T-wave alternans patterns during sleep in healthy, cardiac disease, and sleep apnea patients

T-wave alternans (TWA) activity is known to be a function of heart rate and condition, as well as perhaps physiological state. A recently published nonparametric nonstationary TWA analysis method has been shown to reject nonstationary noise accurately using phase-randomized surrogates and has been shown to estimate TWA accurately. This new method was evaluated on multiple databases over a range of heart rates and in healthy subjects, cardiac disease patients, and obstructive sleep apnea (OSA) patients. We hypothesized that TWA would be lower than previously reported when measured with our new technique and that higher levels of TWA would be observed in OSA patients when compared with healthy subjects.

Five databases were analyzed, as follows: (1) healthy subjects from PhysioNet's Normal Sinus Rhythm Database, (2) arrhythmia patients from PhysioNet's Chronic Heart Failure Database and (3) PhysioNet's Sudden Cardiac Death Database, (4) OSA patients from PhysioNet's MIT-BIH Polysomnographic Database, and (5) 85 subjects from a private Sleep Apnea Database. T-wave alternans magnitudes were calculated for 7 heart rate decades (intervals of 10 beats/min (bpm) between 40 and 110 bpm) for each database. The Mann-Whitney U test and the 2-sample Kolmogorov-Smirnov test were applied to test for significant differences between data from each database in each heart rate decade interval.

In the healthy population, TWA activity level tended to increase with heart rate. Moreover, there appeared to be an unexpected nadir in TWA activity around 60 to 70 bpm and a small but significant rise in TWA above and below these heart rates. The rise in TWA at lower heart rates has not been previously reported, to our knowledge. We also observed that TWA is unexpectedly lower in OSA patients and did not increase with heart rate.

Although the physiological mechanisms underlying our observations are unclear, there may be clinical implications for TWA testing, particularly at low heart rates, a previously overlooked aspect of TWA.