Background While mammalian heart size maintains constant proportion to whole body size, scaling of left ventricular (LV) function parameters shows a more complex scaling pattern. We used 2-D speckle tracking strain imaging to determine whether LV myocardial strains and strain rates scale to heart size.

Methods We studied 18 mice, 15 rats, 6 rabbits, 12 dogs and 20 human volunteers by 2-D echocardiography. Relationship between longitudinal or circumferential strains/strain rates (S_Long/SR_Long, SC_irc/SR_Circ), and LV end-diastolic volume (EDV) or mass were assessed by the allometric (power-law) equation Y = kM^β.

Results Mean LV mass in individual species varied from 0.038 to 134 g, LV EDV varied from 0.015 to 102 ml, while RR interval varied from 81 to 1090 ms. While S_Long increased with increasing LV EDV or mass (β values 0.047±0.006 and 0.051±0.005, p<0.0001 vs. 0 for both) S_Circ was unchanged (p = NS for both LV EDV or mass). Systolic and diastolic SR_Long and SR_Circ showed inverse correlations to LV EDV or mass (p<0.0001 vs. 0 for all comparisons). The ratio between S_Long and S_Circ increased with increasing values of LV EDV or mass (β values 0.039±0.010 and 0.040±0.011, p>0.0003 for both).

Conclusions While S_Circ is unchanged, S_Long increases with increasing heart size, indicating that large mammals rely more on long axis contribution to systolic function. SR_Long and SR_Circ, both diastolic and systolic, show an expected decrease with increasing heart size.