Metabolic syndrome and myocardial contractile reserve.

REFER TO THE PAGE 176-182

Metabolic syndrome (MetS) is a clustering of cardiovascular risk factors including hyperglycemia, dyslipidemia, and hypertension, which have been shown to increase cardiovascular (CV) morbidity and mortality.1)2) In addition, it is well known that the association between the presence of MetS and adverse CV outcomes is independent of diabetes mellitus (DM) and arterial hypertension.3)4) Since the MetS is a strong predictor of future CV events, the changes of left ventricular (LV) structure and function in patients with MetS are considered one of mechanisms explaining the link between the MetS and CV morbidity and mortality. The previous studies have already reported the LV geometry and function by demonstrating increased LV mass and subclinical LV systolic and/or diastolic dysfunctionis impaired in the MetS patients.5-7) However, the most studies were conducted in the resting state, not during exercise. Considering that most patients with type 2 DM and/or hypertension have symptoms during exercise, the study by Ha et al.8) suggests that the patients with MetS might also have decreased LV functional reserve during dynamic exercise. A total of 112 hypertensive patients complaining of exertional dyspnea were included and divided into two groups according to the existence of MetS. Exercise stress echocardiography using supine bicycle was performed and a variety of Doppler parameters were measured at baseline, at each stage of exercise, and during recovery. There was a significant difference in E/E' parameter, that is an index of LV filling pressure, in MetS patients at resting state. The increased E/E' value was maintained during dynamic exercise test, which is similar to the result of a prior study.9) A unique result to this study is that LV longitudinal functional reserve, defined as the change in tissue Doppler imaging (TDI), S' (longitudinal tissue velocity) from baseline to peak exercise was significantly lower in MetS patients even though there was no significant difference in baseline TDI S' velocity between the two. After adjusting for known CV risk factors, an independent relationship between the presence of MetS and LV systolic functional reserve was observed.

The recent use of TDI, strain, and strain rate at resting state allows detecting earlier subclinical dysfunction and providing essential mechanisms in understanding the development of heart failure. In addition, TDI parameters such as S', E', A', and E/E' have been known to have independent prognostic values in various cardiac diseases,10) even in low risk population.11) However, there is a study showing that resting LV TDI parameter did not add any incremental prognostic impact over the clinical data in patients with type 2 DM.12) Thus, even though exercise capacity is already recognized as a significant predictor of CV diseases, the LV TDI measurements acquired by stress echocardiography seem to be more reliable and stronger in detecting the presence of subclinical CV disease13) and predicting clinical outcome,12) compared with resting TDI values. Nevertheless, measuring Doppler parameters including TDI during exercise stress echocardiography for the evaluation of longitudinal diastolic functional reserve is still challenging, especially at peak stress, because the mitral velocities summate as the heart rate increases. In this study, no differences in longitudinal diastolic functional reserve may be due to these limitations in addition to a small sample size. Although the exact mechanism related to abnormal contractile reserve in patients with MetS is still unknown, it seems that multiple factors including sympathetic nervous system, renin-angiotensin-aldosterone system, myocardial metabolism, and coronary flow reserve contribute to subclinical dysfunction of longitudinal myocardial contraction during exercise.

In summary, MetS is associated with LV systolic and diastolic dysfunction at resting state and decreased LV longitudinal contractile reserve during exercise despite of similar values at baseline examination. Although larger studies to confirm the results of this study are needed, the assessment of longitudinal functional reserve in MetS patients, using exercise stress echocardiography, is helpful for earlier identification of subclinical dysfunction and more aggressive treatment strategies to prevent a progression to overt heart failure.