Practical Recommendations for High-Intensity Interval Training for Adults with Cardiovascular Disease

Abstract

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• High-intensity interval training (HIIT) can be implemented safely in adults with cardiovascular disease (CVD) after a graded exercise test assessing exercise responses at a high intensity (e.g., to 85% heart rate peak [HR_peak] or a rating of perceived exertion [RPE] of 15).

• Always involve the patient in the decision-making process for selecting the mode of exercise to perform HIIT to increase adherence, compliance, and enjoyment to the exercise program.

• Given the low exercise tolerance that is typically experienced by patients with CVD, short duration high-intensity bouts (e.g., 30 seconds to 1 minute) may be necessary to increase exercise tolerance and self-efficacy. Active or passive recovery interspersed between high-intensity intervals should be equal to or greater than the duration of the high-intensity intervals. Passive recoveries should be avoided if a person experiences vasovagal symptoms/events.

AN OVERVIEW OF HIGH-INTENSITY INTERVAL TRAINING FOR ADULTS WITH CARDIOVASCULAR DISEASE

Cardiovascular disease (CVD) remains the leading cause of death in the United States (¹). Low levels of exercise are a major risk factor for CVD, and there is irrefutable evidence that regular exercise is effective in managing CVD, lowering the risk of first or subsequent cardiovascular events (¹). Because many adults with CVD consider lack of time a barrier to exercise, high-intensity interval training (HIIT) may be an appealing exercise training paradigm as cardiovascular health benefits can be achieved in less time than traditional moderate-to-vigorous intensity continuous training (MICT). HIIT may also address other frequently reported deterrents to MICT, including lack of interest and motivation, boredom, not challenging enough, and tailored for older people’s needs. For the purposes of this article, sprint interval training (SIT) will not be discussed as a form of interval training. To date, there is no evidence evaluating the safety and efficacy of SIT for cardiovascular health outcomes in people with CVD. Therefore, this mode of interval training is not currently recommended for this population group.

Systematic reviews, meta-analyses, and additional original work in adults with cardiometabolic and CVD have demonstrated comparable or greater improvements after HIIT when compared with MICT in cardiorespiratory fitness (V˙O_2peak) (²), diastolic blood pressure (³), high-density lipoprotein (⁴), vascular function (⁵), and body composition (i.e., total body fat, abdominal fat mass, and body mass index) (⁶). Few studies have addressed quality of life and mental health (i.e., anxiety, depression) (⁷). Readers are referred to Heinz et al. in this issue for more on HIIT and mental health.

SCREENING

When prescribing any exercise program, it is important to consider and obtain information on the patient’s reason for referral, exercise history and preferences (such as modes of exercise), and comorbidities (e.g., osteoarthritis of joints, obesity, etc.). Individuals with established CVD who participate in HIIT should seek medical clearance; the type of medical evaluation is left to the discretion and clinical judgment of the provider (⁸). If a maximal symptom-limited cardiopulmonary exercise test (CPET) is conducted, exercise intensity can be prescribed using measured peak/maximum heart rate (HR_peak/max), peak/maximum exercise capacity (V˙O_2peak/max), or peak/maximum workload. Such individualized exercise prescriptions maximize health benefits through increases in cardiorespiratory fitness, quality of life, mental health, and lowered CVD risk factors, while ensuring the exercise intensity is safely below the threshold for triggering cardiovascular symptoms or events.

When prescribing any exercise program, it is important to consider and obtain information on the patient’s reason for referral, exercise history and preferences (such as modes of exercise), and comorbidities (e.g., osteoarthritis of joints, obesity, etc.). Individuals with established CVD who participate in HIIT should seek medical clearance; the type of medical evaluation is left to the discretion and clinical judgment of the provider (⁸).

