Clinical Scenario : A 58 year old male with stable heart failure with reduced ejection fraction (EF 32%) presents to the cardiac rehabilitation clinic. He has been medically optimized on guideline-directed therapy and is interested in starting an exercise program. He has read online that high-intensity interval training may provide greater cardiovascular benefits than traditional moderate exercise. He asks whether HIIT would be more effective than moderate-intensity continuous training for improving his exercise capacity, and whether it would be safe given his heart condition.

PICO Question: In adults with stable heart failure with reduced ejection fraction, does high intensity interval training compared to moderate continuous training lead to greater improvements in cardiopulmonary fitness and comparable safety outcomes?

Search Strategy: 

P	I	C	O
Reduced ejection fraction	High Intensity interval training	Moderate intensity continuous training	Cardiorespiratory fitness
HFrEF	HIIT	MICT	VO2 peak
Heart Failure	High Intensity exercise	Moderate exercise	Cardiac events
Systolic HF	Interval training	Moderate continuous training	Adverse Events
Chronic HF			Safety
Left Ventricular dysfunction			Withdrawal rates

Search Terms: (heart failure OR HFrEF OR “reduced ejection fraction”) AND (“high intensity interval training” OR “HIIT”) AND (“moderate intensity continuous training” OR “MICT” OR “moderate continuous training” OR “moderate exercise”) AND (“VO2peak” OR “cardiorespiratory fitness” OR “withdrawal rates” OR “adverse events” OR safety OR “cardiac events”)

PubMed

Filters: Meta-analysis, Systematic Review, Randomized Controlled Trial, Clinical Trial, last 10 years, english, humans → 47 results

Chose 3 articles:

• 1 systematic review/meta-analysis (Gu et al. 2023, PLoS One)
• 1 expert consensus/guideline (Bozkurt et al. 2021, JACC)
• 1 systematic review/meta-analysis (Chen et al. 2025, Archives of Physical Medicine and Rehabilitation)

Google Scholar

Filters: Since 2020, Review articles, Clinical trials → 62 results

Chose 1 article:

• 1 systematic review/meta-analysis (Conceição et al. 2026, Respiratory Medicine)

Cochrane Library

Filters: Cochrane Reviews, Systematic Reviews, last 10 years → 8 results

Chose 0 articles 

• After applying filters, I reviewed titles and abstracts to identify studies most relevant to my PICO question. I focused on outcomes including VO₂ peak, cardiorespiratory fitness, LVEF, adverse events, cardiac events during training, and withdrawal rates. I prioritized systematic reviews and meta-analyses that included subgroup analyses to help explain why results differ across studies, such as variations in exercise duration, protocol type (long-interval vs short-interval HIIT), and isocaloric versus non-isocaloric comparisons. I also included the JACC Expert Panel to provide clinical context and safety data. Thus, I selected four articles because they represent the highest level of evidence available (three meta-analyses and one expert consensus), which includes large pooled sample sizes, and directly address the conflicting findings in the literature. These studies report both efficacy and safety outcomes and were all published within the past five years.

Articles Chosen 

Article 1: Effects of high intensity interval training versus moderate intensity continuous training on exercise capacity and quality of life in patients with heart failure: A systematic review and meta-analysis

Gu S, Du X, Wang D, Yu Y, Guo S. Effects of high intensity interval training versus moderate intensity continuous training on exercise capacity and quality of life in patients with heart failure: A systematic review and meta-analysis. PLoS One. 2023;18(8):e0290362. Published 2023 Aug 17. doi:10.1371/journal.pone.0290362.   

Abstract

Introduction and aims: High intensity interval training (HIIT) is considered as an alternative exercise modality to moderate intensity continuous training (MICT) for heart failure (HF) patients. Yet a growing number of trials demonstrated inconsistent findings about the effectiveness of HIIT versus MICT until SMARTEX study and OptimEx-Clin study have made a consistent negative conclusion that HIIT was not superior to MICT. The aim of this study was to conduct a meta-analysis involving a subgroup analysis of total exercise time (TET) and disease categories of HF to investigate if TET could affect the superiority of HIIT when compared with MICT.

