Aim. Respiratory sinus arrhythmia (RSA) may serve an inherent function in optimizing pulmonary gas exchange efficiency via clustering and scattering of heart beats during the inspiratory and expiratory phases of the respiratory cycle. This study sought to determine whether physiological levels of RSA, enhanced by slow paced breathing caused more heart beats to cluster in inspiration. Methods and Results. In 12 human subjects we analyzed the histogram distribution of heart beats throughout the respiratory cycle during paced breathing at 12, 9 and 6 br/min. The inspiratory period to respiratory period ratio was fixed at approximately 0.5. RSA and its relationship with respiration was characterized in the phase domain by average cubic-spline interpolation of electrocardiographic R wave to R wave interval fluctuations throughout all respiratory cycles. Although 6 br/min breathing was associated with a significant increase in RSA amplitude (RMANOVA, p<0.0001), we observed no significant increase in the proportion of heart beats in inspiration (RMANOVA, p=0.25). Conclusion. Contrary to assumptions in the literature, we observed no significant clustering of heart beats even with high levels of RSA enhanced by slow breathing. The results of this study do not support the hypothesis that RSA optimizes pulmonary gas exchange efficiency via clustering of heart beats in inspiration.