EXERCISE PRESCRIPTION FOR HIIT IN CVD

Patients should be included in the decision-making process when devising the HIIT prescription to increase adherence, compliance, and enjoyment (⁹). Individuals with CVD should be advised to monitor for signs and symptoms during an exercise session that may arise such as dizziness, severe shortness of breath, nausea, and angina. Further, it is important to provide patients with the skills and knowledge to perform HIIT in a home-based environment. HIIT sessions should be supervised for the first 1 to 2 wk to (i) monitor physiological responses to HIIT, (ii) familiarize the individual to HIIT, (iii) evaluate compliance, (iv) address any progression or regression to exercise prescription, (v) educate the patient on methods to monitor exercise intensity, and (vi) gradually reduce supervision to increase patient autonomy and self-efficacy (see self-efficacy and barriers to exercise training section below).

WARM-UP AND COOL-DOWN

An aerobic warm-up and cool-down should precede and follow HIIT, respectively. A 5- to 10-minute warm-up that gradually increases from a light to moderate intensity is recommended. However, in patients with angina or with chronotropic incompetence, a longer duration warm-up may be beneficial. A 5- to 10-minute gradual reduction to light-intensity cool-down after HIIT is important to prevent vasovagal events and allow physiological responses to return to near-resting levels.

FITT PRINCIPLES

As per ACSM’s Guidelines for Exercise Testing and Prescription, exercise prescription is composed of four main principles (⁸): (i) frequency (number of sessions per week), (ii) intensity (exertion during exercise), (iii) time (duration of session), and (iv) type (mode of exercise) (FITT). The volume (total amount of exercise) and progression or regression of exercise prescription needs to be considered to ensure that exercise prescription is individually tailored. Tables 1 and 2 provide case study examples on how the FITT principles may be implemented in adults with CVD.

TABLE 1 - Case Study 1

Monday	Wednesday	Saturday
Weeks 1–2
F: 1/3 session (in-clinic supervised)	F: 2/3 session (in-clinic supervised)	F: 3/3 session (unsupervised home based)
I*: high intensity, 15 + active recovery, 9–10	I*: high intensity, 15 + active recovery, 9–10	I*: moderate intensity 14 + active recovery, 9–10
T: 4 × 1 minute at high intensity with 2 minutes active recovery between each work interval	T: 8 × 30 seconds at high intensity with 1 minute active recovery between each work interval	T: 4 × 1 minute at moderate intensity with 2 minutes active or passive (e.g., rest on a bench) recovery between each work interval
T: upright or recumbent cycle ergometer	T: elliptical	T: walking in her neighborhood with a friend or family member
Weeks 3–4
F: 1/3 session (unsupervised home based)	F: 2/3 session (in-clinic supervised)	F: 3/3 session (unsupervised home based)
I*: high intensity, 15 + active recovery, 9–10	I*: high intensity, 15–16 + active recovery, 9–10	I*: moderate intensity, 14 + active recovery, 9–10
T: 4 × 2 minutes at high intensity with 2 minutes active recoveries in between	T: 10 × 30 seconds at high intensity with 1 minute active recovery between each work interval	T: 4 × 2 minutes at moderate intensity with 2 minutes active recoveries in between
T: half the session as aerobics; the other half as air boxing	T: elliptical	T: brisk walking in her neighborhood
Weeks 5–6
F: 1/3 session (unsupervised home based)	F: 2/3 session (supervised)	F: 3/3 session (unsupervised home based)
I*: high intensity, 15–16 + active recovery, 11–12	I*: high intensity, 15–16 + active recovery, 11–12	I: high intensity, 15–16 + active recovery, 11–12
T: 4 × 2 minutes at high intensity with 2 minutes active recoveries in between; can add a repeated block after 3 minutes of passive rest	T: 10 × 30 seconds at high intensity with 1 minute active recovery between each work interval; can add a repeated block after 3 minutes of passive rest	T: 4 × 2 minutes at high intensity with 2 minutes active recoveries in between
T: half the session as aerobics; the other half as air boxing	T: elliptical	T: brisk walking in her neighborhood or air boxing

Patient: 68-year-old female with heart failure; low exercise tolerance (<5 METs); sedentary; body mass index = 27 kg·m⁻²; left knee osteoarthritis; type 2 diabetes; low self-efficacy when performing exercise.

