Echocardiographic study of circadian myocardial function among clinically healthy subjects

Background: Synchronization of organisms with their environment along the 24-hour scale is mediated by circadian clocks. This cell-autonomous mechanism has been identified within all cardiovascular-relevant cell types, including cardiomyocytes. Subjects and Methods: The study aims to assess the circadian pattern in the contractility of the left ventricle (LV), using myocardial deformation imaging. The study involved 11 clinically healthy volunteers (mean age 38y ±10), including 4 men (mean age 30.5y ±12.4) and 7 women (mean age 42.7y ±4.8). Automated quantification of LV systolic function by measurement of LV systolic strain from speckle-tracking echocardiography was used. Echocardiography was performed with a commerciallyavailable standard ultrasound scanner, and a 2.5-MHz transducer. All images were obtained at a rate of 50 to 70 frames/s. Strain and twist/untwist measurements were performed offline with dedicated automated software. Results: In health, global longitudinal strain (GLS) varied from −22.35% to −22.87%, global circumferential strain (GCS) from −18.41% to −21.50%, and global radial strain (GRS) from 36.75% to 42.30%. LV twist (LVT) ranged from 8.33° to 10.77° and untwist rate (LVUR) from −87.45°/s to −71.49°/s. Overall, highest values for GLS, GRS, and LVT occurred around 06:00 and overall lowest values of GRS and LVT around 18:00. Considering results from all 6 walls as independent replications, a statistically significant circadian rhythm was found for both regional circumferential strain (P<0.001) and for radial strain (P=0.002). Discussion and Conclusion: This study assessed global physiological consequences of the circadian clock specifically within the myocardial deformation. Apart from influences of the cardiomyocyte circadian clock on heart rate and the responsiveness of the heart to an increased workload, evidence is provided herein for the circadian stage-dependence of regional deformation, an important result pertaining to global ventricular function.