Figure 3. Effects of cardioselective β-adrenergic blockade (0.2 mg kg⁻¹ atenolol) and double autonomic blockade (atenolol + 0.04 mg kg⁻¹ atropine) on respiratory sinus arrhythmia during controlled frequency breathing from 0.25 to 0.05 Hz in the 40 deg tilt position.

n = 10. β-blockade enhances respiratory sinus arrhythmia at all breathing frequencies, not just at higher breathing frequencies. This undermines the contention that respiratory sinus arrhythmia is a purely vagal phenomenon (redrawn from Taylor et al. 2001).