This was a retrospective mixed-methods analysis to evaluate the feasibility of HIIT in a CR setting. Ethics approval for this study was obtained from the Ottawa Health Sciences Network Research Ethics Board (Protocol #: 20170721-01H). This study was conducted in line with the Declaration of Helsinki.
Patients were referred by a physician or nurse practitioner in the community or using an automatic referral process at the University of Ottawa Heart Institute (UOHI), to an on-site CR program at the UOHI between January 2014 and May 2019. Eligible participants were those who: (i) had CVD (e.g. coronary artery disease, arrhythmias, valvular disease, stroke or transient ischemic attack, spontaneous coronary artery dissection, or heart failure) or CVD risk factors; (ii) had a baseline exercise level ≥ 4 metabolic equivalents (METS [≥ walking pace of 4.0 mph]); (iii) did not have contraindications for participating in high-intensity exercise; (iv) were able to sustain at least 30 minutes of aerobic exercise; and, (v) were able to independently self-monitor and report heart rate and rating of perceived exertion responses during exercise sessions.
High-Intensity Interval Training
Participants attended a group-based HIIT class led by a CR Physiotherapist (MLK) twice weekly for 10 weeks in the Cardiac Prevention and Rehabilitation Centre at the UOHI. The 45-minute classes followed a modified Norwegian HIIT protocol (10) which consisted of: (i) a 10-minute warm-up at 60-70% peak heart rate (HRpeak); (ii) 4 x 4-minutes of high-intensity intervals at 85-95% HRpeak interspersed with 3 minutes of lower intensity intervals at 60-70% HRpeak; and, (iii) 10-minute cool-down at 60-70% HRpeak with resistance and stretching exercises. Peak HR was determined by a graded exercise test (GXT). In cases where GXT data were not available, HRpeak was estimated using the Gellish formula: 207-(0.70 x age) (24). For those taking β-blockers, 30 bpm were subtracted from their estimated HRpeak to address the HR blunting effect of these medications (24).
Patients were provided the option to complete HIIT using: (i) aerobic exercise equipment (treadmill, cycle ergometer, elliptical, etc.) or (ii) dance/movement-based routines. All participants, regardless of exercise choice, completed the HIIT with musical accompaniment of a tempo appropriate for high or lower intensity training. The initial two weeks of the 10-week program were designed to allow the participants to familiarize themselves with the HIIT protocol. All participants monitored their exercise HRs using by wearing a chest strap which displayed HR values through Polar HR monitors (Polar RS800CX, Polar Electro Oy, Kempele, Finland) or on aerobic exercise equipment. For patients using Polar HR monitors, values were displayed on a television in front of the participants in the Polar Team iPad application. Each of the participant’s HR recordings were verified by the supervising physiotherapist.
Participants were instructed to keep their HR within the appropriate target training range (i.e. 85-95% HRpeak or 60-70% HRpeak dependent on the interval), and to adjust movement or workload to stay within these exercise-intensity target ranges. HR was recorded after the first and last HI and LO intervals at each session. Participants were also encouraged to attain a Rating of Perceived Exertion (RPE) (6-20 scale) (25) of 15-17 (“hard to very hard”) during the high-intensity intervals and 11-13 (“light to somewhat hard”) during the low-intensity intervals. Patients were instructed to record a typical RPE representing the effort of all the HI and LO intervals during HIIT. At the end of each session, participants received a 5-minute educational talk addressing the self-management of CVD (e.g. physical activity, diet, medications and stress-management).
To assess the feasibility of HIIT in CR, exercise attendance, compliance, the patient experience and safety were examined.
