Effect of Constant vs. Variable Moderate-Intensity Load on Peak Oxygen Uptake in Outpatient Cardiac Rehabilitation

Abstract

Background: In outpatient center-based cardiac rehabilitation (O-CBCR), moderate-intensity continuous training (MICT) based on the anaerobic threshold (AT) determined by cardiopulmonary exercise stress testing is recommended. However, it is unclear whether differences in exercise intensity within the MICT domain affect peak oxygen uptake (%peakV̇O₂).

Methods and Results: We retrospectively evaluated patients who underwent O-CBCR at Japan Community Healthcare Organization Osaka Hospital. Those treated with the constant-load method were designated as Group A (n=38), whereas those treated with the variable-load method were designated as Group B (n=48). Although the change in exercise intensity was significantly greater in Group B by approximately 4.5 W, the change in %peakV̇O₂ was not significantly different between groups. Group A had a significantly longer exercise time than Group B (by approximately 4–5 min). No deaths or hospitalizations occurred in either group. The percentage of episodes with exercise cessation was similar between the 2 groups, but the percentage of episodes with load reduction was significantly higher in Group B, mostly because of the increased heart rate.

Conclusions: In supervised MICT based on AT, the variable-load method increased exercise intensity more than the constant-load method without severe complications, but did not improve %peakV̇O₂.

Cardiac rehabilitation (CR) improves the prognosis and quality of life of patients with cardiovascular disease.^1,2 Exercise prescription in CR generally consists of 6 items, namely frequency, intensity, time (duration), type, volume of exercise, and progression (FITT-VP).² Sufficient improvement in peak oxygen uptake (peakV̇O₂) is necessary to achieve prognostic benefits, for which the appropriate setting of exercise intensity is essential.^1–6 Exercise intensity is classified into different levels based on the physiological response to exercise: light, moderate, high, severe, extreme.^1,7 Because high-intensity interval training (HIIT) is more effective at improving peakV̇O₂ than moderate-intensity continuous training (MICT), a higher exercise intensity is considered more effective.^8,9 Conversely, HIIT should be performed in selected patients only because of the higher rate of cardiovascular complications compared with MICT.⁶ Therefore, MICT is the standard CR strategy.^2,6 Moderate-intensity exercise is defined as exercise to 46–63% of maximum oxygen uptake, 40–59% of heart rate reserve, and 64–76% of maximum heart rate, and a score of 12–13 on the Borg rating of perceived exertion scale (Borg scale).¹ However, exercise intensity varies depending on the index used as a prescription.^2,7 Exercise prescriptions based on estimated values may not reflect the appropriate exercise intensity, thereby contributing to individual differences in the effectiveness of exercise therapy. Therefore, exercise prescriptions based on anaerobic threshold (AT) determined by cardiopulmonary exercise stress testing (CPX) are recommended whenever possible.^10–12 Furthermore, it is desirable to aim for a more personalized prescription by combining various indices obtained from the CPX with subjective measures of exercise intensity, such as the Borg scale.² However, no internationally established method has been presented for specific load methods.⁵

In Japanese guidelines, MICT is recommended for exercise prescription after the recovery phase (Phase II) in outpatient center-based cardiac rehabilitation (O-CBCR) using indices determined via CPX, such as work rate (WR) and heart rate.¹ There are 2 methods of loading WR using ergometers: constant- and variable-load methods. In the constant-load method, WR is fixed for the first and second halves of a 2-month period when CPX is performed at baseline, mid-term, and at end of the O-CBCR period. Consequently, exercise intensity is not commensurate with exercise capacity, which improves daily. The concepts of overload and progression have not been used.^13,14 However, it is difficult to assess AT using CPX multiple times. Therefore, in the variable-load method, optimal exercise intensity can be achieved by varying the WR each time based on the heart rate and Borg scale score, in addition to the initially prescribed WR obtained via CPX (Figure 1). The question arises whether loading a higher exercise intensity for patients who are not candidates for HIIT provides greater improvement in peakV̇O₂, even within the MICT domain. To the best of our knowledge, no report has compared these 2 methods to answer this question. Therefore, in the present study we report differences in the effects of the 2 methods on WR, peakV̇O₂, complications, and episodes of exercise cessation or load reduction.

