L. N. COTHRAN, E. W. HAWTHORNE AND H. SANDLER 
563 
appear to be meaningful approaches to calcula- 
tion of the internal ventricular volume from 
end-diastole to end-systole in the canine. 
SUMMARY 
Experiments were designed and conducted to 
study the moment-to-moment changes in exter- 
nal and internal dimensions in conscious ani- 
mals. The dimensions which were monitored in- 
cluded left ventricular circumference, external 
diameter, internal diameter and wall thickness 
in the mionor plane at the level of the equator 
of the heart. The chord length of the ventricle 
from the bifurcation of the coronary vessels to 
the apex was recorded simultaneously in five 
dogs by biplane cineradiography. 
Analysis of the data obtained during normal 
cardiac cycles revealed that although the dimen- 
sions of the ventricular chamber decrease in 
both the longitudinal and transverse directions 
the reduction in internal diameter made the 
greater contribution to ejection. The increase in 
wall thickness during ventricular systole was 
found to be a major determinant of the decrease 
in internal cross-sectional area as well as a fac- 
tor modifying the magnitude and time course of 
the myocardial stresses. A linear relationship 
between the internal diameter and stroke vol- 
ume of the left ventricle was found ; and a simi- 
lar linear relationship was found when the in- 
ternal radius was plotted against the time 
course of the changes in axis ratio throughout 
ejection. The equation describing this latter re- 
lationship was determined to be : 
Axis Ratio = -0.6 R; + 3. 
By use of this equation and the equation for vol- 
ume of a prolate ellipsoid (V = 4/377 Rj^AR), 
it was possible to calculate the volume of the 
left ventricle at any point from end-diastole to 
end-systole using the measured internal trans- 
verse radius alone. The stroke volumes obtained 
by this procedure were found to underestimate 
the actual stroke volume by only 3.5%. Calcula- 
tion of the stroke volume in this fashion but 
using the end-diastolic axis ratio as a constant, 
overestimated the actual stroke volume by only 
9.7%. 
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