VO(2) reserve and the minimal intensity for improving cardiorespiratory fitness

Regression to the mean. A threat to exercise science?

Regression to the mean (RTM) can bias any investigation where the response to treatment is classified relative to initial values for a given variable without the use of an appropriate control group. The phenomenon and resulting errors of interpretation have been recognised by clinicians in a number of disciplines. The causes of RTM include both intra-individual variance and measurement error. The magnitude of RTM can be estimated quite simply, given a knowledge of intra- and inter-individual variance. RTM can be avoided by using a fully controlled experimental design. Difficulties can also be minimised by making duplicate measurements prior to the experimental manipulation, the first measurement serving for classification, and the second (with randomly distributed variance) allowing an assessment of the response to treatment. Less satisfactorily, surrogate measurements (for example, plasma volume for maximal oxygen intake [VO2(max)]) can assess the bias introduced by an initial non-random sorting of study participants. The impact of RTM on the design and interpretation of investigations has as yet received little consideration by exercise scientists and sports physicians. The response to training is often related to initial measurements of a dependent variable such as heart size, ST segmental depression, fitness or level of physical activity. In particular, analyses of this type have been adduced to support the belief that the response to aerobic training is inversely related to an individual's VO2(max). In fact, RTM may account for a major part of this apparent relationship.

New insights in the prescription of exercise for coronary patients

Prescribing exercise for cardiac patients is comparable in many ways to prescribing medications; that is, one recommends an optimal dosage according to individual needs and clinical status. Recent research has shown that it is more accurate to prescribe exercise as a percentage of the oxygen uptake reserve (VO2R), which is the difference between resting and maximal or peak oxygen consumption, rather than as a percentage of the VO2 max. Moreover, it appears that a minimum of 1600 kcal/week of leisure-time physical activity may halt the progression of coronary artery disease, whereas regression may be achieved with a gross energy expenditure of 2200 kcal/week. Upper body and resistance training have also been shown to be safe and effective for clinically stable patients. Aerobic capacity serves as an independent predictor of all cause and cardiovascular mortality in patients referred to an outpatient cardiac rehabilitation program, with each 1 metabolic equivalent increase in aerobic fitness conferring an approximate 10% reduction in mortality. The goal of preventing recurrent cardiac events is, to a large extent, based on sustained compliance to multifactorial interventions, which can be influenced by numerous socioeconomic and clinical variables, and enhanced by home-based or group cardiac rehabilitation programs that are designed to circumvent or attenuate barriers to participation and adherence, so that many more individuals may realize the benefits that secondary prevention can provide.

Cardiac rehabilitation secondary prevention programs

Contemporary cardiac rehabilitation programs are more accurately described as "secondary prevention centers." They offer comprehensive care for the patient with cardiovascular disease, resulting in decreased mortality, improvement of most cardiac risk factors, and an enhanced quality of life. Although overall participation has increased with enhanced recognition of the importance of secondary prevention, 80% of eligible patients still do not participate, in part due to lack of insurance reimbursement. This rate can be significantly increased by specific endorsement from the physician.

Is brisk walking an adequate aerobic training stimulus for cardiac patients?

Walking is the most common aerobic training modality utilized in cardiac rehabilitation programs. However, it remains unclear whether or not brisk walking is of a sufficient intensity to improve aerobic fitness in this population. In this study, we investigated whether men and women with coronary artery disease can achieve an exercise intensity that is sufficient to induce a training effect, ie, a training heart rate (THR), defined as >/= 70% of measured maximal heart rate (HRmax), via brisk walking on a flat surface. One hundred forty-two outpatient volunteers from the William Beaumont Hospital Cardiac Rehabilitation Program (Royal Oak, MI) and the University of Wisconsin-La Crosse Exercise and Health Program (La Crosse, WI) were asked to walk one mile as briskly as possible on measured tracks. Heart rate was monitored throughout the walk via radiotelemetry. The percentage of patients within each gender and phase of rehabilitation who attained a THR were assessed using peak or symptom-limited exercise testing to determine the HRmax. All of the women and 90% of the men achieved a THR, averaging 85 +/- 8% and 79 +/- 10% of HRmax, respectively (mean +/- SD). There was no difference in the percentage of phase II or phase III cardiac rehabilitation program patients who achieved a THR. These findings suggest that brisk walking is of a sufficient intensity to elicit a THR in all but the most highly fit patients with coronary disease. Thus, physicians and allied health professionals can prescribe brisk walking on a flat surface to their cardiac patients with confidence that this intensity will achieve cardiorespiratory and health benefits.

Is there a threshold intensity for aerobic training in cardiac patients?

PURPOSE

Recent guidelines have recommended the use of a percentage of oxygen uptake reserve (VO2R) for prescribing aerobic exercise intensity for cardiac patients. Moreover, these guidelines suggest that a threshold intensity may exist, below which no improvement in peak oxygen uptake (VO2peak) occurs. The purpose, therefore, was to translate the intensity of aerobic exercise in previous training studies using cardiac patients into %VO2R units, and determine whether a threshold intensity exists.

METHODS

Twenty-three studies, using 28 groups of aerobically trained cardiac patients, were identified in which VO2peak was measured before and after training by gas exchange. Intensity of exercise was variously described as a percentage of VO2peak, percentage of peak heart rate (HRpeak), percentage of heart rate reserve (HRR), or percentage of peak workload. These intensities were translated into equivalent units of %VO2R.

RESULTS

Of the 28 groups of patients, three failed to show significant improvements in VO2peak. These groups exercised at intensities corresponding to 47-55% of VO2R. However, six other groups exercised at comparable intensities (i.e., 42% to 55% of VO2R) and experienced significant increases in VO2peak. Other confounding variables in these studies were similar, including the initial VO2peak of the subjects, suggesting that the failure of three groups to significantly improve aerobic capacity was due to their small sample size.

CONCLUSION

No threshold intensity for aerobic training was identified in cardiac patients, with the lowest intensity studied being approximately 45% of VO2R. It is possible that intensities below this value may be an effective training stimulus, especially in extremely deconditioned subjects, but further research is needed to test that possibility and to determine whether a threshold exists.