Training prescription in patients on beta-blockers: percentage peak exercise methods or self-regulation?

Abstract

Background: Exercise prescription based on percentage of peak exercise variables has many limitations in patients taking beta-blockers. The aim of this study was to evaluate efficacy and safety of a training protocol based on the rating of perceived exercise (RPE) in patients taking beta-blockers after cardiac surgical revascularization.

Design and methods: 71 patients treated with beta-blockers after recent coronary artery bypass grafting were randomly allocated to two different programmes with training intensity adjusted to keep heart rate close to first ventilatory threshold (36 subjects, AeT group) or RPE between grades 4 and 5 of 10-point category-ratio BORG scale (35 subjects, RPE group).

Results: In the RPE group, mean training workloads and heart rate values were significantly higher than in the AeT group; during the last week of the programme, six RPE patients were training very close to anaerobic threshold. Aerobic peak capacity increased similarly in the two groups. Considering the potential effects on training intensity of prescriptions based on percentages of peak exercise variables, we found that only percentage heart rate reserve and peak workload methods were reliable in defining a safe upper limit of training intensity, with values of 50% and 65% respectively.

Conclusions: Self-regulation of exercise training intensity between grades 4 and 5 of the 10-point category-ratio BORG scale is effective but may promote overtraining in some patients without significant functional advantages. For these reasons, RPE method should be integrated with objective indices based on percentage of heart rate reserve or of peak workload.

Keywords

Anaerobic threshold beta-blocker therapy exercise intensity prescription percentage peak exercise methods rating of perceived exercise

Introduction

In cardiac rehabilitation the use of heart rate (HR) as an estimate of exercise intensity is the common standard. The range of HR considered optimal for aerobic conditioning, with maximal gain and minimal risk, is generally established as a percentage of maximum HR (HR_peak) or heart rate reserve (HRR): training intensities recommended by these two methods are 40–80% of HRR or 70–85% of HR_peak.^1,2 The rationale supporting this practice is the relatively linear relationship existing between HR and oxygen uptake (VO₂) which enables the use of HR as an index of training intensity.

However, aging, disease, medications, and motor skills can displace the HR/VO₂ relationship so that the use of standard HR intensity guidelines can result in very different levels of metabolic stress across subjects.³^–⁶ This problem is amplified in patients on beta-blockers: the blunting of HR response observed in these patients reduces the reliability of HR methods since small HR changes can account for large differences in workload. The findings of recent studies addressing this issue are interesting, but operative recommendations for clinical use are inconsistent.⁷^–⁹ Furthermore, these studies were conducted in healthy volunteers or patients after myocardial infarction; no data exist, to our knowledge, about patients taking beta-blockers after coronary artery bypass grafting (CABG) who often suffer from marked deconditioning as well.

Ideally, training intensity should be adjusted according to metabolic markers such as aerobic and anaerobic threshold,^10,11 but systematic cardiopulmonary exercise testing (CPET) for training prescription cannot be recommended to all cardiac patients because it is not feasible and probably unnecessary, at least for low-risk patients.

In an editorial addressing this issue, Schmid¹² suggested starting training at 65% of HR_peak with adjustments of training intensity based on rating of perceived exercise (RPE) for low-risk patients taking beta-blockers. This approach is supported by the strong relationship between RPE and both VO₂ and lactate concentration;¹³^–¹⁵ moreover, the RPE/power output relationship seems to be unaffected by beta-blockade.¹⁶^–¹⁸ The reliability of perceived exertion ratings in exercise prescription has been evaluated in normal subjects and in some clinical settings.¹⁹^–²² but no specific data are available about the effects of beta-blockers on RPE regulation of exercise intensity in cardiac rehabilitation.

The aim of the present study was to evaluate the efficacy and safety of a training protocol based on RPE in patients on beta-blockers after CABG.

Methods

Patients

Consecutive male patients (from 1 September 2007 to 30 June 2008) referred to the Cardiac Rehabilitation Centre of Istituti Clinici di Perfezionamento in Milan for a phase-II rehabilitation programme were enrolled into this study if the following eligibility criteria were met: recent CABG (<60 days) and beta-blocker therapy. Exclusion criteria were: atrial fibrillation, left ventricular ejection fraction <35%, haemoglobin <10 g/dl, cerebrovascular or musculoskeletal disease preventing exercise testing or training, peripheral arterial occlusive disease limiting exercise, severe respiratory limitations (FEV1 or VC <60% of predicted), residual myocardial ischaemia or exercise induced arrhythmias, and unmeasurable aerobic threshold (AeT) during baseline cardiopulmonary exercise testing (CPET). This study was approved by the institution’s research Ethics Committee and all patients gave written informed consent.

