Spinal anesthesia is a safe and effective technique for a wide variety of procedures, including obstetric surgeries and other surgeries involving the lower abdomen. Spinal anesthesia has a much lower risk of respiratory collapse compared to general anesthesia and does not require intubation/mechanical ventilation. However, roughly 33% of non-obstetric patients experience spinal anesthesia-induced hypotension and this incidence goes up to 70–80% in obstetric patients without pharmacological prophylaxis (1). Due to the complications associated with hypotension, it is critical to improve the prevention and management of hypotension after spinal anesthesia.
Following spinal anesthesia, hypotension mainly occurs secondary to a blockage of sympathetic nervous system activity. This systemic vasodilation can trigger the Von Bezold-Jarisch (VBJ) reflex, resulting in a low heart rate (bradycardia). If untreated, spinal anesthesia-induced hypotension can progress to cardiac arrest (2). Several methods have been described to reduce the incidence of spinal anesthesia-induced hypotension during surgery, including intravenous fluids, vasopressor drugs and lower-leg compression, but no single technique has been confirmed to be completely effective (1). Interestingly, recent studies have suggested that ondansetron, a 5-HT3 serotonin receptor antagonist generally used for the prevention and treatment of nausea and vomiting, may also help to reduce hypotension associated with spinal anesthesia (1).
In a 2015 meta-analysis published in the International Journal of Obstetric Anesthesia, Gao et al. investigated randomized clinical trials that studied the prophylactic effects of ondansetron on spinal anesthesia-induced hypotension in both obstetric and non-obstetric patients. 10 randomized clinical trials with 863 participants were included in the meta-analysis. In 8 out of the 10 studies, ondansetron was administered 5 min before spinal anesthesia, however, the other two studies did not clearly state the timing of ondansetron administration. Based on a definition of hypotension as systolic blood pressure <90 mmHg, Gao et. al found that ondansetron was effective in the prevention of spinal anesthesia-induced hypotension with a relative risk (RR) of 0.40 (P=0.0005). Next, it was found that the relative risk of developing bradycardia after prophylactic ondansetron was 0.27 (P<0.0001) indicating that prophylactic ondansetron also significantly reduced the incidence of bradycardia caused by spinal anesthesia. Last but not least, they also evaluated vasopressor consumption after prophylactic ondansetron. Phenylephrine and ephedrine, both sympathomimetic agents, were the most common vasopressors used in the clinical trials that were analyzed. It was found that the mean difference of ephedrine and phenylephrine use after prophylactic ondansetron administration
was -2.35 mg and −31.16 μg, respectively. These results further support the assertion that ondansetron administration prior to spinal anesthesia reduces the amount of vasopressor needed, and thus, is associated with a lower incidence of post-anesthesia hypotension (1).
All in all, there is ample evidence to suggest that ondansetron can help with the prevention of hypotension and bradycardia induced by spinal anesthesia. Although the mechanism for this action is not fully understood, it is believed that ondansetron may prevent the VBJ reflex from occurring, thus preventing the hemodynamic changes associated with spinal anesthesia (3). Nevertheless, further strict and larger randomized controlled trials are still needed before a clear recommendation can be made for routine prophylactic use of ondansetron for the prevention of spinal anesthesia-induced hypotension.
References
1. Gao L, Zheng G, Han J, Wang Y, Zheng J. Effects of prophylactic ondansetron on spinal anesthesia-induced hypotension: a meta-analysis. Int J Obstet Anesth. 2015;24(4):335-343. doi:10.1016/j.ijoa.2015.08.012
2. Mendonça FT, Crepaldi Junior LC, Gersanti RC, de Araújo KC. Effect of ondansetron on spinal anesthesia-induced hypotension in non-obstetric surgeries: a randomised, double-blind and placebo-controlled trial. Braz J Anesthesiol. 2021;71(3):233-240. doi:10.1016/j.bjane.2020.12.028
3. Campagna JA, Carter C. Clinical relevance of the Bezold-Jarisch reflex. Anesthesiology. 2003;98(5):1250-1260. doi:10.1097/00000542-200305000-00030