Correlation between diastolic function and maximal exercise capacity on exercise test

Abstracts

FUNDAMENTO: O aumento da pressão de capilar pulmonar (PCP) é um dos mecanismos de intolerância ao exercício. A avaliação da função diastólica pelo ecocardiograma (ECO) é capaz de estimar a PCP. OBJETIVO: Identificar variáveis determinantes da capacidade de exercício em paciente submetidos a teste ergométrico (TE) de rotina, ECO convencional e doppler tecidual (DT). MÉTODOS: Foram estudados, retrospectivamente, 640 pacientes submetidos a TE e ao ECO e DT. Pacientes com fração de ejeção < 55% foram excluídos. As velocidades de Doppler mitral convencional foram obtidas em diástole precoce (E) e diástole tardia (A), e o DT do anel mitral mediu as velocidades de diástole precoce (e’) e diástole tardia (a’). E/e’ > 10 foi considerada uma estimativa de aumento da PCP. A capacidade máxima de esforço foi avaliada pelo número de equivalentes metabólicos (MET). Para análise, os pacientes foram divididos em dois grupos: MET < 7 (n = 48) e MET > 7 (n = 572). O escore de Morise demonstrou uma população de baixo risco (60%) para doença coronária (DAC). RESULTADOS: O número de pacientes com E/e’ > 10 foi significativamente maior no grupo MET < 7 em relação a MET > 7(41,7% vs 9,4%, p = 0,001), bem como a presença de algum grau de disfunção diastólica (76,6% vs 34,1% p = 0,001). Pela análise de regressão logística, as variáveis independentes de baixa capacidade de exercício (MET < 7) foram a idade, o sexo feminino e a velocidade de A (diástole tardia). CONCLUSÃO: A disfunção diastólica determinada pelo ECO, sexo feminino e idade estão associados com a menor capacidade de exercício em uma população de baixo risco de DAC.

Disfunção Ventricular; teste de esforço; exercício; diástole

FUNDAMENTO: El aumento de la presión de capilar pulmonar (PCP) es uno de los mecanismos de intolerancia al ejercicio. La evaluación de la función diastólica por el ecocardiograma (ECO) es capaz de estimar la PCP. OBJETIVO: Identificar variables determinantes de la capacidad de ejercicio en paciente sometido a test ergométrico (TE) de rutina, ECO convencional y doppler tisular (DT). MÉTODOS: Fueron estudiados, retrospectivamente, 640 pacientes sometidos a TE y al ECO y DT. Pacientes con fracción de eyección < 55% fueron excluidos. Las velocidades de Doppler mitral convencional fueron obtenidas en diástole precoz (E) y diástole tardía (A), y el DT del anillo mitral midió las velocidades de diástole precoz (e') y diástole tardía (a'). E/e' > 10 fue considerada una estimativa de aumento de la PCP. La capacidad máxima de esfuerzo fue evaluada por el número de equivalentes metabólicos (MET). Para análisis, los pacientes fueron divididos en dos grupos: MET < 7 (n = 48) y MET > 7 (n = 572). El escore de Morise demostró una población de bajo riesgo (60%) para enfermedad coronaria (EAC). RESULTADOS: EL número de pacientes con E/e' > 10 fue significativamente mayor en el grupo MET < 7 en relación a MET > 7(41,7% vs 9,4%, p = 0,001), así como la presencia de algún grado de disfunción diastólica (76,6% vs 34,1% p = 0,001). Por el análisis de regresión logística, las variables independientes de baja capacidad de ejercicio (MET < 7) fueron la edad, el sexo femenino y la velocidad de A (diástole tardía). CONCLUSIONES: La disfunción diastólica determinada por el ECO, sexo femenino y edad están asociados con la menor capacidad de ejercicio en una población de bajo riesgo de EAC.

Disfunción ventricular; test de esfuerzo; ejercicio; diástole

Correlation between diastolic function and maximal exercise capacity on exercise test

Maria Estefânia Bosco Otto; Márcio Mendes Pereira; Adenalva Lima de Souza Beck; Maurício Milani

Instituto de Cardiologia do Distrito Federal, Brasília, DF - Brazil

Mailing address

ABSTRACT

BACKGROUND: Increased pulmonary capillary pressure (PCP) is one of the mechanisms of exercise intolerance. Assessment of the diastolic function by echocardiography (ECHO) enables estimation of PCP.

