Severe hypotension associated with high spinal anesthesia is caused primarily by

Severe hypotension associated with high spinal anesthesia is caused primarily by

• A. Decreased cardiac output secondary to decreased preload
• B. Decreased systemic vascular resistance
• C. Decreased cardiac output secondary to bradycardia
• D. Decreased cardiac output secondary to decreased myocardial contractility

Answer: (A)

High spinal anesthesia blocks sympathetic output, which causes vasodilation of both arterial and venous beds. The most impactful effect on blood pressure is the pooling of blood in the venous capacitance vessels, leading to a marked decrease in venous return to the heart (preload). With less filling, stroke volume falls and so does cardiac output, driving the severe hypotension. While the systemic vascular resistance also falls due to vasodilation, the drop in preload is the primary driver of the reduced cardiac output. Bradycardia from unopposed vagal tone and any effect on myocardial contractility can contribute, but the central, dominant mechanism is decreased preload.

High spinal anesthesia blocks sympathetic output, which causes vasodilation of both arterial and venous beds. The most impactful effect on blood pressure is the pooling of blood in the venous capacitance vessels, leading to a marked decrease in venous return to the heart (preload). With less filling, stroke volume falls and so does cardiac output, driving the severe hypotension. While the systemic vascular resistance also falls due to vasodilation, the drop in preload is the primary driver of the reduced cardiac output. Bradycardia from unopposed vagal tone and any effect on myocardial contractility can contribute, but the central, dominant mechanism is decreased preload.