Change in institutional policy. Change in resources available.

Cesarean deliveries are the most commonly performed surgery in the United States and account for 32.9% of all births.8,9 The ASA recommends the use of neuraxial opioids of post-cesarean analgesia partly because respiratory depression in the obstetric population, as measured by intermittent respiratory rate and pulse oximetry, is reported to be low (0-1.2%).10,11 Respiratory depression lacks a standard definition,12 but the most sensitive means of detecting respiratory depression is hypercapnia.1,3 Two recent studies using continuous hypercapnia (>50mmHg PaCO2) monitoring demonstrated higher rates of respiratory depression (17.8-37%) in healthy, non-obese women receiving intrathecal opioids for post-cesarean analgesia.13,14 In addition, supplemental opioids are required in the majority of women receiving intrathecal morphine and may increase the risk of respiratory depression.11,14 Anesthesiologists debate whether neuraxial opioids or intravenous patient controlled opioid analgesia (PCA) are the safest practice for postoperative analgesia in obese parturients following cesarean delivery. The ASA recommendations to employ neuraxial analgesia post-cesarean delivery does not differentiate between non-obese and obese women who now make up 30.3% in US women of child-bearing age.2 Obesity has been described as a risk factor for respiratory depression in those receiving opioids via any route of opioid administration,11,15, 17 but whether obesity itself is the risk factor or associated co-morbidities such as sleep apnea is debated.

Studies are conflicting whether intrathecal opioids or patient controlled intravenous opioids cause more respiratory depression. Several studies have documented the incidence of respiratory depression with IV PCA; the rates range from 0.19% to 5.2%, which are equivalent or higher than those reported for intrathecal opioids. (Hagle 16). Dalchow et al. demonstrated higher rates of hypercapnia in patients receiving intrathecal opioid compared with those receiving intravenous opioid via patient controlled analgesia in nonobese women following cesarean delivery. (Dalchow)

The Topological Oscillation Search with Kinematical Analysis (TOSCA) monitor allows a noninvasive method to measure transcutaneous carbon dioxide levels, with relative accuracy compared to arterial carbon dioxide monitoring.4-7 No studies have examined transcutaneous carbon dioxide levels in obese women following cesarean delivery using any form of postoperative analgesia. The investigators propose a randomized controlled trial using continuous transcutaneous carbon dioxide monitoring to evaluate the degree of respiratory depression in obese women receiving neuraxial opioid compared to intravenous opioid via PCA for post-cesarean analgesia.

Two studies have demonstrated high rates of hypercapnia in non-obese women following administration of intrathecal morphine for cesarean delivery in the postpartum period. (Dalchow, Bauchat) Dalchow et al. demonstrated higher rates of hypercapnia in women receiving intrathecal diamorphine than intravenous morphine delivered via patient controlled analgesia. It is unclear whether intrathecal morphine causes more or less respiratory depression than intravenous opioid delivered via patient-controlled analgesia in obese women.

This study will add to the understanding of respiratory function in the immediate postpartum period in obese women using opioids via intrathecal or intravenous routes. This study will better inform guidelines for the postpartum analgesic route of choice in the obese obstetric population and allow the investigators to make recommendations for the detection and prevention of respiratory depression after opioid administration in the obstetric population.

Objective is to examine the transcutaneous carbon dioxide levels in obese women using either intrathecal morphine or intravenous patient-controlled hydromorphone for post-cesarean analgesia.

The hypothesis is carbon dioxide levels will be significantly higher in obese women receiving intrathecal morphine versus obese women receiving intravenous patient controlled intravenous hydromorphone.

Inclusion Criteria:

• Term (≥37 week's gestation)
• Healthy
• ASA class 2-3
• BMI ≥40kg/m2 scheduled for elective cesarean section whose anesthetic plan is for neuraxial anesthesia (spinal or combined-spinal epidural technique)

Exclusion Criteria:

• Women with ASA >3,
• BMI <40 kg/m2
• Allergy to any of the medications used for pain control
• Contraindication to the spinal anesthetic technique
• Known sleep apnea or other sleep disordered breathing
• Regular use of other medications that cause respiratory depression (ie. benzodiazepines).