CLINICAL STUDY: HEART FAILURE
Dobutamine echocardiographic study in patients with nonischemic dilated cardiomyopathy and prognostically borderline values of peak exercise oxygen consumption
18-month follow-up study
Ioannis A. Paraskevaidis, MDa,
Stamatis Adamopoulos, MDa and
Dimitrios Th Kremastinos, MDa
a Second Department of Cardiology, Onassis Cardiac Surgery Center, Athens, Greece
Manuscript received June 21, 2000;
revised manuscript received December 28, 2000,
accepted January 18, 2001.
Reprint requests and correspondence: Dr. Ioannis A. Paraskevaidis, Onassis Cardiac Surgery Center, 356 Sygrou Avenue, 176 74 Athens, Greece
elbee{at}ath.forthnet.gr
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Abstract
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OBJECTIVES
We sought to study the prognostic value of dobutamine echocardiography in patients with nonischemic dilated cardiomyopathy (DCM) and prognostically borderline values of peak oxygen consumption (VO2max) during exercise.
BACKGROUND
Changes in echocardiographic variables assessed by dobutamine echocardiography can be used to evaluate the functional status of patients with chronic heart failure (CHF) and DCM.
METHODS
In 27 consecutive patients (mean age 55 ± 15 years) with VO2max values between 10 and 14 ml/kg body weight per min, a low infusion rate (10 µg/kg per min) dobutamine echocardiographic test was performed. The induced changes in echocardiographic variables were measured, and an 18-month follow-up study was done.
RESULTS
At the end of the protocol, 9 patients (group I) had died from cardiac reasons, whereas the remaining 18 patients (group II) survived. After dobutamine infusion, the left ventricular end-systolic diameter (LVESD) was smaller in group II (6.22 ± 0.94 cm) than in group I (6.99 ± 0.76 cm; p < 0.05), whereas end-systolic wall stress (ESWS) was higher in group I (1030.66 ± 193.98 g/cm2) than in group II (691.57 ± 297.06 g/cm2; p < 0.05). The changes in LVESD and ESWS were greater in group I (0.75 ± 0.36 cm and 463.11 ± 159.87 g/cm2, respectively) than in group II (–0.04 ± 0.36 cm and 83.16 ± 291.74 g/cm2, respectively; p < 0.01 for both).
CONCLUSIONS
In the "gray" zone of VO2max, dobutamine echocardiography seems to be a valuable prognostic indicator in patients with CHF and DCM.
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Abbreviations and Acronyms
| | CHF | = chronic heart failure | | DCM | = dilated cardiomyopathy | | ESWS | = end-systolic (meridional) wall stress | | FS | = fractional shortening | | LVEDD | = left ventricular end-diastolic diameter | | LVESD | = left ventricular end-systolic diameter | | PWTs | = posterior wall thickness in systole | | SBP | = systolic blood pressure | | Vcfc | = left ventricular heart rate-corrected mean velocity of circumferential fiber shortening | | VO2max | = peak exercise oxygen consumption |
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Recent American Heart Association consensus statements have recommended that peak oxygen consumption (VO2max) during exercise can be used to objectively evaluate the functional status of patients with heart failure (1). However, several groups have found no statistical difference in survival between patients with VO2max levels in the range of 10 to 14 ml/kg body weight per min and those with levels in the range of 14 to 18 ml/kg per min (2). Although the decision for heart transplantation in patients with chronic heart failure (CHF) is mainly based on VO2max values, it has been suggested that patients who present with an increase in VO2max of at least 2 ml/kg per min at follow-up have a short-term survival, similar to those who had received a heart transplant (3), suggesting that this variable may vary throughout the course of the disease.
Recently, we have found that the changes in echocardiographic variables assessed by dobutamine echocardiography are well correlated with VO2max and seem to be accurate for evaluating the functional status of patients with CHF and nonischemic dilated cardiomyopathy (DCM) (4).
The aim of this study was to investigate the prognostic value of dobutamine echocardiography in patients with nonischemic DCM and prognostically borderline values of VO2max (10 to 14 ml/kg per min).
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Methods
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Patients.