Figure 1.

Illustration of the constant- and variable-load methods. (Top) In Group A, a constant work rate (WR), the initially prescribed WR, was applied strictly during the first and second halves of each period. (Bottom) In Group B, the initially prescribed WR was applied as in Group A, but the WR varied using the Borg scale score to aim for higher exercise intensity. CBCR, center-based cardiac rehabilitation; CPX, cardiopulmonary exercise test; ∆WR, difference between the initially prescribed WR and the actual loaded WR.

Methods

Patients

We retrospectively evaluated consecutive patients who underwent O-CBCR in Japan Community Healthcare Organization Osaka Hospital between January 2016 and December 2019. Patients who were prescribed the constant-load method between January 2016 and March 2018 were designated as Group A, whereas those prescribed the variable-load method between April 2018 and December 2019 were included in Group B. Patients prescribed moderate- to high-intensity constant training (M-HICT), HIIT, or low-intensity training (LIT) were excluded from the study. Patients for whom 3 sets of CPX data were unavailable were also excluded. Because only medical practice data were used, no consent form was required for this study. However, all patients provided written informed consent regarding the handling of their personal data. This study was approved by the Ethics Committee of the Japan Community Healthcare Organization Osaka Hospital (Reference no. B-2022-008).

Assessments

The actual loaded WR and exercise time for every CR session during O-CBCR were obtained, and the difference between the actual loaded WR and the initially prescribed WR (∆WR) was calculated. The percentage of estimated peakV̇O₂ (%peakV̇O₂), considering age, sex, and body weight, is often used to evaluate the effectiveness of CR.^2,3 The following equations were used to determine predictive peakV̇O₂ values:¹⁵

peakV̇O₂ = −0.36 × Age + 46.6 in males

peakV̇O₂ = −0.23 × Age + 35.3 in females

Therefore, to evaluate the effect of O-CBCR, the percentage change in %peakV̇O₂ ((post-CR − baseline value) / baseline value × 100) was calculated. To assess the degree of agreement with the predictive value, the achievement rate ((post-CR − baseline value) / (100 − baseline value)) was also calculated. Safety was assessed in terms of all-cause mortality and the number of patients with complications requiring hospitalization or emergency treatment during O-CBCR. The number and proportion of episodes with exercise cessation and load reduction are also presented, and the reasons for these episodes were assessed.

Cardiopulmonary Exercise Stress Test

CPX was performed using a cardiopulmonary exercise stress machine (AEROMONITOR SE310S; Minato Medical Science, Osaka, Japan). All patients were subjected to the symptomatic limit with a ramp load using a standard incremental protocol on a bicycle ergometer with an increase of 10 W/min starting at 10 W. Measurements were taken using the breath-by-breath method, and data are presented as a moving average of 9 breaths and a moving average of 10 s.

Outpatient Center-Based Cardiac Rehabilitation

O-CBCR was performed using a standard program implemented under Japanese medical insurance. Exercise duration was 60 min per session, frequency was up to 3 sessions per week, and the CR period was 150 days from the start of treatment, including in-hospital and outpatient times. One session consisted of 30 min of endurance exercise, 10 min of warm-up and cool-down exercises, and 20 min of resistance training; however, the therapist adjusted the time allocation to suit individual patients. Endurance exercise was performed using a bicycle ergometer, and exercise intensity was set by maintaining a constant rotation rate of 50 r.p.m. for the prescribed WR.

CR Protocol

The LIT protocol was for patients who were unable to perform baseline CPX or finished it during warm-up. The protocols of the moderate and high intensity domain were changed from the continuous-load method to the variable-load method and from M-HICT to HIIT, respectively, in April 2018 after the addition of a team member with experience with these protocols. The M-HICT protocol was used for patients (Group A) with low Borg scale scores and acceptable heart rate and blood pressure even at the updated WR with mid-term CPX in the second half of the program. The HIIT protocol was designed for patients (Group B) who were able to briefly increase exercise intensity beyond the WR at the AT in the second half of the program.

