Reply to: 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?

We appreciate Prof. Gunaydin’s¹ interest in our article² and for highlighting his concerns regarding the optimal dose (weight and height adjusted dose versus fixed ED95 dose of intrathecal hyperbaric bupivacaine) and using a fixed dose of adjuvant intrathecal opioids for cesarean delivery in parturients with different Body Mass Index (BMI) in our study.

As this was a retrospective study, we were not able to control the dose of hyperbaric bupivacaine in women of higher BMI category. Our institution uses a standard ED95 dose of hyperbaric bupivacaine (12 mg) for all patients irrespective of their BMI similar to the recommendation from dose finding studies in morbidly obese women^3,4.

Our dosing is based on several factors. A weight and height-adjusted (lower) dose may achieve an adequate initial sensory level in morbidly obese patients and decrease maternal hypotension, but it does not guarantee successful intraoperative anesthesia because of the longer duration of cesarean delivery in these patients. In the study by Harten and colleagues⁵, the patients in the adjusted dose group had a mean (SD) body weight of 76.2 (± 9) kg. compared with our study population² with a mean body weight of 116 (± 23) kg. There is a greater prevalence of obesity in our patient population, and it is doubtful that we would have similar outcomes to their study if we used a height and weight adjusted dosing. Furthermore, the study by Carvalho and colleagues, reported a higher incidence of inadequate block duration in patients that received a lower dose of hyperbaric bupivacaine even though these patients had a successful induction defined as loss of pinprick sensation at T6 dermatome⁴. Adjusting the dose of hyperbaric bupivacaine for weight and height may carry the risk of failure of spinal anesthesia in an unacceptable number of our morbidly obese patients. Nevertheless, we also believe that a “one-size-fits-all” approach may not be ideal for every patient. For example, in patients with dwarfism it is reasonable to administer adjusted dose. Fortunately, none of the patients in our study had high spinal anesthetic experience despite the use of a fixed ED 95 dose of hyperbaric bupivacaine.

We acknowledge the criticism regarding not using a fixed dose of adjuvant intrathecal opioids in our study. There were practice changes during our study period, which resulted in the different dosing of adjuvant intrathecal opioids. Patients who delivered before mid-2016 received morphine 250 mcg and fentanyl 25 mcg as a standard, and after this time, all patients received the lower dose stated in our article. Very few studies have investigated the dose-response relationship of intrathecal morphine for post-cesarean analgesia. Palmer et al. and Girgin et al. reported intrathecal morphine dose of 0.1 mg or less produced analgesia comparable to doses as high as 0.5 mg^6,7. Although there was no difference among their treatment groups with respect to nausea and vomiting, pruritus and the need for treatment interventions increased in direct proportion to the dose of intrathecal morphine. Milner et al. concluded that the use of 0.1 mg morphine intrathecally produced comparable analgesia to 0.2 mg after cesarean delivery with significantly less nausea and vomiting⁸. Our current standard intrathecal narcotic dose is 15 mcg of fentanyl with 0.15 mg of preservative free morphine for cesarean delivery.

In conclusion, we agree that a weight and height adjusted dosing of hyperbaric bupivacaine may reduce the incidence of spinal anesthesia induced maternal hypotension but may not provide an adequate duration of anesthesia for our patient population. Future studies comparing weight and height adjusted dosing versus fixed dosing in morbidly obese patients are needed to guide clinicians caring for these patients.