The aim of this study was to quantify changes in cardiorespiratory fitness in patients receiving cardiac rehabilitation in the UK. The results of this study suggest that the supervised exercise training prescribed as part of UK cardiac rehabilitation does not produce changes in cardiorespiratory fitness comparable with international studies. It also appears that changes in fitness vary greatly between UK cardiac rehabilitation centres, all of which adhere to the same practice guidelines, and that the assessment protocols used to measure change in fitness may account for at least some of this heterogeneity.
Compared with a recent meta-analysis of international studies, which reported a mean increase of 1.55 METs, the overall increase reported here (0.52 METs) is very conservative. The meta-analysis was based on treadmill test data, the gold standard assessment for cardiorespiratory fitness, and concluded that the treadmill test protocol was a significant source of the between-trial heterogeneity. There are very few comparable UK studies of change in fitness due to cardiac rehabilitation, but survey data suggest this gold standard measure is rarely used in routine assessment of patients both pre- and post-cardiac rehabilitation. One study, which was not included in the recent meta-analysis due to omissions in data reporting, demonstrated a 90 s increase in treadmill time for CABG patients assessed using the Bruce protocol; the same protocol used to assess as post-MI patients in centre A. This is equivalent to an increase of approximately 0.60 METs, which is less than the value reported here. This was despite patients receiving 16 supervised exercise sessions compared with only eight in the present study. One reason for the larger increase in fitness observed in the present study may be differences in the clinical characteristics of the patient groups. Post-MI patients increased their fitness by more than elective revascularisation patients in our recent meta-analysis (d=2.08 vs d=0.97). Both these effect sizes are well in excess of that reported presently (d=0.77), but our estimate is actually quite similar to that which we previously reported for change in studies using the Bruce treadmill protocol (d=0.79). The Bruce protocol is characterised by large increments and may not be the most suitable protocol for the assessment of older patients, such as cardiac rehabilitation recipients who often present with orthopaedic co-morbidities.
Treadmill testing is also costly and time consuming, which has lead to many UK cardiac rehabilitation centres employing field-based estimates of cardiorespiratory fitness. There are relatively few data regarding the value of incremental cycle ergometry in assessing cardiac rehabilitation patients. Exercise test performance during cycle ergometry appears to be sensitive to changes in fitness due to cardiac rehabilitation and can provide important prognostic information. Di Valentino et al reported differences of 148 (±47) versus 124 (±38) W between surviving and non-surviving cardiac rehabilitation patients. They also reported an improvement in work capacity of 24 W; more than twice that observed in the patients from centre B (10.4 W). Differences in the test protocol may account for some of the variation in improvement, but the very large differences in exercise training seem more likely. Di Valentino et al describe in detail the exercise training protocol in this German rehabilitation programme as comprising a 4-week build-up phase with daily rehabilitation activities taking up to 3 h per day, then a consolidation phase of 8 weeks, exercising 3×2 h per week. The total exercise dose over 12 weeks was, therefore, 132 h (84 intense plus 48 h consolidation). In comparison, patients at centre B received either 1- or 2-hourly sessions over a maximum of 8 weeks, equating to only 8–16 h total exercise.
The 6MWT is commonly used in the assessment of patients with heart failure, but few data are available in typical cardiac rehabilitation patients and we found no data comparable to those for changes in fitness due to exercise programmes like those reported here. Even in patients with heart failure, the 6MWT does not appear to be as sensitive to changes in fitness as the ISWT; this lack of sensitivity may explain the small effect size for change in test performance observed here.
The most commonly used field test in UK cardiac rehabilitation centres is the ISWT, and a number of studies have reported changes in ISWT performance of 60–100 m due to cardiac rehabilitation. Little has been reported on the potential dose–response between exercise and changes in fitness test performance using this protocol. Arnold et al recently reported no difference in a sample of cardiac patients exercising once or twice per week, but their results are difficult to interpret due to differences in test performance at baseline. A more recent study in patients with very similar baseline scores also reported no differences in training response of patients exercising either once or twice per week. Of interest, this study is one of the few to report ISWT performance in METs as opposed to the distance walked during the test. This detail may be of some importance and deserves elucidation.
