Letter Regarding Article by Scharf et al, “Diagnosis of Sleep-Related Breathing Disorders by Visual Analysis of Transthoracic Impedance Signals in Pacemakers”
To the Editor:
We congratulate Scharf et al on their innovative and stimulating article about the feasibility of detecting apnea/hypopnea events by analyzing transthoracic impedance signals in pacemaker patients.1 There are some questions we would like the authors to address.
The pacemaker lower rate was programmed to 70 bpm in 50% of patients. It would be of interest to know the percentage of atrial and ventricular pacing during the study because pacing may influence sleep apnea syndrome in selected patients.2 Were there any changes in the apnea-hypopnea index before and during pacing?
The differentiation between the obstructive and central type of sleep-related breathing disorders was based on thoracic and abdominal excursions during polysomnography. Another differentiation between the obstructive and central type of sleep-related breathing disorders would be the occurrence of Cheyne-Stokes respiration pattern, characterized by periodic oscillation of hyperventilation and apnea, which is seen in ≈30% of patients with central sleep-related breathing disorders. Detection of the Cheyne-Stokes respiration pattern by transthoracic impedance measurement is being used successfully in applying adaptive ventilation therapy.3 Did Scharf et al systematically analyze transthoracic impedance signals for the Cheyne-Stokes respiration pattern? Did these findings correspond to the polysomnographic determination of central and obstructive types of sleep-related breathing disorders? Detection and monitoring of the Cheyne-Stokes respiration pattern by transthoracic impedance measurement may be used as a valuable surrogate for the efficacy of cardiac resynchronization therapy in heart failure patients; a recent study showed that cardiac resynchronization therapy improved the apnea-hypopnea index and the Cheyne-Stokes respiration pattern in these patients.4
We thank Dr Sinha and colleagues for their interest and comments on our study.1 The purpose of the study was to evaluate pacemakers for diagnosis of sleep-related breathing disorders, not to test an intervention to reduce them. Therefore, pacemaker parameters were not changed. The apnea/hypopnea index was not related to nocturnal heart rate (r=0.06) nor to percentage of pacing (r=0.13, range 0.1% to 100%, mean 40±40%). Of note, the therapeutic effect of atrial overdrive pacing in patients with sleep apnea has not been duplicated.2
The differentiation between central and obstructive events should not be based on periodicity because obstructive events typically occur periodically as well. In addition, mixed respiratory disturbances (with central and obstructive components) are often observed in patients without heart failure.
Another issue is periodic respiration and Cheyne-Stokes respiration in patients with heart failure. In patients with central sleep apnea, the occurrence of Cheyne-Stokes respiration, especially during the daytime, has been associated with a worse prognosis.3 Interestingly, central sleep apnea patients without Cheyne-Stokes respiration had a prognosis similar to that of age-matched controls in that study.3 Unfortunately, the differentiation between Cheyne-Stokes respiration and Central sleep apnea is not always made.4 Our study was designed to study sleep apnea in pacemaker patients and not Cheyne-Stokes respiration in heart failure patients. This is illustrated by the lack of correlation between the apnea/hypopnea index and ejection fraction in our patients. We agree with Dr Sinha and colleagues that the transthoracic impedance signal may be an ideal tool in cardiac resynchronization therapy to monitor Cheyne-Stokes respiration.
The transthoracic impedance signal used by the pacemaker minute ventilation sensor is obtained by injecting an electrical current across the thoracic tissue. This should not be confounded with respiratory inductive plethysmography, in which no electricity passes through the individual, and which was employed by Teschler et al.5
Scharf C, Cho YK, Bloch KE, Brunckhorst C, Duru F, Balaban K, Foldvary N, Liu L, Burgess RC, Candinas R, Wilkoff BL. Diagnosis of sleep-related breathing disorders by visual analysis of transthoracic impedance signals in pacemakers. Circulation. 2004; 110: 2562–2567.
Pepin JL, Defaye P, Garrigue S, Poezevara Y, Levy P. Overdrive atrial pacing does not improve obstructive sleep apnoea syndrome. Eur Respir J. 2005; 25: 343–347.