In UK cardiac rehabilitation programmes, exercise training is often set at a percentage of maximal heart rate or heart rate reserve, either predicted or measured. Problems may arise when using this method for chronic heart failure (CHF) patients who often have chronotropic incompetence and are treated with beta blockers. A safer approach is to use cardiopulmonary exercise testing to prescribe training below the ventilatory threshold, thus ensuring that the exercise is moderate. The aim of this study was to determine whether British Association for Cardiac Rehabilitation (BACR) heart rate guidelines prescribe moderate intensity exercise for CHF patients. The only target heart rate range to prescribe exercise below the ventilatory threshold was 60–80% measured maximum heart rate. Target heart rates calculated from predicted maximum values were higher than those from measured values, and the heart rate reserve method resulted in the highest target heart rates. Cardiac rehabilitation exercise practitioners should be aware that these methods may well result in CHF patients performing heavy rather than moderate exercise.
Introduction
Exercise training is recommended for chronic heart failure (CHF) patients in New York Heart Association (NYHA) Class II and III,1 and can be successfully incorporated into general cardiac rehabilitation programmes in the UK.2 Prescribing exercise of an appropriate intensity can be a challenge. Guidelines for defining exercise intensity include percentages of maximum oxygen uptake (VO2max) or heart rate (HRmax), and VO2 reserve (VO2R) or heart rate reserve (HRR). However, there is considerable variability among methods of expressing exercise intensity in elderly subjects and cardiac patients, including CHF.3-5 In addition, chronotropic incompetence is common in CHF, and the widespread use of beta blockers, which reduce resting and maximum HR, has confounded the issue further.6
The European Society of Cardiology recommends that cardiopulmonary exercise testing should be used to objectively evaluate functional capacity and prescribe exercise.1 However, this is not feasible on many cardiac rehabilitation programmes, and although individual clinical history and symptoms are taken into account, exercise is commonly prescribed from generic HR guidelines. The British Association for Cardiac Rehabilitation (BACR) recommends target HR ranges of 60–80% predicted HRmax or 40–70% HRR, combined with rating of perceived exertion (RPE) of 12–15 on the Borg 6–20 scale.7
The thresholds that determine exercise intensity occur at a different percentage of maximal capacity in individuals,8 hence a group of CHF patients exercising at 50% VO2max may be working at different relative intensities. For that reason, intensities based solely on a percentage of HRmax or HRR are also likely to impose variable cardiovascular and metabolic demands in individuals,9 including CHF patients.10
The ventilatory threshold marks the transition from moderate sustainable exercise to heavy exercise, and generally occurs between 40% and 60% VO2max. The respiratory compensation point marks the transition from heavy to severe exercise, which will quickly result in exhaustion, and generally occurs between 70% and 90% VO2max (figure 1). There is some concern that exercising above the ventilatory threshold increases left ventricular wall stress in CHF,11 and may induce deterioration in left ventricular function.12 Hence, it has been suggested that an intensity at or below ventilatory threshold is appropriate for CHF.13 It is questionable whether conventional HR guidelines are effective at prescribing exercise at this intensity, particularly in cardiac patients, including CHF.
The aim of this study was to determine whether current HR guidelines prescribe moderate intensity exercise for CHF patients.
Methods
Eighteen clinically stable CHF (NYHA Class II–III, left ventricular ejection fraction [LVEF] <40%) patients were recruited from Eastbourne District General Hospital. Patients in atrial fibrillation and those whose medication had been changed during the preceding four weeks were excluded from the study. They performed an incremental exercise test, with increments of 10 W/min until volitional exhaustion, on a cycle ergometer (Lode Corival, Groningen, The Netherlands). All patients had previously performed at least one similar exercise test to exhaustion. HR and respiratory gas exchange were recorded throughout. VO2peak was expressed as the highest value from a 30 second moving average during the final stage, or within 30 seconds of completion, of the exercise test. Ventilatory threshold was identified by the V-slope method and confirmed by plots of the ventilatory equivalent and end tidal pressure for oxygen and carbon dioxide. Respiratory compensation point was determined as the point when ventilation increased disproportionately to carbon dioxide output, confirmed by plots of the ventilatory equivalent and end tidal pressure for oxygen and carbon dioxide.8
Target exercise training HRs were calculated according to BACR guidelines (60–80% HRmax and 40–70% HRR) from both measured and predicted HRmax values:
- HRmax = highest HR recorded during the incremental exercise test
- Predicted HRmax = 220 – age, with a further 30 beats/min deducted for patients on beta blockers, according to BACR guidelines
- HRR = HRmax– resting HR
- Predicted HRR = predicted HRmax – resting HR.
