Use of Intravenous Dexmedetomidine as an Adjuvant to Extend the Duration of Spinal Anesthesia With Hyperbaric Bupivacaine in Orthopedic Surgery in Hospital Central "Dr. Ignacio Morones Prieto": Double Blind Randomized Trial.

Neuraxial blockade is produced by interrupting nervous impulse transmission with the administration of a local anesthetic. When the drug in injected directly into the subarachnoid space, it passes through the pia mater and enters the Virchow-Robin orifices all the way to the nerve ganglion of the deepest dorsal roots. This nerve endings are highly accessible and easily anesthetized, even with a low dose of local anesthetic, in comparison with the extradural nerves. The speed of the spinal blockade depends on the size, surface, and degree of myelination of the fibers exposed to the anesthetic. However, the regression or termination of the desired effect appears because of the decrease in drug concentration within the cerebrospinal fluid, which happens when the blood vessels inside the pia mater absorb the substances into the systemic circulation to be metabolized and excreted. The rate of elimination depends on the distribution of local anesthetic; while the diffusion surface is larger, the drug exposure to the vessels increases, and thus the time of action will be shorter.

Bupivacaine is one of the most used local anesthetics for spinal anesthesia, with high protein binding and a slow onset because of it relatively elevated pKa. The standard doses for clinical practice make it appropriate for surgical procedures that take 2.5 to 3 hours. The administration of complementary drugs such as dexmedetomidine altogether with the local anesthetic allows a reduction in the dosage, as well as in the requirements for analgesics and sedative drugs along the procedure and through the first hours of the postoperative period.

Dexmedetomidine properly carries its mechanism of action in the pre and post synaptic alpha-2 receptors along the dorsal horn of the spinal cord, decreasing the release of neurotransmitters and thus the pulsatile transmission of impulses. On the other hand, when administered as an IV infusion, this alpha agonist has shown the ability to reach light sedation levels, easily reversed by verbal or tactile stimulus. When acting at the locus coeruleus presynaptic spots, the effect of trans and postoperative analgesia can also be consistently observed. Because of these characteristics, the benefits of intravenous dexmedetomidine have been recently tested over the length and quality of spinal anesthesia with local anesthetics, by being administered as bolus and then as maintenance infusion at variable doses per kilogram.

This therapeutic alternative has been applied during several types of infraumbilical surgeries, such as varicocelectomy, hernioplasty, orchiectomy, and lower limb interventions, without the risk of producing nausea, vomiting, headache, or shivering. The use of this adjuvant provides longer lasting nervous blockade, as well higher time for postoperative analgesia, which helps to reduce the consumption of sedative drugs and opioids since the beginning of the intervention until up to the following first 24 hours. Finally, the undesired effects that have been reported in some populations when receiving this drug (hypotension and bradycardia), appear unfrequently and can be easily reversed with the use of common prescription hemodynamic drugs (atropine, ephedrine).

However, it is common to observe differing therapeutic responses following drug administration during daily clinical practice, which can be often attributed to interindividual differences in drug pharmacokinetics, linked to race, ethnicity, and social habits, that can develop enzymatic variations or genetic polymorphisms. Currently, there are many bibliographic references that support the theories that punctually describe these changes in pharmacological responses between ethnic groups, which is why there is a constant need for testing the efficacy and safety of certain drugs, to improve patient safety and clinical outcomes in all the medical fields.