Twenty-nine consecutive patients with documented DCM were studied. Included were patients with VO2max values between 10 and 14 ml/kg per min, measured on the day before the dobutamine echocardiographic study. Twenty-seven patients (98%; 20 men and 7 women, mean age 57 ± 8 years) with good-quality M-mode and two-dimensional echocardiograms were recruited for analysis. The diagnosis of DCM was based on the echocardiographic findings of 1) a dilated left ventricle (left ventricular end-diastolic diameter [LVEDD] >60 mm), with severely affected systolic function; 2) fractional shortening [FS] <20%; and 3) ejection fraction <35%. Dilated cardiomyopathy was classified as idiopathic in all patients on the basis of their clinical history (no history of alcohol consumption, myocardial infarction, hypertension, valvular heart disease or familial hypertrophic cardiomyopathy and no evidence of restrictive cardiomyopathy), echocardiogram, coronary angiogram (angiographic lesions <30% lumen diameter stenosis) and biopsy. All patients were in New York Heart Association functional class III. All patients were in sinus rhythm and were taking digoxin, angiotensin-converting enzyme inhibitors and diuretic drugs in adequate doses. All of them received the same level of care and equal access to heart transplantation. None of them had placement of a left ventricular assist device, and the dobutamine echocardiographic results were not used to determine the type of care these patients received. Patients with rhythm disturbances, ischemic cardiomyopathy, more than mild valvular heart disease or regional wall motion abnormalities were excluded. Transthoracic echocardiography was performed, and the echocardiographic variables were measured at baseline and after dobutamine infusion. Each patient was followed for 18 months by monthly clinical visits, and in case of a terminal event, information was acquired from either the physician or the hospital archives. All patients were prospectively identified into two groups: those who died (group I) or survived (group II) after the 18-month follow-up period. The groups were analyzed after completion of the study to evaluate the prognostic importance of dobutamine echocardiographic variables. All patients gave written, informed consent.
Echocardiography.
Measurements and tracings were carried out according to the principle of the leading edge, in accordance with the recommendations of the American Society of Echocardiography (5). Left ventricular dimensions and wall thickness were measured from parasternal targeted M-mode echocardiographic recordings. Care was taken to record the largest and smallest left ventricular dimensions present between the tips of the mitral valve leaflets and the superior aspect of the papillary muscles. The LVEDD was taken at the Q-wave of the electrocardiogram (ECG). The left ventricular end-systolic diameter (LVESD) was determined to be the shortest distance between the walls (6). Using a Hewlett-Packard (Sonos 1000 or 2500; Palo Alto, California) ultrasound device, the following echocardiographic variables were measured at baseline and at the end of 10 µg/kg per min of dobutamine infusion: 1)  ; 2) ventricular septal and posterior wall thickness in systole (PWTs); 3) end-systolic (meridional) wall stress (ESWS), calculated using the formula:  (7) (SBP was represented by brachial artery systolic pressure and was measured every minute with a cuff sphygmomanometer); 4) left ventricular heart rate–corrected mean velocity of circumferential fiber shortening (Vcfc), calculated as follows:  (LVET represents left ventricular ejection time in milliseconds from the opening to the closing clicks of the aortic valve flow velocity envelope, as measured by continuous wave Doppler imaging); 5) Vcfc/ESWS ratio; 6) cardiac output, calculated as the product of stroke volume x heart rate (stroke volume was calculated according to the formula:  ) (aortic diameter was measured in a two-dimensional parasternal long-axis view, just below the aortic orifice [8]; aortic velocities were measured by continuous wave Doppler imaging in an apical five-chamber view); and 7) left ventricular ejection fraction, calculated using the modified Simpson’s rule.
Left ventricular myocardial reserve was defined as the changes in the aforementioned echocardiographic variables after dobutamine infusion. The Vcfc/ESWS ratio was also measured as an index of myocardial contractile reserve. Changes represent the values obtained after inotropic stimulation, minus those obtained at baseline. All measurements were made at a paper speed of 100 mm/s and represent the average of the measurements of five consecutive beats. All of the patients’ echocardiograms were analyzed by two independent, expert observers. In cases of discrepancy, the average was calculated, and the mean value was reported.
Dobutamine infusion.
Dobutamine was infused intravenously in two steps after establishment of a stable hemodynamic state for each step (heart rate and blood pressure had achieved a plateau for the last 2 to 3 min). The duration of each step was 5 min, and the maximal end dose of infused dobutamine was 10 µg/kg per min (9). At each step, dobutamine infusion was increased by 5 µg/kg per min, reaching 10 µg/kg per min at the second step. Every minute during the protocol, systolic, diastolic and hence mean arterial blood pressures (Sirecust 888 device, Siemens), heart rate and a 12-lead ECG were recorded.
Cardiopulmonary exercise testing.
Exercise testing with respiratory gas exchange measurements was performed by using the Medgraphics CPX/MAX measuring system, while patients exercised on a treadmill according to the Dargie protocol (10). Blood pressure was measured with a mercury sphygmomanometer, and the ECG was continuously monitored with a computer-assisted system (Marquette Electronics Inc., Milwaukee, Wisconsin). All patients quit the test because of dyspnea or fatigue, and in all patients, the gas exchange anaerobic threshold and a respiratory exchange ratio >1.0 were reached. Peak oxygen consumption (VO2max, ml/kg per min) during exercise was reported as the mean value during the last minute of exercise.
