Pethidine meperidine

Pharmacology

Because of its unparalleled spasmoanalgesic action during labor, pethidine is the analgesic of choice for this indication. When it is used during labor, the labor is not lengthened and the strength of the contractions is not diminished. Pethidine does not appear to influence either the severity of bleeding or the involution of the uterus postpartum.

During pregnancy, pethidine passes unimpeded to the fetus and can reach higher concentrations there than in the maternal serum. Because of the limited metabolic capacity in newborns, pethidine is decomposed only slowly and has a considerably lengthened halflife (18 hours as compared to 3-4 hours in adults; Caldwell 1978).

Toxicology

Pethidine is one of the most thoroughly studied spasmoanalgesics used in labor. The metabolic acidosis described after parenteral application (De Boer 1987, Kariniemi 1986) can probably be explained by individual overdose and the resultant hypotonic circulatory reaction in the mother. Epidemiologic studies have not uncovered an association between the use of pethidine during the first trimester and congenital malformations (Heinonen 1977). In newborns, pethidine can induce respiratory depression. Behavioral (for instance sucking behavior) and EEG disturbances, which may continue beyond the first few days of life, have been observed. The magnitude of these effects will vary with timing and dosing (Kansjci-Arvidson 2001, Hafstrom 2000, Nissen 1997). Because of the pharmacokinetics, observable effects are to be expected when maternal administration occurs between 1 and 4 hours before delivery. Multiple doses to the mother over a longer period of time result in accumulation of pethidine and its metabolite (Nissen 1997, Kuhnert 1985). Premature infants are at greater risk. Clinical studies have not uncovered lasting impairments of neonatal function.

The safety and efficacy of the different strategies for labor analgesia have been discussed extensively in the literature during the last decade. There arc reports claiming that systemically administered pethidine (and other opioids) lack analgesic effectiveness for labor pain, but serve primarily to sedate the mother and, inadvertently, the neonate (Reynolds 1997, Olofsson 1996). Other strategies discussed include paracervical block, spinal blockade and epidural analgesia with local anesthetics or opioids. Combinations of epidural with parenteral analgesia and of epidural with spinal analgesia are also used (Eberle 1996). Epidural analgesia seems to be very effective in reducing pain during labor, but may also have some adverse effects.

Recommendation. The spasmoanalgesic, pethidine, can be used during labor when there are critical indications for it; with premature births, however, it is relatively contraindicated. Use as an analgesic during pregnancy is not recommended; inadvertent use does not require either a termination of the pregnancy or additional diagnostic procedures.

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