Chagas ’ Cardiomyopathy Prognosis Assessment through Cardiopulmonary Exercise Testing

In Brazil , a frequent cause of HF is Chagas’ cardiomyopathy, which in 2007 accounted for 39.4% of all hospital admissions for decompensated HF and 7.8% of deaths from HF2. It is estimated that 12 million Brazilians are infected and about 1.9 million will develop Chagas’ cardiomyopathy. Life expectancy in patients with advanced HF due to Chagas’ disease is lower than that observed in other diseases4-6. This is because it is a chronic myocarditis, which leads to myocardial fibrosis, with a higher incidence of complex ventricular arrhythmias7 and biventricular failure8, and it is the third largest indication for heart transplant in Brazil9.


Introduction
Heart failure (HF) is the natural progression of many diseases that damage the heart muscle either directly or indirectly, leading to its failure.Its prevalence is increasing worldwide.It is estimated to affect about 23 million individuals and that each year two thousand new cases may occur 1 .
In Brazil, a frequent cause of HF is Chagas' cardiomyopathy, which in 2007 accounted for 39.4% of all hospital admissions for decompensated HF and 7.8% of deaths from HF 2 .It is estimated that 12 million Brazilians are infected and about 1.9 million will develop Chagas' cardiomyopathy.Life expectancy in patients with advanced HF due to Chagas' disease is lower than that observed in other diseases [4][5][6] .This is because it is a chronic myocarditis, which leads to myocardial fibrosis, with a higher incidence of complex ventricular arrhythmias 7 and biventricular failure 8 , and it is the third largest indication for heart transplant in Brazil 9 .
Cardiopulmonary exercise testing (CPET) has proven to have value in determining prognosis of chronic HF 10 .The use of CPET in the stratification of prognosis of Chagas' cardiomyopathy is still scarce and, therefore, it is necessary to specifically define the data that would indicate the best time for more complex interventions, probably earlier, in order to prevent deaths and improve the use of resources.
The objective of this study was to determine whether CPET can discriminate the prognostic differences in mortality among patients with HF with severe systolic left ventricular dysfunction in Chagas' disease compared to non-Chagas heart disease and determine which CPET variables, and at which cutoff points, are independent predictors of death in two years in these two populations.

Methods
Retrospective analysis of a cohort of patients seen in Chagas disease clinics and in the heart failure clinic of a single tertiary cardiology institution in Rio de Janeiro referred to their first CPET between June 2005 and December 2011.
The patients were followed for two years after this CPET and mortality during this period was analyzed.Inclusion criteria, besides the completion of CPET, were age ≥18 years, severe HF diagnosis characterized by NYHA functional class III or IV, left ventricular ejection fraction (LVEF) ≤35% on echocardiography, and ischemic or idiopathic dilated Chagas disease.The latter two made up the non-Chagas group.
Patients which, in the two years of follow-up, had pacemaker, cardiac resynchronizer, cardioverter or cardiopulmonary device implanted, or those who underwent heart transplantation and those who did not have any follow-up completed were excluded from the study.Mortality was evaluated by searching patient records and taking note of any deaths occurring.When there were notes about doctor visits after two years of completing the CPET, he was considered a survivor, and when there was none of these notes, the patients were excluded from the study.
This study was approved by the Research Ethics Committee of Instituto Nacional de Cardiologia under no.18967113.7.0000.5272according to Resolution CNS 466/12.Because it is a retrospective study, Informed Consent Form was not required.

Cardiopulmonary Exercise Testing
All tests were performed and analyzed by a single examiner using the VO 2000 gas analyzer (MedGraphics® -St Paul -USA) coupled to the computerized a n a l y s i s s y s t e m E r g o P C E l i t e (Micromed® -Brasília -Brazil) and treadmill (Inbramed® -Porto Alegre -Brazil).The tests were performed at ramp protocol to achieve maximum fatigue around ten minutes, according to the story of stress tolerance collected before the test.All patients were encouraged to make the maximum possible effort.The recovery phase was active with 2 km/h walk and 2% slope for all patients.
The CPET variables studied were maximum heart rate (HR peak ); maximum systolic blood pressure (Spe ak ); drop in heart rate in the first minute of recovery (DHR1R), not evaluated in patients with pacemaker or atrial fibrillation; respiratory exchange ratio (VCO 2 /VO 2 ); oxygen intake at peak exercise (VO2 peak ); anaerobic threshold oxygen intake (VO 2 AT); oxygen pulse (O 2 Pulse), not analyzed in patients with atrial fibrillation; ventilatory equivalent slope for carbon dioxide output (VE/VCO 2 slope); circulatory power (CP); ventilation power (VP); presence of oscillatory ventilation in exercise (OVE); and complex ventricular arrhythmias (VA comp ).