OBJECTIVE: To identify variables that determine the exercise capacity in patients undergoing routine exercise test (ET), conventional ECHO, and tissue Doppler imaging (TD).

METHODS: A total of 640 patients undergoing ET, ECHO, and TD were retrospectively studied. Patients with ejection fraction < 55% were excluded. Mitral annulus velocities by conventional Doppler imaging were obtained in early diastole (E) and late diastole (A), and TD of the mitral annulus measured early diastole (e') and late diastole (a') velocities. E/e' > 10 was considered an estimate of increased PCP. Maximal exercise capacity was analyzed by the number of metabolic equivalents (MET). The patients were divided into two groups for analysis: MET<7 (n=48) and MET>7 (n=572). Morise score showed a population at low risk (60%) for coronary artery disease (CAD).

RESULTS: The number of patients with E/e' > 10 was significantly higher in the MET < 7 group in relation to the MET > 7 group (41.7% vs 9.4%, p=0.001), and so was the presence of any degree of diastolic dysfunction (76.6% vs 34.1% p=0.001). Using logistic regression analysis, age, female gender and A velocity (late diastole) were the independent variables related to a low exercise capacity (MET < 7).

CONCLUSION: Diastolic dysfunction as determined by ECHO, female gender, and age are associated with a lower exercise capacity in a population at low risk for CAD.

Keywords: Ventricular dysfunction; exercise test; exercise; diastole.

Introduction

Exercise capacity is influenced by innumerous factors, such as advanced age¹, comorbidities (metabolic syndrome and hypertension)², obesity³ and previous cardiorespiratory fitness¹. Identification of other factors that modify the exercise capacity is highly relevant, because its reversal can improve the quality of life of the patients and have an effect on survival, since exercise capacity is associated with prognosis both in healthy individuals^4-6 and in patients with heart diseases⁷.

In an attempt to explain the cardiac mechanisms that decrease exercise capacity, ejection fraction did not show a significant correlation, perhaps for being dependent on the preload⁸. However, maximal exercise capacity, as well as the symptoms triggered by exercise, are directly related to increased pulmonary capillary pressure and, therefore, to increased left ventricular filling pressures. Filling pressures, in turn, are directly related to the left ventricular diastolic function^9,10.

Recent clinical studies observed a significant correlation between exercise capacity and diastolic function parameters, especially in patients with cardiovascular disease¹¹.

Recent advances in tissue Doppler imaging improved the accuracy of echocardiography in the identification of abnormalities of left ventricular early-diastole¹², including the estimation of filling pressures¹³. Consequently, the use of tissue Doppler imaging and of the analysis of diastolic function may be useful to explain the correlation between the presence of diastolic dysfunction and maximal exercise capacity on exercise test.

The objective of the present study was to analyze the exercise capacity and its correlation with diastolic function, as well as with the different clinical parameters in a population predominantly consisting of individuals at a low risk for coronary artery disease.

Methods

This is a retrospective cross-sectional study conducted from July 2007 to January 2008, which evaluated 640 individuals with a mean age of 49±13 years. The study was approved by the Research Ethics Committee of Instituto de Cardiologia do Distrito Federal.

All individuals underwent exercise test and transthoracic echocardiography within an interval of up to 30 days¹⁴. Patients with ejection fraction < 55% were not included in the analysis.

Functional capacity was assessed by the peak metabolic equivalent (MET peak), indirectly obtained by formulas¹⁵, according to the maximum slope and speed achieved in an incremental exercise treadmill test, with the ramp protocol adjusted to the individual.

Echocardiographic study was performed with two-dimensional images of the M mode and with pulsed Doppler using a echocardiographic equipment (Philips HDI 5000) with a 2 to 4 mHz multifrequency transducer. Mitral inflow velocities were recorded using pulsed Doppler in the apical 4-chamber view, with a sample volume of 5 mm at the tip of the mitral leaflets. Early diastolic velocity (E), late diastolic velocity (A) and deceleration time were measured; the E/A ratio was also calculated¹⁶. Using the apical 4-chamber view, a 2-mm sample volume was placed in the junction of the LV wall with the septal mitral annulus in order to record tissue Doppler, deriving the tracing velocities during systole (S), early diastole (e'), and late diastole (a')¹². The E/e' ratio was calculated to estimate LV filling pressures, and values greater than ten (10) were considered increased filling pressures¹². The cut-off point for the E/e' ratio was set at 10 based on data from Burgess et al's study, in which E/e' > 10 ratios ( with e' of the septal annulus) were the optimal index for the identification of patients whose filling pressures increase with exercise, with sensitivity of 71% and specificity of 69%¹⁷.