Statistical analysis.
All data are expressed as the mean value ± SD. The data were processed and analyzed using the programs Statistica and SPSS for Windows 98. At this stage, we added categorical and grouping variables. Every variable was analyzed with regard to its statistical power, producing tables of descriptive statistics according to the group (I = terminal; II = survival). The Kolmogorov-Smirnov procedure was applied to determine normality. Because a number of variables did not follow the normal distribution, we opted for use of nonparametric tests in paired and independent sample comparisons. The Wilcoxon test was used for paired comparisons, such as comparisons of echocardiographic variables before and after dobutamine infusion. The Kolmogorov-Smirnov test was used for independent sample comparisons, such as comparisons of echocardiographic variables between the two groups. Pairwise comparisons were used to assess: 1) the effect of dobutamine infusion (before and after the end of 10 µg/kg per min of dobutamine infusion); and 2) the potential of any given echocardiographic variable to serve as a prognostic factor. Variables that exhibited statistically significant differences between the terminal and survival groups were considered for inclusion in a prediction model constructed according to the logistic regression estimation method. Interobserver and intraobserver variabilities of our laboratory have been reported recently (4). A p value <0.05 was considered statistically significant.
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Results
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No major side effects were reported during dobutamine infusion. The VO2max measured in all patients ranged from 10.2 to 13.9 ml/kg per min (mean 12.4 ± 1, median 12.2). The VO2max and anaerobic threshold were similar between groups (12.1 ± 1.3 and 7.35 ± 1.1 ml/kg per min for group I and 12.53 ± 0.8 and 7.35 ± 0.89 ml/kg per min for group II, respectively).
Echocardiographic measurements.
The echocardiographic measurements of the entire study group, before and after dobutamine infusion, are shown in Table 1. After dobutamine infusion, an increase in LVEDD was observed in the entire study group (p < 0.05), as well as in both subgroups. However, only in group I was this significantly increased (p < 0.05). This might be due to the selection of a particular group of patients in whom the effect of dobutamine infusion on ESWS exceeded its effect on contractility. Therefore, left ventricular stroke volume seems to be maintained, probably by enhancement of LVEDD.
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Table 1 Echocardiographic and Other Measurements Before and After Dobutamine Infusion in the Entire Study Group (n = 27)
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18-month follow-up.
All patients were prospectively identified into two groups according to whether they died (group I, n = 9) or survived (group II, n = 18) after 18 months of follow-up.
The values of the echocardiographic variables for groups I and II, before and after dobutamine infusion, are shown in Table 2 and Figures 1 and 2. Although, at baseline, all echocardiographic variables were similar between groups after inotropic stimulation, LVESD and ESWS were higher in group I than in group II (p < 0.05 for both). Systolic and mean blood pressures, as well as heart rate, were similar between groups, both before and after dobutamine infusion. Interestingly, the changes in left ventricular posterior wall thickness (p < 0.05), LVESD (p < 0.01), ESWS (p < 0.01) and Vcfc/ESWS ratio (p < 0.05) were statistically different between the groups (Table 3, Fig. 3, 4).
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Table 2 Echocardiographic Variables Before and After Dobutamine Infusion in the Terminal (Group I, n = 9) and Survival (Group II, n = 18) Groups
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Figure 1 The left ventricular end-systolic diameter (LVESD) response to dobutamine in the terminal (group I, n = 9) and survival (group II, n = 18) groups.
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Figure 2 The end-systolic (meridional) wall stress (SWS) response to dobutamine in the terminal (group I, n = 9) and survival (group II, n = 18) groups.
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Table 3 Changes and Percent Changes in Echocardiographic Variables in the Terminal (Group I, n = 9) and Survival (Group II, n = 18) Groups
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Prediction model.
In constructing a prediction model for a dependent dichotomous variable such as survival (only two possible outcomes), without affecting the continuity of the independent variables, one must use a special regression model. The logistic regression model describes the relationship between a group of independent variables that may be continuous or categorical and one binary (dichotomous) dependent variable. In our case, the dependent binary variable is the possible outcome (death or survival), and the candidate independent variables for inclusion in the model are the changes in LVESD, ESWS, Vcfc/ESWS ratio and PWTs. The single independent variable that explained most of the dependent variable’s variability is the change in LVESD, with the change in ESWS a close second (66.6% for positive and 94.4% for negative predictive accuracy for both). The model we derived, using the Hooke-Jeeves pattern moves and quasi Newton approach, involves three variables—changes in PWTs, LVESD and ESWS—and explains most of the dependent variable’s variability. According to this model, the dependent variable is predicted by the function: Outcome = exp (b0 + b1 x change in PWTs + b2 x change in LVESD + b3 x change in ESWS)/ {+ exp (b0 + b1 x change in PWTs + b2 x change in LVESD + b3 x change in SWS)} where b0 = 5.75; b1 = –19.72; b2 = –3.17; and b3 = –0.02. This model delivers 88.9% predictive accuracy, both positively and negatively.