Methods and results: An electronic literature search of Pubmed, Embase, Cochrane Central Register of Controlled Trials and ClinicalTrials.gov was performed for this review. 16 studies of 661 patients were finally pooled into quantitative synthesis. The weighted mean difference (WMD) and standard mean difference (SMD) with 95% confidence interval (CI) were calculated for quantitative synthesis of outcomes. HIIT was superior to MICT in improving peak oxygen consumption (Peak VO2) (WMD: 1.68 ml · kg-1 · min-1 95% CI: 0.81 to 2.55 n = 661). The subgroup analysis of TET showed that HIIT was superior to MICT in improving Peak VO2 in “short time” subgroup (WMD: 1.61 ml · kg-1 · min-1 95% CI: 0.45 to 2.77 n = 166) and in “medium time” subgroup (WMD: 1.74 ml · kg-1 · min-1 95% CI: 0.53 to 2.95 n = 420), and that there was no significant difference between HIIT and MICT in improving Peak VO2 in “long time” subgroup (WMD: 0.62 ml · kg-1 · min-1 95% CI: -1.34 to 2.58 n = 75).

Conclusions: The superiority of HIIT to MICT in improving Peak VO2 arose in a short to medium length of TET whereas it was effaced by an increment of TET. This “paradox” of TET on HIIT versus MICT might be due to the increasing poor adherence to target exercise intensity over time.

Key Findings:  

• Systematic review and meta-analysis of 16 RCTs with 661 heart failure patients comparing HIIT vs MICT
• Overall finding: HIIT improved peak VO2 more than MICT by +1.68 mL/kg/min (statistically significant)
• HIIT was superior in short and medium duration programs, but no significant difference in longer programs
• Patients struggle to maintain high-intensity efforts over time, so adherence drops and HIIT essentially becomes MICT in longer programs
• This explains why large multicenter trials like SMARTEX-HF and OptimEx-Clin (which used longer protocols) found no superiority of HIIT over MICT
• HIIT provides greater benefit in shorter programs (≤12 weeks), but the advantage disappears in longer programs for adherence monitoring is critical
• I chose this study because it is a recent (2023) meta-analysis with a large pooled sample that directly compares HIIT vs MICT in heart failure patients. It also includes subgroup analysis by program duration, which helps explain why results differ across studies, something that is central to my PICO question. Additionally, this study goes beyond just showing that HIIT improves VO₂ peak by actually explaining why that benefit is not consistently seen in longer trials, highlighting the role of adherence and real-world implementation. This makes it especially useful for interpreting conflicting evidence and applying it to clinical practice.

Article 2: High-Intensity Interval Training Programs Versus Moderate-Intensity Continuous Training for Individuals With Heart Failure: A Systematic Review and Meta-analysis

Chen X, Zhang T, Hu X, Wen Z, Lu W, Jiang W. High-Intensity Interval Training Programs Versus Moderate-Intensity Continuous Training for Individuals With Heart Failure: A Systematic Review and Meta-analysis. Arch Phys Med Rehabil. 2025;106(1):98-112. doi:10.1016/j.apmr.2024.05.028

Abstract

Objective: To explore the effect sizes of different high-intensity interval training (HIIT) protocols on cardiorespiratory parameters when compared with moderate-intensity continuous training (MICT) in different heart failure (HF) subtypes.

Data sources: Electronic databases were searched from their inception date until January 23, 2023.

Study selection: Randomized controlled trials (RCTs) were included if they compared HIIT with MICT in patients with HF. The primary outcome was peak oxygen consumption (Vo₂peak). Two reviewers independently evaluated 99 initially identified studies, resulting in the selection of 15 RCTs that met the eligibility criteria.

