The key finding of our analysis is that a 12-week Tai Chi exercise program improves sleep stability as assessed by a novel, fully automated, ECG-based sleep spectrogram technique. This improvement was found in patients with chronic heart failure already on maximized medical management. Furthermore, an interesting correlation between improved sleep stability and improved quality of life was demonstrated. In the control group, there was a possible worsening of sleep stability measures. With traditional heart rate variability measures, there was a trend towards increased short-term heart rate variability during sleep in the Tai Chi group, suggesting a possible improvement in cardiac vagal modulation. While there were no changes in resting catecholamines, a trend in improvement in serum BNP suggested possible decreases in neurohumoral activation.
An increase in stable sleep may be a robust marker of overall improved sleep quality. For patients with heart failure, unstable sleep may cause excessive hemodynamic stress through respiratory and non-respiratory mechanisms, and may be associated with ventricular arrhythmias [30
]. In contrast, stable sleep is associated with stable respiration and hemodynamics and may protect from triggered arrhythmias, thus demonstrating potential restorative effects for the deranged cardiopulmonary physiology of heart failure. Improvements in sleep stability may, therefore, promote acute beneficial changes in hemodynamic indices during sleep, as well as chronic changes in the overall hemodynamic profile. This study suggests that Tai Chi may be a potential adjunctive therapeutic option for patients with heart failure that can improve sleep stability. Sleep stability, as reflected in CAP/non-CAP sleep, has not previously been described with Tai Chi, although one prior clinical trial has reported improvements in sleep quality after 24 weeks of Tai Chi as compared to conventional low-impact exercise. Improvements were seen in self-reported sleep quality (Pittsburgh Sleep Quality Index and Epworth Sleepiness Scale), as well as decreased sleep latency and increased sleep duration [15
]. Changes in sleep quality have also been demonstrated with other meditative practices, such as mindfulness-based stress reduction and yoga [31
Mechanisms of improvement in sleep stability with Tai Chi are speculative but biologically plausible. Certainly, exercise has been shown to improve sleep quality. In both humans and animal models, regular physical activity increased EEG delta power during stage 3/4 NREM sleep and improved overall sleep quality [33
]. Exercise has also been shown to affect the neurohormonal axis, which could contribute to improved stable sleep state. Increased physical activity has also been shown to improve sleep hygiene [33
]. In addition, the meditative practice inherent in Tai Chi may also contribute to increased sleep stability. Reduced respiratory chemoreflex sensitivity [38
] and increases in baroreflex sensitivity have been described in practitioners of yoga and meditation[42
]. These improvements in respiratory control could lead to improvements in sleep-disordered breathing. Furthermore, Tai Chi includes a component of respiratory rhythm training with slow, deep, diaphragmatic breathing, which could play a role in retraining breathing patterns during sleep. Finally, the Tai Chi exercise program may have indirectly promoted better sleep hygiene, improved mental health (e.g., decreased anxiety/depression), and provided a social group for participants, which each may have contributed to improved sleep quality and increased spectrographic sleep stability. Of note, we also observed a possible deterioration of spectrographic sleep stability in the control group. Possible explanations include natural progression of disease, night-to-night variability in sleep quality, sleep stability, and disordered breathing, and group variability in sleep hygiene.
There may be an association between sleep stability and quality of life, with stable sleep (high-frequency cardiopulmonary coupling) correlating with better quality of life, and unstable sleep (low-frequency cardiopulmonary coupling) correlating with worse quality of life. Interestingly, unstable EEG sleep is seen in several chronic disease states, including depression, fibromyalgia, primary insomnia, sleep apnea and epilepsy [44
]. This disrupted sleep is typically associated with a worse perceived quality of life [48
]. The preliminary findings here suggest that Tai Chi-related improvements in patients may, in part, be mediated by effects on sleep.
We recognize several inherent study limitations. First, this was a retrospective analysis of existing data and polysomnographic evaluation of patients was not done in the original study. Thus, changes in conventional sleep stages, especially slow-wave sleep, and the exact reasons for sleep disruption in the study subjects are not known. In addition, the sample size is small and likely underpowered to detect relationships between change in sleep stability measures and markers of neurohormonal activation, or to detect changes in standard HRV measures or in serum BNP. For example, in the original cohort of 30 patients,[21
] we reported a significant improvement in BNP in the Tai Chi versus control group (p
= 0.03). With the subset of 18 patients in this analysis, trends toward improvement in BNP still exist (p
= 0.08); however, power becomes limited. As with the original study, we also cannot determine what component of Tai Chi is responsible for the observed benefits. Physical activity may have important effects, and it is unclear what added role meditation and relaxation may play. Finally, we have no data on direct hemodynamic effects during sleep or respiratory chemoreflexes, so that possible mechanistic effects remain speculative. Despite these limitations, the data presented provide valuable preliminary information for generating testable hypotheses on the potential effects on sleep of Tai Chi exercise in patients with heart failure.
In conclusion, we report preliminary findings suggesting that a 12-week Tai Chi exercise program may improve sleep stability in patients with heart failure on maximal medical therapy. As stable sleep has the potential to improve the sleep-related hemodynamic profile in heart failure and have at least theoretical cardioprotective effects, further evaluation of this safe approach seems justified. Further research should include polysomnographic testing to document sleep stages and patterns of sleep disruption, active controls (e.g., conventional exercise) in order to better understand component effects of meditative exercise, and outcomes directly assessing cardiac function and hemodynamic changes during sleep. In addition, further characterization of the study population, such as heart failure with and without Cheyne-Stokes respiration and central sleep apnea, may provide insights into the mechanisms of the effects of Tai Chi on sleep.