What causes post spinal hypotension?

Post-Spinal Hypotension: Causes and Mechanisms

Post-spinal hypotension is primarily caused by sympathetic blockade leading to arterial and venous vasodilation, with decreased systemic vascular resistance being the dominant mechanism rather than reduced cardiac output. 1, 2

Primary Pathophysiologic Mechanisms

Sympathetic Blockade and Vascular Effects

• The fundamental cause is sympathetic nervous system blockade from local anesthetic spread in the subarachnoid space, resulting in arterial and venous vasodilation. 1, 3
• Arterial dilation and reduction in systemic vascular resistance (SVR) are the major contributors to spinal-induced hypotension, not cardiac output reduction. 4
• In elderly patients specifically, a decrease in SVR—not cardiac output—is the main mechanism of hypotension during spinal anesthesia. 2

Cardioinhibitory Component

• Paradoxical activation of cardioinhibitory receptors (Bezold-Jarisch reflex) contributes to hypotension and can cause bradycardia. 1
• Bradycardia after spinal anesthesia must always be treated as a warning sign of important hemodynamic compromise. 1

Dose-Related Factors

Local Anesthetic Dosing

• Higher doses of intrathecal bupivacaine (≥10 mg) are associated with increased hypotension risk. 5
• Lower doses of intrathecal bupivacaine (<10 mg) appear to reduce associated hypotension in elderly patients undergoing hip fracture surgery. 5
• The amount of plain bupivacaine 0.5% used for spinal anesthesia is independently associated with hypotension occurrence. 6

Sensory Block Height

• Higher sensory block levels significantly increase hypotension risk (odds ratio = 2.32). 6
• The extent of sympathetic blockade correlates directly with the cephalad spread of local anesthetic. 6

Patient-Related Risk Factors

Demographic and Physiologic Factors

• Chronic alcohol consumption increases hypotension risk threefold (OR = 3.05). 6
• History of hypertension more than doubles the risk (OR = 2.21). 6
• Higher body mass index incrementally increases risk (OR = 1.08 per unit increase). 6
• Male gender, obesity, and vascular disease risk factors (hypertension, peripheral vascular disease) increase perioperative hypotension risk. 5

Age-Related Considerations

• Elderly patients have limited physiological reserve and increased susceptibility to hypotension from spinal anesthesia. 5
• In patients over 80 years, the mechanism is predominantly SVR decrease rather than cardiac output reduction. 2

Procedural and Surgical Factors

Urgency and Surgical Context

• Emergency surgery increases hypotension risk nearly threefold (OR = 2.84) compared to elective procedures. 6
• Simultaneous administration of spinal and general anesthesia is associated with precipitous falls in intraoperative blood pressure and should be avoided. 5

Positioning Effects

• Patient positioning (lateral vs. supine) affects local anesthetic distribution and sympathetic block extent. 5
• Attempted lateralization of subarachnoid anesthesia using hyperbaric bupivacaine may ameliorate hypotension. 5

Volume Status and Preload

Fluid Management Considerations

• Venous vasodilation reduces venous return and cardiac preload, though this is not the primary mechanism. 1, 4
• Strategies aimed solely at expanding intravascular volume or increasing venous return have limited effectiveness. 4
• The amount of colloid infusion before puncture is associated with hypotension occurrence. 6

Clinical Significance

Incidence

• Clinically significant hypotension (>30% decrease in mean arterial pressure requiring intervention) occurs in approximately 5.4% of patients receiving spinal anesthesia. 6
• The incidence is higher in specific populations, particularly parturients undergoing cesarean delivery and elderly patients. 4

Risk Stratification

• The risk of hypotension increases two- to threefold with each additional risk factor present. 6
• Independent risk factors include chronic alcohol consumption, history of hypertension, body mass index, sensory block height, and urgency of surgery. 6

Important Clinical Caveats

• Hypotension after spinal anesthesia is a physiological consequence of sympathetic blockade, not a complication per se, but requires prompt recognition and treatment. 3
• The frequency of dural puncture attempts is associated with hypotension risk, suggesting technical factors matter. 6
• Intraoperative hypotension, whether deliberate or unintentional, may increase the risk of perioperative ischemic optic neuropathy in high-risk spine surgery patients, though evidence is equivocal. 5