If the patient tolerates the sessions well toward the end of week 2, progress by increasing the duration of the moderate-intensity interval, reducing the active recovery time, or increasing the intensity to high. Note that the prescription can be regressed if the patient finds it too difficult. Given a patient’s low self-efficacy, small short-term goals should be set, and reinforcements with positive feedback should be provided when they are achieved.

If tolerated well by the patient toward the end of week 4, increase the intensity to reach high intensity and progressively reduce the active recovery time. To continue to increase patient’s self-efficacy, the regular use of behavioral change techniques is encouraged.

*Intensity based on 6–20 Borg RPE scale.

TABLE 2 - Case Study 2

Monday	Wednesday	Thursday	Saturday
Weeks 1–2
F: 1/4 session (in-clinic supervised)	F: 2/4 session (unsupervised home based)	F: 3/4 session (unsupervised home based)	F: 4/4 session (in-clinic supervised)
I*: high intensity, 15–16 + active recovery, 9–10	I*: high intensity, 15–16 + active recovery, 9–10	I*: high intensity, 15–16 + active recovery, 9–10	I*: High intensity, 15–16 + active recovery, 9–10
T: 4 × 2 minutes at high intensity with 2 minutes active recoveries in between	T: 4 × 2 minutes at high intensity with 2 minutes active recoveries in between	T: 4 × 2 minutes at high intensity with 2 minutes active recoveries in between	T: 2 blocks of 8 × 30 seconds at high intensity with 30 seconds active recovery between intervals and 1 minute of passive recovery between blocks
T: upright cycle ergometer	T: jogging outdoors	T: jogging outdoors	T: boxing and elliptical
Weeks 3–4
F: 1/4 session (in-clinic supervised	F: 2/4 session (unsupervised home based	F: 3/4 (unsupervised home based)	F: 4/4 session (in-clinic supervised)
I*: high intensity, 15–16 + active recovery, 9–10	I*: high intensity, 15–16 + active recovery, 9–10	I*: high intensity, 15–16 + active recovery, 9–10	I*: high intensity, 15–16 + active recovery, 9–10
T: 4 × 3 minutes at high intensity with 2 minutes active recoveries in between	T: 2 blocks of 8 × 30 seconds at high intensity with 30 seconds active recovery between intervals and 1 minute of passive recovery between blocks	T: 4 × 3 minutes at high intensity with 2 minutes active recoveries in between	T: 2 blocks of 8 × 1 minute at high intensity with 1 minute active recovery between intervals and 1 minute of passive recovery between blocks
T: upright cycle ergometer	T: outdoor jogging or walking on hilly terrain	T: outdoor jogging or walking on hilly terrain	T: boxing and elliptical

Patient: 65-year-old male with moderate to high exercise tolerance (>5 METs); physically active lifestyle; body mass index = 24 kg·m⁻²; high self-efficacy when performing exercise.

*Intensity based on 6–20 Borg RPE scale.

FREQUENCY

Although it is recommended that patients with CVD engage in aerobic exercise 3 to 5 days per week (⁸), most studies have implemented HIIT 2 to 3 days per week (^2,10), which appears to lead to high adherence (¹¹). For patients with low exercise tolerance, the frequency of HIIT should be 2 to 3 days per week until self-efficacy with exercise increases. Before introducing an additional session per week, monitor tolerance (e.g., reduction or elevation in Borg ratings of perceived exertion [RPE], ability to comply to exercise intensity and duration) and adherence to the frequency of HIIT sessions prescribed. For patients with a moderate to high exercise tolerance, frequency may start at ≥3 days per week. Exercise volume may be increased by including an additional HIIT session per week if HIIT is well tolerated and feasible (e.g., schedule availability) for the patient.