Exercise Prescription

In Group A, a constant WR at 1 min before AT obtained from CPX (initially prescribed WR) at the baseline and mid-term of the O-CBCR period was strictly applied during the first and second halves of each period. In Group B, the patient started based on the initially prescribed WR but aimed for the highest possible intensity using the heart rate at AT and a Borg scale score of 11–13 as a reference (Figure 1). Exercise intensity was resumed from the intensity at the end of the previous session and increased by 1–5 W. The increased load for patients with the ST-segment depression on CPX did not exceed the ischemic threshold. The instructions were reviewed at weekly multidisciplinary conferences.

Electrocardiogram monitors were used during exercise, and blood pressure and oxygen saturation were measured as appropriate. Borg scale scores and subjective symptoms were monitored during exercise. Exercise was stopped or WR was reduced based on guideline criteria for exercise cessation, such as heart rate or blood pressure exceeding the prescription, Borg scale score exceeding 14, subjective symptoms, and complications.¹

Statistical Analysis

Patient numbers are presented as percentages in each group, whereas episode numbers are presented as the percentage of the total sessions in each group. Continuous data are presented as the median and interquartile range (IQR). The Mann-Whitney U test was used to compare quantitative variables between the 2 groups using SPSS version 27 (IBM Corp., Armonk, NY, USA). Median values and 95% confidence intervals (CIs) between the 2 groups were estimated using Hodges-Lehmann estimator. Missing values were noted for grip strength, knee extension muscle strength, and 6-min walking distance, which were excluded from the calculations and are shown as reference values. For comparisons of categorical variables, the Chi-squared test or Fisher’s exact test was used. Statistical significance was set at P<0.05. To evaluate the correlation between change rate and achievement rate in Group B, a single regression analysis was performed for age, sex, baseline %peakV̇O₂, ∆WR, and total time, whereas multiple regression analysis was performed on variables that showed a significant correlation in the single regression analysis.

Results

Of the 172 patients, 119 completed the O-CBCR program (54 in Group A, 64 in Group B); 1 patient who underwent the constant-load method in the first half and the variable-load method in the second half was excluded. In Group A, 3 patients with LIT, 9 patients with M-HICT, and 4 patients for whom 3 sets of CPX data were not available were excluded. In Group B, 2 patients with LIT, 8 patients with HIIT, and 6 patients for whom 3 sets of CPX data were not available were excluded. This left 38 patients in Group A and 48 patients in Group B for evaluation (Figure 2).

Figure 2.

Flowchart showing study design and patient enrolment. CBCR, center-based cardiac rehabilitation; HIIT, high-intensity interval training; LIT, light-intensity training; M-HIT, moderate-to-high-intensity training.

The mean patient age was 75 years, and approximately 60% of patients were male. Indications for CR after angina pectoris and cardiac surgery were differed significantly between the 2 groups (Table 1). The duration of O-CBCR was approximately 5 months, the actual number of days of participation was 27–28 days, and the percentage participation was approximately 85% in both groups. The frequency of participations changed in 7 (18.4%) patients in Group A and in 10 (20.8%) patients in Group B. The number of days per week increased by 1 day in 3 patients in Group B, decreased by 1 day in 7 patients in Group A and 6 patients in Group B, and decreased by 2 days in 1 patient in Group B. No significant intergroup differences were observed in other background diseases, physical function, or CPX results.

