RESPIRATORY AND CARDIAC EFFECTS ON VENOUS RETURN

Cardiac activity and respiration markedly alter the venous flow patterns in the major thoracic systemic veins. Cardiac activity (vis à fronte) is associated with two periods of increased and two periods of decreased venous flow, corresponding to changes in atrial pressure. Respiratory activity predominates in most instances, however, producing a large increase in venous return during inspiration. Alterations in respiratory activity produced by procedures such as positive pressure ventilation, airway occlusion, whining, etc., cause much greater changes in venous return and cardiac output than do changes in posture or passive tilting. Excitement, exercise, and pentobarbital anesthesia can produce marked changes in venous return. Pentobarbital can increase the percentage of blood returning to the right heart via the inferior vena cava. Gravity apparently is relatively unimportant in returning blood to the heart. © 1969.