Obstetric analgesia. Clinical pharmacokinetic considerations

Academic Detailing of Meperidine at a Teaching Hospital

Meperidine (Demerol) is an opiate analgesic that is not considered first-line therapy for most pain management indications because of concerns about its safety and efficacy. Inpatient data from a 417-bed community teaching hospital revealed high use of meperidine in oral, IM, and IV forms. A multifaceted academic detailing approach was employed to change prescribing behavior and decrease meperidine use. This approach included conducting two concurrent Medication Use Evaluations; Grand Rounds presentations for pharmacy staff, nurses, and medical residents; solicitation of opinion leaders; pocket and table-top cards; newsletter articles; and provision of pharmaceutical care. Comparing the number of meperidine doses dispensed per adjusted patient day before and after the intervention, use was reduced by 0.0966 doses per patient (P < 0.05: 95% CI, 0.0955 to 0.0977). The number of patients receiving meperidine was reduced by 2.43% (P < 0.05: 95% CI, 1.97 to 2.88). This translates into a relative reduction of 29.5% in patients receiving meperidine and a relative reduction of 31% in meperidine doses dispensed per patient after academic detailing initiatives vs before. Eighty-five percent of standard orders were changed to improve therapy; these changes included converting meperidine to morphine or hydromorphone, decreasing cumulative acetaminophen daily dosages, using controlled-release and immediate-release opioids for pain management when oral therapy was tolerated, and combining modalities with different mechanisms of action for synergy and to decrease potential adverse effects from larger dosages of single entities. Academic detailing of meperidine resulted in short-term changes in prescribing patterns and decreased meperidine use at this institution. Long-term implications for pain management have not yet been assessed.

Could local anesthesia while breast-feeding be harmful to infants?

BACKGROUND

Few studies have been carried out on the levels and possible toxicity of local anesthetics in breast milk after parenteral administration. The purpose of this study is to determine the amount of lidocaine and its metabolite monoethyl-glycinexylidide (MEGX) in breast milk after local anesthesia during dental procedures.

METHODS

The study population consisted of seven nursing mothers (age, 23-39 years) who received 3.6 to 7.2 mL 2% lidocaine without adrenaline. Blood and milk concentrations of lidocaine and its metabolite MEGX were assayed using high-performance liquid chromatography. The milk-to-plasma ratio and the possible daily doses in infants for both lidocaine and MEGX were calculated.

RESULTS

The lidocaine concentration in maternal plasma 2 hours after injection was 347.6 +/- 221.8 microg/L, the lidocaine concentration in maternal milk ranged from 120.5 +/- 54.1 microg/L (3 hours after injection) to 58.3 +/- 22.8 microg/L (6 hours after injection), the MEGX concentration in maternal plasma 2 hours after injection was 58.9 +/- 30.3 microg/L, and the MEGX concentration in maternal milk ranged from 97.5 +/- 39.6 microg/L (3 hours after injection) to 52.7 +/- 23.8 microg/L (6 hours after injection). According to these data and considering an intake of 90 mL breast milk every 3 hours, the daily infant dosages of lidocaine and MEGX were 73.41 +/- 38.94 microg/L/day and 66.1 +/- 28.5 microg/L/day respectively.

CONCLUSIONS

This study suggests that even if a nursing mother undergoes dental treatment with local anesthesia using lidocaine without adrenaline, she can safely continue breastfeeding.

Intrapartum pain: pharmacologic management

Even with the tremendous therapeutic benefit of nonpharmaceutical pain relief measures for laboring women, pharmaceutical therapies often are needed. Nurses and other health care providers need to be familiar with the differing pharmaceutical properties of commonly prescribed pain-relieving drugs. The pharmaceutical properties of sedatives and hypnotics, opioids, and local anesthetic agents used to relieve pain during labor and delivery are reviewed. Individualization of drug therapy and maximal therapeutic effects result when the health care provider is informed about the pharmaceutical properties of analgesic and anesthetic agents so that a wise choice can be made.

