Weight and height adjusted dose regimen or fixed ED95 dose of intrathecal hyperbaric bupivacaine for cesarean delivery in parturients with different BMIs: which would be optimal?

Dear Editor,

I have read with great interest the manuscript entitled “The association of maternal obesity with foetal pH in parturients undergoing caesarean delivery under spinal anaesthesia” which is to be published in Current Medical Research and Opinion¹.

Hyperbaric bupivacaine is one of the most commonly used local anaesthetics for spinal anaesthesia in parturients undergoing caesarean delivery (CD)². The dose of intrathecal hyperbaric bupivacaine for single shot spinal block to provide a 95% successful surgical anaesthesia (ED95) in normal weight women is well documented/accepted³. However, it is difficult to make a decision to select best optimal dose for intrathecal use in obese parturients. In previous studies, obesity has been shown as an independent risk factor for lower foetal pH, where foetal pH declines with increasing body mass index (BMI) in women having CD under spinal anesthesia⁴. There has also been an association between ephedrine administration and lower umbilical arterial pH compared with phenylephrine^5,6.

In the Ituk et al. paper¹, the relationship between maternal obesity and foetal umbilical arterial pH in a cohort of parturients that received a prophylactic phenylephrine infusion in the management of spinal anaesthesia induced hypotension during CD was investigated at a single academic tertiary care institution retrospectively between January 2012 and March 2019. It has been hypothesized that obesity does not increase the risk of lower umbilical arterial pH in women undergoing CD with spinal anaesthesia and receiving a prophylactic phenylephrine infusion. In this respect, this study seemed to be unique because parturients were categorized according to BMI (kg/m²) as <25 (normal), 25–29.9 (overweight), 30–34.9 (class I obesity), 35–39.9 (class II obesity), and ≥40 (class III obesity) and all of them received prophylactic phenylephrine infusion in order to prevent spinal anaesthesia-induced hypotension. As a result, although it was shown that maternal BMI was not associated with a decrease in umbilical arterial pH with the clear methodology of the study that ruled out confounding factors such as ephedrine use and/or longer delivery times, I have a concern about fixed ED95 dose of hyperbaric bupivacaine (12 mg) was used for all three BMI category groups. Regarding the anaesthetic management of morbidly obese pregnant patients, dose finding studies have failed to demonstrate differences in ED50 and ED95 of local anaesthetics for CD in morbidly obese vs non-obese patients^7–9. However, a more optimal option could be if Ituk et al.¹ had chosen a weight and height adjusted dose regimen table as described by Harten et al.¹⁰

According the present study of Ituk et al.¹ there was no difference in the mean induction of anaesthesia to delivery time (IADT) and uterine incision to delivery time (UDT) across BMI categories based on the regression model depicting a decrease in umbilical arterial pH to predict by a longer UDT and IADT. IADT (p = .04) and UDT (p < .001) remained significant predictors of umbilical arterial pH.

Dose finding studies recommend ED95 doses of intrathecal hyperbaric (11.2 mg) and isobaric bupivacaine (13 mg) with fentanyl (10 µg) and morphine (200 µg) provide successful surgical anaesthesia for CD¹¹. Therefore, I have another concern related to the current paper with the authors¹ not choosing a standard dose of intrathecal adjuvant opioids (fentanyl 10 µg plus morphine 200 µg instead of fentanyl 15–25 µg plus morphine 15–250 µg) while choosing ED95 standard dose hyperbaric bupivacaine in the patients with varying BMI. I believe it would be much more reasonable to prefer weight and height adjusted intrathecal local anaesthetic dose regimen co-administered with fixed dose of opioids in obese patients undergoing CD.