Statistical analysis
The categorical variables were expressed as frequency and percentage and were analyzed using the chisquare test or Fisher's exact test.Continuous variables were analyzed using the Shapiro-Wilks test as to their distribution and were expressed as mean and standard deviation, except for the time of death, measured by the median, and all variables were analyzed using the Mann-Whitney U test.A statistical significance level of 5% was adopted.The survival curve was built using the Kaplan-Meier method and the survival probability among groups was determined by the log-rank test.Logistic regression analysis was performed using the step-by-step model with a significance level of 5% to assess the influence of each clinical variable and CPET over death.Its coefficient and standard error, p value, relative risk and 95% confidence interval were expressed.The variables considered as independent predictors were studied by ROC (Receiver Operating Characteristic) curve to determine their accuracy by determining the area under the curve (AUC).All statistical analyses were performed using the software SAS ® System, version 6.11 (SAS Institute Inc. -Cary, USA).

Results
The study included 153 patients: 28 in the Chagas group (CG) and 125 in the non-Chagas group (NCG).Of these, 56 patients were excluded (Figure 1).The study evaluated 97 patients with mean age 50.0±12.0years, LVEF 26.3±6.0%, of which 70.0% were men.Of these, 21 patients belonged to the Chagas group (21.6%) and 76 belonged to the non-Chagas group (78.4%).Table 1 presents the characteristics of the two groups.
Table 2 shows the CPET data.The CG presented a higher VO 2peak , with no difference in the other variables.Due to atrial fibrillation and pacemaker, pulse O 2 and DHR1R were not analyzed in all patients, which occurred respectively in 19.0% and 14.3% of CG and 11.8% and 1.4% of NCG, showing that the sinus rhythm was significantly higher in the NCG (p=0.019).Failure to determine the first ventilatory threshold in many patients also prevented the analysis of VO 2 AT, which occurred in 47.6% of the GC and 51.3% of the NCG.In those patients where ventilatory threshold was identified, there was no significant difference between the two groups (p=0.64).Specifically in the CG, there were no significant differences between survivors and non-survivors for HR peak , respectively, 110.0±21.0bpm vs. 87.0±17.0bpm, p=0.026); and for VO 2 AT (11.1±1.7 mL.kg -1 .min - vs. 6.5±0.4 mL.kg -1 .min - , p=0.016) (Table 3).In the NCG, there was no significant single variable.In non-surviving patients, it was found that the CG had lower HR peak (87.0±17.0bpm vs. 112.0±22.0bpm, p=0.019); and VO 2 AT (6.5±0.4 mL.kg -1 .min - vs. 8.5±2.2 mL.kg -1 .min - , p=0.05) (Table 4).
Kaplan-Meier survival curves were not significantly different between the CG and the NCG (Figure 2).
In the logistic regression analysis, LVEF was the only clinical variable independently associated with death and, exclusively in the NCG, with relative risk ratio 0.89 (95% CI 0.80-0.98),p=0.021.Circulatory power was the only CPET variable independently associated with death in both groups, with a relative risk ratio 17.3 (95% CI 1.39-217.0) in the CG and 4.8 (95% CI 1.59-14.6) in the NCG, with p=0.027 and p=0.005.
After identifying this CPET variable, a cutoff point was sought in the circulatory power cutoff for both groups through the ROC curve.In the CG, there was an area under the curve of 0.91 with 95% CI 0.78-1.00and p=0.006, and a cutoff value to predict death ≤1280, with sensitivity of 80% and specificity of 81%.The NCG presented an area under the curve of 0.75 with 95% CI of 0.64-0.86 and p<0.0001, and a cutoff value to predict death ≤1245 with sensitivity of 72% and specificity of 63%.Then the survival curve was based on the CP in each group using these cutoff points.A significant difference was observed for both groups, with p=0.012 for the CG and p=0.004 for the NCG (Figures 3 and 4).

