Introduction 

Spinal anaesthesia is preferred anaesthesia technique for elective caesarean section. The main disadvantage of spinal anaesthesia is maternal hypotension, incidence of which is around 90% of women and can cause dizziness, nausea and vomiting, fetal acidosis and in severe cases fetal bradycardia and cardiovascular collapse. The Bezold–Jarisch reflex has been proposed as a mechanism for the accompanying bradycardia and hypotension post spinal anaesthesia. This reflex is mediated by serotonin receptors (5-HT3 subtype). Ondansetron, a potent 5-HT3 receptor antagonist commonly used as an antiemetic drug, is potentially useful to attenuate this response. Therefore we have designed this study to test our hypothesis that prophylactic ondansetron (8 mg) administered 5 minutes prior to spinal anaesthesia induction would result in greater attenuation of post spinal hypotension when combined with 500 ml co-load of ringer’s lactate. Primary aim will be to assess the severity of hypotension amongst the three groups while secondary aim will be to assess total phenylephrine consumption, neonatal cord blood base excess.

Material methods 

After obtaining ethics committee approval and a written informed consent, this study will be performed in one hundred twenty (n=40) American Society of Anesthesiologists (ASA) physical status II patients, women aged 18 yr or older, BMI less than 40, height between 150-180 cm, non-laboring, at term with singleton uncomplicated pregnancies, scheduled for elective caesarean delivery under spinal anaesthesia. Patients will be randomised and allotted one of the three groups using opaque sealed envelope method. Patients in group A will be administered 500 ml of ringer’s lactate as pre-load infused over 10 minutes in O.R, 10 minutes prior to administration of spinal anaesthesia. as soon as CSF appears during administration of spinal anaesthesia. Patients in group B will be administered 500 ml of ringer’s lactate as co-load over 10 minutes as soon as CSF appears during administration of spinal anaesthesia. Patients in group C will be administered 500 ml of ringer’s lactate as co-load over 10 minutes as soon as CSF appears during administration of spinal anaesthesia. Patients in groups A and B will be administered 8 mg of ondansetron i.v, over one minute, five minutes before administration of spinal anaesthesia while in group C, 8 mg of ondansetron will be administered prior to completion of caesarean section. SBP, diastolic blood pressures (DBP), mean arterial pressures (MAP), HR, and SpO2 values will be monitored throughout the operation and recorded intraoperatively with 1-min intervals till baby is delivered and thereafter every 3 minutes until the end of the operation. A HR lower than 60 beats/min will be defined as bradycardia and will be treated with 0.5 mg of atropine sulphate, intravenously. Hypotension, defined as SBP less than 80% of baseline (prenatal), will be treated with 100 μg phenylephrine i.v bolus. The results will be presented as mean ± standard deviation, median (range) or n (%) as appropriate. Patient’s characteristics, onset, duration and severity of hypotension, cord blood base excess among the groups will be compared using one-way analysis of variance followed by Bonferroni correction. For haemodynamic changes within the groups, repeated measures analysis of variance followed by post hoc Bonferroni tests will be performed. Apgar scores and phenylephrine doses will be analysed by Kruskal-Wallis and Mann- Whitney U tests. The incidence of hypotension and adverse effects will be analysed using chi-square test and Fisher’s exact test. A P value <0.05 will be considered statistically significant.