Table 1. Baseline Characteristics and Exercise Measurements

	Group A	Group B	P value
No. patients	38	48
Age (years)	75.0 [67.3–80.0]	75.0 [67.3–79.0]	0.804
Male sex	24 (63.2)	28 (58.3)	0.650
Indication for CR
Acute myocardial infarction	10 (26.3)	10 (20.8)	0.550
Angina pectoris	3 (7.9)	12 (25.0)	0.038*
Cardiac surgery	15 (39.5)	7 (14.6)	0.009*
Aortic disease	2 (5.3)	3 (6.3)	1.000
Chronic heart failure	8 (21.1)	15 (31.3)	0.289
Peripheral arterial disease	0 (–)	1 (2.1)	1.000
Participation
Period (months)	5.0 [5.0–5.0]	5.0 [4.3–5.0]	0.376
Total no. participation days	28.5 [19.8–35.3]	27.0 [17.3–37.5]	0.931
No. days/week	2.0 [1.0–2.0]	2.0 [1.0–2.0]	0.561
% Participation	85.3 [74.8–94.4]	87.1 [78.8–93.9]	0.608
Comorbidities
Hypertension	27 (71.1)	30 (62.5)	0.405
Dyslipidemia	17 (44.7)	24 (50.0)	0.627
Diabetes	15 (39.5)	19 (39.6)	0.992
Pacemaker rhythm	2 (5.3)	3 (6.3)	1.000
Persistent AF/Afl/tachycardia	7 (18.4)	6 (12.5)	0.447
Medications
β-blocker	33 (86.8)	34 (70.8)	0.076
ACEI/ARB	16 (42.1)	29 (60.4)	0.091
Statin	18 (47.4)	25 (52.1)	0.664
Diuretics	19 (50.0)	17 (35.4)	0.173
Clinical data
Height (cm)	159.3 [153.0–166.3]	160.0 [153.1–164.8]	0.893
Weight (kg)	58.0 [51.4–64.6]	58.3 [58.3–65.5]	0.951
BMI (kg/m2)	23.0 [20.3–24.8]	22.9 [20.5–25.5]	0.924
LVEF (%)	56.5 [45.0–67.8]	61.0 [48.3–73.0]	0.175
Handgrip strength (kg)	24.8 [17.0–29.8]	22.8 [18.3–31.3]	0.675
Knee extension strength (kgf/kg)	0.38 [0.28–0.43]	0.40 [0.30–0.51]	0.262
6MWD (m)	380 [313–438]	406 [320–470]	0.333
CPX
Peak WR (W)	54.0 [42.0–69.3]	58.0 [41.3–76.0]	0.731
PeakV̇O2 (mL/kg/min)	11.8 [9.9–15.3]	12.0 [10.2–15.1]	0.629
%peakV̇O2	51.5 [41.8–64.0]	50.0 [43.0–62.8]	0.865
Peak respiratory exchange ratio	1.12 [1.08–1.17]	1.17 [1.07–1.24]	0.081
AT (mL/kg/min)	9.6 [7.6–10.6]	9.2 [7.9–10.6]	1.000
%AT	58.5 [46.0–64.0]	55.0 [49.3–64.8]	0.969
V̇O2/WR slope (mL/min/W)	6.7 [5.8–8.2]	7.1 [5.4–8.7]	0.455
V̇E/V̇CO2 slope	33.5 [29.1–38.7]	34.4 [30.6–40.2]	0.590

Unless indicated otherwise, data are given as the median [interquartile range] or n (%). *Differences between groups were considered significant at P<0.05. 6MWD, 6-min walking distance; %peakV̇O₂, peak oxygen uptake; ACEI, angiotensin-converting enzyme inhibitor; AF, atrial fibrillation; Afl, atrial flutter; ARB, angiotensin receptor blocker; AT, anaerobic threshold; BMI, body mass index; CPX, cardiopulmonary exercise test; CR, cardiac rehabilitation; Group A, those treated with the constant-load method; Group B, those treated with the variable-load method; LVEF, left ventricular ejection fraction; V̇E, ventricular equivalence; V̇CO₂, carbon dioxide exhaustion; V̇O₂, oxygen uptake; WR, work rate. 1 kgf/kg means Newton (N) × 9.8 ÷ body weight (kg).

