Methoxyflurane metabolism and renal function after methoxyflurane analgesia during labor and delivery

Analgesic use of inhaled methoxyflurane: Evaluation of its potential nephrotoxicity

Methoxyflurane is a volatile, halogenated analgesic, self-administered in a controlled low dose from the Penthrox(®) inhaler for short-term pain relief. It was formerly used in significantly higher doses to produce anaesthesia, when it caused a specific type of dose-related renal tubular damage. The pathogenesis of the renal damage and clinical use of methoxyflurane are discussed here with evidence that a low but effective analgesic dose is not associated with the risk of renal adverse effects. The maximum dose employed to produce analgesia is limited to methoxyflurane 6 mL/day and 15 mL/week, producing a minimum alveolar concentration (MAC) of 0.59 MAC-hours. Renal damage is due to the metabolism of methoxyflurane and release of fluoride ions. Exposure of humans to methoxyflurane ≤2.0 MAC-hours, resulting in serum fluoride ≤40 µmol/L, has not been associated with renal tubular toxicity. The safety margin of analgesic use of methoxyflurane in the Penthrox ((®)) inhaler is at least 2.7- to 8-fold, based on methoxyflurane MAC-hours or serum fluoride level, with clinical experience suggesting it is higher. It is concluded from clinical experience in emergency medicine, surgical procedures and various experimental and laboratory investigations that the analgesic use of methoxyflurane in subanaesthetic doses in the Penthrox inhaler does not carry a risk of nephrotoxicity.

Inhaled analgesia for pain management in labour

BACKGROUND

Many women would like to have a choice in pain relief during labour and also would like to avoid invasive methods of pain management in labour. Inhaled analgesia during labour involves the self-administered inhalation of sub-anaesthetic concentrations of agents while the mother remains awake and her protective laryngeal reflexes remain intact. Most of the agents are easy to administer, can be started in less than a minute and become effective within a minute.

OBJECTIVES

To examine the effects of all modalities of inhaled analgesia on the mother and the newborn for mothers who planned to have a vaginal delivery.

SEARCH METHODS

We searched the Cochrane Pregnancy and Childbirth Group's Trials Register (31 January 2012), ClinicalTrials.gov, and Current Controlled Trials (2 June 2012), handsearched conference proceedings from the American Society of Clinical Anesthesia (from 1990 to 2011), contacted content experts and trialists and searched reference lists of retrieved studies.

SELECTION CRITERIA

Randomised controlled trials comparing inhaled analgesia with other inhaled analgesia or placebo or no treatment or other methods of non-pharmacological pain management in labour.

DATA COLLECTION AND ANALYSIS

Review authors independently assessed trials for eligibility, methodological quality and extracted all data. Data were double checked for accuracy.

MAIN RESULTS

Twenty-six studies, randomising 2959 women, were included in this review.Inhaled analgesia versus a different type of inhaled analgesia Pain relief was measured using a Visual Analogue Scale (VAS) from 0 to 100 mm where 100 corresponds to the most relief. Pain intensity was measured using a VAS from 0 to 100 mm, where 0 corresponds to no pain at all and 100 corresponds to the worst pain. The highest score for pain relief is the most positive in contrast to 'pain intensity' in which the higher score is more negative. Flurane derivatives were found to offer better pain relief than nitrous oxide in first stage of labour as measured by a lower pain intensity score (average mean difference (MD) 14.39, 95% confidence interval (CI) 4.41 to 24.37, three studies, 70 women), also a higher pain relief score for flurane derivatives compared with nitrous oxide (average MD -16.32, 95% CI -26.85 to -5.79, two studies, 70 women). Substantial heterogeneity was found in the analyses of pain intensity (P = 0.003) and in the analysis of pain relief (P = 0.002).These findings should be considered with caution because of the questionable design of the included cross-over trials. More nausea was found in the nitrous oxide group compared with the flurane derivatives group (risk ratio (RR) 6.60 95% CI 1.85 to 23.52, two studies, 98 women).Inhaled analgesia versus placebo or no treatment Placebo or no treatment was found to offer less pain relief compared to nitrous oxide (average RR 0.06, 95% CI 0.01 to 0.34, two studies, 310 women; MD -3.50, 95% CI -3.75 to -3.25, one study, 509 women). However, nitrous oxide resulted in more side effects for women such as nausea (RR 43.10, 95% CI 2.63 to 706.74, one study, 509 women), vomiting (RR 9.05, 95% CI 1.18 to 69.32, two studies, 619 women), dizziness (RR 113.98, 95% CI 7.09 to 1833.69, one study, 509 women) and drowsiness (RR 77.59, 95% CI 4.80 to 1254.96, one study, 509 women) when compared with placebo or no treatment.There were no significant differences found for any of the outcomes in the studies comparing one strength versus a different strength of inhaled analgesia, in studies comparing different delivery systems or in the study comparing inhaled analgesia with TENS.Due to lack of data, the following outcomes were not analysed within the review: sense of control; satisfaction with childbirth experience; effect on mother/baby interaction; breastfeeding; admission to special care baby unit; poor infant outcomes at long-term follow-up; or costs.

AUTHORS' CONCLUSIONS

Inhaled analgesia appears to be effective in reducing pain intensity and in giving pain relief in labour. However, substantial heterogeneity was detected for pain intensity. Furthermore, nitrous oxide appears to result in more side effects compared with flurane derivatives. Flurane derivatives result in more drowsiness when compared with nitrous oxide. When inhaled analgesia is compared with no treatment or placebo, nitrous oxide appears to result in even more side effects such as nausea, vomiting, dizziness and drowsiness. There is no evidence for differences for any of the outcomes comparing one strength verus a different strength of inhaled analgesia, comparing different delivery systems or comparing inhaled analgesia with TENS.

Anesthesia for cesarean section--VII. Early effects on neonatal renal function of enflurane anesthesia for cesarean section

Fourteen women undergoing elective cesarean section were anesthetized with enflurane for induction and maintenance of anesthesia. Blood enflurane and serum inorganic fluoride levels were studied in the mothers at delivery and in the neonates at delivery and in the early neonatal period. Also neonatal urinary excretion of inorganic fluoride was determined. Renal function of the neonates was tested with a sodium load at the age of 1 day. Enflurane showed the expected distribution between mothers and neonates. The enflurane level of the neonates declined rapidly after delivery. Serum inorganic fluoride level increased significantly in the mothers from start of anesthesia until delivery. The neonatal level was lower and did not reach the level of neonates in areas with a high natural fluoride content in drinking water. The level of inorganic fluoride in urine was higher than the corresponding serum level, indicating an inorganic fluoride concentrating effect of the neonatal kidney. The difference between urinary excretion and serum content of inorganic fluoride showed neonatal metabolism of enflurane. The urinary sodium excretion did not differ from that of infants delivered without halogenated inhalation agents given to the mothers.

Placental transfer of intravenous fluoride in the pregnant ewe

Fluoride ion is produced with the biotransformation of two commonly used anesthetics, methoxyflurane and enflurane. Fluoride ion is added to prenatal vitamin preparations and water to prevent carious teeth. Very high levels of fluoride assailure. Previous studies of placental transfer of fluoride ion were conflicting. Our study in pregnant ewes revealed rapid transfer of the fluoride ion across the placenta with high fetal to maternal ratios as early as 1 minute.