THE INFLUENCE OF AFTERLOAD ON UNIFORMITY OF SEGMENT SHORTENING IN FELINE LEFT-VENTRICLES - IMPORTANCE OF CROSS-FIBER CONTRACTION

Previous studies have shown that the combined increase of preload and afterload leads to a more uniform wall contraction. As our previous in formation with a stable afterloaded situation showed considerable incr ease of left ventricular end-diastolic pressure, we have as yet no inf ormation of how afterload per se effects uniformity of segment shorten ing. We therefore analysed maximal systolic shortening of cross-orient ed segments for three consecutive beats during abrupt elevation of aft erload 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 pre ssure 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 fo r circumferential segments (CIRC) from 12.2 +/- 0.7 to 8.3 +/- 0.9% (P < 0.0005). Uniformity of maximal systolic shortening, LONG/CIRC, incr eased 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 ventr icular wall is increased during increased afterload. We propose that t his can be explained by reduction of the effect of cross-fibre contrac tion with increasing afterload.