How similar are the mice to men? Between-species comparison of left ventricular mechanics using strain imaging

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, S_Circ/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.