Reduced venous return

Phase 1 During phase 1, the increased thoracoabdominal pressure transiently increases venous return, thereby raising BP and reflexly lowering heart rate. Phase 2 During phase 2, the sustained rise in intrathoracic pressure reduces venous return VR and so BP falls until a compensatory tachycardia restores it. Phase 3 The release of pressure in phase 3 creates a large empty venous reservoir, causing BP to fall. Show that the heart rate remains elevated. Phase 4 The last phase shows how the raised heart rate then initially leads to a raised BP as venous return is restored. This is followed by a reflex bradycardia before both parameters eventually return to normal. 

Mention should also be made of the possibility of affecting cardiac function in angina pectoris (p. 306) or congestive heart failure (p. 132) by reducing venous return, peripheral resistance, or both, with the aid of vasodilators and by reducing sympathetic drive applying 3-blockers. 

Dilatation of venous capacitance vessels reduced venous return to the heart (preload) leads to reduced cardiac output, especially in the upright position... 

A 68-year-old man was given 0.5% bupivacaine 4 ml or spinal anesthesia, and 5 minutes later complained of nausea and developed hypotension, loss of consciousness, and a tonic-clonic seizure. He had first-degree heart block 4 minutes after subarachnoid injection, followed 1 minute later by third-degree heart block, and then asystole. He was successfully resuscitated. Proposed theories included a reflex bradycardia resulting from reduced venous return and/or unopposed... 

Nitroglycerin is only marketed for the prevention of angina pectoris in humans and the US Food and Drug Administration did not approve its use for the treatment of CHF. If therapeutic plasma concentrations can be achieved in horses, nitroglycerin would be useful for reducing venous return and preload in horses with acute CHF. However, there are no data to support its use in CHF in any veterinary species. 

Relative hypovolemia can occur when a large portion of the body s fluids escape into the tissues, most often owing to low oncotic pressures as a result of decreased protein. The circulating blood volume is decreased, which reduces venous return and results in symptoms similar to those of inadequate intake or fluid loss from the body. Symptoms of hypovolemia can occur when the total volume of fluid in the body is adequate. If an excess amount of fluid volume remains in the tissues, decreased volume in the blood vessels will result. 

Circulatory system—maternal blood volvune rises about 30 percent and cardiac output rises to 30-40 percent above normal until about 27 weeks. The pregnant uterus presses on the large pelvic blood vessels, reducing venous return and causing edema of the feet, along with varicose veins and... 

P-blockers were among the first agents used to prevent NCS and are presumed to work owing to their negative inotropic actions that lessen the degree of cardiac mechanoreceptor activation during periods of reduced venous return. Although p-blockers were reported to be effective in several uncontrolled studies, they did not demonstrate any benefit in five out of seven controlled studies (68-74). 

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