With regard to residual coronary artery stenosis, there was no significant difference between the 2 groups (Supplementary Table 1). Morphologic evaluation using coronary angiography or cardiac computed tomography and evaluation of ischemia using cardiac scintigraphy were performed before CR in 84.2% of patients in Group A and in 93.8% of patients in Group B. The percentage of patients in Group A and B with severe stenosis, ischemia, or both was 26.3% and 31.3%, respectively. The percentage summed difference score (%SDS) of patients with positive ischemia on cardiac scintigraphy was 7%, in Group A and 4% in Group B (P=0.468). One patient in Group A (2.6%) and 1 patient in Group B (2.1%) showed ST-segment depression at baseline CPX. Three patients in Group B showed new ST-segment depression at mid-term CPX (Supplementary Table 2).

The actual loaded WR did not differ significantly between Group A and Group B in either the first half of the O-CBCR period (22.4 vs. 22.5 W, respectively; P=0.277) or the second half (30.9 vs. 33.3 W, respectively; P=0.188; Figure 3A). The ∆WR was significantly higher in Group B than Group A in both the first half (+1.1 vs. −1.8 W, respectively; 95% CI −6.4, −2.1; P<0.001) and the second half (+2.8 vs. −0.4 W, respectively; 95% CI −7.8, −1.8; P<0.001; Figure 3B). Exercise time was significantly longer in Group A than Group B in both the first half (27.0 vs. 20.0 min, respectively; 95% CI 2.7–6.0; P<0.001) and second half (29.5 vs. 22.7 min, respectively; 95% CI 3.9–7.2; P<0.001; Figure 3C).

Figure 3.

Results for (A) actual loaded work rate (WR), (B) the difference between initially prescribed WR and actual loaded WR (∆WR), (C) time per session during outpatient center-based cardiac rehabilitation (O-CBCR), (D) percentage change in peak oxygen uptake (%peakV̇O₂), and (E) achievement rate in Groups A and B in the first and second halves of the center-based cardiac rehabilitation period. The boxes show the interquartile range, with the median value indicated by the horizontal line; whiskers show the range. *Significant intergroup differences (P<0.05).

No significant intergroup differences were observed in CPX results after O-CBCR (Table 2). There were no significant differences between Groups A and B in the percentage change in %peakV̇O₂ (25.6% vs. 22.7%, respectively; P=0.845; Figure 3D) or achievement rates (51.8% vs. 53.8%, respectively; P=0.835; Figure 3E). A subgroup analysis of the percentage change in %peakV̇O₂ by indication cardiovascular disease showed no significant difference between the 2 groups for all diseases (Supplementary Table 3).

Table 2. Results of Cardiopulmonary Exercise Tests

	Group A	Group B	P value
At the end of O-CBCR
Peak WR (W)	68.5 [58.3–90.3]	71.0 [56.0–103.5]	0.509
PeakV̇O2 (mL/kg/min)	14.8 [12.6–17.7]	15.2 [12.4–18.8]	0.476
%peakV̇O2	65.5 [52.3–74.3]	64.5 [53.5–79.5]	0.648
Peak respiratory exchange ratio	1.16 [1.07–1.20]	1.21 [1.15–1.29]	0.001*
AT (mL/kg/min)	11.0 [9.3–11.9]	10.1 [9.1–11.1]	0.110
%AT	67.0 [56.5–74.3]	61.5 [55.0–67.8]	0.164
V̇O2/WR slope (mL/min/W)	8.0 [7.0–8.7]	8.1 [6.8–8.7]	0.617
V̇E/V̇CO2 slope	30.4 [28.0–37.3]	32.8 [29.3–36.6]	0.244
Change before and after O-CBCR
PeakV̇O2 (mL/kg/min)	2.9 [1.7–4.1]	2.7 [0.9–4.5]	0.787
METs	0.83 [0.52–1.17]	0.78 [0.28–1.28]	0.676
%peakV̇O2	24.6 [12.8–35.6]	21.7 [7.1–38.3]	0.725

Unless indicated otherwise, data are given as the median [interquartile range]. *Differences between groups were considered significant at P<0.05. METs, metabolic equivalents; O-CBCR, outpatient center-based cardiac rehabilitation. Other abbreviations as in Table 1.

