From a recent study:
CRF, as assessed by maximal oxygen uptake (VO2max) during exercise testing, was measured at baseline in 1950 middle-aged men (mean age 52.6 years, SD 5.1) from the Kuopio Ischaemic Heart Disease (KIHD) study.
During average follow-up of 19.5 years, there were 305 incident AF cases (annual AF rate of 65.1/1000 person-years, 95% confidence interval [CI] 58.2–72.8). Overall, a nonlinear association was observed between CRF and incident AF. The rate of incident AF varied from 11.5 (95% CI 9.4–14.0) for the first quartile of CRF, to 9.1 (95% CI 7.4–11.2) for the second quartile, 5.7 (95% CI 4.4–7.4) for the third quartile, and 6.3 (95% CI 5.0–8.0) for the fourth quartile. Age-adjusted hazard ratio comparing top vs bottom fourth of usual CRF levels was 0.67 (95% CI 0.48–0.95), attenuated to 0.98 (95% CI 0.66–1.43) upon further adjustment for risk factors. These findings were comparable across age, body mass index, history of smoking, diabetes, and cardiovascular disease status at baseline.
Improved fitness as indicated by higher levels of CRF is protective of AF within a certain range, beyond which the risk of AF rises again. These findings warrant further replication.