Neuraxial anesthesia (NA) works by blocking sympathetic, motor, and sensory nerves, leading to a range of physiological effects due to the interruption of normal autonomic reflexes and unopposed parasympathetic activity. These effects vary based on the level and extent of the block, as well as patient-specific factors, and impact multiple systems, including the cardiovascular, pulmonary, central nervous, thermoregulatory, gastrointestinal, and renal systems.
Cardiovascular Effects
The most common cardiovascular effects of neuraxial anesthesia are hypotension and bradycardia, which result from sympathetic blockade. Hypotension occurs in nearly half of spinal anesthesia cases due to decreased systemic vascular resistance and pooling of blood in the periphery, leading to reduced venous return. This sympathetic block also decreases adrenal medullary secretion, further lowering blood pressure. Blocks above T4 pose a higher risk of hypotension, while blocks below T4 may prompt compensatory vasoconstriction above the block level.
Bradycardia, which occurs in 10-15% of spinal anesthetics, can arise from two mechanisms. The direct mechanism involves the blockade of sympathetic cardioaccelerator fibers (T1-T4), while the indirect mechanism involves reduced venous return and activation of mechanoreceptors in the heart, triggering the Bezold-Jarisch reflex. Factors that increase the risk of bradycardia include younger age, a low baseline heart rate (<60 beats per minute), and high spinal blocks above T6. In rare cases, severe bradycardia can lead to cardiac arrest.
Pulmonary Effects
While neuraxial anesthesia can impact respiratory function, the effects are typically minimal in healthy individuals. A high thoracic block can impair the function of accessory respiratory muscles, but diaphragmatic function usually remains intact, preserving adequate ventilation. However, in patients with chronic respiratory conditions or those who rely on accessory muscles for breathing, neuraxial anesthesia can impair active exhalation and coughing, reducing peak expiratory flow and minute ventilation.
Though it is theoretically possible that neuraxial anesthesia could cause bronchospasm by blocking sympathetic innervation, this effect is not significant in clinical practice. Studies have shown that even in patients with chronic obstructive pulmonary disease (COPD), thoracic epidural anesthesia does not increase airway resistance.
Central Nervous System Effects
Patients often experience a sense of sedation under neuraxial anesthesia due to reduced afferent input to the brain’s reticular activating system. This sedative effect is especially common during spinal anesthesia, and sedation tends to increase during the first hour after the anesthetic is administered. The reduction in sensory input to the brain likely explains this effect, leading to less need for additional sedative medications. Importantly, cerebral blood flow generally remains stable unless blood pressure falls below the autoregulatory threshold.
Thermoregulatory Effects
Similar to general anesthesia, neuraxial anesthesia can cause hypothermia, particularly with higher-level blocks. The sympathetic blockade shifts blood flow from the core to the periphery, promoting heat loss. In addition, metabolic heat production decreases, and the body's natural heat-preserving mechanisms—like shivering and vasoconstriction—are impaired below the level of the block. Blocked afferent nerves also reduce the body’s ability to regulate temperature centrally. To counter these effects during surgery, active warming strategies, such as heated blankets and warmed IV fluids, are often used.
Gastrointestinal Effects
Neuraxial anesthesia affects the gastrointestinal (GI) system by blocking sympathetic input to the abdominal viscera while sparing parasympathetic activity, which is controlled by the vagus nerve. This results in a contracted gut, relaxed sphincters, and normal peristalsis, leading to faster recovery of GI motility following abdominal surgeries. Research shows that epidural analgesia can significantly shorten the time to the first flatus and bowel movement compared to opioid-based analgesia.
Renal Effects
The renal effects of neuraxial anesthesia are minimal in healthy individuals, provided that blood pressure and intravascular volume are maintained. The kidneys receive sympathetic innervation from nerves originating between T10 and L1. Any impact on renal function typically stems from changes in blood pressure or cardiac output rather than direct neural effects. Hypotension may temporarily decrease urine output, but renal oxygenation is generally preserved, and normal urine output resumes once blood pressure is stabilized.
| System | Effect | Mechanism or caveat |
|---|---|---|
| Cardiovascular | Hypotension in nearly half of spinal anesthetics; bradycardia in 10–15% | Fall in systemic vascular resistance with peripheral pooling; blockade of cardioaccelerator fibers (T1–T4) or the Bezold-Jarisch reflex |
| Pulmonary | Minimal in healthy patients | A high thoracic block impairs accessory muscles while the diaphragm is preserved; active exhalation and coughing suffer in patients who depend on accessory muscles |
| Central nervous | Sedation, especially during spinal anesthesia and in the first hour | Reduced afferent input to the reticular activating system; cerebral blood flow stays stable unless pressure falls below the autoregulatory threshold |
| Thermoregulatory | Hypothermia, more so with higher blocks | Core-to-periphery redistribution, reduced metabolic heat production, and impaired shivering and vasoconstriction below the block |
| Gastrointestinal | Contracted gut, relaxed sphincters, normal peristalsis; faster return of motility | Sympathetic input to the viscera is blocked while vagal activity is spared |
| Renal | Minimal if blood pressure and volume are maintained | Sympathetic supply arises from T10–L1; hypotension may transiently reduce urine output, which resumes once pressure is restored |
