model. The model chosen was one in which com- 
plete heart block was produced in anesthetized 
dogs, and the ventricle could be paced at any de- 
sired constant frequency. 
METHODS 
Mongrel dogs were anesthetized with mor- 
phine sulfate, 2 mg/kg i.m., followed 30 min 
later by chloralose, 60 mg/kg i.v. A tracheal 
cannula was inserted, and artificial respiration 
was instituted. The chest was opened trans- 
versely at the level of the fourth intercostal 
space. Ethanol, 95%, was injected into the re- 
gion of the A-V node in order to produce com- 
plete heart block. 
A bipolar electrode catheter was inserted into 
the right atrium to permit registration of the 
atrial electrogram on an eight-channel direct- 
writing oscillograph (Brush Mark 200). The 
