Fibre orientation in the left ventricle and its influence on local pressure-length loop analysis in cats.

Deviation from local fibre orientation may seriously affect quantification of myocardial segment work, as measured by implanted ultrasound crystals. Crystal pairs were positioned in mid-wall layers with known directions to the long axis of the heart. One transmitter and four receiving crystals covered a sector on the anterior ventricular wall of 100 degrees. The results show a strong influence of alignment to fibre direction on the pressure-length loop area; the relation followed a cos2 psi function where psi denotes the angle between the crystal pair direction and fibre direction established by microscopy. The angle between the mid-wall muscle fibres on the anterior aspect of the left ventricle and the long axis of the heart varied between 62 degrees and 90 degrees with a mean value of 76 degrees. This study shows that pressure-length loop quantitation depends strongly on crystal alignment to cardiac muscle fibres through which the ultrasound passes.