QT restitution properties of middle-aged women with different exercise capacities

Framingham Study data indicate an incremental risk of cardiovascular death in relation to low exercise capacity in women. The reason for death is still not clear.

QT restitution properties in 80 middle-aged women were investigated to confirm whether the cardiac restitution property was affected by the exercise capacity. Exercise tests were performed according to the Bruce protocol. Seventy cases were divided into the low-exercise capacity group (LEC group) and high-exercise capacity group (HEC group) by median exercise capacities. Sequential QT intervals and their preceding TQ intervals were measured, and the QT restitution curve (QTRC) was constructed. Two exponential equations were used to match the data and calculate the maximum slope (Smax) of QTRC, respectively.

With elevation in the exercise level, the women in the LEC group had a higher change rate of QT intervals (41 ± 10 vs 29 ± 3 ms/Met, P = .001) and TQ intervals (46 ± 12 vs 41 ± 7 ms/Met, P = .046); and the ratio of QT interval alterations to TQ interval alterations increased (0.41 ± 0.09 vs 0.36 ± 0.07, P = .003). The Smax of the QTRC in the LEC group was higher than that in the HEC group (1.43 ± 0.44 vs 1.13 ± 0.34, P = .002). There was an inverse relationship between Smax and exercise capacity (r = -0.43, P = .001).

Middle-aged women with low exercise capacity have steeper QTRCs than those with high exercise capacity, denoting a more unstable alternation of QT interval with elevation in exercise level.