BACKGROUND: Atrial fibrillation (AF) is the most common sustained dysrhythmia and appears to be an independent predictor of sudden cardiac death. The irregular ventricular rhythm contains both linear and non-linear patterns; however, it remains unclear whether vagally mediated effects are present within these patterns. OBJECTIVE: We sought to determine if (1) power spectral analysis of heart rate can detect changes in vagal activity in patients with AF and (2) if the vagus modulates ventricular response during AF. METHODS: Time and frequency domain parameters of heart rate variability (HRV) were calculated during forced vagal oscillations at 0.125 and 0.25 Hz imposed by neck suction and deep breathing in five AF patients. RESULTS: There was a significant increase in SDRRI during deep breathing/neck suction combined compared to baseline (p=0.01) and deep breathing (p=0.03). Neck suction significantly increased SDRRI compared to baseline (p=0.03). Deep breathing/neck suction significantly increased spectral power compared to baseline (p=0.02) and deep breathing (p=0.03). Neck suction significantly increased spectral power compared to baseline (p=0.03). Deep breathing did not significantly increase HRV compared to baseline (p>0.20). In addition, SDRRI and spectral power were significantly correlated during deep breathing (r=0.91, p=0.03) and deep breathing/neck suction combined (r=0.92, p=0.02). INTERPRETATION: These data suggest that (1) power spectral analysis can detect vagal influences on heart rate in AF patients, and (2) oscillatory vagal maneuvers produce ventricular entrainment during AF.