Study design

After an initial assessment (clinical evaluation, cardiopulmonary exercise test, standard echocardiogram, and routine laboratory tests) patients were randomly allocated to one of two different programmes of endurance exercise training: (1) AeT group: intensity of training adjusted to keep HR at the aerobic threshold (AeT) ±2 bpm; and (2) RPE group: training intensity adjusted to keep RPE between grades 4 and 5 of 10-point category-ratio (CR-10) BORG scale.²³^–²⁴ AeT group patients were used as controls since exercise intensity prescription based on AeT determination may be considered a reference method, in view of its capacity to elicit uniform metabolic responses across subjects during aerobic training.^5,6,25

We considered the range of workload (WL) between the individual AeT and anaerobic threshold (AnT) as the optimal training zone. In the RPE group, an exercise intensity persistently lower than workload at AeT or higher than individual workload at AnT were considered indicative of undertraining or overtraining, respectively. Initial training data were compared with the results of the first CPET and final training data with the results of the last CPET. Physiotherapists were blinded to CPET results.

Rating of perceived exertion

Subjective regulation of training intensity in the RPE group was based on the CR-10 BORG scale, currently used for training monitoring at our centre: grades 4 and 5 correspond to effort levels perceived as ‘somewhat hard’ and ‘hard’ respectively and are equivalent to 13–15 of 6–20 on the Borg scale. The reliability of the Borg scale as a means of regulating high levels of exercise intensity in repeated trials has been previously confirmed.¹⁹

Standardized instructions about correct usage of CR-10 scale¹ were given to every patient before starting the rehabilitation programme; in addition the first two training sessions were ‘learning’ sessions during which the patients were taught to exercise using a self-regulated intensity between a rating of 4 and 5. In the following sessions each subject of the RPE group continued to exercise at this intensity and was provided with no other feedback regarding HR or WL; thus, RPE work demand was used as an independent variable to control work intensity.

Training programme

Patients trained in hospital as out-patients, three times a week for 5 weeks. Every training session consisted of flexibility and stretching exercises (20 min), dynamic resistance training on multifunctional fitness equipment (UNICA, Technogym) at 30% of one repetition maximum (10 min) and cycling: 5 min of warming-up, 30 min of endurance training, and 5 min of cool-down. Training sessions were supervised by experienced physiotherapists; ECG and workload during bicycle training were centrally monitored by an automatized system (CUSTO-MED).

Besides exercise training, patients underwent an educational programme based on individual and group sessions with medical staff, a dietician, and a psychologist.

At the end of the training programme, patients underwent a final CPET.

Cardiopulmonary exercise test

Symptom-limited cardiopulmonary exercise test was performed on a bicycle ergometer (Ergoselect 200P; Ergoline), under medication, with 10–15 W/min increments until exhaustion. Twelve-lead ECG and heart rate were recorded continuously (Cardiosoft; GE Medical Systems); blood pressure was measured every 2 min, at peak exercise, and during recovery. Respiratory gas exchange measurement involved use of the VMAX 29C Sensor Medics system. VO₂, VCO₂, and VE were measured breath by breath and data were averaged every 15 s; peak oxygen consumption (VO_2peak) was expressed as the highest attained VO₂ during the final 30 s of exercise. The AeT (first ventilatory threshold) was measured using the V-slope method and, if necessary, ventilatory equivalent and end-tidal gas concentration curves for O₂.²⁶ Only patients in whom AeT was clearly determined were included in the study. The AnT (second ventilatory threshold), corresponding to maximal lactate steady state, was assessed using one of the following methods: inflection of VE vs. VCO₂, nadir or nonlinear increase of VE/VCO₂ vs. workload or deflection point of the PETCO₂.²⁷ Unmeasurable AnT was not considered an exclusion criterion. All measurements were reviewed by the same experienced investigator.

Statistical analysis

Data are reported as mean ± SD. Differences between groups or between pre- and post-training values for continuous variables were tested with unpaired or paired Student’s t-test. For all statistical analyses, a p-value <0.05 was considered to indicate statistical significance.

Results

Study population

Between September 2007 and June 2008 we screened 90 consecutive male patients with recent CABG treated with beta-blockers. Sixteen were excluded for: left ventricular ejection fraction <35% (n = 6), atrial fibrillation (n = 4), unmeasurable AeT (n = 3), Hb <10 g/dl (n = 2), and lower extremity arterial disease (n = 1). Out of the remaining patients, three completed <12 training sessions and were excluded a posteriori. The data of 71 patients are reported in the present study; the beta-blockers used were: atenolol (n = 27, mean daily dose 52 ± 26 mg), bisoprolol (n = 33, mean daily dose 3.2 ± 1.3 mg), and metoprolol (n = 11, mean daily dose 98 ± 49 mg). The mean interval between heart surgery and first CPET was 37 ± 10 days; in 11 patients CABG was associated with valve surgery and 22 patients had a history of myocardial infarction (inferior or infero-lateral n = 9, anterior or antero-lateral n = 8, no Q wave n = 5).

