Pharmacokinetics of Oxytocin at Cesarean Delivery

Study Overview
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Brief Summary
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The primary objective is to obtain data to inform the design of a population pharmacokinetic study of oxytocin after administration at CD as per standard institutional practice.

Detailed Description
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In this pilot study the investigators will compare plasma oxytocin concentrations measured in arterial and venous blood, after oxytocin administration at cesarean delivery, in order to assess the validity of venous samples. They will also measure oxytocinase to explore how levels vary around the time of delivery of the placenta and to assess the effect that this may have on the metabolism of oxytocin.

Official Title
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The Pharmacokinetics of Oxytocin in Pregnant Women Undergoing Cesarean Delivery - a Feasibility Pilot Study

Conditions
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Postpartum Hemorrhage

Intervention / Treatment
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* Drug: oxytocin
* Device: Shore durometer


Participation Criteria
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Eligibility Criteria
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Inclusion Criteria: Term singleton pregnancy Age 18-45 years of age ASA classification 2 or 3 Scheduled cesarean delivery under neuraxial anaesthesia Exclusion Criteria: Age or ASA classification outside of inclusion criteria Need for general anaesthesia Absence of consent Inability to communicate in English or other barrier to providing informed consent

Ages Eligible for Study
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Minimum Age: 18 Years
Maximum Age: 45 Years

Sexes Eligible for Study
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Female

Accepts Healthy Volunteers
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Accepts Healthy Volunteers

Study Plan
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How is the study designed?
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Arms and Interventions

| Participant Group/Arm | Intervention/Treatment |
| --- | --- |
| BMI 20 - 29.9 kg/m2<br> | Drug: oxytocin<br>* PK measurements of oxytocin<br>Device: Shore durometer<br>* The investigators will take serial readings from a digital durometer at the uterine fundus at 3, 6, 9 and 12 minutes following OXT administration to provide data that will inform an estimate of the timing of peak effect. Shore durometers measure the hardness of materials on a scale of 0-100 Shore Units (SU). Preliminary (unpublished) data provided a range of 10-56 SU after oxytocin administration at cesarean delivery. Durometer readings were also shown to correlate positively with the obstetrician's manual assessment of uterine contractility, which is traditionally used to determine the success or failure of uterotonic therapy.<br>|
| BMI > 30 kg/m2<br> | Drug: oxytocin<br>* PK measurements of oxytocin<br>Device: Shore durometer<br>* The investigators will take serial readings from a digital durometer at the uterine fundus at 3, 6, 9 and 12 minutes following OXT administration to provide data that will inform an estimate of the timing of peak effect. Shore durometers measure the hardness of materials on a scale of 0-100 Shore Units (SU). Preliminary (unpublished) data provided a range of 10-56 SU after oxytocin administration at cesarean delivery. Durometer readings were also shown to correlate positively with the obstetrician's manual assessment of uterine contractility, which is traditionally used to determine the success or failure of uterotonic therapy.<br>|

What is the study measuring?
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Primary Outcome Measures

| Outcome Measure | Measure Description | Time Frame |
| --- | --- | --- |
| Arterial [OXT] | Concentration of oxytocin in arterial samples | Scheduled samples during the first 30 minutes after oxytocin administration | 
| Venous [OXT] | Concentration of oxytocin in venous samples | Scheduled samples during the first 30 minutes after oxytocin administration | 

Secondary Outcome Measures

| Outcome Measure | Measure Description | Time Frame |
| --- | --- | --- |
| Arterial [OXTase] | Concentration of oxytocinase in arterial samples | Scheduled samples during the first 30 minutes after oxytocin administration | 
| Uterine tone | Uterine tone measured using a Shore Durometer or assessed by obstetrician using numerical rating score (NRS, 0-10) | Scheduled measurements during the first 30 minutes after oxytocin administration | 
| Hypotension | Incidence of systolic BP reduced by 20% from baseline | Measured intraoperatively | 
| Nausea & Vomiting | Incidence of nausea and vomiting | Recorded intraoperatively | 
| EBL | Estimated blood loss in milliliters | Estimated at the end of surgery |