The influence of adrenaline on the maternal plasma levels and placental transfer of lignocaine following lumbar epidural administration

Developments in labour analgesia and their use in Australia

Since the introduction of chloroform for labour analgesia in 1847, different methods and medications have been used to relieve the pain of labour. The use of heavy sedative medication in the early 1900s was encouraged by enthusiastic doctors and by women empowered by the women's suffrage movement in America. Nitrous oxide by inhalation has been used in Australia since the 1950s and improved methods of administration have made this method of analgesia safe and practical. Caudal epidural analgesia and lumbar epidural analgesia were first made popular in America and by the 1970s these techniques were more widely available in Australia. In 1847, physicians and the public were unsure whether relieving labour pains was the 'right' thing to do. However, many medical and social changes have occurred thanks to the clinical connection between Australia and the United Kingdom and those first settlers to land on Australian shores. Thanks to this historical connection, in today's Australia there is no question that women should use analgesia as a pain relief if they wish. Currently, the majority of women worldwide use some form of analgesia during labour and different methods are widely available. This paper discusses the four milestones of the development of obstetric analgesia and how they were introduced into patient care in Australia.

Placental transfer of drugs administered to the mother

Drugs administered to mothers have the potential to cross the placenta and reach the fetus. Under particular circumstances, the comparison of the drug concentration in the maternal and fetal plasma may give an idea of the exposure of the fetus to the maternally administered drugs. In this review drugs are classified according to their type of transfer across the placenta. Several drugs rapidly cross the placenta and pharmacologically significant concentrations equilibrate in maternal and fetal plasma. Their transfer is termed 'complete'. Other drugs cross the placenta incompletely, and their concentrations are lower in the fetal than in maternal plasma. The majority of drugs fit into 1 of these 2 groups. A limited number of drugs reach greater concentrations in fetal than maternal plasma. It is said that these drugs have an 'exceeding' transfer. The impression prevails that suxamethonium chloride (succinylcholine chloride) and doxorubicin do not cross the placenta. However, a careful analysis of the literature suggests that this impression is wrong and that all drugs cross the placenta, although the extent transfer varies considerably. The following parameters were considered as possible factors determining the extent of placental transfer: (i) the molecular weight of the drug; (ii) the pKa (pH at which the drug is 50% ionised); and (iii) the extent of drug binding to the plasma protein. Drugs with molecular weights greater than 500D have an incomplete transfer across the human placenta. Strongly dissociated acid drug molecules should have an incomplete transfer, but this does not seem to be an absolute rule. For example, ampicillin and methicillin transfer completely and they are strongly dissociated at physiological pH. The extent of drug binding to plasma protein does not influence the type of drug transfer across the human placenta.

Extending epidural blockade for emergency caesarean section Evaluation of 2% lignocaine with adrenaline

Pre-existing epidural analgesia was rapidly extended in 36 consecutive patients presenting for emergency caesarean section by using 20 ml of 2% lignocaine + 1/200,000 adrenaline freshly prepared given by slow bolus injection. Despite a wide range of initial analgesic sensory levels the technique produced blocks that were dense with adequate anaesthesia for surgery in all patients within 12.5 min. The plasma local anaesthetic concentrations were well below the toxic range. Neonatal condition was good.

Obstetric analgesia. Clinical pharmacokinetic considerations

All drugs used in obstetric analgesia are more or less lipophilic, their site of action is in the central nervous system, and they have good membrane penetrability in the fetomaternal unit. Thus the dose and method of administration as well as the duration of treatment are important clinical determinants of drug effects in the fetus and newborn. In the past, too much emphasis has been placed on fetomaternal blood concentration ratios of different agents; it is now appreciated that the extent of fetal tissue distribution and the neonatal elimination rate are pharmacokinetically much more important. Extensive fetal tissue distribution is reflected in a low fetomaternal drug concentration ratio, which may be followed by prolonged neonatal elimination of the drug. Currently, the most effective and safest method for obstetric analgesia is regional epidural administration of bupivacaine or lignocaine (lidocaine); only low doses are needed and the newborn is able to handle these agents efficiently. On the basis of pharmacokinetic and neurobehavioural assessments, inhalational anaesthetic agents appear to be more attractive than pethidine (meperidine) or benzodiazepines. Intermittent administration and fast pulmonary elimination of inhalational agents ensure that long-lasting residual effects are unlikely to occur. The kinetics of epidural and intrathecal opiates explain the problems associated with their use in obstetrics. Among the newer drugs used in obstetric analgesia, the properties of meptazinol and isoflurane appear interesting and these agents warrant further study. All drugs used in obstetric analgesia have a potentially detrimental effect on the neonate and, therefore, knowledge of fetal and neonatal pharmacokinetics is of importance to the clinician.

[Maternal and fetal levels of local anaesthetics after epidural injection of a triple mixture]

Twenty-four parturients received an epidural injection of a 5.10(-6) adrenaline mixture containing: lidocaine 295 +/- 19 mg, bupivacaine 58 +/- 4 mg and etidocaine 58 +/- 4 mg. The mean serum levels measured in the mother (peripheral vein) and in the fetus (umbilical vein) and in the fetus (umbilical vein) and the fetus/mother ratios were, respectively:--1.05 +/- 0.47 micrograms . ml-1, 0.68 +/- 0.34 micrograms . ml-1 and 0.66 +/- 0.15 for lidocaine; --0.25 +/- 0.10 micrograms . ml-1, 0.14 +/- 0.06 micrograms . ml- and 0.51 +/- 0.16 for bupivacaine; ---0.27 +/- 0.10 micrograms . ml-1, 0.17 +/- 0.10 micrograms . ml-1 and 0.63 +/- 0.23 for etidocaine. From the sum of these concentrations the maternal and fetal serum levels and the fetus/mother ratio could be calculated in lidocaine equivalents. The values obtained were: 3.14 +/- 0.74 . ml-1, 1.90 +/- 0.68 micrograms . ml-1 and 0.60 +/- 0.13 respectively. A comparison of these data with those found in the literature led to the following conclusions: (1) The maternal serum levels of each anaesthetic drug in the mixture are the same as if it had been injected separately; (2) owing to tissue competition, the transplacental passage is increased by majoration of the free form, this being more pronounced with bupivacaine and etidocaine; (3) the circulating drug concentrations evaluated as lidocaine equivalent in the mother and fetus are comparable to those determined at the end of conventional local anaesthetic procedures.

The binding of bupivacaine to maternal and foetal plasma proteins

The binding of bupivacaine to maternal and foetal plasma proteins has been investigated using a three-compartment dialysis apparatus, in which maternal and foetal plasma could be examined simultaneously. Maternal protein bound approximately twice as much bupivacaine as foetal protein per gram of protein at drug concentrations in the range 0·05 to 5·0 μg/ml. Over the same range of concentrations, 92–78% of bupivacaine was bound to maternal plasma protein but only 35–31% was bound to human albumin indicating that other proteins are involved in the binding of the drug to plasma proteins.