Respiratory Depression in Women With BMI≥30 Underwent Spinal Anesthesia With Intrathecal Morphine in Elective C-section

Since 1979, intrathecal morphine combined with spinal anesthesia has been shown to provide effective postoperative analgesia in a variety of surgeries and more specifically in caesarean sections.

The advantages of addition of morphine to spinal analgesia are in prolonged analgesia during the operation and post operatively. In addition the analgesia is provided using a single injection during the time of surgery with no further need for complex and costly instruments like an opioid pump .

The side effects and risks of intrathecal morphine include: nausea, vomiting, pruritus urinary retention and the most severe being respiratory depression (immediate or delayed). The mild side effects were higher postoperatively in patients receiving spinal anesthesia combined with morphine. In a meta-analysis study of different surgeries performed under spinal anesthesia, it has been shown that at low doses (<0.3mg) intrathecal morphine does not increase the rate of respiratory depression. Nonetheless higher doses (>0.3mg) were associated with more incidents of respiratory depression. In another study there was no evidence of respiratory depression following caesarean section under spinal anesthesia combined with morphine. Yet, in a prospective study that reviewed 856 cases of caesarean sections under spinal anesthesia combined with morphine a rate of 8 incidences of respiratory depression were found. In a retrospective study of 1915 cases of women undergoing caesarean section with spinal anesthesia combined with morphine a rate of 0.25% of respiratory depression was reported.

The definition of respiratory depression is the lack of response to hypercapnia and/or hypoxia and it has been shown that intrathecal morphine depresses both responses. The common parameters that are monitored in order to detect respiratory depression include respiratory rate, sedation score, pulse oximetry, high PCo2 levels and the need for naloxone treatment. However, the overall measurement of frequency of respiratory depression postoperatively is variable. Accounting for this variability is the fact that hypercapnia may occur in patients with normal respiratory rate, normal sedation score and normal oxygen saturation. In addition, respiratory depression detected by high levels of PCo2 may be recognized earlier than with other parameters.

The risk factors that were found to be associated with respiratory depression included high BMI, prior opioid use, magnesium infusion and respiratory comorbidities In a study of 856 women 8 of them experienced oxygen desaturation during sleep. All of these women were extremely obese with a prior history of deep snoring.

In this study the investigators would like to examine whether there is a difference in the frequency of episodes of respiratory depression between obese (BMI≥30) and women with normal BMI receiving spinal anesthesia combined with intrathecal morphine during elective caesarean section. In previous studies it was reported that obese and non-obese women do not respond differently to modest doses of spinal anesthesia. Hence in this study the investigators can be certain that the investigators are examining the influence of addition of morphine to the spinal anesthesia.