Exercise Attendance and Compliance
Exercise attendance was assessed by the number of classes participants attended. High attendance to the CR program was defined as being present at ≥ 70% of the classes based on a previous protocol paper examining the feasibility of HIIT in CR (17). Exercise compliance was assessed as the ability to complete the prescribed intensity for the high (HI) and low (LO) intervals. The HRs across all classes for the HI and LO intervals for each patient were averaged and compared to their target HR prescription. For instance, where patients exercised below, within or above the prescribed HR ranges, these were coded as “does not comply”, “complies” and “exceeds”. As patients were also encouraged to aim for a target HI and LO RPE range, we assessed the compliance to these ranges using the same approach as the HR data.
Participants were asked, upon program completion, to complete a feedback questionnaire which comprised of 20 questions regarding their experience with HIIT. The questionnaire was developed by scientists and clinicians involved in CR at the UOHI. For the purposes of this study, we analyzed questions regarding exercise intensity using a 10-point Likert scale with “0” being “not difficult at all” to “10” being “extremely difficult. Patients were asked if HIIT was challenging using a Yes or No question, and whether patients were satisfied with the program that was offered using a Yes or No question. To further assess exercise intensity, RPEs across all classes for the HI and LO intervals, respectively, were averaged.
Safety was assessed by enumerating reported adverse events during the study period and the response to a single question regarding the participant’s perceptions of the safety of the program using a Yes or No question.
The feedback questionnaire also comprised open-ended questions regarding the HIIT program. The questions that were analysed for themes associated with the attendance, compliance, patient experience and safety were: (i) “Satisfied with the program and would recommend to others”; (ii) “Favourite part of the program”; and, (iii): “Additional comments or concerns”. Data analysis was undertaken using an inductive thematic analysis approach. This involved identifying repeated comments/experiences that were described by patients and coding these responses to determine themes.
Estimated Cardiorespiratory Fitness
A symptom-limited peak GXT on a treadmill using an individualized ramp protocol (i.e. treadmill stress test) was completed at baseline and following the CR program by cardiac stress technologists in the Department of Cardiac Imaging at the UOHI. The ramp protocol involves walking or jogging at a constant speed (e.g. 2.0, 3.0, or 4.0 mph) dependent on participants’ functional abilities with a 1.7% increase in grade every minute until volitional fatigue is achieved. HR was measured throughout the test using an electrocardiogram. Estimated peak exercise capacity (V̇O2peak) was calculated using the ACSM Walking equation which takes into consideration the speed and grade reached in the final stage of the test: V̇O2 peak (mL/kg/min) = Final speed (m/min) x 0.1 + final grade x final speed (m/min) x 1.8 + 3.5.
Research assistants extracted demographic and clinical information from the CR clinical database including age, ethnicity, marital status, education, smoking status, medication use and cardiovascular diagnoses.
Height was measured to the nearest 0.5 cm, body mass was measured to the nearest 0.1 kg, and body mass index (BMI) was calculated (kg/m2). Waist circumference was measured to the nearest 0.5 cm at the midpoint between the lower costal margin and iliac crest while participants stood with arms at their sides, feet 25-30 cm apart and abdomen relaxed.
Resting Blood Pressure and Heart Rate
Resting blood pressure and HR were measured using an automated blood pressure monitor (Bp-TRU, Canada; or, Welch Allyn, Canada) by CR staff at baseline and following the CR program. These measures followed standardized procedures (24).
Analyses were performed using SPSS for Windows (Version 26; IBM Corp, Armonk, NY, USA). All outcome variables were tested for normality using Shapiro-Wilk tests. For feasibility outcomes, descriptive statistics were used to describe the attendance, compliance, patient experience and safety. To assess sex-differences in participant characteristics and feasibility outcomes, independent t-tests and Mann-Whitney U tests were used for continuous variables for normally distributed and non-normally distributed data, respectively. Chi-square tests were used for categorical variables. A sensitivity analysis was performed to ascertain where differences in compliance to target HR ranges existed between participants whose HRpeak was determined by a GXT or the Gellish equation. Data are reported as means ± standard deviations, unless otherwise noted, and p < 0.05 was considered statistically significant.