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

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... . 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 Ϸ30% 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