- The prevalence of chronotropic incompetence, defined as <80% age-predicted maximum HR or HRR,6 was also calculated.
Statistical analysis was performed by SPSS for Windows (v15). Paired t-tests were used to see if there was a difference between predicted and measured values for HRmax and HRR, and target HR derived from them. One-way ANOVA followed by post hoc tests with the Bonferroni adjustment were performed to see if there was a difference between the methods for calculating upper and lower BACR target HR ranges. Data are presented as mean ± standard error of the mean (SEM) unless otherwise indicated. The level of significance was set at p<0.05.
Results
Patient characteristics are presented in table 1. VO2peak was 1240 ± 100 ml/min (14.4 ± 0.88 ml/kg/min), peak work rate was 80 W, and all patients achieved a peak respiratory exchange ratio >1.0 (mean 1.12 ± 0.02, range 1.01–1.24). Details of ventilatory threshold are shown in table 2. Respiratory compensation point was identified in nine patients and HR was on average 10 ± 2 beats/min higher than HR at ventilatory threshold.
Predicted HRmax was significantly higher than measured HRmax, therefore, target training HR were significantly higher when calculated from predicted HRmax (table 3). There were also significant differences in the lower and upper HR targets between the %HRmax and %HRR methods (p<0.01).
Target HR ranges calculated by the different methods are illustrated in figure 2, which shows that the only method to prescribe exercise entirely below ventilatory threshold was 60–80% measured HRmax.
Table 4 shows the number of patients meeting the criteria for chronotropic incompetence.
Discussion
The aim of this study was to investigate if current exercise training HR guidelines correspond to moderate intensity exercise training for CHF patients. The only target HR range to prescribe exercise below the ventilatory threshold was 60–80% measured HRmax. Target HR ranges calculated from predicted maximum values were significantly higher than those calculated from measured maximum values and may result in overestimation of exercise intensity prescription in CHF. In addition, the two methods recommended by BACR did not correspond: 40–70% HRR was significantly higher than 60–80% HRmax.
HRmax measured during the exercise test was 12% lower than predicted HRmax after adjusting for beta-blockade. Although VO2peak values are generally 10% lower in cycle compared with treadmill exercise, due to the lower exercising muscle mass, HRmax does not differ between exercise modes in CHF.14 All patients achieved a maximal respiratory exchange ratio value >1.0, confirming adequate effort.15 Nevertheless, patients may have been limited by local muscular fatigue or dyspnoea before the cardiovascular system was maximally stressed, or by chronotropic incompetence.16 In agreement with previous findings,6 using 80% age-predicted HRR rather than 80% age-predicted HR as a cut-off point results in more patients being identified with chronotropic incompetence, and patients taking beta blockers were more likely to meet the criteria. An arbitrary deduction of 30 beats/min without consideration of different types and doses of medication, or age and individual response, is unlikely to accurately reflect the HR-lowering effect of beta blockers. Anti-arrhythmic medication may also reduce HR,7 but the extent of the effect remains unclear. Although beta-blockade in CHF lowers HR, it does not reduce exercise capacity,17 or limit the benefits of exercise training in CHF.18 Indeed, maximum HR may increase following training, hence, target HR may need to be adjusted as training progresses.19
Exercise training at an intensity no higher than the ventilatory threshold has been recommended, yet the upper end of BACR target HR ranges may well exceed this, thus prescribing heavy rather than moderate exercise. Using 40–70% predicted HRR may exceed the respiratory compensation point, since HR at this threshold was only 10 beats/min higher than at ventilatory threshold. Previous studies have also demonstrated that conventional exercise prescription methods result in HR ranges above ventilatory threshold20 and may compromise left ventricular function.21 Despite the methodological differences in the determination of ventilatory threshold and training intensities, these studies demonstrate the variable HR response in CHF.
This study indicates that predicted HR is an unreliable exercise prescription method for CHF, and that the different BACR target HR calculation methods do
not agree. Exercise practitioners should be aware that the HRR method may prescribe heavy, or even severe, intensity exercise. An incremental cardiopulmonary exercise test to determine ventilatory threshold is recommended to ensure that moderate intensity exercise is prescribed.
Acknowledgements
This research was supported by Eastbourne Hospital Research and Development Fund, and The Peel Medical Trust.
Conflict of interest
None declared.
Key messages
- British Association for Cardiac Rehabilitation (BACR) exercise heart rate guidelines may prescribe heavy rather than moderate exercise
- Predicted heart rate methods are unreliable due to the effects of medication and chronotropic incompetence
- Cardiopulmonary exercise testing is recommended in order to prescribe moderate intensity exercise for chronic heart failure patients
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