Main anthropometric and baseline functional data of patients are shown in Table 1: parameters measured in basal conditions and at maximal exercise capacity are reported for all patients and separately for the two different training groups. No significant differences were found between the two groups; only a trend towards higher HR_resting and HR_peak in the AeT group was observed. AnT was detected in 65 subjects during CPET1 and in 66 during CPET2.

Table 1.

Main clinical and functional characteristics of patients

Characteristic	All patients (n = 71)	RPE group (n = 35)	AeT group (n = 36)	p-value
Age (years)	62.3 ± 8.8	60.5 ± 9	63.9 ± 8.5	0.107
Body mass index (kg/m²)	26.5 ± 3.2	26.6 ± 3.3	26.4 ± 3.2	0.711
Ejection fraction (%)	53.7 ± 8.7	52.9 ± 8.2	54.5 ± 9.1	0.423
Peak systolic BP (mmHg)	152.5 ± 23.8	148.6 ± 23.8	156.1 ± 23.7	0.187
Peak diastolic BP (mmHg)	85 ± 10.5	82.9 ± 10.6	86.9 ± 10.1	0.112
HR_resting (bpm)	64.7 ± 11.8	62.1 ± 10	67.1 ± 12.9	0.076
HR_peak (bpm)	106 ± 19.6	101.7 ± 18.7	110 ± 19.7	0.076
Maximal workload (watt)	92.2 ± 20.6	93.8 ± 20	90.7 ± 17.6	0.532
VO_2peak (ml/kg/min)	17.3 ± 3,7	16.9 ± 3,7	17.6 ± 3,6	0.377
%VO_2max predicted	70.6 ± 17.7	69.8 ± 16.9	72.3 ± 17.9	0.412
RER_max (VCO₂/VO₂)	1.1 ± 0,09	1.09 ± 0.09	1,11 ± 0.09	0.505

Values are mean ± SD. Functional parameters refer to the first cardiopulmonary exercise test. T test for comparison between RPE and AeT groups.

AeT, aerobic threshold; BP, blood pressure; HR, heart rate; RER, respiratory exchange ratio; RPE, rating of perceived exercise.

In Table 2, HR and WL values at AeT and AnT found in the two groups are reported both as absolute values and as percentage of peak exercise values. The only difference between the two groups was found for HR, which at AnT was significantly higher in the AeT group than in the RPE group (p = 0.046), but the relative percentage values of the two groups were very similar (AeT 90 ± 5% vs. 89 ± 6%).

Table 2.

Absolute and relative values of heart rate and workload at AeT and AnT

Characteristic	All patients (n = 71)	RPE group (n = 35)	AeT group (n = 36)	p-value
HR_AeT (bpm)	82.6 ± 12.6	80.7 ± 13.1	84.3 ± 12	0.220
%HR_peak at AeT	79 ± 8	80 ± 8	77 ± 8	0.219
%HRR at AeT	43 ± 14	47 ± 14	40 ± 13	0.051
HR_AnT (bpm)	95.1 ± 30.7	91.1 ± 26.8	98.9 ± 34	0.046
%HR_peak at AnT	89 ± 5	89 ± 6	90 ± 5	0.922
%HRR at AnT	73 ± 11	73 ± 10	73 ± 12	0.972
Workload at AeT (watt)	46.8 ± 10.4	48.8 ± 12.2	45 ± 8.1	0.116
% maximum workload at AeT	52 ± 10	52 ± 10	51 ± 10	0.491
Workload at AnT (watt)	74 ± 25,2	75 ± 23,4	73 ± 26,7	0.521
% maximum workload at AnT	79 ± 8	79 ± 9	78 ± 8	0.875

Values are mean ± SD. Functional parameters refer to the first cardiopulmonary exercise test. T test for comparison between RPE and AeT groups.

AeT, aerobic threshold; AnT, anaerobic threshold; HR, heart rate; RPE, rating of perceived exercise.

Effects on training intensity and functional outcome of the two different training protocols

In Table 3, training parameters of the two groups are compared at the beginning and at the end of the training programme. Mean training WL and corresponding mean RPE values in the RPE group were significantly higher than in the AeT group. In addition, we observed a significant increase of training intensity in both groups as programme went on.

