Foetal Fentanyl Exposure and Ion Trapping after Intravenous and Transdermal Administration to the Ewe

Analgesia for fetal pain during prenatal surgery: 10 years of progress

Some doubts on the necessity and safety of providing analgesia to the fetus during prenatal surgery were raised 10 years ago. They were related to four matters: fetal sleep due to neuroinhibitors in fetal blood, the immaturity of the cerebral cortex, safety, and the need for fetal direct analgesia. These objections now seem obsolete. This review shows that neuroinhibitors give fetuses at most some transient sedation, but not a complete analgesia, that the cerebral cortex is not indispensable to feel pain, when subcortical structures for pain perception are present, and that maternal anesthesia seems not sufficient to anesthetize the fetus. Current drugs used for maternal analgesia pass through the placenta only partially so that they cannot guarantee a sufficient analgesia to the fetus. Extraction indices, that is, how much each analgesic drug crosses the placenta, are provided here. We here report safety guidelines for fetal direct analgesia. In conclusion, the human fetus can feel pain when it undergoes surgical interventions and direct analgesia must be provided to it. IMPACT: Fetal pain is evident in the second half of pregnancy. Progress in the physiology of fetal pain, which is reviewed in this report, supports the notion that the fetus reacts to painful interventions during fetal surgery. Evidence here reported shows that it is an error to believe that the fetus is in a continuous and unchanging state of sedation and analgesia. Data are given that disclose that drugs used for maternal analgesia cross the placenta only partially, so that they cannot guarantee a sufficient analgesia to the fetus. Safety guidelines are given for fetal direct analgesia.

Assessing the impacts on fetal dosimetry of the modelling of the placental transfers of xenobiotics in a pregnancy physiologically based pharmacokinetic model

The developmental origin of health and diseases theory supports the critical role of the fetal exposure to children's health. We developed a physiologically based pharmacokinetic model for human pregnancy (pPBPK) to simulate the maternal and fetal dosimetry throughout pregnancy. Four models of the placental exchanges of chemicals were assessed on ten chemicals for which maternal and fetal data were available. These models were calibrated using non-animal methods: in vitro (InV) or ex vivo (ExV) data, a semi-empirical relationship (SE), or the limitation by the placental perfusion (PL). They did not impact the maternal pharmacokinetics but provided different profiles in the fetus. The PL and InV models performed well even if the PL model overpredicted the fetal exposure for some substances. The SE and ExV models showed the lowest global performance and the SE model a tendency to underprediction. The comparison of the profiles showed that the PL model predicted an increase in the fetal exposure with the pregnancy age, whereas the ExV model predicted a decrease. For the SE and InV models, a small decrease was predicted during the second trimester. All models but the ExV one, presented the highest fetal exposure at the end of the third trimester. Global sensitivity analyses highlighted the predominant influence of the placental transfers on the fetal exposure, as well as the metabolic clearance and the fraction unbound. Finally, the four transfer models could be considered depending on the framework of the use of the pPBPK model and the availability of data or resources to inform their parametrization.

Comparison of analgesic efficacy and fetal effects between transdermal administration of fentanyl and intramuscular administration of buprenorphine in pregnant sheep

OBJECTIVE

To compare analgesic efficacy and fetal effects between transdermal administration of fentanyl and IM administration of buprenorphine in pregnant sheep.

ANIMALS

12 healthy pregnant ewes.

PROCEDURES

Before study initiation, each ewe was confirmed pregnant with a single fetus between 113 and 117 days of gestation. Ewes were randomly assigned to receive buprenorphine (0.01 mg/kg, IM, q 8 h for 48 hours beginning 1 hour before anesthesia induction; n = 6) or fentanyl (a combination of transdermal fentanyl patches sufficient to deliver a dose of 2 μg of fentanyl/kg/h applied between the dorsal borders of the scapulae 24 hours before anesthesia induction; 6). Ewes were anesthetized and underwent a surgical procedure to instrument the fetus with an arterial catheter and place a catheter in utero for collection of amniotic fluid samples. Physiologic variables and behavioral changes indicative of pain were assessed, and amniotic fluid and blood samples from ewes and fetuses were collected for determination of drug concentrations at predetermined times.

RESULTS

Both protocols provided acceptable postoperative analgesia with no adverse effects observed in the ewes or fetuses. Compared with the buprenorphine protocol, the fentanyl protocol induced more profound analgesia, decreased the requirement for isoflurane during surgery, and was associated with a shorter anesthesia recovery time. Fetal indices did not differ significantly between the 2 analgesic protocols.