Figure 4
Kaplan-Meier survival curve for the NCG according to the circulatory power.

Discussion
Chagas cardiomyopathy seems to cause early mortality compared to dilated and ischemic cardiomyopathy 4,5 and CPET -a test of great prognostic value in these diseases 6 -can demonstrate differences between these groups.
As for the average of results found in the CPET, comparing the CG and the NCG, there was a significant difference only in VO 2peak , which was slightly lower in the NCG, which may have contributed to the lower mortality observed in the latter.The VCO 2 /VO 2 ratio was similar and above 1.05 in both groups, demonstrating that similar and adequate maximal exercise intensity levels have been achieved.
As expected in patients with severe HF, we found chronotropic incompetence, low SBP peak values, VO 2peak , VO 2 AT, CP and VP, and high slope of the VE/VCO 2 ratio.In those patients, in which the variables DHR1R (78.0%), O 2 Pulse (82.0%) and VO 2 AT (51.0%) were examined, low values were also obtained in both groups.Surprisingly, there was a very low occurrence of OVE, described at around 30% in this type of population 11 , which can be explained by the type of gas analyzer that was used, which does not make measurements at every breath, a fact that was found in another recent study that used the same equipment 12 .There was also a low occurrence of VA comp during testing, being even smaller in the CG than in the NCG.The occurrence of VA comp is common in patients with Chagas' heart disease due to aneurysm of the left ventricle and multiple zones of fibrosis 13 and are part of the risk score of Rassi et al. 14 .A possible explanation for this low incidence is the fact that 52.4% of the CG and 15.8% of the NCG use amiodarone.
When the results of CPET were analyzed among survivors and non-survivors, four variables that showed significant differences with lower values among nonsurvivors in both groups were found: SBP peak , whose incompetence in rising reflects the failure to increase systolic debt through the great arteries; VO 2peak , classically a great marker of mortality 15,16 ; CP and VP, which consist, respectively, in the product of SBP peak by VO2 peak and in the division of SBP peak by the VE/VCO 2 slope, both with recognized prognostic value 17,18 .
As for the VE/VCO2 slope, there was a tendency to higher values in non survivors, but with no significant expression.It was found that the two NCG groups of survivors and non-survivors were in class III of the Arena et al. 19 classification, while CG survivors were class II, confirming the impression that this group consisted of less severe patients.Finally, comparing the data of non-survivors, only a greater chronotropic incompetence was observed with significant difference, which is explained by the greater autonomic dysfunction found in the Chagas' heart disease 20 .
In the logistic regression analysis, circulatory power emerged as an independent predictor for mortality in this population and for both groups.CP is a variable still little used, the design of which is due to the fact that VO 2peak is influenced by extracardiac factors such as deconditioning and motivation, in addition to inherent factors such as sex and age, being a more accurate measure of heart function in the assessment of prognosis of patients with HF than VO 2pico 21 .
Before, using the invasive method, cardiac power was determined through intracardiac measurement of maximum ventricular pressure divided by end-diastolic volume squared.Subsequently, the measurement of cardiac output through rebreathing of carbon dioxide enabled the non-invasive analysis of cardiac output.
Multiplying cardiac output by the arteriovenous oxygen difference, that is, the equation that quantifies VO 2 , and by the SBP can be considered a reflection of the cardiac output since there is no major variation in the arteriovenous oxygen difference 22 .Thereby, circulatory power is calculated by multiplying VO 2peak by SBP peak .
Cohen-Solal et al 17 .showed the great prognostic value of circulatory power in HF when they studied 175 patients with LV systolic dysfunction and found that those who died or needed heart transplant showed significantly lower CP values than those who survived, respectively: 2,567±984 mmHg.mL.kg -1 .min - and 2,402±843 mmHg.mL.kg -1 .min - vs. 3,573±1,273 mmHg.mL.kg -1 .min - ; p<0.001.Tabet et al. 23 demonstrated that CP values lower than 2,389 mmHg.mL.kg -1 .min - in patients not using beta-blockers or lower than 1,530 mmHg.mL.kg -1 .min - in those taking beta-blockers were independent predictors of poor prognosis in HF.
Another study evaluated 631 patients with severe HF using the beta-blocker carvedilol and found that SBP and CP were, along with OVE, the strongest predictors of mortality in a mean time of 3.8 years 24 .Analyzing the response of cardiac resynchronization therapy, Jassaud et al. 25 found that patients with clinical criteria of improvement also showed significant increase of CP, from 1,663±494 mmHg.mL.kg -1 .min - to 2,125±1.014mmHg.mL.kg -1 .min - (p=0.009),regardless of LVEF increase.In this study, a significant variation was found between survivors and non-survivors, both from the Chagas' disease group and from the non-Chagas disease group, with a high risk ratio for the Chagas' group, but with a wide confidence interval range, pointing to an inaccuracy of the logistic regression model for this group, probably due to the small sample size.However, in line with the literature, in the patients studied, nearly all of which were taking beta-blockers, CP values were found to be lower than 1,280 mmHg.mL.kg -1 .min - for patients with Chagas' disease and lower than 1,245 mmHg.mL.kg -1 .min - for patients without Chagas' disease, hence indicating a significant higher mortality in two years.
The unexpected absence of higher mortality in the Chagas group, as described in the literature, may be due to its lower severity, characterized by greater LVEF and VO 2peak .Besides, it was made up by a higher percentage of women.It is believed that if these data were similar in both groups, mortality in the CG would have been higher than in the NCG.This is important, since while it is waited for Chagas' disease patients to reach the cut-off values currently used for other patients with severe HF, the chances to establish therapies that could modify this higher mortality, such as heart transplantation, are missed out.

