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Kinetics of Left Ventricular Mechanics during Transition from Rest to Exercise.

PURPOSE
At the onset of physical exercise, oxygen (O2) transport adapts to meet the working muscle O2 demands. Cardiac output abruptly increases through the concomitant changes of heart rate and stroke volume (SV), which is conditioned by the left ventricular (LV) function. The purpose of this study was to investigate the contribution of many LV diastolic and systolic function parameters, including twist-untwist mechanics, to SV adaptation during the first minutes after exercise onset.


METHODS
Diastolic and systolic myocardial strains and twist were monitored by two-dimensional (2D) speckle-tracking echocardiography with high temporal resolution in 28 young men (mean age: 23 ± 4 years) who performed five similar constant work-load exercises on a cycloergometer (target heart rate: 125 bpm). 2D cine-loops were recorded every 15 seconds during the first minute of exercise, and then every 30 seconds for the next 3 minutes.


RESULTS
During the first 60 seconds of exercise, SV (from 104 ± 15 to 126 ± 21 mL, p<0.001) increased concomitantly with LV strain and strain rates. Early filling was the main SV determinant during this phase, probably linked to the increase of venous return (at the very beginning of exercise), LV relaxation (from 1.5 ± 0.3 to 2.5 ± 0.4 s, p<0.001) and untwisting (from -78 ± 34 to -165 ± 61 deg.s, p<0.001). After the first minute, SV remained constant, while LV untwisting continued to increase (from -165 ± 61 to -187 ± 60 deg.s, p<0.001) and the other systolic and diastolic parameters reached a plateau.


CONCLUSION
This study gives new mechanical insights into LV kinetics to address the challenge of SV response at the onset of exercise.