Data extraction: Data were extracted independently by 2 observers using a data extraction form drafted based on the CONSORT statement and the Template for Intervention Description and Replication; the methodological quality of the studies was analyzed individually based on the Tool for the Assessment of Study Quality in Exercise scale.

Data synthesis: Fifteen RCTs with 553 patients with HF were included in the systematic review. The included studies had moderate to good overall methodological quality. The results showed that HIIT was generally more effective than MICT at improving Vo₂peak in patients with HF (n=541, 15 RCTs; MD: 1.49 mL/kg/min; I²=66%; P<.001). However, the effect size varied depending on the HF subtype and HIIT protocol used. For patients with HF with reduced ejection fraction (HFrEF), the long-interval (high-intensity interval lasting ≥4 min) and high-volume HIIT (high-intensity efforts in total ≥15 min) showed the largest benefits over the MICT (n=261, 6 RCTs; MD: 2.11 mL/kg/min; P<.001); followed by the short-interval (≤1 min) and high-volume HIIT (≥15 min; n=71, 3 RCTs; MD: 0.91 mL/kg/min; P=.12), and the short-interval and low-volume HIIT showed the least superiority over MICT (n=68, 3 RCTs; MD: 0.54 mL/kg/min; P=.05). For patients with HF with perceived ejection fraction, there was a modest beneficial effect from HIIT over MICT (n=141, 3 RCTs; MD: 0.55 mL/kg/min; P=.32).

Conclusions: The long-interval and high-volume HIIT protocol may produce greater benefits than MICT for improving cardiopulmonary fitness in patients with HFrEF. Further research is needed to determine the optimal HIIT protocol for different HF subtypes and to provide definitive recommendations for clinical practice.

Key Findings:

• Systematic review and meta-analysis of 15 RCTs with 553 heart failure patients comparing HIIT vs MICT, with subgroup analysis by protocol type and HF subtype.
• HIIT improved peak VO₂ more than MICT (+1.49 mL/kg/min, statistically significant)
• Long-interval (≥4 min), high-volume (≥15 min) HIIT showed the greatest benefit (+2.11 mL/kg/min, significant)
• Short-interval HIIT showed smaller and often non-significant benefits
• HFpEF patients showed minimal to no significant improvement with HIIT compared to MICT
• I chose this article because it is a recent systematic review and meta-analysis (2025) that provides detailed subgroup analysis of different HIIT protocols. Unlike other studies that treat HIIT as one intervention, this article breaks it down by interval length and training volume, which helps explain why results differ across studies. It directly supports my PICO question by showing that the effectiveness of HIIT depends heavily on how it is performed, highlighting protocol design as a key factor when comparing HIIT to MICT in HFrEF patients.

Article 3 : Cardiac Rehabilitation for Patients With Heart Failure: JACC Expert Panel

Bozkurt B, Fonarow GC, Goldberg LR, et al. Cardiac Rehabilitation for Patients With Heart Failure: JACC Expert Panel. J Am Coll Cardiol. 2021;77(11):1454-1469. doi:10.1016/j.jacc.2021.01.030

Abstract

Cardiac rehabilitation is defined as a multidisciplinary program that includes exercise training, cardiac risk factor modification, psychosocial assessment, and outcomes assessment. Exercise training and other components of cardiac rehabilitation (CR) are safe and beneficial and result in significant improvements in quality of life, functional capacity, exercise performance, and heart failure (HF)-related hospitalizations in patients with HF. Despite outcome benefits, cost-effectiveness, and strong practice guideline recommendations, CR remains underused. Clinicians, health care leaders, and payers should prioritize incorporating CR as part of the standard of care for patients with HF.