Study limitations
This was a retrospective study of patients from a single cardiology center, where the number of patients with Chagas' disease was much lower than patients without Chagas disease, which also showed significant differences in the clinical variables.Patients whose follow-up records were considered lost, in fact may have died, without any records of death.All CPET were evaluated by the same examiner, who was not "blind" regarding the etiology of the heart disease.Furthermore, the medical groups that handled each of the groups were different, which may have caused differences in aspects of conducts not analyzed in this study.

Figure 2
Figure 2Kaplan-Meier survival curve for Chagas and non-Chagas groups.

Figure 3
Figure 3Kaplan-Meier survival curve for the CG according to the circulatory power.CG -Chagas group

Table 3 CPET variables in survivors and non-survivors from the Chagas group
CPET -cardiopulmonary exercise testing; HR peak -peak heart rate; SBP peak -peak systolic blood pressure; DHR1R -drop in heart rate in the first minute of recovery; VCO 2 /VO 2 -respiratory exchange ratio; VO 2peak -peak oxygen consumption; VO 2 AT -anaerobic threshold oxygen intake; O 2 pulse -oxygen pulse; VE/VCO 2 -ventilatory equivalent of carbon dioxide; CP -circulatory power; VP -ventilatory power; OVE -oscillatory ventilation in exercise; VA comp -complex ventricular arrhythmias (*) significant p value

Table 4 CPET variables in non-survivors from the groups studied CPET variables Chagas group (n=21) Non-Chagas group (n=76) p value n Values n Values
CPET -cardiopulmonary exercise testing; HR peak -peak heart rate; SBP peak -peak systolic blood pressure; DHR1R -drop in heart rate in the first minute of recovery; VCO 2 /VO 2 -respiratory exchange ratio; VO 2peak -peak oxygen consumption; VO 2 AT -anaerobic threshold oxygen intake; O 2 pulse -oxygen pulse; VE/VCO 2 -ventilatory equivalent of carbon dioxide; CP -circulatory power; VP -ventilatory power; OVE -oscillatory ventilation in exercise; VA comp -complex ventricular arrhythmias (*) significant p value