INTENSITY

The high-intensity bouts should be prescribed at 80% to 100% HR_peak/max or V˙O_2peak/max (⁸) or an RPE of 15 to 18/20 (¹²), interspersed with active or passive recovery (i.e., rest). Previous work has shown that the inclusion of short passive recovery (~30 seconds) increases exercise tolerance in heart failure patients (¹³). However, passive recovery may increase the risk of vasovagal events because of blood pooling into the extremities. Careful monitoring by the exercise professional for signs and symptoms of a vasovagal event is imperative. Furthermore, in patients with ischemic heart disease, exertional angina is attenuated or dissipated if intensity is briefly reduced before resuming exercise at the same or higher levels of exercise intensity (⁸). As most patients with CVD are prescribed β-blockers with HR blunting effects, RPE is the preferred method of monitoring intensity. For patients who have a low exercise tolerance or lack self-efficacy with high-intensity exercise, higher-intensity bouts should be gradually increased from moderate intensity (e.g., 64% to 76% HR_peak/max or RPE, 12 to 13/20) (⁸) to high intensity. Intensity should be progressed as tolerated by the patient until the high-intensity range is achieved. Active recoveries should be reduced to low-intensity (RPE, 9 to 10; 50% to 55% HR_peak/max) to allow patients to recover and be familiarized with HIIT protocols.

When progressing from a supervised to home-based HIIT program (see Figure), patients should be encouraged to use the Talk Test—a practical and feasible tool for monitoring exercise intensity for HIIT in supervised and home settings. While using the Talk Test, when the patient is aiming for the high-intensity range, intensity, or workload should be increased if holding a conversation feels comfortable. Patients should be able to hold a comfortable conversation during light to moderate exercise (i.e., active recoveries) but not during high-intensity bouts. The Talk Test may not be appropriate for shorter duration high-intensity intervals (e.g., 30 seconds to 1 minute) because of the lack of sensitivity to detect rapid changes in exercise intensity (¹⁴). The Talk Test should therefore be implemented with longer duration intervals (>1 minute).

Figure: Recommended exercise intensity and symptom monitoring tools to transition from supervised to unsupervised high-intensity interval training.

Wearables (e.g., smartwatches, smart patches, smart phones, etc.) may be helpful for monitoring intensity when transitioning from a supervised to home-based HIIT program. Output from these devices should be reviewed during a supervised session with a patient to educate them on which physiological responses to HIIT should be monitored using such devices and how to modify HIIT accordingly.

TIME

Several HIIT protocols have been studied with different durations for the work and recovery bouts, including the 4 × 4 protocol (i.e., 4 bouts of 4-minute high-intensity exercise interspersed with 3-minute recovery periods) and the 10 × 1 minute protocol (i.e. 10 bouts of 1-minute high-intensity exercise interspersed with 1-minute recovery periods). Previous studies have predominately prescribed HIIT sessions up to 30 minutes in duration, consisting of 4 to 10 high-intensity bouts for 1 to 4 minutes with active recoveries for 1 to 3 minutes. Meyer et al. (¹³) found that shorter duration high-intensity bouts (i.e., 30 seconds to 1 minute) with equal duration of passive recovery were the most well-received HIIT protocol for heart failure patients when compared with longer duration intervals (i.e., 90 seconds). Interestingly, patients were unable to maintain exercising at high intensity when asked to cycle for longer than 90 seconds (¹³). Despite evidence suggesting that the 4 × 4 protocol is well tolerated by patients with CVD, commencing HIIT with shorter duration high-intensity bouts may be necessary given their low exercise tolerance and to increase self-efficacy. Further, to assist with the initial integration of HIIT into an exercise program, shorter HIIT sessions, with a total duration of ≤10 minutes, also could be implemented. Duration may be progressed by increasing the number of intervals completed or the length of time spent at high intensity. Alternatively, HIIT may be regressed by reducing the time spent at high intensity and increasing the duration of recovery between bouts.

TYPE

To increase exercise enjoyment and adherence to HIIT, patient preference should be considered upon exercise selection. Where possible, exercise involving large muscle groups, such as walking, jogging, swimming, cycling, rowing, dancing, boxing, or aerobics, should be encouraged.