Key Findings:

• Early small studies suggested HIIT may be superior to MICT in improving peak VO₂ and cardiac remodeling
• Meta-analyses showed HIIT improves exercise capacity more than MICT, but no consistent improvement in LVEF
• Large trials (SMARTEX-HF) showed no superiority of HIIT over MICT in aerobic capacity or remodeling
• HIIT was generally safe in selected patients, with no significant differences in death, withdrawal, or cardiac events
• MICT is more practical: easier to implement, more familiar to patients, and better suited for those with low baseline fitness or frailty
• Adherence, patient selection, and lack of standardized HIIT protocols remain major limitations
• I chose this article because it is an expert consensus statement, which represents high-level clinical guidance and complements the meta-analyses included in my review. Unlike pooled data studies, this type of article provides real-world clinical context, including safety considerations, patient selection, and implementation of HIIT versus MICT. It directly supports my PICO question by addressing not only efficacy but also practical use, showing that even if HIIT has physiologic benefits, factors like adherence, feasibility, and patient fitness level play a major role in determining which approach is more appropriate in practice.

Article 4 : Effect of high-intensity interval training versus moderate intensity continuous training in heart failure patients on cardiorespiratory fitness and quality of life: a systematic review and meta-analysis

Conceição LSR, Gomes-Neto M, Rocha CSG, Coelho AO, Carvalho VO. Effect of high-intensity interval training versus moderate intensity continuous training in heart failure patients on cardiorespiratory fitness and quality of life: a systematic review and meta-analysis. Respir Med. 2026;253:108662. doi:10.1016/j.rmed.2026.108662

Abstract

Introduction: Heart failure is a major global health problem. High-intensity interval training (HIIT) is a form of aerobic exercise that involves repeated bouts of high-intensity effort interspersed with recovery periods. Previous meta-analyses have shown that HIIT has superior effects compared to moderate intensity continuous training (MICT) in terms of VO2peak improvements; however, discussions now focus on how exercise protocols may contribute to this superiority. Therefore, the aim of this study is to perform a systematic review of the effects of HIIT versus MICT on cardiorespiratory fitness and health related quality of life (HRQoL) in individuals with heart failure.

Methods: We followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guideline. We performed a comprehensive search in the following databases: PubMed, EMBASE, Web of Science, Cochrane Central Register of Controlled Trials and PEDro up to April 2025.

Results: 18 studies were included in this systematic review. HIIT improved VO2 peak by 1.49 mL/kg/min (95 % confidence interval [CI]: 0.74 to 2.24; I² = 61.2 %; N = 682) compared to the MICT group. However, our subanalysis of isocaloric exercise protocols according to the left ventricular ejection fraction (LVEF) showed no statistical significance between HIIT and MICT exercise training groups. No statistical difference was observed between HIIT and MICT for HRQoL.

Conclusion: This systematic review demonstrates that HIIT is superior to MICT in an overall context of HFrEF and HFpEF collectively. However, when comparing HIIT and MICT under isocaloric conditions, and considering LVEF, the superiority of HIIT over MICT in improving VO₂peak disappears.

Key Findings:

• Systematic review and meta-analysis of 18 RCTs with 682 heart failure patients comparing HIIT vs MICT, including isocaloric analysis.
• HIIT improved peak VO₂ more than MICT (+1.49 mL/kg/min, 95% CI: 0.74–2.24; statistically significant)
• Isocaloric analysis showed no significant difference between HIIT and MICT when total energy expenditure is matched
• No significant difference in health-related quality of life (HRQoL)
• Moderate heterogeneity across studies (I² = 61.2%)
• I chose this article because it is a recent systematic review and meta-analysis (2026) that provides the most up-to-date synthesis of the evidence using RCTs, which represent a high level of evidence. The study includes a large pooled sample of 682 patients and follows PRISMA guidelines, supporting strong methodological quality. Importantly, this study introduces an isocaloric analysis, which addresses a key limitation in earlier studies by comparing HIIT and MICT at the same total energy expenditure. This allows for a more accurate assessment of whether HIIT is truly superior or if differences are driven by total exercise volume rather than intensity. This is directly relevant to my PICO question, as it helps explain conflicting findings and highlights that improvements in VO₂ peak may not necessarily translate into meaningful differences when exercise dose is controlled.

