632 
PHYSIOLOGY 
essed by means of a Hewlett-Packard 2116B 
digital computer, with 16K word of main mem- 
ory (16 bit words) and two direct memory ac- 
cess channels allowing data transmission rates 
up to 625 KHz by interleaving the channels. Pe- 
ripheral devices include a multiplaced 16 chan- 
nel, 14 bit resolution A to D converter capable 
of throughput rates of 18 KHz, two moving 
head disc drives each with a fixed disc a remov- 
able cartridge providing a peripheral storage ca- 
pability of 2.6 million words, accessible at a 
transfer rate of 45 KHz, a 7 channel digital 
magnetic tape unit (200 bits/in., 30 in./sec.) ; a 
high speed printer (300 lines/min.) ; and a Tek- 
tronix 4002A graphic terminal display unit. 
The hardware system is supervised by a HP- 
DOSM operating system with special in- 
put/output routines to facilitate continuous 
sampling and storage of large blocks of data 
(up to 500,000 words). 
The first derivative of the left ventricular in- 
tracavitary pressure (dP/dt) was obtained by 
pacing the amplified signal from a pressure cell 
and feeding it into a Biotronex laboratory ac- 
tive differentiator, (Model 621) where the 
phase angle was 90° and the center frequency 
(fo) was set such that the corner frequency was 
150 Hz. Amplitude was estimated through the 
active differentiation of a triangular wave which 
was adjusted so that the peak to peak amplitude 
was equivalent to 160 mm Hg and the frequency 
was varied from 4 to 16 Hz. The resultant 
square wave at each step served to calibrate the 
output of the differentiator. 
The mean velocity of circumferential fiber 
shortening (mean Vcf (EJ) ) during ejection 
was defined as the extent of shortening of the 
left ventricular internal circumference, in the 
midplane normal to the long axis, during the 
period between end isovolumic contraction and 
end ejection, divided by the duration of ejection. 
These points in the cardiac cycle were identified 
by an algorithm using the computer. 
In previous studies we have made measure- 
ments of the simultaneous changes in base to 
apex, length, external circumference and wall 
thickness in the plane of the minor axis. These 
measurements have also been made along with 
estimation of left ventricular length of the 
major axis using biplane cinefluorography. 
From these studies we have observed that the 
shape of the left ventricle can most closely be ap- 
proximated using geometry of an ellipsoid of 
revolution. In addition it has been observed that 
the change in internal axis ratio during ejec- 
tion is very small. The volume of an ellip- 
soid of revolution is given by the expression: 
V = 4/3 7rRi3B (1) 
V = volume where R = length of V2 the minor 
axis ; B = axis ratio. 
In these studies R was measured directly and 
the axis ratio was established at end diastole 
and end systole for each dog. The mean of these 
two values was then used in equation (1) for 
estimation of internal volume during the phase 
of ejection. 
Left ventricular stroke work for each peak 
was computed through integration of the left 
ventricular pressure times the change in intra- 
cavitary volume throughout the phase of ejec- 
tion. The expression used for computation was : 
N 
SW = 1.36x10-2 ^ P) + P, + 1 _ y. + 1) 
i = 1 
(2) 
where N = total number of samples taken by 
computer during the phase of ejection. The esti- 
mation of left ventricular endocardial wall 
force is based on the assumption that in a plane 
normal to the long axis, at the middle of the left 
ventricle, the rim of muscle is circular. Then 
the force acting on the endocardial surface at 
any given instant is a function of the intracavi- 
tary pressure and the internal radius and can 
be computed from the expression : 
Wall force = 1.36 x LVP x tt ^? (3) 
The average wall force during ejection is then 
equal to the sum of forces acting during ejec- 
tion divided by the duration of ejection. In this 
study wall force was estimated at 5 msec inter- 
vals throughout the cardiac cycle. 
A Grass stimulator (Model S8) was used to 
stimulate the atrium and ventricle. The fre- 