CONCLUSIONS AND CLINICAL RELEVANCE

Results indicated that both protocols provided acceptable analgesia. However, the fentanyl protocol was superior in regard to the extent of analgesia induced, inhalant-sparing effects, and anesthesia recovery time.

Oxycodone pharmacokinetics and fetal exposure after intravenous or epidural administration to the ewe

INTRODUCTION

There are limited data on oxycodone pharmacokinetics during pregnancy and on fetal exposure after maternal administration. The present study describes the pharmacokinetics of intravenous (i.v.) oxycodone in pregnant sheep and fetal exposure after intravenous and epidural administration.

MATERIAL AND METHODS

Ten pregnant sheep received 0.1 mg·kg^-1 oxycodone intravenously, and blood samples were collected up to 24 hours. Seven days later, the ewes were randomized to receive 0.5 mg·kg^-1 oxycodone intravenously (n = 5) or epidurally (n = 5) as a single bolus, before laparotomy for placement of catheters into the fetal superior vena cava and carotid artery. Paired maternal and fetal blood samples were taken when the fetal arterial catheter was in place and at the end of surgery. Maternal blood samples were taken up to 24 hours.

RESULTS

After 0.1 mg·kg^-1 oxycodone intravenously, the median clearance was 5.2 L·h^-1 ·kg^-1 (range 4.6-6.2), but the volume of distribution varied between 1.5 and 4.7 L·kg^-1 . The area under the curve was 17 h·ng·mL^-1 (range 14-19) and the plasma concentration at 2 minutes 60 ng·mL^-1 (range 50-74). Following administration of 0.5 mg·kg^-1 intravenously or epidurally, oxycodone concentrations were similar in the maternal and the fetal plasma. Accumulation of oxymorphone in the fetus occurred; fetal-to-maternal ratios were 1.3-3.5 (median 2.1) in the i.v.-group and 0.9-3.0 (1.3) in the Epidural-group.

CONCLUSIONS

We determined the pharmacokinetics of oxycodone in pregnant sheep. We showed accumulation of oxymorphone, which an active metabolite of oxycodone, in the fetus. Further studies in human pregnancies are required to evaluate the safety of oxycodone.

The uptake of transdermal fentanyl in a pregnant sheep model

OBJECTIVE

To evaluate the maternal and foetal uptake of transdermal fentanyl patch applied to the groin of pregnant sheep following surgery.

STUDY DESIGN

Prospective series.

ANIMALS

A group of 16 singleton pregnant sheep underwent anaesthesia for laparotomy, hysterotomy and instrumentation of the foetus. Of these ewes 10 (101 ± 12 days of gestation) were used to evaluate the maternal uptake of transdermal fentanyl, and the efficacy of the drug in the postoperative period (n = 10). To determine the extent of transplacental transfer of fentanyl, six ewes from the group of 10, and six other ewes (92 ± 1 days' gestation) were studied.

METHODS

A 75 μg hour^-1 fentanyl patch was placed onto the woolless skin of the medial thigh close to the groin at the end of surgery. Maternal blood samples were collected from the cephalic or jugular vein, and pain and sedation scores were determined, prior to application of the patch (time 0) and at 3, 6, 12, 24, 36 and 48 hours after. A commercial Fentanyl ELISA kit was used to determine the concentration of fentanyl. Paired maternal and foetal blood samples were collected 48 hours after surgery. Animals were euthanized at the end of the study. Data were tested for normality and compared with Student t test or one-way anova and are expressed as mean ± standard deviation or median (range).

RESULTS

Recovery from anaesthesia and surgery was uneventful in all ewes. The dose of fentanyl was 1.4 ± 0.2 μg kg^-1 hour^-1. The maximum maternal plasma concentration of fentanyl was 0.547 ng mL^-1 (range, 0.349-0.738 ng mL^-1) at 12 hours. After 48 hours, the concentration of fentanyl was 0.381 ng mL^-1 (range, 0.211-0.487 ng mL^-1; maternal) and 0.295 ng mL^-1 (range, 0.185-0.377 ng mL^-1; foetal; p = 0.175). The placental transfer rate of fentanyl was 77%.

CONCLUSIONS AND CLINICAL RELEVANCE

The uptake of fentanyl varied between animals. The placental transfer rate of fentanyl was 77%.