Cardiac output is often higher in obesity, due to an augmented stroke volume and an increase in heart rate.2
Ventricular systolic function as assessed by ejection fraction or with load independent measures such as mid wall fractional shortening is usually normal in obesity.9
The increased resting LV end diastolic volume in obesity has been interpreted as evidence of enhanced recruitment of preload reserve (Frank-Starling mechanism). Obese people, however, may fail to increase their ejection fraction with exercise.
Obesity adversely impacts cardiac diastolic function. In a recent large community based investigation, body mass index (BMI) emerged as a key correlate of impaired diastolic filling indexes.10
Obesity can alter LV filling indexes because of altered loading conditions (see above), as well as due to an increased LV mass that can adversely impact the passive filling properties of the ventricle.3
In this context, the study by Pascual and colleagues in this issue of the Heart
The investigators observed an increased mitral valve pressure half-time and decreased deceleration slope of the early transmitral filling wave in obese women. They interpret these observations as indicating subclinical diastolic dysfunction representing impaired LV relaxation. They also note an increase in ejection phase indices, consistent with some prior reports.
An important strength of the investigation by Pascual and colleagues11
is the selection of obese women without co-morbid conditions such as hypertension, diabetes or dyslipidaemia, a design that eliminates confounding by sex and these disease conditions. However, important methodological and conceptual limitations of their approach should be noted. The sample is presumably hospital based, small sized, and exclusively female. Hence, the findings may not be generalisable to the average obese person in the community in whom risk factors frequently cluster. The authors do not provide data on the duration of obesity and its pattern, the presence of impaired glucose tolerance, and LV mass (all factors that can influence diastolic function), or information about the reproducibility of the echocardiographic measurements. Furthermore, the investigators compared 21 different echocardiographic measurements across the four BMI categories, raising the issue of multiple statistical testing.
A more serious limitation is the interpretation that changes in LV diastolic “filling” indexes constitute alterations in myocardial diastolic “function”. Transmitral Doppler flow patterns are dependent on multiple interrelated factors, including the rate and extent of ventricular relaxation, ventricular suction, net atrioventricular compliance, mitral valve inertance, atrial systolic function, and left atrial pressure.12
Thus, the observation of an increased mitral pressure half-time with increasing BMI in the present report may be explained partly by the increase in LV end diastolic volume in obese individuals,13
and need not indicate any change in intrinsic myocardial relaxation. Additionally, as noted by the authors, prior cross sectional studies of obese individuals have reported increased, decreased or unchanged transmitral E wave velocities, with variable effects on the derived E/A ratios. Such disparities in results can arise because transmitral flow indexes are notoriously sensitive to loading conditions, and may change over time according to the degree and stage of diastolic dysfunction.12
To address some of these challenges in the assessment of ventricular diastolic function, investigators have used elaborate research protocols that combine measurement of transmitral flow patterns with evaluations of pulmonary venous flows, mitral annular motion on tissue Doppler imaging and/or the velocity of transmitral flow propagation with colour M mode echocardiography.12
Pascual and colleagues11
did not perform such a detailed characterisation of ventricular diastolic filling. So while it can be inferred from their study that diastolic filling patterns are modified in obesity, it is unclear if ventricular diastolic function per se is altered. Future investigations that perform more comprehensive non-invasive evaluation of LV diastolic filling along with assessment of long axis ventricular function, and that include an exercise component, are warranted to understand better the impact of obesity on cardiac function.