Wall thickness referenced to myocardial volume: A new noninvasive framework for cardiac mechanics

Dimensional variables measured for study of left ventricular mechanics are subject to errors arising from difficulty in determining zero-stress dimensions for use as a reference. Based on a method validated for measurements within individuals, we have devised an approach that facilitates comparison between individuals while minimizing random scatter. We define an exact mathematical index of strain, ln(h₀/h), using wall thickness (h) referenced to extrapolated wall thickness at zero-luminal volume (h₀). Noninvasive data from rabbits, pigs, and humans all yielded highly similar myocardial stress, ln(h₀/h), and work values. The stress-ln(h₀/h) relationship during afterload variation was constant among individual pigs with a twofold variation in ventricular mass. Stress-ln(h₀/h) data from our analysis displayed lower scatter than either pressure-volume data normalized to myocardial mass or stress-ln(h₀/h) data referenced to end-diastolic dimensions. A Frank-Starling-like curve with high correlation (r² = 0.96) was constructed from single points from different pigs, suggesting a low level of size and intersubject scatter. This method offers high precision for noninvasive characterization of ventricular and myocardial mechanics and for comparisons between subjects and between species.