Table 3.

Training parameters and functional outcomes of the two groups

Parameter	AeT group (n = 36)		RPE group (n = 35)
Parameter	Initial	Final	Initial	Final
Training intensity (watt)	42.9 ± 12.5	55 ± 11.8^a	50.5 ± 12.7^b	65.6 ± 19^a,c
HR_AeT (bpm) CPET	84.3 ± 12	83.5 ± 9.8	80.7 ± 13^b	78.3 ± 10.1^b
Training HR (bpm)	85.6 ± 10.2	86.5 ± 12	81.8 ± 11.2	87.3 ± 12^a
Mean RPE score	3.8 ± 0.4		4.1 ± 0.3^b
Watt/kg CPET	1.2 ± 0.3	1.38 ± 0.3^d	1.22 ± 0.3	1.4 ± 0.3^d
VO_2peak (ml/kg/min) CPET	17.4 ± 3.6	19.4 ± 4.7^d	16.8 ± 3.7	19.2 ± 4.6^d

Values are mean ± SD.

AeT, aerobic threshold; CPET, cardiopulmonary exercise test; HR, heart rate; RPE, rating of perceived exercise.

<0.01 t-test for initial vs. final mean values.

<0.05 t-test for AeT group vs. RPE group mean values.

<0.01 t-test for AeT group vs. RPE group mean values.

<0.05 t-test for initial vs. final mean values.

In the initial phase, mean training HR values in both groups were slightly higher than HR_AeT but while in the AeT group training HR increased minimally during the programme, in RPE patients a significant increase of this parameter was found between the initial and final phase of the training programme.

Working and aerobic peak capacity increased significantly after training and the magnitude of the improvement was similar in the two groups: VO_2peak 11.5 ± 16% in the AeT group vs. 14.3 ± 14% in the RPE group (p = 0.46). This finding is clinically remarkable considering the short period of time elapsed between first and final CPET: 32.4 ± 4.4 days in the RPE group and 33.4 ± 5.5 days in the AeT group.

Regarding individual data of RPE patients, their training intensity expressed as difference in watts between actual training WL and AeT and AnT workloads is reported in Figure 1. Considering the area between AeT and AnT (dotted lines of the figure) as ideal training zone, in the initial phase of the training programme 11 subjects were training below the AeT, while the others were in the optimal training zone. Four exercised at a load very close to AnT, without exceeding it.

During the last week of the rehabilitation programme most patients (87%) exercised in the optimal zone; two of the remaining patients exercised just below AeT and another two were slightly above AnT. No adverse events were reported in either group during the rehabilitation programme.

Comparison with percentage peak exercise methods

In Table 4, the effects on training prescription of different percentages of peak exercise variables are examined with reference to individual AeT and AnT values. The numbers of patients training below AeT, between AeT and AnT, and above AnT are reported for different percentages of these parameters.

Table 4.

Training intensity at different percentages of peak exercise variables

Variable	<AeT	between AeT and AnT	>AnT
HR_peak
70%	52	13	0
75%	44	19	2
80%	28	33	4
85%	16	37	12
90%	4	31	30
HRR
40%	38	27	0
50%	15	50	0
60%	1	57	7
70%	0	38	27
80%	0	20	45
WL_peak
50%	29	36	0
55%	26	39	0
60%	10	55	0
65%	6	58	1
70%	5	48	12

Values are n.

Data relative to 65 patients with AnT detectable at first cardiopulmonary exercise test.

AeT, aerobic threshold; AnT, anaerobic threshold; HR, heart rate; HRR, heart rate reserve; WL, workload.

Regarding %HR_peak method, training at the habitually used upper limit (85%) made 12 patients (18%) exercise above AnT; with lower percentages of HR_peak the number of these subjects diminished but the proportion of undertrained patients rose sharply: at 75% of HR_peak about two-thirds of subjects exercised below AeT.

The %HRR method seemed to work better, since training intensity eliciting HRR values of about 50% made almost 77% of subjects exercise between AeT and AnT; at this percentage value, none exceeded AnT.

Concerning %WL_peak, exercise training around 60% of maximum workload placed 85% of patients between AeT and AnT.

Figure 2 shows the distribution of individual percentage values of peak exercise variables at AeT and AnT; for each variable the optimal cut-off percentage value corresponding to the maximum number of subjects training between AeT and AnT is reported.

As far as %HR_peak values are concerned, we observed significant overlapping between individual data at AeT and AnT. Consequently, to avoid the risk of overtraining, patients should work at very low %HR_peak values.

