Cardiac structure and cardiorespiratory fitness. When size matters!

During the last decades, it has become obvious that left ventricular ejection fraction (LVEF) is no longer the central parameter used for diagnosis and management in patients with heart failure. This is mainly accepted since LVEF is both preload- and afterload-dependent and can change depending on loading conditions. Thus, LVEF does not represent stroke volume which is influenced by hemodynamic conditions and related to exercise tolerance [1]. Furthermore, symptoms of exercise intolerance are connected with right ventricular function, that acts as a reservoir for left ventricular end-diastolic (LVEDV) volume and left ventricular diastolic function pattern.

In the current issue of the European Journal of Preventive Cardiology, Rowe S et al., in a total cohort of 2876 individuals [2], including athletes, healthy non-athletes and heart failure patients with preserved EF, evaluated the relationship between resting echocardiographic measures of left ventricular size and cardiorespiratory fitness (CRF) in individuals with normal LV EF (≥50%). It is well known that CRF is an independent predictor of morbidity and all-cause and cardiovascular disease mortality. Left ventricular size was the strongest independent echocardiographic predictor of CRF across the health–disease continuum. Indeed, ventricular size and its relationship to cardiac output is only one metric involved in CRF, and abnormalities in peripheral factors are known to play a significant role in exercise intolerance in HFpEF patients [3]. What is striking more important is that cardiac structure and especially left ventricular dimensions may determine exercise capacity through all the spectrum of health and disease with preserved left ventricular function. In this study, among various imaging indices, small resting left ventricular size was associated with a higher probability of low CRF and functional disability. Individuals with the smallest left ventricle volumes had the highest probability of achieving a peak VO2 below the required for daily tasks and functional independence. A cut-off point for ventricular size associated with CRF needed for functional independence 88mL for LVEDV and 57 mL/m2 for LVEDVi. In the case of HFpEF, small ventricle size with increased wall stiffness is related with lower stroke volume at rest and impaired reserve during exercise. Thus, HFpEF patients exercise capacity is dependent on chronotropic response during exercise. In those cases, right ventricular function, oxygen extraction and utilization by skeletal muscle and heart rate increase during exercise are related with exercise tolerance and compensated left ventricular diastolic function [4]. In a parallel situation, healthy non-trained individuals may present decreased exercise capacity due to smaller left ventricular cavities and lower left ventricular preload, in combination with untrained peripheral muscles that show reduced oxygen extraction and utilization. The weaker relationships between peak VO₂ with left ventricular diastolic volume seen in HFpEF compared with athletes, may be due to the impairment of right ventricular function which is related with all peripheral adaptations. On the other hand, in elite athletes, exercise induces cardiac remodeling as an adaptation to an increased reserve of stroke volume during exercise in combination with a slower chronotropic response [5]. The question raised by authors of which individuals may gain benefit from the physiological adaptations of exercise on heart structure, has been limited addressed in literature, as sex differences, age of onset regular exercising and atrial function reflecting ventricular filling pressures, exist.

It seems that in health and in disease similar adaptations in cardiac structure are related with exercise tolerance, where cardiac output reserve along with muscles oxygen extraction have the most important role. A role that is moderated by left atrial reverse remodeling, right ventricle performance and left ventricle size.

Lifelong exercise can mitigate the cardiac changes of aging, including the changes in ventricular size and compliance and therefore it is important in reducing the chance of disability associated with lower CRF. The article supports clinicians in recommending physical activity to patient with unexplained dyspnea and small LV volume.