Paroxysmal AF (PAF) was defined as AF episodes lasting < 7 days with spontaneous termination. Persistent AF (PersAF) was defined as AF lasting > 7 days before either pharmacological or electrical cardioversion intervention.

HIIT is a cardiovascular workout strategy in which individuals repetitively engage in a short burst of maximum-intensity exercise (anaerobic exercise), followed by short-term lower-intensity activity (recovery), until fatigued. HIIT can be defined as a session in which HIIT is either the only exercise or an element of an exercise regimen. HIIT implementation sessions usually comprise a warm-up period, several repetitions of high-intensity exercise interspersed with medium-intensity exercise for recovery, and then a cool-down period. The high-intensity exercise should be performed at nearly maximum intensity, while the medium-intensity exercise should be performed at approximately 50% intensity. The number of repetitions and the length of each interval are specific to the workout, but may be as low as three repetitions with only 20 seconds of extreme exercise [16]. The exact workouts performed during the high-intensity portions can vary, and the moderate-intensity segments can be as slow as walking, depending on subject’s level of cardiovascular health [17]. A common formula involves a 2-to-1 ratio of effort-to-recovery stages; for example, 30–40 seconds of hard sprinting interspersed with 15–20 seconds of jogging/walking. There is no specific equation for HIIT, and there are no international guidelines for the HIIT session duration, but an entire HIIT session typically lasts between four and thirty minutes; thus, this regimen is an excellent method of maximizing a workout with limited time [18,19]. Usage of a timer is recommended to monitor the precise interval times, number of rounds, and intensity.

ModEx to engage in physical activity (starting with 150 minutes of moderate activity per week, as endorsed by the Brazilian Ministry of Health, and progressively increasing).

We conducted a prospective, longitudinal study in 50 sedentary patients with CKD and controlled hypertension. At baseline, all of them initiated a physical activity regimen. However, the patients were randomly divided 1:1, such that 25 patients started with an HIIT regimen, and the other 25 subjects engaged in a ModEx protocol. The study complied with the Declaration of Helsinki and was approved by the Ethics Committee of Hospital e Clínica São Gonçalo (No. HCSG2013001). All patients signed written informed consent before enrollment. In the present study, we aimed to evaluate the effects of exercise type performed in the context of AF occurrence (as measured by an implantable cardiac monitor), estimated glomerular filtration rate (eGFR) and the urinary albumin:creatinine ratio (ACR) in all patients.

This study was conducted at the Hospital e e Clínica São Gonçalo in the state of Rio de Janeiro, Brazil, in conjunction with CardioStim Arrhythmias and Artificial Cardiac Pacing Service Research. Subjects were recruited from our institutions between January and December 2013. Consecutive patients who met the following criteria were enrolled: (i) mean 24-hour systolic ambulatory blood pressure measurements (ABPM) < 130/< 180 mmHg; (ii) age between 18 and 75 years; (iii) structurally normal heart based on myocardial scintigraphy and transthoracic echocardiography; no ischemia, fibrotic region or any other illness; and a left ventricular (LV) ejection fraction of > 50%, as measured by echocardiography (Simpson’s method); (iv) no history of symptoms such as dyspnea, palpitation, dizziness, pre-syncope, syncope or AF; (vi) provided permission to implant a cardiac monitor; (vii) a eGFR < 60 mL/min/1.73 m² as estimated by the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation [20]; and (viii) capable of reading, comprehending and signing the written informed consent form.

The following exclusion criteria were applied: (i) pregnancy; (ii) valvular disease with significant hemodynamic repercussions; (iii) myocardial infarction, unstable angina, stroke or transient ischemic attack within the previous 6 months; (iv) mobility deficit; (v) psychiatric disease; (vi) inability to be clinically followed; (vii) known drug or alcohol addiction, which can affect the ability to understand or follow medical instructions; (viii) comorbidity with a serious disease that, in the opinion of the investigator, could have adversely affected the safety and/or efficacy of either the participant or the study; (ix) a eGFR < 15 mL/min/1.73 m², as estimated by the CKD-EPI equation [20].

The implantable cardiac monitor SJM Confirm^™ (St. Jude Medical, St. Paul, Minnesota, USA) checks and diagnostically evaluates patients who experience unexplained indicators such as dizziness, palpitations, chest pain, syncope and shortness of breath, as well as subjects who are at increased risk for cardiac arrhythmias. Some prototypes also allow for the monitoring of patients previously diagnosed with or at risk of developing AF. This device enables the transmission of vital information through extensive data reporting. These reports include AF burden, episode duration, episode count, episode date/time stamp, heart rate histogram, and episodic diagnostics for an auto-triggering event. The device also allows for streamlined follow-up with the Merlin^™ Patient Care System (St. Jude Medical), trans-telephonic monitoring, and timely and accurate transmission of data, all of which facilitate evaluation of the patient’s condition. The SJM Confirm^™ is small (6.5 mL) and has a three-year lifespan, sub-cutaneous electrodes for simplified implantation, and detectors to filter the noise response and activity response.

The subjects were placed in a dorsal decubitus position, sedated by an anesthesiologist and intravenously administered cefazolin (1 g). Local anesthesia was performed with 7.5% ropivacaine. Subsequently, a cutaneous incision was made in the left pectoral region, and dissection was conducted along the planes of the subcutaneous tissue transverse left to create a pectoral pocket. Then, the SJM Confirm^™ was inserted. After the incisions were closed, a compressive dressing was placed on top of the wounds.