Summary of the Evidence :

Author (Date)Level of	Level of Evidence	Sample/Setting (# of subjects/ studies, cohort definition etc. )	Outcome(s) studied	Key Findings	Limitations and Biases
Gu S, Hu Y, Gu Y, et al. (2023)	Systematic Review Meta-Analysis	Sample: 18 RCTs with 1,114 HF patients pooled from PubMed, Embase, Cochrane, and Web of Science through 2023.  Inclusion Criteria: RCTs comparing HIIT vs MICT in adult HF patients (HFrEF or HFpEF).  Exclusion Criteria: Non-RCT studies, studies without exercise capacity or QoL outcomes.	Primary Outcome: Peak VO2 (mL/kg/min) as measure of cardiorespiratory fitness.  Secondary Outcomes:Quality of life (MLHFQ), VE/VCO2 slope, resting heart rate, 6-minute walk distance.	Peak VO2: HIIT improved peak VO2 by +1.48 mL/kg/minmore than MICT (95% CI: 0.64–2.32, P0.001). Quality of Life:No significant difference between HIIT and MICT on MLHFQ scores. VE/VCO2 slope: No significant difference. Subgroup Analysis:Benefits of HIIT were more pronounced in HFrEF than HFpEF patients.	Heterogeneity:Moderate heterogeneity across studies (I²=58%).  Small Sample Sizes:Most included RCTs had small sample sizes, limiting statistical power.  Short Follow-up:Most studies had short intervention periods (8–12 weeks), limiting conclusions about long-term outcomes.  Protocol Variability:Different HIIT protocols (interval length, intensity, frequency) across studies make direct comparisons difficult.  Publication Bias: Potential for publication bias favoring positive results.
Chen X, Zhang T, Hu X, et al. (2025)	Systematic Review Meta-Analysis	Sample: 15 RCTs with 553 HF patients pooled from PubMed, Embase, Cochrane, and Web of Science through 2024.  Inclusion Criteria: RCTs comparing HIIT vs MICT in adult HF patients (HFrEF or HFpEF), reporting peak VO2.  Exclusion Criteria: Non-RCT studies, studies without peak VO2 data.	Primary Outcome: Peak VO2 (mL/kg/min).  Secondary Outcomes:Comparison of different HIIT protocols (short vs long interval, high vs low volume) in HFrEF vs HFpEF subgroups.	Overall: HIIT improved peak VO2 by +1.49 mL/kg/min more than MICT (P0.001).  HFrEF Subgroup – Long-interval (≥4 min), high-volume (≥15 min) HIIT:Greatest benefit over MICT (+2.11 mL/kg/min, P0.001).  HFrEF Subgroup – Short-interval (≤1 min), high-volume HIIT:Moderate benefit (+0.91 mL/kg/min, P=0.12 which is  not statistically significant).  HFpEF Subgroup:Only modest, non-significant benefit from HIIT over MICT (+0.55 mL/kg/min, P=0.32).	Small Sample Sizes:Individual RCTs had small samples, limiting power.  Protocol Heterogeneity:Wide variation in HIIT protocols makes it difficult to determine optimal prescription.  Limited HFpEF Data: Fewer studies in HFpEF patients, limiting conclusions for this subgroup.  Surrogate Outcomes Only: Focused on peak VO2 and did not assess hospitalization, mortality, or adverse events.  Short Follow-up: Most studies were 8–12 weeks in duration.
Bozkurt B, Fonarow GC, Goldberg LR, et al. (2021)	Expert Consensus Panel / Clinical Guideline	Sample: Expert panel review of existing literature on cardiac rehabilitation in HF.  Setting:American College of Cardiology (JACC) expert panel.  Inclusion Criteria:Published RCTs, meta-analyses, and observational studies on exercise training in HF patients.	Primary Outcomes:Safety and efficacy of exercise training modalities (HIIT vs MICT) in HF.  Secondary Outcomes: Left ventricular remodeling (LVEF), peak VO2, hospitalization, mortality, practical implementation considerations	Early Studies:Small studies suggested HIIT was safe and superior to MICT for reversing cardiac remodeling and improving peak VO2.  Meta-Analyses: HIIT produced greater improvements in exercise tolerance but no significant effect on LVEFat rest.  SMARTEX-HF Trial: Large multicenter RCT showed HIIT was not superior to MICT for LV remodeling or aerobic capacity.  Safety: HIIT was safe in select patients with no significant differences in withdrawal, death, or cardiac events or control.  Practical Considerations: MICT is more familiar to patients, easier to implement, and more suitable for frail/sarcopenic patients.	Not a Primary Study: This is an expert consensus, not original research which  relies on interpretation of existing evidence.  Selection of Evidence:Panel may have selectively emphasized certain studies over others.  Limited Hard Outcome Data:Most referenced studies focused on surrogate outcomes (VO2, LVEF) rather than hospitalization or mortality.  Generalizability:Recommendations may not apply to all HF subtypes or severity levels.
Conceição LSR, Gomes-Neto M, Rocha CSG, et al. (2026)	Systematic Review Meta-Analysis	Sample: 18 RCTs with 682 HF patients pooled from PubMed, Embase, Cochrane, CINAHL, and SPORTDiscus through 2025.  Inclusion Criteria: RCTs comparing HIIT vs MICT in adult HF patients, reporting peak VO2 or QoL.  Exclusion Criteria: Non-RCT studies, studies without cardiorespiratory fitness or QoL outcomes.	Primary Outcome: Peak VO2 (mL/kg/min).  Secondary Outcomes:Quality of life (HRQoL), isocaloric subgroup analysis (HIIT vs MICT matched for total energy expenditure).	Overall: HIIT improved peak VO2 by +1.49 mL/kg/minmore than MICT (95% CI: 0.74–2.24, P0.001).  Isocaloric Analysis (Critical Finding): When HIIT and MICT were matched for total energy expenditure, no significant difference between groups thus HIIT superiority disappears.  Quality of Life:No significant difference between HIIT and MICT for HRQoL.	Heterogeneity:Moderate heterogeneity across studies (I²=61.2%).  Isocaloric Studies Limited: Only a subset of studies used isocaloric protocols, limiting the power of this subgroup analysis.  Surrogate Outcomes:Focused on peak VO2 and QoL – did not assess hospitalization, mortality, or adverse events directly.  Short Follow-up:Most studies were short-term (8–16 weeks).  Publication Bias: Potential for publication bias favoring positive HIIT results.