The diastolic function was analyzed according to the protocol used in our laboratory, starting with the assessment of the E and A velocities of the mitral pulsed Doppler, and E/A ratio calculation. Next, the tissue Doppler velocities of the septal annulus (e' and a') are obtained, and the E/e' ratio is calculated. Using the two-dimensional image, the left atrial (LA) volume is evaluated. When in doubt about the degree of diastolic dysfunction using the methods previously described, the pulmonary vein flow velocities or Valsalva maneuver are performed. Based on these parameters, the diastolic function is classified as: normal (E/A>0.8, e'>8 cm/s, normal LA volume; E/e' ratio was not considered); diastolic dysfunction with abnormal relaxation pattern (E/A<0.8, e'<8 cm/s, variable LA volume, and variable E/e'); pseudonormal diastolic dysfunction (E/A>0.8, e'<8 cm/s, LA volume usually increased, and E/e'>15); and restrictive diastolic dysfunction (E/A> 1.8, e'<8 cm/s, LA volume usually increased, and E/e'>15). This methodology is based on several classifications of analysis of the diastolic function^12,18,19 and is relatively easy to use in the daily practice of echocardiography laboratories.

For the statistical analysis, the individuals were divided into two groups: MET<7 (n=48) and MET >7 (n=572), according to their exercise capacity.

Individuals with MET peak < 7 show low physical capacity, usually associated with functional limitations and symptoms, in addition to being exposed to a higher risk of cardiovascular events^4-6,15.

The chi square test and Fisher test were used to analyze the association between the groups and the classificatory variables. The Wilcoxon test was used for comparison of the means between the two groups. The variables showing statistical significance in the univariate analysis were used in the adjustment of the logistic regression model. P values < 0.05 were considered statistically significant. The Stata 8.0 and SAS software programs were used for processing and statistical analysis.

Results

The baseline clinical characteristics and main echocardiographic measurements are described in Tables 1 and 2, respectively.

Analysis of the diastolic function

Normal diastolic function was observed in 62% of the individuals; 30.4% showed diastolic dysfunction with relaxation abnormality; 6.1% showed pseudonormal diastolic dysfunction; 0.32%, restrictive diastolic dysfunction; and 1.1%, undetermined degree of diastolic dysfunction. Only 12.7% had increased filling pressures (as assumed by an E/e' ratio >10). Of these 12.7%, 17 patients (2.6%) had normal diastolic function and normal exercise capacity (MET > 7).

Risk of CAD in the population

The probability of CAD was calculated based on the Morise score²⁰. We found 60% of individuals with low probability, 39% with moderate, and 1% with high probability of CAD. Therefore, most of the patients had low probability of CAD.

Determinants of functional exercise capacity on exercise test

Univariate analysis of the exercise capacity in MET was carried out in relation to clinical and echocardiographic parameters. We observed that the clinical parameters that correlated with a low functional capacity (MET < 7) were female gender, age, presence of DM, presence of hypertension, and obesity (Table 3). In relation to the echocardiographic parameters, we observed that the LV mass index, A wave velocity of the mitral flow velocity, E/A ratio of the mitral flow, e' wave velocity of tissue Doppler of the mitral annulus, S wave velocity of tissue Doppler of the mitral annulus, and the E/e' ratio were associated with MET < 7 (Table 4).

The clinical and echocardiographic parameters which were independently associated with a low exercise capacity in the adjusted logistic regression model were age, female gender, and A wave velocity of the mitral flow (Table 5).

The comparative analysis of the MET < 7 and MET > 7 groups in relation to the presence of increased filling pressures (Graph 1) showed that 41.6% of the individuals with low functional capacity had a E/e' ratio > 10, whereas only 9.4% of the individuals had a good functional capacity. The same was observed in relation to the presence of diastolic dysfunction: 76.6% of the patients with low functional capacity had some degree of diastolic dysfunction in comparison to 34.1% of the patients with a better functional capacity (Graph 2).