The ISWT, as the name implies, is an incremental test in which the patient is required to walk faster during each sequential stage. The use of distance to describe fitness test performance is clearly useful in the clinical setting. Distance is a metric readily understood by practitioners and patients and changes in shuttle walking tests can be easily monitored and expressed as change in m. The magnitude of change in walking distance is commonly reported to be around 100 m in patients receiving outpatient cardiac rehabilitation. Cardiorespiratory fitness itself, however, represents the ability to produce and maintain a given work rate, not a work capacity. From an exercise physiology perspective therefore, it would be more correct to express ISWT performance as walking speed (m/s or km/h), or better still, as estimated Vo2peak (ml/kg/min) or METs. This practice is uncommon within the scientific literature and to our knowledge only two studies have reported such metrics. There are additional drawbacks with the use of distance as an expression of an individual's fitness. While study results are often highly statistically significant (p<0.01), there is little evidence of power analyses in any of the published data. Due to this, such changes have been interpreted as clinically significant despite no clinically meaningful lower cut-off being available. Like any fixed stage time incremental protocol, there are many more shuttles per level in the latter stages of the test than there are early on. This means that changes in distance walked in excess of 100 m can be attained without the need for and individual to increase walking speed (work rate). Changes in walking speed (and therefore estimated METs) are therefore commonly much smaller than changes in distance walked, with many patients not increasing work rate (fitness) at all. In the quest for easily interpretable values and statistical significance, it appears that some authors have neglected to assess whether the changes in distance walked during the ISWT represent a clinically meaningful improvement in cardiorespiratory fitness.
Our data suggest that patients attending UK cardiac rehabilitation centres can only expect a 0.59 MET increase in their cardiorespiratory fitness over a typical 6–8-week programme. This value may be slightly smaller if their fitness is assessed using the ISWT (0.54 METs). These values represent approximately one third the improvement in fitness reported in the literature. While testing modality may account for some of this difference, it is of interest that even in the group who improved most in the present study (post-MI patients tested on a treadmill), the magnitude of change was less than half the value synthesised from international studies.
The starkest difference between the studies analysed in the recent systematic review and the present groups is the total volume of exercise completed during outpatient cardiac rehabilitation. We previously found that total number of exercise bouts was a significant mediator of change in fitness. Using a median split of the number exercise sessions, we have previously reported greater gains in fitness for patients receiving >36 exercise sessions than in those receiving 36 or less. Brodie et al reported that UK patients undertake a mean of 11.6 exercise sessions. In common with the magnitude of change in fitness, the UK exercise dose is also one third that reported in the literature. Patients in the present study received a modal value of 8 exercise sessions (range 6–16); the lower end of this range is similar to the exercise prescription which had no significant impact on clinical outcomes in RAMIT.
There is a clear dose–response between increases in fitness and reductions in mortality. Given that a 1 MET increase in fitness is needed to elicit a 12% reduction in mortality, it seems unsurprising therefore that the UK cardiac rehabilitation services examined in RAMIT did not significantly reduce mortality.
Strengths and Limitations
The strengths of the present study lie in the large sample size and the inclusive nature of the study population afforded by the retrospective design. Conversely, a major limitation is the lack of any control group. The cohort design means we cannot quantify how much of the reported improvement in fitness is due to cardiac rehabilitation per se. There is evidence of some spontaneous recovery in fitness in patients following revascularisation and MI, suggesting that the current study is likely to overestimate the real effect that cardiac rehabilitation has on patients' cardiorespiratory fitness. The use of patient records allowed us to quickly gather a large sample without the bias of requiring consent to participate in a research study; it was also a limitation as there were differences in the recording methods used between centres. All centres described the testing protocols as 'symptom limited' and all included attainment of 85% peak predicted maximum heart rate as termination criteria. While not reported here, patient records indicated that volitional termination was by far the most common reason for stopping the incremental exercise tests. All centres reported that they applied the ISWT according to national guidance, but this was often performed by a variety of individuals (nurses, physiotherapists, exercise physiologists) within each centre. Our retrospective design does not allow us to standardise test variables such as 'encouragement' or 'patient motivation' either within or between centres.
For the purpose of this study we only included patients with complete data necessary for this analysis. This meant the rejection of >1200 patient records due primarily to having either incomplete or insufficient data, but also due to the patients' failure to complete the rehabilitation programme or to return for retesting.