Neuraxial anesthesia prevents the transmission of painful sensation and abolishes the tone of skeletal muscle, enhancing operating conditions for the surgeon and provides a comfortable experience for the patient. For instance, in surgeries such as cesarean sections as well as total joint replacements in orthopedic surgeries, somatic blockade provides adequate muscle relaxation. This allows the surgeons to safely make incisions and reposition the patient for optimal access to the site of interest. Due to heavily myelinated large axonal fibers, somatic nerves will be the last to be blocked but first to return to function.
Neuraxial blockade effectively blocks efferent autonomic transmission of the spinal nerve roots, producing a sympathetic block and a partial parasympathetic block. Preganglionic sympathetic nerve fibers are small, myelinated nerve fibers and postganglionic sympathetic nerve fibers are small and unmyelinated, resulting in rapid sympathectomy. The sympathetic block will be followed by a sensory blockade, then motor blockade ensues. Sympathetic fibers exit the cord from T1 to L2 (thoracolumbar) and parasympathetic fibers exit in the cranial and sacral regions (craniosacral). With this in mind, the autonomic effects following neuraxial blockade are a decrease in sympathetic tone with unopposed parasympathetics. However, it is important to note that the neuraxial blockade does not have any effect on the vagus nerve. Due to this imbalance, the anesthesia provider will observe many expected physiologic changes exhibited by the patient after epidural or spinal anesthesia.
Due to the autonomic imbalance where the parasympathetic nervous system prevails or is temporarily unopposed, there are cardiovascular changes that are depicted in multiple ways. Providers should be prepared to observe hypotension due to predominantly venodilation, bradycardia due to blockade of cardioaccelerator fibers exiting from T1 toT4 (13% nonpregnant patients), and a decrease in inotropy. These signs and symptoms should be expected by the provider administering neuraxial anesthesia. Sympathectomy is the term used to describe blockade of sympathetic outflow. Sympathectomy is directly related to the height of the block and results in venous and arterial vasodilation. Dilation of the venous system is predominantly responsible for decreases in blood pressure since the arterial system is able to maintain much of its vascular tone. Heart rate may also decline as a result of a decrease in SVR, decreased right atrial filling, and decreases in the intrinsic chronotropic stretch receptor response resulting in a decrease in heart rate. Aggravating factors such as a head-up position or the weight of a gravid uterus on venous return in the parturient may cause further declines in blood pressure. Occasionally sudden cardiac arrest may be seen with spinal anesthesia due to unopposed vagal stimulation.
Volume loading the patient with 10–20 mL/kg of crystalloid fluid or appropriate amount of colloid immediately prior and during the administration of a neuraxial anesthesia may be helpful. The patient’s cardiac function and medical history should be taken into account prior to this measure. Trendelenburg position ...