Effects of baseline heart rate at sea level on cardiac responses to high-altitude exposure

High-altitude (HA) exposure has been widely considered as a cardiac stress, and associated with altered cardiac function. However, the characteristics of cardiac responses to HA exposure are unclear. In total, 240 healthy men were enrolled and ascended to 4100 m by bus within 7 days. Standard echocardiography and color tissue Doppler imaging were performed at sea level and at 4100 m. In all subjects, HA exposure increased HR [65 (59, 71) vs. 72 (63, 80) beats/min, p < 0.001] but decreased the stroke volume index (SVi) [35.5 (30.5, 42.3) vs. 32.9 (27.4, 39.5) ml/m², p < 0.001], leading to an unchanged cardiac index (CI). Moreover, baseline HR was negatively correlated with HA exposure-induced changes in HR (r = − 0.410, p < 0.001) and CI (r = − 0.314, p < 0.001). Following HA exposure, subjects with lowest tertile of baseline HR showed an increased HR [56 (53, 58) vs. 65 (58, 73) beats/min, p < 0.001], left ventricular ejection fraction (LVEF) [61.7 (56.5, 68.0) vs. 66.1 (60.7, 71.5) %, p = 0.004] and mitral S′ velocity [5.8 ± 1.4 vs. 6.5 ± 1.9 cm/s, p = 0.040]. However, subjects with highest tertile of baseline HR showed an unchanged HR, LVEF and mitral S′ velocity, but a decreased E′ velocity [9.2 ± 2.0 vs. 8.4 ± 1.8 cm/s, p = 0.003]. Our findings indicate that baseline HR at sea level could determine cardiac responses to HA exposure; these responses were characterized by enhanced LV function in subjects with a low baseline HR and by reduced LV myocardial velocity in early diastole in subjects with a high baseline HR.