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Background: The ventriculophasic response (VR) refers to shortening of sinus cycle length during heart block when a QRS complex is interposed between 2 P waves. Our purpose was to analyze its relationship to respiratory sinus arrhythmia (SA) and to compare VR in relation to paced versus intrinsic QRS complexes., Methods: Patients with advanced heart block had their pacer devices temporarily programmed to ventricular inhibited mode at 30 ppm. In 35 subjects, we analyzed VR and SA before, during and after 3 cycles of deep breathing. In 16 other patients we compared VR in the presence of paced versus narrower intrinsic QRS complexes., Results: The magnitude of P-P interval shortening surrounding QRS complexes during inspiration correlated with SA (r = 0.36, P = 0.03). The prevalence of VR increased from 37% at baseline to 77% of subjects during deep breathing (P = 0.02). The mean P-P interval shortening was greater surrounding intrinsic QRS complexes than paced QRS complexes (3.6 ± 3.6% vs. 1.4 ± 1.1%, P = 0.02). The prevalence of VR increased from 25% during paced rhythm to 56% when intrinsic complexes were present., Conclusion: VR, like SA, increases with deep breathing and likely reflects intact parasympathetic nervous system function. Its increase in the presence of narrower beats suggests it may reflect ventricular synchrony., (©2013, Wiley Periodicals, Inc.)