29-06-2012, 11:56 AM
Characterizing Differences in Mortality at the Low End of the Fitness Spectrum
Characterizing Differences in Mortality at the Low End of the Fitness Spectrum.docx (Size: 454.43 KB / Downloads: 24)
Abstract
Purpose: A graded nonlinear relationship exists between fitness and mortality with the most remarkable difference in mortality rates observed between the least-fit (first, Q1) and the next-least-fit (second, Q2) quintile of fitness. The purpose of this study was to compare clinical characteristics, exercise test responses, and physical activity patterns in Q1 versus Q2 in apparently healthy individuals.
Conclusions: Reduced physical activity patterns rather than differences in clinical characteristics contribute to the striking difference in mortality rates between the least-fit and the next-least-fit quintile of fitness in healthy individuals.
Introduction
Numerous epidemiological studies in recent years have reported an inverse, graded, and dose-dependent relationship between physical fitness and mortality in individuals with and without cardiovascular disease.[2,9,18,21] Accumulating evidence suggests that this relationship is nonlinear, with the greatest decline in risk of mortality occurring between the least-fit (first, Q1) and the next-least-fit (second, Q2) quintile of fitness.[2,8,18,21,24] These findings have led the current consensus statements on physical activity to emphasize that the greatest health benefits are achieved by increasing physical activity among the least-fit individuals.[20,25] However, factors that may explain the steep mortality gradient at the lowest end of the fitness spectrum have not been explored. Therefore, it remains unknown whether differences in clinical characteristics (reflecting differences in severity of underlying disease), physical activity patterns, other behaviors, environmental, or other factors such as genetics could explain the steep mortality gradient between Q1 and Q2. The purpose of this retrospective study was to compare clinical characteristics, exercise test responses, and physical activity patterns between Q1 and Q2 in apparently healthy individuals.
Methods
Experimental Design and Approach
The study population consisted of 4384 veterans (200 women) referred for treadmill testing for clinical reasons at the Long Beach and VA Palo Alto Health Care System (Palo Alto, CA) from 1987 to 2006. Detailed clinical history, current medications, risk factors, and cardiovascular disease were recorded prospectively on computerized forms.[5,6] The study was approved by the Stanford Investigational Review Board, and all subjects signed written informed consent.
All subjects had normal exercise test results and no history of cardiovascular disease. We excluded individuals with a history of cardiovascular disease, chronic obstructive pulmonary disease, cancer, chronic renal insufficiency, endocrine, liver, or neurological disease, or abnormal exercise test results (defined as exercise-induced angina and/or ST-segment depression ≥1 mm that was horizontal or downsloping during exercise, in recovery, or both).
The population was divided into quintiles of fitness on the basis of metabolic equivalents (METs) achieved. Cutoff points between the categories were set at approximately every 20th percentile of the population to yield similar sample sizes in each quintile.
Exercise Testing
After providing written informed consent, the subjects underwent symptom-limited treadmill testing according to standardized graded[29] or individualized ramp[15] treadmill protocols. Before testing, exercise capacity was estimated by a questionnaire that allowed individualization of the ramp protocol such that maximal exercise capacity was achieved within 8 to 12 min in most subjects.[16] Subjects were encouraged to exercise until volitional fatigue in the absence of symptoms or other signs of ischemia. The use of handrails during exercise was discouraged. Target heart rate (HR) were not used as predetermined end points. A 12-lead ECG was monitored throughout the test. After exercise, subjects were placed in a supine position. Medications were not changed or stopped before testing. The exercise tests were performed, analyzed, and reported according to a standardized protocol and with the use of a computerized database.[22]