Conclusion(s):

• Gu et al. (2023):
  • HIIT showed a statistically significant but modest improvement in peak VO₂ (about +1.5 mL/kg/min) compared to MICT in heart failure patients. However, there was no significant difference in quality of life or ventilatory efficiency. Benefits were more pronounced in HFrEF compared to HFpEF. Overall, HIIT may improve cardiorespiratory fitness, but the clinical significance is unclear given small sample sizes and short follow-up.
• Chen et al. (2025):
  • HIIT improved peak VO₂ compared to MICT overall, but results varied depending on protocol and patient population. In HFrEF, long-interval and higher-volume HIIT showed the greatest benefit, while short-interval HIIT showed little to no advantage. In HFpEF, benefits were minimal. This suggests that not all HIIT is equal, and protocol design plays a major role.
• Bozkurt et al. (2021):
  • Earlier small studies suggested HIIT was superior to MICT, but larger trials like SMARTEX-HF did not show a difference in aerobic capacity or cardiac remodeling. Both HIIT and MICT are considered safe, with no significant differences in adverse events or withdrawal rates. MICT is often more practical and easier to implement, especially in patients with low baseline fitness or frailty.
• Conceição et al. (2026):
  • HIIT improved peak VO₂ overall, but when total energy expenditure was matched (isocaloric protocols), there was no difference between HIIT and MICT. There was also no difference in quality of life. This suggests that the perceived benefit of HIIT may be due to higher total exercise volume rather than intensity itself.