SPECIAL CONSIDERATION FOR PATIENTS ON β-BLOCKERS

Most patients with CVD are prescribed medications that can affect their cardiac output, blood pressure, cardiac electrophysiology, and cardiorespiratory fitness. The effects of β-blockers on exercise responses are important to consider. HR response is blunted in patients treated with β-blockers, which limits exercise performance (i.e., decreases V˙O_{2 peak}) (¹⁵). When conducting CPET for the purposes of exercise prescription, the timing of β-blockers with respect to exercise testing and training participation should be considered and standardized to prescribe efficacious and safe HIIT protocols. Alternatively, exercise intensity for HIIT may be prescribed using estimated HR_peak/max adjusted for β-blockers (HR_peak/max, −30 bpm) or RPE (¹⁴). Caution is warranted, however, when prescribing exercise intensity based on estimated HR, as its accuracy is influenced by the method used. Direct measurement of the physiological responses to exercise through incremental CPET is preferred whenever possible.

Most patients with CVD are prescribed medications that can affect their cardiac output, blood pressure, cardiac electrophysiology, and cardiorespiratory fitness. The effects of β-blockers on exercise responses are important to consider. HR response is blunted in patients treated with β-blockers, which limits exercise performance (i.e., decreases V˙O_{2 peak}) (¹⁵).

There are no clear guidelines or data to suggest that patients without a baseline CPET should not participate in HIIT. However, when CPET screening is not available, as in many clinical settings, patients who seek to participate in HIIT should be clinically stable and complete a submaximal exercise test (¹⁶) to evaluate physiological responses to high-intensity exercise to ensure HIIT is a safe option (¹⁷). Thereafter, patients may incorporate a small number of short high-intensity bouts (e.g., initially 2 to 4 bouts, 30 seconds to 1-minute per bout) separated by sufficient recovery periods (≥1:1 work–rest ratio) under the supervision of certified exercise professionals.

CONTRAINDICATIONS

There are no specific contraindications for prescribing HIIT in adults with CVD. Considering the greater stress high-intensity exercise imposes on the cardiovascular system, applying the exercise contraindications for patients with CVD may be the most conservative approach (⁸).

PACEMAKER AND IMPLANTABLE CARDIOVERTER DEFIBRILLATORS

Exercise professionals should follow the exercise considerations for those with pacemakers or implantable cardioverter defibrillators (ICDs) as provided in ACSM’s Guidelines for Exercise Testing and Prescription when prescribing HIIT. There is no evidence of increased risk of shocks in patients with a pacemaker or ICD who engage in exercise training (¹⁸).

Exercise professionals should obtain the HR threshold for an ICD and prescribe exercise 10 bpm below the HR threshold. A systematic review showed that supervised exercise training at moderate to high intensity (including a study prescribing HIIT) is a safe and effective intervention for patients with a pacemaker or ICD in improving cardiopulmonary outcomes without adverse events (¹⁹). Given the limited evidence, the same precautions as other cardiovascular conditions (i.e., medical clearance from a physician, exercise testing to evaluate HR and rhythm responses, and beginning with lower-intensity exercise) should be practiced.

SURGICAL INTERVENTIONS

Many patients with CVD undergo percutaneous transluminal coronary intervention (PCI) or coronary artery bypass graft (CABG) surgery for the treatment of atherosclerosis. Exercise professionals should monitor any discomfort or pain at the incision sites (i.e., the radial or femoral artery) after a PCI or a CABG when performing HIIT. Any exercises that cause pain at the incision site should be avoided. A patient who reports the same or worsening symptoms experienced before PCI or CABG (e.g., angina, unusual shortness of breath, and fatigue) during HIIT should be referred to their specialist for further evaluation before commencing or continuing with an exercise program. ACSM’s special considerations for sternotomy should be followed for any patient recovering from a CABG (⁸).