For the percentage change in %peakV̇O₂ in Group B, single regression analysis showed a significant correlation between baseline %peakV̇O₂ and ∆WR. Achievement rate was significantly correlated with sex, ∆WR, and total exercise time. Multiple regression analysis showed significant correlations between percentage change in %peakV̇O₂ and both baseline %peakV̇O₂ and ∆WR, but only between ∆WR and achievement rate (Table 3; Supplementary Figure).

Table 3. Results of Univariate and Multivariate Analyses of Factors Associated With the Percentage Change in %peakV̇O₂ and Achievement Rate

	Percentage change in %peakV̇O2		Achievement rate
	Univariate	MultivariateA	Univariate	MultivariateB
Age	0.080		0.191
Male sex	0.138		0.015*	0.238
Baseline %peakV̇O2	0.004*	0.003*	0.865
ΔWR	0.006*	0.005*	0.001*	0.022*
Total time	0.074		0.003*	0.055

^AMultivariate analysis of the percentage change in %peakV̇O₂ showed that the model was significant (P=0.001; adjusted R²=0.272), and that the variance inflation factor (VIF) of all values was <1.1. ^BMultivariate analysis of the achievement rate showed that the model was significant (P=0.002; adjusted R²=0.274), and the VIF of all values was <1.2. *Significant correlation (P<0.05). %peakV̇O₂, peak oxygen uptake; ΔWR, difference between the initially prescribed work rate and actual loaded work rate.

Regarding safety, no deaths or complications requiring hospitalization or emergency treatment were observed in either group, but exacerbations of bronchial asthma and paroxysmal atrial fibrillation were observed in Group A. With regard to patients with residual coronary stenosis, the final WR did not exceed the ischemic threshold, even for the 4 patients in Group B (Supplementary Table 2).

The total number of sessions was 1,094 in Group A and 1,351 in Group B. Exercise cessation rates were similar in Groups A and B (2.5% vs. 2.3%, respectively; P=0.725), but episodes of load reduction were significantly higher in Group B than Group A (5.1% vs. 0.5%, respectively; P<0.001; Table 4). The main reason for exercise cessation was a significantly higher heart rate increase in Group A. Dyspnea and chest discomfort were more common in Group B, although the differences between the 2 groups were not significant. The most common reason for load reduction in Group B was increased heart rate.

Table 4. Episodes and Reasons for Exercise Cessation and Load Reduction

	Group A	Group B	P value
Episodes
Exercise cessation	27 (2.5)	30 (2.3)	0.725
Load reduction	5 (0.5)	68 (5.1)	<0.001*
Reasons for exercise cessation
Increased heart rate	9 (0.8)	1 (0.1)	0.007*
Increased blood pressure	1 (0.1)	5 (0.4)	0.232
Increased Borg scale score	2 (0.2)	1 (0.1)	0.452
Arrhythmia	3 (0.3)	1 (0.1)	0.229
Dizziness	2 (0.2)	1 (0.1)	0.452
Dyspnea/chest discomfort	2 (0.2)	10 (0.8)	0.047*
General fatigue/leg fatigue	1 (0.1)	8 (0.6)	0.047*
Back pain/joint pain	1 (0.1)	0 (–)	0.451
Pedal under-rotation	0 (–)	0 (–)	–
Other	6 (0.5)	3 (0.2)	0.315
Reasons for Load reduction
Increased heart rate	2 (0.2)	27 (2.0)	<0.001*
Increased blood pressure	0 (–)	5 (0.4)	0.068
Increased Borg scale score	0 (–)	3 (0.2)	0.257
Arrhythmia	0 (–)	0 (–)	–
Dizziness	0 (–)	3 (0.2)	0.257
Dyspnea/chest discomfort	0 (–)	5 (0.4)	0.068
General fatigue/leg fatigue	2 (0.2)	16 (1.2)	0.004*
Back pain/joint pain	0 (–)	6 (0.5)	0.036*
Pedal under-rotation	0 (–)	3 (0.2)	0.257
Other	1 (0.1)	0 (–)	0.451

Unless indicated otherwise, data are given as n (%). *Differences between groups were considered significant at P<0.05. Group A, those treated with the constant-load method; Group B, those treated with the variable-load method.

