Respiratory sinus arrhythmia: laws derived from computer simulation

Dynamic mathematical relations describing respiratory sinus arrhythmia were derived through analogue computer simulation. If a signal proportional to thorax circumference is fed into the analogue computer, it calculates with differential equations the complex heart rate changes in real time and records them along with those of the real heart. Close correspondence of the predicted and actual changes of heart rate for a wide variety of modes of breathing, and for different individuals, proves the validity of the nonlinear differential equations describing the phenomenon. The respiratory effects are shown to be caused by two separate reflexes each producing biphasic heart rate transients in the same directions. The observed effects are the result of superposition of those transients. Previous paradoxical results in attempting to relate heart rate to respiration on a steady-state, nondynamic basis are thus explained. The laws indicate that stretch receptors and not hemodynamic or central nervous factors initiate the changes in heart rate. The analysis also allows heart rate effects of exercise and emotional stresses to be more precisely perceived, as clearly separated from respiratory effects. Submitted on June 16, 1959