Discussion

The novel and important finding of the present study is that even in patients at low risk of CAD there is a significant correlation between diastolic function and exercise capacity. Another significant finding corroborating other studies^1,10,21 is that clinical parameters such as advanced age and female gender are independent predictors of a low functional capacity.

Undoubtedly, the presence of diastolic dysfunction has a physiological ground on the decreased exercise capacity since, during exercise, the maximal cardiac output is dependent on the diastolic filling. In exercise-induced tachycardia, the diastole is shortened. If diastole is accompanied by left ventricular relaxation and filling abnormalities, the lowest filling rates are achieved and become inadequate to supply the cardiac output required during exercise, with a subsequent increase in filling pressures and decrease in the maximal capacity²². The more abnormal the baseline diastolic function, the lower the exercise capacity.

Several studies have demonstrated a good correlation between tissue Doppler of the mitral annulus and functional exercise capacity^23,24, probably due to the association of the E/e' ratio with left ventricular filling pressures^19,25. However, most of these studies were conducted in populations at high cardiovascular risk or in patients with proven cardiovascular disease. Recently, Grewal et al¹⁰ observed a good correlation between the E/e' ratio and exercise capacity in a population at a moderate cardiovascular risk.

Our study population was characterized as at a low cardiovascular risk; 62% of the individuals had a normal diastolic function, and 60% showed a low probability of CAD. Nonetheless, we could find correlations between diastole and exercise capacity.

In our protocol model, the number of patients with normal diastolic function was high, and the fact that the predominant diastolic dysfunction was relaxation abnormality (30% of the patients) may explain why the low functional capacity, but not the E/e' ratio, is associated with increased A wave velocity in conventional mitral Doppler. A wave velocity increases in patients with diastolic dysfunction with relaxation abnormality and becomes a significant parameter in a population with 30% of the patients classified in this degree of diastolic dysfunction.

Firstenberg et al's study²⁶ showed absence of correlation between E/e' ratio and left ventricular filling pressures in individuals without cardiovascular diseases, and this corroborates the findings of the present protocol, in which most of the patients did not present with cardiovascular disease (60%). The physiological basis for this lack of correlation may be reinforced by recent evidences that suggest that the early diastolic velocity of the septal annulus (e') is significantly influenced by the preload in normal ventricles, but this does not occur in ventricles with abnormal diastolic function, especially abnormalities in ventricular relaxation²⁷.

Advanced age was another independent factor related to low functional capacity. Decrease in functional capacity is evidenced as from 50 years of age, and becomes more evident after 75 years of age in both genders²⁸. In our study, the mean age of the individuals in the MET < 7 group was 62.1 years, thus corroborating the findings previously described. Multiple factors are related to this decreased functional capacity, such as decrease in joint motion, muscle mass, strength, coordination, and endurance, as well as the presence of chronic diseases^28,29.

Another clinical variable related to low functional capacity in our study was the female gender. The factors that explain this difference between genders are a reduced capacity of oxygen transportation, smaller muscle mass, higher percentage of fat, and lower cardiorespiratory fitness observed in women³⁰.

The major limitations of this study are its retrospective design; the use of only some parameters in the analysis of the diastolic function on echocardiogram; indirect assessment of physical capacity by exercise test instead of cardiopulmonary exercise test; and the fact that the interval between tests was 30 days at most. We should also mention that laboratory data and pulmonary functional capacity data were not available, since innumerous individuals were being followed-up in other services. However, the population studied is at a low risk for CAD and is representative of patients who undergo frequent assessments of the cardiovascular risk, which makes the parameters found useful in the clinical practice.

Conclusion

Diastolic dysfunction as determined by echocardiography, female gender, and age are associated with a lower exercise capacity in a population at a low risk for CAD with preserved left ventricular function.

Potential Conflict of Interest

No potential conflict of interest relevant to this article was reported.

Sources of Funding

There were no external funding sources for this study.

Study Association

This study is not associated with any post-graduation program.

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Publication Dates

• Publication in this collection
  14 Jan 2011
• Date of issue
  Feb 2011

History

• Reviewed
  16 Nov 2009
• Received
  08 Oct 2009
• Accepted
  11 Mar 2010