Frequência cardíaca e exercício: aspectos prognósticos e interações com tônus vagal cardíaco

Abstract

Heart rate (HR) at rest (Rest HR), in maximal exercise (HR Max) and after exercise (HR Rec) provides important health information. Part of this behavior is modulated by the cardiac vagal tone (CVT), which offers cardiac protection as well. For a better understanding of the HR prognostic aspects and its interactions with the CVT, three studies were developed, two retrospective and one randomized clinical trial. The first study verifyied if CVT, estimated using the cardiac vagal index (CVI), contributes to HR Max expressed as % predicted for age (208 to 0.7 x age (years)) in 1000 healthy subjects (39 ± 14 years , 719 men). Linear regression analysis identified that CVT explains only 1 % of HR Max variability expressed as % predicted for age with a high standard error of estimate (~ 6.3%), pointing complementary clinical role for those two exercise-related variables. The second study verified if mortality risk using combined HR reserve (HR Res) and HR Rec would discriminate better mortality compared to the analysis of one of these items alone. Data from HR Res and HR Rec of 1,476 individuals (41 to 79 years, 937 men) were calculated and divided into quintiles, which provided added risk categories ranging from 2 to 10, producing a HR exercise gradient (EHRG), reflecting the magnitude of the initial and final maximum transient period. Survival analyzes were performed using the scores of EHRG, HR Res and HR Rec in the lower quintiles (Q1). During an average follow-up of 7.3 years, 44 participants died (3.1%). There was an inverse trend for scores of EHRG and mortality rate (p < 0.05), which increased from 1.2% to 13.5 %, respectively, for scores 10 and 2. A score in EHRG 2 was a better predictor of mortality from all causes, compared to the Q1 of HR Res and HR Rec, with adjusted relative risk by age : 3.53 (p=0.01) , 2.52 (p<0.05) and 2.57 (p<0.05), respectively. It was concluded that the EHRG is a better predictor of mortality risk than either HR Res or HR Rec alone. The third study verified the possibility of increasing CVT through specific training, using the rest-exercise transition in the protocol called vagal training (VT), for participants in a supervised exercise program (SEP) with low CVI (&#8804; 1.30). They were randomized to a study with crossover design (two stages of eight weeks), with or without three weekly VT sessions. There was a slight improvement in CVI at 16 weeks (1.19 vs 1.22, p = 0.02) of the 44 patients (64 % men; 65.5 ± 11.4 years) that completed the study, but we could not affirm that the difference was due to the period which the VT was performed (p = 0.36). So 16 weeks of SEP, including eight weeks with VT, increase vagal response to rest-exercise transition, although it was not possible to attribute the results solely to the VT. The three studies performed contribute for a better understanding of the relevant HR, CVT and exercise interaction