Labour analgesia. A risk-benefit analysis

The pain associated with labour can be severe. The ideal labour analgesic does not exist and systemic opioids provide little relief. Nausea, vomiting and sedation are common adverse effects of systemic opioids. Paracervical block can relieve only the pain of the first stage of labour. The duration of analgesia obtained using paracervical block is limited and repeat blocks increase the risk of direct fetal injection. Epidural analgesia effectively relieves labour pain. The insertion of an epidural catheter can provide continuous analgesia throughout labour. In addition, the catheter can be used to provide surgical anaesthesia, should operative delivery be required. Epidural local anaesthetics commonly produce maternal hypotension and motor blockade. However, opioids potentiate the effect of epidural local anaesthetics. Thus, concomitant epidural opioid injection allows the use of lower concentrations of local anaesthetics, decreasing the frequency and severity of hypotension and motor blockade. Epidural analgesia has other, potentially catastrophic, adverse effects but, with safe clinical practice, these problems are extremely rare. Intrathecal injection of opioids or local anaesthetics also effective labour analgesia. However, no single intrathecal drug or drug combination reliably provides analgesia for the duration of labour. Many clinicians use both intrathecal and epidural analgesia as a combined spinal-epidural technique. This approach provides the rapid onset of intrathecal drugs and the flexibility of continuous epidural block. Fetal heart rate decelerations occasionally follow the use of any of the above labour analgesic techniques. Most studies of the aetiology of fetal heart rate decelerations have focused on factors unique to each analgesic technique. However, the similar timing and appearance of fetal bradycardia suggests a common cause. Induction of maternal analgesia may transiently alter the balance between factors encouraging and inhibiting uterine contraction. A temporary increase in the uterotonic effects of endogenous or exogenous oxytocin may then produce a tetanic uterine contraction with subsequent decrease fetal oxygen delivery and resultant fetal bradycardia. Regardless of aetiology, these bradycardias are transient and should not produce maternal or fetal morbidity. Much controversy surrounds the effects of analgesia, especially epidural block, on the course and outcome of labour. Various studies have reported that epidural analgesia slows labour, increases the incidence of malposition of the fetal head, increases the need for forceps delivery and increases the risk of caesarean delivery. Most of the studies reporting these effects are retrospective and nonrandomised. More careful studies suggest that specific anaesthetic techniques (i.e. local anaesthetic-opioid mixtures) or obstetrical management can limit or eliminate these 'risks' of epidural labour analgesia.

Anaesthetic agents and excretion in breast milk

This review is an update on anesthetic agents and their excretion into breast milk; it presents the reported effects on suckling infants, and discusses the precautions which should be considered. For most anaesthetic agents, there is very sparse information about breast milk excretion and even less published knowledge about the possible effects on the suckling infant. Generally, when an anaesthetic agent is given on a single-dose basis, there is no evidence that it is excreted in breast milk in clinically significant amounts, even if there are detectable concentrations of the drug in the milk. Most anaesthetics are rapidly cleared from the mother, and, consequently, it should be possible to allow suckling as soon as practically feasible after surgery. However, repeated administration of certain opiates and benzodiazepines has been reported to cause adverse effects in neonates, with premature neonates apparently being more susceptible. Thus, in long-term treatment with these drugs, the importance of uninterrupted breast feeding should be assessed against possible adverse drug effects in the neonate.

Early respiratory depression during caesarean section following epidural meperidine

A case of respiratory depression which occurred following administration of epidural meperidine during Caesarean section is described. Epidural meperidine, 75 mg (10 mg.ml-1) was given after delivery of the infant to provide postoperative analgesia. Oxygen desaturation (SaO2 90%) and a decrease in respiratory rate (4.min-1) were noted 30 min after epidural meperidine was administered. Naloxone, 0.1 mg, was given iv which resulted in prompt improvement in both respiratory rate and oxygen saturation. Vascular absorption of meperidine from the epidural venous plexus is the most probable explanation for this case of early respiratory depression. We recommend a maximum bolus dose of 50 mg of epidural meperidine for pain management after Caesarean section. It is also important to monitor oxygen saturation continuously during the intraoperative period, and to monitor the patient closely in the recovery room for at least one hour for evidence of respiratory depression.