ARRHYTHMIAS

Abnormalities of cardiac rhythm are associated with substantial economic costs, morbidity, and mortality (²⁰). Atrial fibrillation (AF), bradyarrhythmias, and conduction system diseases are the most frequently observed rhythm abnormalities in the community (¹⁹). Their risk factors include older age, male sex, white race, and multiple cardiovascular comorbidities (²⁰). The response to exercise training is presumably different in those with arrhythmias because of their unique rate and/or rhythm control issues that are not seen in other cardiovascular disorders. Consequently, HR is not advisable to prescribe HIIT in adults with arrhythmias. Other methods such as % of V˙O_2peak or V˙O₂ reserve (although not practical for many clinical settings to obtain given the time, costs, equipment, and trained personnel required to measure), workload, RPE, or the Talk Test are advised. HIIT has been shown to be beneficial in patients with nonpermanent AF. Malmo et al. (²¹) demonstrated reductions in time in AF (−3.3%) after a 12 week HIIT (4 × 4-minute intervals at 85% to 95% of HR_peak) program with sessions three times per week.

SEX DIFFERENCES

Females with CVD tend to be older in age, have more comorbidities, and have poorer lifestyle behaviors (e.g., females tend to be less physically active) than males; this will impact the selection of the most appropriate HIIT prescription and progression.

Whether HIIT elicits similar adaptations in females and males with CVD remains unclear. Postmenopausal females, who represent the majority of females with CVD, have been shown to improve V˙O_2peak to a lesser extent than males following the same exercise stimulus (²²); thus, females with CVD may benefit from the higher exercise stimulus elicited by HIIT. A systematic review (¹⁰) did not find sex differences in the relative (%) improvement in V˙O_2peak in cardiac patients after HIIT. HIIT may be a more appropriate alternative to traditional MICT to avoid further broadening of sex differences in V˙O_2peak in females and males with CVD.

SAFETY OF HIIT IN INDIVIDUALS WITH CVD

The risks associated with HIIT will likely be influenced by the intensity and duration of the high-intensity work bouts, as well as the duration of recovery periods. A meta-analysis in adults with coronary artery disease or heart failure has shown that the risk of a major or minor cardiovascular event was very low when HIIT sessions were supervised and preceded by CPET with electrocardiography (1 per 17,083 HIIT sessions and 1 per 8,541 HIIT sessions, respectively) (²³). Exercise professionals should be reassured that CVD events are rare during HIIT, and HIIT appears safe in people with CVD.

SELF-EFFICACY AND BARRIERS TO EXERCISE TRAINING

Barriers to participation in regular exercise training in the general community are wide ranging, with the most common being lack of time, inaccessibility to equipment and associated costs, and lack of skills, education, motivation, and low self-efficacy. Exercise professional supervision of home-based HIIT via telehealth has promise for lowering the barriers of accessibility and time constraints in a cost-effective way (²⁴); however, it has a downside for application with HIIT because of the decreased ability to monitor patients. The remote application of HIIT via telehealth may be most appropriate after stable physiological responses to HIIT have been established in face-to-face sessions, as well as after successful patient education on how to appropriately conduct unsupervised HIIT at home or in the community. Wearable technology, especially the monitoring of HR and the incidences of AF via smart watches, can have use with telehealth to remote monitor HIIT sessions in these patients.

A meta-analysis in adults with coronary artery disease or heart failure has shown that the risk of a major or minor cardiovascular event was very low when HIIT sessions were supervised and preceded by CPET with electrocardiography (1 per 17,083 HIIT sessions and 1 per 8,541 HIIT sessions, respectively) (²⁴). Exercise professionals should be reassured that CVD events are rare during HIIT, and HIIT appears safe in people with CVD.

SUMMARY

HIIT appears to be a safe, feasible, and effective aerobic exercise modality that can be included into the exercise prescription for people with CVD. This may be particularly useful for younger individuals and females with CVD who may need more aggressive exercise approaches to elicit increases in cardiorespiratory fitness and other cardiovascular health benefits. Exercise professionals should aim to educate and provide their patients with the appropriate tools to complete HIIT unsupervised to enable and empower people with CVD to integrate HIIT into their lifestyle.

BRIDGING THE GAP

This article equips exercise professionals with practical instructions on how to implement a safe and evidence-based approach to HIIT in adults with CVD while considering the lack of specialized equipment in most commercial gyms or cardiovascular rehabilitation facilities. Exercise professionals are further provided with a guide on how to safely transition a patient from a supervised to a home-based HIIT program to integrate such exercise into their physical activity routines.