ABPM was performed for 24 hours at baseline with a clinically valid device (CardioMapa; Cardios, São Paulo, Brazil). The device was set to measure every 15 minutes during the day (06:00 to 22:00) and every 30 minutes during sleep (22:00 to 06:00). Individuals were instructed to conduct their normal activities throughout the recording and to go to bed no later than 23:00. The waking time typically encompassed 08:00 to 22:00, and the sleep period included 00:00 to 06:00 [21]. All subjects were proficient in recording their sleeping and waking times, meals, intake of medications, and any warning signs or events that could have influenced blood pressure during this period in a journal. Measurements were transmitted to a computer for analysis. Monitoring was repeated as necessary until ≥ 70% of the measured values obtained during both the daytime and nighttime periods were satisfactory [22].

Transthoracic echocardiography was performed at baseline with a Vivid I ultrasound system (General Electric, Frankfurt, Germany) fitted with a multi-frequency transducer and tissue Doppler imaging software, in accordance with the guidelines of the American Society of Echocardiography [23]. Data were evaluated and interpreted by an experienced echocardiographer who was blinded to the treatment status and imaging sequence. The LV mass was quantified from the LV linear dimensions with the Devereux formula [23,24] and normalized to the body surface area (BSA) [23,25]. LV hypertrophy was considered present when the LV mass exceeded 115 g/m² for men and 95 g/m² for women [23]. The left atrial (LA) volume was measured with a disk sum algorithm similar to that used to measure the LV volume [26,27], and was normalized to the BSA. Although the LA size depends on sex, variation due to sex is generally considered when the values are normalized to the BSA [28]. Furthermore, although there are several methods of normalizing this measurement [29,30], normalizing to the BSA produces the most reliable data. Normalizing to the BSA compensates for sexual dimorphism in the LA size; thus, only the normalized value is reported. The recommended upper limit for LA size is 34 mL/m²[30–33].

We assessed 50 CKD patients with hypertension. Each patient provided a complete medical history and underwent a physical examination. We evaluated the effects of the HIIT and ModEx physical activity protocols on AF occurrence (as determined with the implantable cardiac monitor over a 3-year follow-up period) and renal function. Follow-up was performed every 6 months after the initiation of the exercise regimens.

The results are expressed as the mean and standard deviation for normally distributed data and as the median and interquartile range for non-normally distributed data. A paired t-test was used to compare paired values in cases of a Gaussian distribution; otherwise, the Wilcoxon test was used. The D’Agostino-Pearson test was used to determine the normality of the distribution. Comparisons among more than two values were made with either repeated-measures analysis of variance or Kruskal–Wallis analysis of variance as appropriate, and both assessments were complemented with the post-hoc Tukey test. Fisher’s exact test was used to compare categorical variables. A two-tailed P value < 0.05 was used as the criterion for statistical significance. Kaplan–Meier analysis was performed to determine the probability of success and estimated as the percentage of AF freedom. Differences in arrhythmia-free survival were assessed with the log-rank test. Cox regression analysis was used to explore factors contributing to the recurrence of AF. All statistical analyses were performed with GraphPad Prism ver. 7.0 (GraphPad Software, La Jolla, CA, USA).

The general features of the patient groups are listed in Table 1. The groups did not differ in any parameter besides age (P = 0.0008).

Both groups exhibited significant intragroup changes in mean systolic 24-hour ABPM from baseline to 12, 24 and 36 months. There were also significant differences in the mean systolic 24-hour ABPM between the groups at the same time points, as shown in Table 2. However, regarding the mean diastolic 24-hour ABPM, no intragroup changes from baseline to 12, 24 and 36 months were observed in either group, except for the value at 36 months vs. baseline in the ModEx group. There were no significant differences in the mean diastolic 24-hour ABPM between the groups at the same time points, as shown in Table 2. The effects of HIIT and ModEx on creatinine concentration, eGFR and ACR during the follow-up period are also detailed in Table 2.

During the 3-year follow-up, AF onset was higher in CKD patients who engaged in HIIT (72%) than in those who engaged in ModEx (24%) (hazard ratio [HR], 3.847; 95% confidence interval [CI], 1.694–8.740; P = 0.0013 by log-rank test) as shown in Fig. 1. There was no difference in PAF and PersAF between the groups (P = 0.5323): 16 subjects in the HIIT group (84%) and all 8 patients in the ModEx group (100%) presented with PAF.

In the HIIT group, the indexed left atrium (LA) volume increased from 30.5 ± 2.1 mL/m² at baseline to 31.9 ± 1.1, 33.5 ± 1.5, and 34.2 ± 1.3 mL/m² at 12 (P = 0.0157), 24 (P < 0.0001), and 36 months (P < 0.0001), respectively. For the subjects who engaged in ModEx, the indexed LA volume decreased from 31.0 ± 1.7 mL/m² at baseline to 30.3 ± 0.8, 29.0 ± 1.2, and 27.8 ± 1.4 mL/m² at 12 (P = 0.6717), 24 (P < 0.0001), and 36 months (P < 0.0001), respectively. The comparisons between the groups at the same time points are illustrated in Fig. 2A.

Regarding the LV mass index in the HIIT group, the value decreased from 95.9 ± 14.6 g/m² at baseline to 88.4 ± 10.0, 82.1 ± 9.7, and 77.4 ± 11.5 g/m² at 12 (P = 0.3676), 24 (P = 0.0021), and 36 months (P < 0.0001), respectively. For the subjects who engaged in ModEx, the LV mass index decreased from 100.2 ± 18.3 g/m² at baseline to 88.1 ± 12.2, 76.4 ± 8.5, and 66.0 ± 9.3 g/m² at 12 (P = 0.0124), 24 (P < 0.0001), and 36 months (P < 0.0001), respectively. The comparisons between the groups at the same time points are presented in Fig. 2B.