F. KRAFT-HUNTER AND E. W. HAWTHORNE 
633 
quencies used were 2.0, 2,5, 3.0, 3.5, 4.0 ; the du- 
ration was 3 msec, and voltage 3-5. The fre- 
quencies were used only as a reference for the 
relative level of stimulation. Heart rate and 
cycle length was calculated by the computer on 
the basis of definition of the cardiac cycle. 
RESULTS 
A typical recording from a conscious instru- 
mented dog, 45 days after surgery for implanta- 
tion of transducing devices, is shown in Figure 2. 
This recording was taken during the control 
period for one experiment where the dog is 
standing quietly in a stand as described above. 
Of interest is the characteristic fidelity of the 
recordings of the primary variables being moni- 
tored. Such detail is seen for example on the left 
atrial pressure pulse tracing where the distinct 
components of the wave form is easily discerned. 
All phases of the cardiac cycle can be readily 
identified from these data for specific analysis. 
In addition, the low heart rate and marked 
sinus arrhythmia is a characteristic finding of 
the conscious dog in good health in our labora- 
tory. These recordings are stable and allow per- 
formance of a variety of experiments. 
Pacing and Left Ventricular Size 
The Atrium 
The typical response of one dog to different 
frequencies of atrial pacing is shown in Figures 
3 and 4. Left ventricular end-diastolic internal 
diameter consistenly decreased with successive 
increases in frequency of stimulation of the 
atrium. However there was no significant 
change in end-systolic diameter. Figure 4 is a 
faster recording which shows some of the detail 
attending atrial pacing. In addition to the 
changes seen in heart size — at high atrial pac- 
ing rates* there is demonstrated in left atrial 
pressure pulse tracing the presence of fusion of 
"V" and "A" waves. Figure 5 shows the re- 
sponses during sequential pacing of the left 
atrium and ventricle. During these studies, the 
interval between atrial systole and ventricular 
systole (AS-VS interval) was 100 msec. 
Sequential pacing of the heart resulted in a per- 
sistent decrease in end-diastolic and end-sys- 
tolic diameters. As shown in Figure 6, ventricu- 
lar pacing resulted in a decrease in end-diastolic 
internal diameter with little change at end-sys- 
tole. Tables I and II show the changes in end- 
diastolic volume (EDV) observed during atrial 
and ventricular pacing. 
Pacing and Myocardial Contractility 
In Figures 3, 4, 5, are shown the changes in 
peak dP/dt and peak d InP/dt observed during 
atrial and sequential pacing. It is to be noted 
that although the mean LVP during ejection is 
significantly increased over the control level 
with atrial pacing, the mean ventricular pres- 
sure is not significantly different from one pac- 
ing rate to the other. With each increase in fre- 
quency of stimulation there is an increase in 
peak dP/dt and peak d InP/dt. In addition, as 
shown in Table II, the average wall force dur- 
ing ejection is decreased with each frequency of 
stimulation while there is a persistent increase 
in peak dP/dt and mean Vcf above the control 
values for these variables. Similar observations 
are made during periods of sequential pacing. 
In contrast, pacing of the left ventricle resulted 
in a decrease in mean LVP, with no significant 
difference from one pacing rate to the other. 
There is also significant difference in the aver- 
age wall force during ejection with each incre- 
ment in frequency of stimulation of the ventri- 
cle. Further, peak dP/dt and mean Vcf tend to 
remain constant. 
Pacing and the Stroke Work (SW) 
End-Diastolic Volume (EDV) Relation 
In these studies we have found it difficult to 
always capture and pace the atrium. The typical 
effects of atrial pacing in one dog is shown in 
Table II. A facsimile of the computer plot of the 
stroke work EDV relation is shown in Figure 7 
along with its computed linear regression equa- 
tion labeled (1). There is some scatter of the 
data around the regression line. Figure 8 shows 
the regression line for atrial pacing (1) along 
with the data for ventricular pacing (2) in the 
same dog. It can be seen that there is a linear 
relation between stroke work and end-diastolic 
volume. The profile af the curve unlike that 
curvelinear relation that is usually seen for the 