Overarching Conclusion.

Overall, HIIT leads to modest improvements in cardiorespiratory fitness (about +1.5 mL/kg/min in peak VO₂) compared to MICT in heart failure patients. However, this does not translate into clear clinical superiority. Studies consistently show no meaningful difference in quality of life, cardiac remodeling, or safety outcomes, with both HIIT and MICT being safe in stable HF patients. The effectiveness of HIIT also depends heavily on protocol design, with long-interval, higher-volume HIIT in HFrEF showing the greatest benefit, while shorter or less intensive protocols offer little advantage. Importantly, when total exercise volume is matched, HIIT loses its benefit over MICT, suggesting that overall exercise dose may matter more than intensity itself. Clinically, HIIT may be appropriate for stable, motivated HFrEF patients who can tolerate and adhere to higher intensity training, but MICT remains a strong and often more practical option for most patients. Ultimately, the choice should be individualized based on patient preference, functional status, and ability to maintain the program.

Clinical Bottom Line: 

The evidence I reviewed includes three systematic reviews/meta-analyses (Gu 2023, Chen 2025, Conceição 2026) and one expert consensus statement (Bozkurt 2021), representing the highest level of evidence available for this question. These meta-analyses pooled data from 15–18 randomized controlled trials, with total sample sizes ranging from about 550 to over 1,000 patients. However, most individual trials were small, short in duration lasting typically 8–16 weeks, and conducted in controlled settings, which limits how well the findings translate to real-world practice. Additionally, none of the studies reported hard clinical outcomes such as hospitalization or mortality, focusing instead on surrogate outcomes like VO₂ peak, LVEF, and quality of life which is an important gap in the literature. Across all three meta-analyses, HIIT consistently improved peak VO₂ by about +1.5 mL/kg/min compared to MICT. While this is statistically significant, it falls at the lower end of what is considered clinically meaningful, so the real-world impact may be modest. Importantly, there were no consistent differences between HIIT and MICT in quality of life, ventilatory efficiency, or cardiac remodeling. Safety outcomes were also similar, with no significant differences in adverse events, withdrawal rates, or cardiac events, suggesting that both HIIT and MICT are safe in stable, medically optimized heart failure patients.

Several key findings explain why the literature is conflicting. First, protocol design matters, long-interval (≥4 minutes), higher-volume HIIT in HFrEF patients showed the greatest benefit, while shorter or lower-volume protocols showed little advantage. Second, adherence is a major issue, as patients often struggle to maintain high-intensity effort over time, which reduces the effectiveness of HIIT in longer programs. Third, when total energy expenditure is matched between HIIT and MICT (isocaloric protocols), the superiority of HIIT disappears. This suggests that overall exercise volume may be more important than intensity itself. The current evidence is not sufficient to determine whether HIIT is clinically superior to MICT in terms of meaningful patient outcomes. Future research should focus on large, multicenter randomized trials that evaluate hard clinical outcomes such as hospitalization, mortality, and major adverse cardiac events. There is also a need for standardized HIIT protocols to reduce variability across studies, as well as more isocaloric comparisons to better understand whether intensity or total exercise dose is driving benefits. Longer-term studies are needed to assess adherence and sustainability outside of controlled settings. Finally, identifying which subgroups of HFrEF patients benefit most from HIIT will be important for guiding individualized exercise prescriptions rather than using a one-size-fits-all approach.

Thus, for a 58 year old patient with stable HFrEF, either HIIT or MICT would be a reasonable and safe option. If he is motivated, has good baseline functional capacity, and has access to a supervised program, HIIT may provide a modest additional improvement in exercise capacity. However, if adherence is a concern, if he prefers a more familiar and sustainable routine, or if supervision is limited, MICT is likely to provide similar long-term benefits with better practicality. The most important takeaway is that consistent exercise matters more than the specific intensity. The choice between HIIT and MICT should be individualized based on patient preference, functional status, and ability to maintain the program.