The distribution of individual %HRR and WL_max values is more favourable, since the overlapping of data at AeT and AnT is small; this enables the detection of acceptable cut-off values for effective and safe management of most patients as reported in Figure 2.

Discussion

In low-risk cardiac patients, the option of training prescription based on RPE regulation of effort intensity is very attractive for several reasons. First of all, after CABG many patients undergo cardiac rehabilitation without a true maximal exercise test owing to their low fitness, which would prevent the achievement of a true peak heart rate. Furthermore, even if we could base our prescription on the results of a preliminary exercise test, in patients on beta-blockers with very low chronotropic reserve, the definition of a training zone based on percentage HR methods is of limited utility, because very small HR changes correspond to large differences in workload: in 14% of our patients the HR difference between AeT and AnT was ≤5 bpm. Finally, learning to self-regulate training intensity properly may favour long-term patient compliance and is essential for those participating in home rehabilitation programmes or when monitoring facilities are not available.

The present study demonstrated that, in patients on beta-blockers after CABG, self-regulation of exercise intensity between grades 4 and 5 of the CR-10 BORG scale makes most patients train between AeT and AnT. During the first week of the programme, 65% of subjects exercised in the optimal training zone, while less fit patients set their training WL below AeT. As rehabilitation proceeded, patients increased their training intensity progressively, so that during the last week of the programme about 90% of them were exercising between AeT and AnT; worthy of note, six of them trained very close to AnT.

Since training WL for AeT patients was adjusted to keep training HR close to HR at AeT, mean training HR and WL in the RPE group were significantly higher than in the AeT group.

Aerobic capacity improvement of RPE subjects was a little higher, but the difference vs. AeT patients was not statistically significant. This finding was probably due to the small sample size and to the short duration of the programme.

On the other hand, the significant improvement in VO_2peak observed in the AeT group confirms the effectiveness of training WL close to AeT. Since every patient of the RPE group was able to reach this level of training intensity in the course of the programme, the choice of AeT as lower limit of the optimal training zone seems reasonable, as it is effective and can be proposed even to less fit patients.

Comparing these results with those of a prescription based on percentage peak exercise variables we found that:

In our clinical setting the %HR_peak method is absolutely inadequate owing to the broad overlapping between individual AeT and AnT values, which prevents the definition of acceptable upper and lower limits for exercise intensity optimization.

The %HRR method works better, but the upper limit should be reset to 50% to avoid overtraining of a significant proportion of patients.

Even better, the use of %WL_peak method with a cut-off value of 64% enables effective and safe treatment of about 90% of subjects.

As mentioned previously, when chronotropic reserve is very reduced, among the objective methods of exercise prescription we think that the %WL_peak method should be preferred to %HRR method in view of the very low reliability of HR as indicator of effort intensity in these patients.

A few considerations should be borne in mind for the assessment of differences between our data and conclusions of previous studies addressing this issue. Exercise training intensity is usually evaluated with reference to the concept of anaerobic threshold. However, this term has been used very often as synonym of the first and second ventilatory threshold: this is a source of growing conceptual and methodological confusion and can make comparisons among different studies very difficult, as recently outlined by Binder et al.²⁷

Chaloupka et al.⁸ and Tabet et al.⁹ proposed values between 70 and 80% of HRR as upper limits for training intensity in patients on beta-blockers. These values are notably higher than our cut-off value for this parameter. However, our data arise from direct measurement of the true AnT, whereas their optimal criterion was a training intensity corresponding to HR at first ventilatory threshold±10%. Limitations and potential pitfalls of such a methodological approach have already been pointed out: in some patients the lower limit may be considerably below AeT, while the upper limit may correspond to the peak WL. Very different conclusions were drawn by Wonisch et al.,⁷ who studied the influence of beta-blockers on percentage HR prescription methods in healthy males. Using AnT as upper limit for safe exercise training this author demonstrated that the usual upper values that are considered safe for %HR_peak and %HRR are too high for patients on beta-blockers. This is consistent with our findings.

Finally, we used the direct measurement of training WL to evaluate training intensity instead of a ‘surrogate’ load indicator, as HR has many limitations in patients taking beta-blockers.

In conclusion, our findings suggest that self-regulation of exercise training intensity between grades 4 and 5 of the 10-point category-ratio BORG scale is effective and safe in the initial phase of the rehabilitation programme in patients on beta-blockers after CABG. As some patients tend to increase effort intensity as the programme continues and reach training WL very close to AnT, we think that after the initial phase subjective methods should be integrated with objective indices. According to our data, the %HRR or %WL_peak method with upper limits of 50% and 65%, respectively, can prevent the subject from exceeding AnT during exercise training.

Footnotes

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Conflict of interest

None.

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