Respiratory Effects

Alterations in pulmonary variables in healthy patients during neuraxial block are usually of little clinical consequence. Tidal volume remains unchanged during high spinal anesthesia, and vital capacity decreases a small amount from 4.05 to 3.73 L.^[46] This decrease in vital capacity is a result of a decrease in expiratory reserve volume related to paralysis of abdominal muscles necessary for forced exhalation, rather than a decrease in phrenic or diaphragmatic function.^[14] This minimal impact on pulmonary function also holds for elderly patients undergoing lumbar and thoracic epidural anesthesia.^[47] Pulmonary function testing in cesarean section patients undergoing epidural anesthesia shows that using lidocaine provides larger decrements in peak expiratory pressure (abdominal musculature-dependent maneuver) than using bupivacaine.^[48]

The rare respiratory arrest associated with spinal anesthesia is also unrelated to phrenic or inspiratory dysfunction, but rather to hypoperfusion of the respiratory centers in the brainstem. Supportive evidence for this concept is observed after resuscitation, when apnea almost always disappears as soon as pharmacologic and fluid therapies have restored cardiac output and blood pressure. This would not be the case if phrenic paralysis due to high levels of local anesthetic was the cause of apnea.^[14] There is experimental evidence from rabbits during epidural anesthesia that their response to hypoxia results in apnea, which is different from the response without neuraxial anesthesia.^[49] This finding may have implications for understanding some episodes of cardiovascular depression during neuraxial blocks in humans, although confirmation is needed. Conversely, in a canine model of hypoxia epidural anesthesia and hypoxic challenge results in the same outcome but with more normal acid-base function compared with animals receiving general anesthesia alone.^[50]

Neuraxial block should be used cautiously in respiratory cripples because of paralysis of respiratory muscles. Except for the severely compromised patient with respiratory failure, inspiratory muscle function during neuraxial blocks should be adequate to maintain ventilatory function. The physiologic consideration related to muscle paralysis with neuraxial block should focus on the expiratory muscles in these severely compromised patients because these muscles are important for effective coughing and clearing of intrapulmonary secretions.^{[38] [46]}