What is the mechanism of hypotension after spinal anesthesia?

From the Guidelines

Hypotension after spinal anesthesia primarily occurs due to sympathetic nervous system blockade, resulting in vasodilation and decreased systemic vascular resistance. When local anesthetic is injected into the subarachnoid space, it blocks sympathetic nerve fibers that maintain vascular tone, leading to vasodilation of both arteries and veins 1. This vasodilation causes decreased systemic vascular resistance and venous pooling, reducing venous return to the heart. The reduced preload leads to decreased cardiac output and subsequent hypotension.

Key Factors

• The extent of hypotension depends on the level of the block, with higher blocks (T4 and above) potentially affecting cardioaccelerator fibers (T1-T4), which can cause bradycardia and further reduce cardiac output.
• Risk factors include hypovolemia, advanced age, high sensory block level, and pre-existing cardiovascular disease.

Prevention and Treatment Strategies

• Prevention strategies include adequate prehydration with crystalloids (10-15 ml/kg), left uterine displacement in pregnant patients, and prophylactic vasopressors like phenylephrine (50-100 mcg bolus) or ephedrine (5-10 mg bolus) 1.
• Treatment involves fluid administration, vasopressors, and positioning the patient to optimize venous return by elevating the legs or placing in Trendelenburg position.

Importance of Euvolemia

• Preoperative and intraoperative euvolemia are important factors in patient perioperative care and appear to lead to improved maternal and neonatal outcomes after cesarean delivery, as noted in the guidelines for intraoperative care in cesarean delivery 1.
• A combination of vasopressors and adequate fluid therapy could be effective in reducing the incidence and severity of hypotension, as supported by recent studies 1.

From the Research

Mechanism of Hypotension after Spinal Anesthesia

The mechanism of hypotension after spinal anesthesia is complex and involves several factors. Some of the key factors include:

• Arterial and venous vasodilatation resulting from the sympathetic block along with a paradoxical activation of cardioinhibitory receptors 2
• Decrease in systemic vascular resistance (SVR), which is the main mechanism of hypotension seen during spinal anesthesia in elderly patients 3
• Bradycardia, which can occur after spinal anesthesia and must always be treated as a warning sign of an important hemodynamic compromise 2

Risk Factors for Hypotension and Bradycardia

Several risk factors have been identified for hypotension and bradycardia after spinal anesthesia, including:

• Increasing age 3, 4
• Analgesia level > or = T4 dermatome 4
• Body mass index > or = 30 4
• Cesarean section 4
• Prehydration fluid < 500 mL 4

Prevention and Treatment of Hypotension

Several methods have been studied to prevent and treat hypotension after spinal anesthesia, including:

• Fluid preloading with colloids such as hydroxyethyl starch (HES) 2
• Co-loading with crystalloid or colloid 2
• Use of vasoconstrictors such as ephedrine and phenylephrine 2, 5
• Combination of preventive methods, such as crystalloid preload with ephedrine 5