Medicine Poster Session
Discussion and Conclusion
Patients with LV dysfunction and heart failure have a limited reserve to exercise. Therefore it appears unsatisfactory to use maximal exercise tests in such patients. Different submaximal exercise tests are proposed to assess exercise capacity in patients with LV dysfunction. There is no consensus on which is the best. All tests have their advantages and limitations. In recent years, interest has grown to peak oxygen consumption directly measured by using respiratory gas analysis. However, this techniques is still expensive and complex for the physician and is somewhat uncomfortable for the patient (3).
The 6 MW test proved to be an appropriate exercise test in elderly, frail and severely limited patients with heart failure (9,10). The SOLVD trials showed that the 6 MW test is a safe and simple clinical tool that strongly and independently predicts morbidity and mortality in patients with LV dysfunction (5). Gualeni et al. (11) reported that the 6 MW test is a simple, inexpensive, and well-tolerated test to measure changes in exercise tolerance induced by pharmacologic interventions, even on a short-term basis.
Recently Delahaye et al. (12) compared upright maximal graded bicycle exercise, stair climbing and a 6 MW test. Exercise capacity indices (peak oxygen consumption, maximal bicycle work rate, stair climbing time, and the distance covered during the 6 MW test) correlated significantly with one another. There was no correlation between submaximal exercise tolerance indexes and resting or exercise LV ejection fraction.
Despite the 6 MW test has been proved to be a useful measure of functional exercise capacity and a suitable measure of outcome for clinical trials in patients with heart failure and LV dysfunction, data on determinants of the 6 MW test distance are limited. In the SOLVD trials, distance walked decreased with age and NYHA class, and was shorter for women and black participants. Six MW test distance was only weakly related to resting LV ejection fraction (5). Indices of LV diastolic function were not measured in the SOLVD trials.
Exercise capacity has long been considered to be closely related to LV systolic function. Several lines of evidence, however, suggest that exercise intolerance in patients with LV dysfunction is multifactorial. Such mechanisms as central hemodynamic abnormalities, pulmonary abnormalities, and alterations in peripheral blood flow, skeletal muscle biochemistry and histology appear to be involved (13).
The results of this study have showed that diastolic dysfunction is one of the main factors limiting exercise performance in patients with LV dysfunction as assessed by 6 MW test. Exercise capacity was diminished in patients with a predominant early (restrictive) LV filling and baseline tachycardia. These data are consistent with the results of several other studies, in which different exercise tests have been used.
Spiranova et al. (14) with the use of isometric exercise (handgrip) and bicycle exercise found that the functional capacity of patients with heart failure is influenced not only by LV ejection fraction and other systolic variables, but also by filling conditions. The duration of effective diastole may be one of the most important of them. Sumimoto et al. (15), who used supine bicycle ergometry, demonstrated that LV diastolic function rather than LV systolic function is an important factor in determining exercise capacity in patients with LV dysfunction after myocardial infarction. Xie et al. (16) having used a modified Naughton treadmill protocol, revealed that patients with LV dysfunction, who had nonrestrictive patterns of LV filling exercised significantly longer than those with restrictive patterns despite comparable reductions in ejection fraction. Davies et al. (17) with the use of progressive treadmill exercise with respiratory gas analysis found that peak oxygen consumption in patients with congestive heart failure negatively correlated with the PE/PA ratio and with the Doppler estimate of isovolumic relaxation time.
The mechanism underlying the relationship between abnormalities of diastolic function and exercise intolerance is not clearly determined. Kitzman et al. (18) demonstrated that exercise limitations in patients with heart failure with preserved LV systolic function are related to a failure to increase LV end-diastolic volume despite marked elevation in LV filling pressure. Cheng et al. (19), who used an experimental model of heart failure, showed that LV pressure increases during exercise in chronic heart failure. This observation suggested that the failure of the enhancement of LV relaxation and an increase in early diastolic LV pressure with exercise may contribute to exercise intolerance in congestive heart failure. We believe that in patients with predominant early (restrictive) pattern of LV filling the failure to further increase the early diastolic atrioventricular pressure gradient is likely to be a mechanism responsible for exercise intolerance.
Potential limitations of this study should be addressed. Doppler-derived indices of LV filling are influenced by multiple factors, including age, heart rate, mitral regurgitation, loading conditions and pharmacological treatment. In the present study, age was not a determinant of 6 MW test distance. Heart rate was higher in patients with poor exercise performance. However tachycardia is known to increase PA velocity. In this study, PA velocity was in a positive relation to 6 MW test distance, which was independent of a negative relationship between heart rate and exercise capacity. Patients with severe mitral regurgitation were excluded from the study. Loading conditions and medical treatment were not characterized in this study. However, the majority of study patients were taking the same therapy, including angiotensin-converting enzyme inhibitors and diuretics.
Conclusion. This study has showed that Doppler-derived indices of LV diastolic filling (PA wave velocity and PE/PA ratio) and heart rate are the main determinants of exercise capacity in patients with LV dysfunction as assessed by 6 MW test.
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|Nikitin, NP.; (1998). Determinants Of Exercise Capacity In Patients With Left Ventricular Dysfunction As Assessed By Six-Minute Walk Test. Presented at INABIS '98 - 5th Internet World Congress on Biomedical Sciences at McMaster University, Canada, Dec 7-16th. Available at URL http://www.mcmaster.ca/inabis98/medicine/nikitin0578/index.html|
|© 1998 Author(s) Hold Copyright|