Discussion

In the present study, regarding supervised MICT based on CPX in O-CBCR for patients without HIIT indication, the variable-load method allowed a significantly greater change in exercise intensity than the constant-load method without serious complications. However, this advantage in exercise intensity did not lead to a significant improvement in the %peakV̇O₂.

There was no significant difference in actual loaded WR in both groups. This lack of difference in absolute exercise intensity may be the reason for the similar %peakV̇O₂ at the end of O-CBCR between the 2 groups. Focusing on the change in these values, there was a significant difference of over 4.5 W in ∆WR. However, there was no significant intergroup difference in the percentage change in %peakV̇O₂, even though these values achieved the expected effect (15–25%) with CBCR in general.^1,10 Therefore, this difference in exercise intensity with a ∆WR of approximately 4.5 W may not be enough to have a significant effect on the percentage change in %peakV̇O₂.

In the present study, exercise time was almost 4–5 min shorter in Group B than in Group A. Because the degree of increase in exercise intensity and time was reversed between the 2 groups, it seems difficult to explain these results purely in terms of changes in exercise intensity alone. To assess the impact of exercise time, exercise volume was considered.^2,5,6 Exercise volume is expressed as energy consumption (EC), and is determined by the product of exercise intensity and exercise time.¹⁶ The oxygen consumption of 1 metabolic equivalent (MET) corresponds to 3.5 mL/kg/min, and the consumption of 1 L of oxygen is equivalent to approximately 5 kcal.⁷ Because the relationship between oxygen uptake and WR increased linearly from the start to around AT in CPX, the oxygen uptake corresponding to the actual loaded WR was determined using the V̇O₂/WR slope, AT, and WR. Therefore, the EC can be estimated using the following equation:

EC (kcal) = Intensity (MET) × Time (h) × Body weight (kg) × 1.05

The EC per week did not differ significantly between Groups A and B (132.4 vs. 108.9 kcal, respectively; P=0.134). Compared with the EC that would be expected if WR strictly adhered to the initial prescribed WR, the intergroup difference was also not significant (+5.7 vs. +8.2 kcal, in Group A and Group B, respectively; P<0.063; Figure 4A). If both groups exercised for 30 min in all sessions, the estimated difference was significantly higher in Group B than Group A (+11.1 vs. +6.1 kcal, respectively; 95% CI 1.7–12.7; P<0.007; Figure 4B). However, this difference of only 6.6 kcal per week is not expected to be effective, because exercise therapy for health management purposes generally recommends 1,000–2,000 kcal per week.^2,17 Therefore, it was considered that the 4–5 min time difference had little effect on the overall EC of the O-CBCR. Conversely, multivariate analysis showed that the percentage change in %peakV̇O₂ was significantly correlated with ∆WR, although very weakly (adjusted R²=0.2). Similar results were obtained with the achievement rate, which was not affected by baseline %peakV̇O₂. These results reflect the need for exercise intensity to improve %peakV̇O₂ and the limitations of increasing exercise intensity within the MICT and extending exercise time during O-CBCR. Expanding the adaptation of HIIT and home-based CR using new technologies, such as telerehabilitation, should be considered to take advantage of the small differences in exercise intensity within MICT.^18,19 Although home-based CR is not currently covered by health insurance in Japan, the results of the present study show its importance as an alternative to O-CBCR.

Figure 4.

Estimated energy consumption per week in Groups A and B. WR, work rate. *Significant intergroup differences (P<0.05).

No fatal complications or those requiring hospitalization or emergency treatment were observed during the O-CBCR period in either group. The rate of serious cardiovascular events during exercise training was reported to be 1/50,000,¹¹ but could not be assessed in the present study.

