The aim of this study was to verify even if there is an association between autonomic cardiac dysfunction and walking test performance in hemodialysis patients. Autonomic dysfunction (AD) in chronic renal patients is characterized by increased sympathetic activity and impairment of parasympathetic activity and may contribute to several cardiovascular clinical manifestations in such patients as resistant hypertension, orthostatic and/or intradialytic hypotension decreased heart rate variability and impaired activity baroreflex with increased risk of sudden death13.
In the present study, a cardiorespiratory performance test was used in this population and the correlation between worse cardio-respiratory performance and worse heart rate variability was verified, showing that the autonomic dysfunction of the chronic renal patient is worse in those with worse walking test performance.
The 6-minute walk test is a useful test with prognostic value for morbidity and mortality. A 2012 study of chronic renal patients showed the prognostic value of 6-MWT in a group of 55 patients followed for 144 months, with a 5% increase in survival for every 100 meters walked, including a correlation with peak VO2 improvement, being one of the first studies that associated survival with the 6-MWT in this population14.
In 2014, a study of 296 patients from various hemodialysis centers in Italy found a positive correlation between poorer physical performance in the walk-at-risk-of-death walk test and found that a 20-meter increase in the 6-MWT was associated with a decrease in the 6-MWT 6% risk mortality and hospitalization, denoting the importance of the test15.
In the present study, laboratory variables were not correlated with the distance covered in the walking test, even hemoglobin (related to anemia), potassium (related to muscle weakness), phosphorus or albumin. This finding may be partially explained by the fact that the mean values of these variables were within the expected range for chronic dialytic renal patients, and perhaps due to a sample bias (patients in a better general condition felt after to perform the walking test). Similar data were found in other studies16,17.
A study conducted in a Japanese dialysis population found a positive correlation between walking test performance and transferrin saturation only, even in a sample of older individuals with lower mean hemoglobin and higher serum phosphorus17. In a Slovenian study, it was found that the worst performance in the walking test was associated with higher serum levels of ultrasensitive CRP (variable inflammation cellular), a variable not measured in the present study and perhaps explaining the difference in performance between hemodialysis patients - degree of subclinical inflammation presented and not measured in clinical practice 18.
The physical performance of these patients is also much lower than normal for age and gender, and it has been verified by previous studies showing that the average 6-MWT for these patients is 400 meters. In the present study, the average 6-MWT was 423.5 ± 115 meters and obtained a positive correlation with the patients' age, similar data observed 16. In study, an improvement in physical performance was verified by the walk test and the sit-to-test stand and also by tests that assess quality of life ratifying the importance of these tests as evaluators of cardiorespiratory performance 19.
In a study of chronic renal patients with peritoneal dialysis, it was found that patients with shorter 6-minute walk distance were older, had a lower left ventricular ejection fraction, and had a higher failure rate with hemodialysis transfer, reflecting lower cardiac performance. even within normal ranges 20, which may be a useful tool in predicting patients who would need more intervention and possibly more effort in indicating the physical training program.
In other patient populations such as heart disease, the walking test has been widely used and has been proposed as a tool to follow up these patients, being a strong predictor of frailty and morbidity and mortality, correlating better aerobic capacity (better 6-MWT) with better cardiac output and lower blood pressure. Pulmonary artery average 21. Being readily applicable even to a group of octogenarians undergoing hemodialysis, which after a low-intensity physical training program during the intradialytic, improved physical performance was verified by the walking test 22.
Regarding the variables of the autonomic nervous system, this study shows that higher SD1 and RMSSD values that show better parasympathetic nervous system integrity and, therefore, lower autonomics, were more prevalent in those patients who had the greatest distance covered in the test walk. In a Brazilian study with hemodialysis patients, it was found in the exercise treadmill test that those patients with worse cardiorespiratory performance had lower RMSSD (higher degree of dysautonomia) than controls 23. It is noteworthy that this test is performed in cardiac clinics, in a separate environment from the hemodialysis clinic, on a treadmill, and that many patients have difficulty accessing the exam, with tc6min and heart rate variability electrocardiogram being a more alternative applicable to hemodialysis clinics.
Some limitations of this study are the fact that it is a cross-sectional study with a small sample and no inflammatory markers were dosed, requiring further studies to verify the clinical applicability of these tools.