The influence of afterload on uniformity of segment shortening in feline left ventricles. Importance of cross‐fibre contraction

Previous studies have shown that the combined increase of preload and afterload leads to a more uniform wall contraction. As our previous information with a stable afterloaded situation showed considerable increase of left ventricular end-diastolic pressure, we have as yet no information of how afterload per se effects uniformity of segment shortening. We therefore analysed maximal systolic shortening of cross-oriented segments for three consecutive beats during abrupt elevation of afterload in 12 open-chest cats. Peak left ventricular systolic pressure increased during the three beats from 154 +/- 5 mmHg (mean +/- SEM) to 167 +/- 5 mmHg and 186 +/- 5 mmHg. Left ventricular end-diastolic pressure remained unchanged. Maximal systolic shortening was reduced for both segments with increasing afterload, in longitudinal segment (LONG) from 7.6 +/- 1.1 to 6.1 +/- 1.0% (P < 0.005), and more pronounced for circumferential segments (CIRC) from 12.2 +/- 0.7 to 8.3 +/- 0.9% (P < 0.0005). Uniformity of maximal systolic shortening, LONG/CIRC, increased from 0.63 +/- 0.08 in the first beat to 0.73 +/- 0.10 and 0.82 +/- 0.12 in the following beats (P < 0.02). We conclude that uniformity of contraction for cross-oriented segments in the anterior left ventricular wall is increased during increased afterload. We propose that this can be explained by reduction of the effect of cross-fibre contraction with increasing afterload.