The proportion of exercise cessation was the same in both groups, but the main cause differed, with an increased heart rate in Group A and subjective symptoms in Group B. Conversely, the proportion of episodes of load reduction was significantly higher in Group B, primarily because of an increase in heart rate. Heart rate cannot increase theoretically in Group A because of a constant load below AT. However, it can sometimes change due to physical conditions, such as ill-health or insomnia. In response to such unusual reactions, in Group A, ill-health was suspected and exercise cessation was recommended. Conversely, in Group B, it was difficult to determine whether the reason was ill-health or overload; therefore, the load was first reduced, and the exercise was stopped when subjective symptoms appeared. Thus, the need to adjust exercise intensity or decide to discontinue exercise suggests that monitoring by medical staff and patient self-management are more important in Group B. Of course, due to the issue of the accuracy of the AT, careful attention was required even in Group A, especially in the early days of the O-CBCR period.^20,21

In Group B, episodes of load reduction were as few as 5% of all sessions, despite aiming for the highest possible load. This indicates that the results converged to a comfortable exercise intensity for the patients because of the limitation of the Borg scale.²² Moreover, this could be the reason for the increased variability in ∆WR in Group B. Therefore, it is important to instruct patients not to go below the AT prescription, as in Group A. However, exceeding comfortable exercise intensity tends to cause discomfort and may contribute to patients discontinuing CR.^12,23 Considering that a higher exercise intensity increases heart rate and symptoms even within the MICT domain, despite having no significant effect on the percentage change in %peakVO₂, the choice of the loading method may be based on patient comfort with exercise in terms of adherence at home or after the O-CBCR program.^6,24

Study Limitations

Because this was a single-center retrospective observational cohort study, it has several biases. First, the background cardiovascular disease was heterogeneous within and between the 2 groups. Second, there are no uniform criteria for the indications of low- and high-intensity exercise training. Third, changes in medical staff throughout the study period may have influenced the decisions regarding intensity, exercise duration, and exercise discontinuation or load reduction. Fourth, no special monitoring or intervention was provided for home life activities.¹⁷ Therefore, a prospective randomized trial is required to validate the results of the present study.

Finally, the present study evaluated %peakV̇O₂ as a surrogate indicator of life expectancy, which was the original purpose of CBCR. Previous studies that showed little improvement in oxygen uptake after CBCR (0.5 MET) did not find any prognostic benefit,²⁵ in contrast with studies that showed improvement (1.55 MET).²⁶ The improvement in oxygen uptake in the present study was also approximately 0.8 MET, indicating that no prognostic improvement can be expected within the MICT range, regardless of the loading method. Doubling of the %peakV̇O₂ change in this study suggests 100% of the achievement rate; that is, 100% of %peakV̇O₂. However, for elderly patients with many underlying diseases, prognosis should be assessed using cardiopulmonary function, as well as broader concepts, such as frailty.^27,28 Further studies are needed to evaluate the prognostic value, including the method of evaluation, and the study setting.

Conclusions

In MICT in O-CBCR for patients without indications for HIIT, the variable-load method, aiming for a higher exercise intensity, increased exercise intensity compared with the constant-load method. However, it did not significantly improve %peakV̇O₂. There was no difference in safety between the 2 methods, but responses to heart rate and subjective symptoms during exercise were greater with the variable-load method. This study only tested a hypothesis, but did demonstrate one aspect of the difficulty in improving %peakV̇O₂ with MICT in the current O-CBCR and the need for further research in CBCR, such as expanding the indications for HIIT and combining home-based CR using new technologies.

Acknowledgments

None.

Sources of Funding

This study did not receive any specific funding.

Disclosures

The authors declare that there are no conflicts of interest.

IRB Information

The study protocol was approved by the Ethics Committee of the Japan Community Healthcare Organization Osaka Hospital, Osaka, Japan (Reference no. B-2022-008).

Data Availability

The data that support the findings of this study are not openly available because they are human data.

Supplementary Files

Please find supplementary file(s);

https://doi.org/10.1253/circrep.CR-23-0024

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