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Discussion
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The results of this study showed that inotropic stimulation with dobutamine, in patients with nonischemic DCM and borderline values of VO2max during exercise, leads to an improvement in indexes of left ventricular performance. In addition, this study shows that the changes in ESWS and the Vcfc/ESWS ratio observed after inotropic stimulation may represent an alternative way of assessing the functional status of the heart. Importantly, it seems that the changes in these echocardiographic variables can identify high risk patients.
Echocardiographic changes after inotropic stimulation.
In keeping with previous reports (11), we observed that after low-dose dobutamine infusion, left ventricular performance indexes were increased. Numerous attempts have been made to assess ventricular performance noninvasively. However, left ventricular ejection fraction, though a good index of left ventricular performance, is affected mainly by afterload and does not adequately reflect exercise capacity or severity of functional class (12).
In this study, dobutamine infusion did not affect Vcfc, whereas the Vcfc/ESWS ratio, an index that incorporates afterload, was significantly decreased, suggesting that the contractility indexes are mainly affected by afterload (13). Because left ventricular afterload increased after dobutamine infusion, it seems likely that myocardial contractility in these patients did not result in a decrease in left ventricular internal load (14). In this respect, concerning this group of patients, the observed rise in ESWS implies that the effect of dobutamine infusion on ESWS exceeded its effect on contractility. Accordingly, the results of this study indicate that the changes in ESWS induced by dobutamine infusion were greater in the patients who died during the 18-month follow-up period than in those who survived, suggesting a different unloading response to dobutamine. The ability of the ventricle to unload itself is crucial to the maintenance of normal myocardial mechanics, because it is the wall stress that determines the overall extent and mean velocity of fiber shortening (15). Because wall stress incorporates LVESD and end-systolic PWT, is not surprising that the changes in these variables can also determine patients who will be alive after 18 months. The importance of LVESD and PWT have been well documented by other investigators (16,17). However, it should be pointed out that the specific dobutamine echocardiographic variables investigated in this study might provide valuable prognostic information for the group of patients with VO2max values of borderline significance, rather than for any individual patient.
Study limitations.
1) The heterogeneous contractile responses to dobutamine observed in this study, among patients with CHF, have been reported previously and may reflect differences in beta-1-adrenoreceptor density, which was not investigated in this study. Although the response to dobutamine may underestimate the contractile reserve of the failing myocardium, the magnitude of the drug’s effect appears to decline as left ventricular function deteriorates, and this is accompanied by a reduction in beta-1-adrenoreceptor density (18). 2) Brachial artery pressure was used to calculate peak SBP. Although numerous validation studies have been performed (19), we recognize that brachial artery pressure is only an estimate of left ventricular ejection pressure, and not a direct measurement. In addition, there is a lack of synchrony because of estimates of ventricular pressure (which occurred at early or mid ejection) and those of cavity dimensions and wall thickness (which occurred at the end of ejection). However, because the main purpose of this study was to estimate wall stress before and after inotropic stimulation, we assumed that the method used in this study for the measurement of peak SBP had little influence on the final results. Although the peak (as opposed to end-systolic) pressure was used in this study, previous reports have shown that this substitution is reasonable (20). However, measurements of peak SBP are less accurate than those of end-systolic pressure, most likely because there is a variable degree of neurohumoral activation and responsiveness in this selected group of patients.
Clinical implications.
The induced changes in LVESD, ESWS and the more sophisticated Vcfc/ESWS ratio seem to represent the simplest, easiest way to detect the functional status of patients with CHF, and, more importantly, these changes represent indexes of survival. This might be of great importance, because in this category of patients, VO2max might be influenced by noncardiac factors, and VO2max, per se, cannot provide significant information.
Conclusions.
In the "gray" zone of VO2max, dobutamine echocardiographic variables seem to be valuable prognostic indicators in patients with CHF and nonischemic DCM.
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Acknowledgments
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We thank Miss Eleni Binou for her expert secretarial assistance, Mr. George Tentis for his valuable contribution in the statistical analysis and Miss Polymnia Anthopoulou for her meticulous nursing assistance.
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References
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