Risk-benefit assessment of anaesthetic agents in the puerperium

A critical evaluation of anaesthetic agents in the puerperium is difficult because systematic, relevant studies are still lacking. Current knowledge of the effects of different agents used in labour and caesarean section indicates that significant residual effects on the mother and newborn are limited. In the early puerperium, based on physiological and/or hormonal changes, the mother could be more sensitive to inhalational anaesthetic agents and local analgesics. To date there is no evidence that any anaesthetic agent is excreted in breast milk in clinically significant amounts when given as a single dose. The only exception is perhaps in the case of very premature neonates whose mothers have had multidrug therapy before labour. Even then the importance of breast milk should be carefully assessed against possible adverse drug effect. However, repeated administration of long-acting benzodiazepines and continuous epidural administration of pethidine (meperidine) can have adverse effects on the neonate. The essential conclusion of this review is that breast-feeding is best. The different anaesthetic agents are excreted in the milk in amounts so low that detrimental effects on the neonate should not be expected.

Comparison of propofol and thiopentone for induction of anaesthesia for elective caesarean section

Propofol 2.5 mg/kg was compared with thiopentone 5 mg/kg as an induction agent for elective Caesarean section. Thirty-two healthy women with cephalopelvic disproportion were included in an open randomised study. The placental transfer of propofol was also studied in 10 other mothers given a single dose of 2.5 mg/kg. The induction characteristics and haemodynamic response to propofol and thiopentone were similar. Side effects were rare with both agents, but propofol caused more discomfort on injection compared to thiopentone. Recovery times were shorter after propofol as evaluated by time to orientation, recovery scoring after anaesthesia and measurements with the Maddox wing. Rapid placental transfer and significant fetal uptake were detected for propofol. There was no significant neonatal depression as assessed by Apgar scores and blood gas analyses. Propofol appears to be a suitable alternative to thiopentone as an induction agent for anaesthesia in elective Caesarean section.

Epidural analgesia with a bupivacaine-fentanyl mixture in obstetrics: comparison of repeated injections and continuous infusion

We compared the efficacy and side-effects of continuous infusion versus repeated injections of epidural bupivacaine-fentanyl during labour. Forty-four parturients were randomly distributed into two groups balanced for population size, morphology and parity. Analgesia was begun at the same stage of labour with a mixture of 20 ml 0.25 per cent plain bupivacaine and 2 ml (100 micrograms) fentanyl. In Group I the initial dose ranged from 8-12 ml as a function of height; an injection of the same dose was repeated immediately upon recurrence of pain. In Group II, after an initial dose of 5-7 ml, a continuous infusion of 3 ml.h-1 was begun, and continued until full dilatation. Analgesia was rated using a pain scale; effects on maternal blood pressure, respiratory rate and neonatal status were noted. Bupivacaine and fentanyl assays were carried out on maternal venous blood in 30 parturients during the course of labour. There was a longer latency to onset of analgesia in Group II (approximately five minutes), followed by a more constant degree of analgesia. This better analgesia cannot be accounted for by a difference in dosage; doses were significantly lower in Group II, despite the fact that labour was of the same duration. The course of labour, and maternal and neonatal status were comparable in the two groups. Assays showed no difference in bupivacaine blood concentrations between the two groups nor signs of drug accumulation. The constant infusion technique is advantageous since it provides a more regular degree of analgesia with lower doses than those required for patients having repeated injections.

The transfer of ketorolac tromethamine from maternal to foetal blood

Thirty two women who were participating in an efficacy study comparing 10 mg ketorolac with 50 mg or 100 mg of pethidine in the relief of labour pain, underwent sampling of vein blood, for determination of plasma ketorolac concentrations. The sample was withdrawn at delivery and a sample of umbilical cord blood was withdrawn at the same time. The ratio of ketorolac concentrations in the cord blood sample: the maternal venous sample were calculated and plotted against the time elapsed between drug administration and sampling. Samples for one patient, withdrawn 24 min after dosing, had ketorolac concentrations below the quantification limit. The ratios in the remaining patients were all low and showed a tendency to increase with time. The mean ratio was 0.116 with a range of 0.04 in 2 patients, at 43 min and 1 h 6 min, to 0.25 at 6 h 34 min.