A Simulation Study of Factors Influencing Respiratory Variations in Blood Flow with Special Reference to the Effects of the Phase Delay Between the Respiratory and Cardiac Cycles

Respiratory variations in blood flow are initiated by the changing pressures within the intrathoracic and abdominal cavities, thereby altering the pressure gradients for blood flow into and out of the thoracic cavity. The respiratory venous return variations alter the right ventricular volume which directly affects left ventricular function through ventricular interdependence. Variations in right ventricular output affect, after transmission through the pulmonary circulation, left atrial inflow and hence left ventricular (LV) output. LV output is also directly affected by respiratory variations in the pressure gradient between the left ventricle and the systemic circulation. Any respiration-induced variations in the haemodynamic impedance to blood flow (collapse of the great veins, variations in pulmonary vascular impedance) will also contrib ute to flow variations. The heart rate changes with respiration affecting heart chamber filling and output. The wide variety of factors contributing to respira tory blood flow variations complicates its analysis. However, by enabling the individual factors to be separately considered, model studies provide a convenient method of analysing the effects of the different factors involved.