p. KRAFT-HUNTER AND E. W. HAWTHORNE 
629 
(1971) reported that while the calculated aver- 
age slope for the ventricular function curve 
relating stroke volume to left ventricular end- 
diastolic pressure was steeper in normal sub- 
jects than in a group of patients with elevated 
left ventricular end-diastolic pressure, there 
was considerable overlap between the two 
groups. Because of this observation, they felt 
that "pacing" ventricular function curves had 
their greatest value in assessing the effects of 
various interventions (such as drugs) which 
augment or depress ventricular function in an 
individual patient. They were not impressed 
with the adequacy of these curves as a means of 
discriminating between patients or detecting 
early signs of ventricular dysfunction. 
In 1970 Daggett, Bianco, Powell and 
Austen^^ showed that both the AS-VS interval 
and the pattern of ventricular activation impor- 
tantly influence ventricular function during 
cardiac pacing. It was noted, however, that 
when the A-V delay during sequential pacing 
was adjusted so as to match the AS-VS interval 
prevailing in the unpaced dog, the hemodyn- 
amic state of the ventricle was apparently unaf- 
fected by the aberrant nature of ventricular 
depolarization that occurs during direct ventric- 
ular electrical stimulation. They therefore, con- 
cluded that with properly timed sequential pac- 
ing, ventricular function closely approximates 
that obtained during a normally propagated 
wave of depolarization. 
Some questions emerge when considering the 
usefulness of "pacing" ventricular function 
curves for continued evaluation of ventricular 
performance in conscious dogs. These are: (1) 
what effect does left atrial or left ventricular 
pacing at different rates have on myocardial 
contractility in conscious dogs; (2) what is the 
character of the function curves when stroke 
work is plotted against end-diastolic volume 
and not end-diastolic pressure; (3) can these 
curves be made to shift to the left when con- 
scious dogs are infused with either norepineph- 
rine or to the right when the animal has been 
given a beta blocking dose of propanolol; (4) 
how reproducible are these effects; (5) is there 
a clear difference between the hemodynamic ef- 
fects produced by atrial pacing as compared to 
left ventricular pacing. 
Since we have developed in our laboratory 
conscious instrumented dogs from which the 
changes in aortic, left atrial and left ventricular 
pressures, left ventricular internal diameter, 
the left atrial and left ventricular electrograms 
can all be monitored directly, simultaneously 
and continuously, we felt it appropriate to con- 
duct a systematic study on the effect of atrial 
and ventricular pacing on ventricular function. 
The monitored primary variables as listed 
above when fed thru an A to D converter to 
our digital computer (HP2116B (16K) and pe- 
ripherals) provides practically on-line computa- 
tion of a variety of derived variables for each 
chosen cardiac cycle. By utilizing this data ac- 
quisition and reduction system we have been 
able to follow the changes occurring in dP/dt, 
heart rate, total cycle length, end-diastolic, 
end-systolic and stroke volumes, stroke work and 
stroke power along with changes in the primary 
variables and a variety of other derived data 
for a series of consecutive cycles at different 
atrial and ventricular pacing rates. 
This study presents our findings on the 
efficacy of using "Pacing" Ventricular Function 
Curves as one means of evaluating ventricular 
performance in conscious instrumented dogs. 
METHODS 
The Conscious Instrumented Dog 
Dogs weighing approximately 25 kilograms 
and of either sex were used in this study. Dogs 
were obtained in groups of 10 and housed in the 
Animal Section for at least one month during 
which time they were improved nutritionally, 
dewormed and their general health status deter- 
mined. Those animals found to be in good clini- 
cal condition at the end of this period were then 
operated and instrumented as shown diagra- 
matically in Figure 1. All dogs were instru- 
mented in this way. In some of the animals ei- 
ther an inflatable silastic aortic cuff or a flow 
probe was placed about the ascending aorta 
after the latter has been covered with a segment 
of Dacron. 
A considerable period of time has been re- 
quired to develop our techniques and procedures 
before finally arriving at the stage of develop- 
