Methoxyflurane anesthesia in pediatric patients: evaluation of anesthetic metabolism and renal function

A pilot study of inhaled methoxyflurane for procedural analgesia in children

BACKGROUND

Methoxyflurane (MF), a potent volatile anesthetic, can be used as an analgesic in subanesthetic concentrations. In Australia, MF is extensively used in children and adults as an analgesic in the prehospital setting via a hand-held inhaler device. We conducted a pilot study to explore its use as a patient controlled analgesic for painful procedures in children in the emergency department (ED).

METHODS

This is a prospective observational case series of children aged 5 years and older requiring procedural analgesia for brief painful procedures. Pain scores, depth of sedation, adverse events and patient, parent and staff satisfaction were assessed as well as consumption of MF measured.

RESULTS

Fourteen patients (aged 6-13 years) received MF mainly for extremity injuries. Amount of MF consumed ranged from 0.36 to 3.06 g per patient inhaled over 4-25 min. There were no serious adverse events. No patient was deeply sedated. Five patients had mild brief self-resolving adverse events including agitation, euphoria, blurry vision, dizziness and cough. Four patients with fractures with initial high pain scores (> or =6) received MF for bridging analgesia with large drops in pain scores. Four patients who required fracture reductions with initial low scores did not achieve adequate analgesia. The remaining six patients had painful procedures undertaken with satisfactory analgesia.

CONCLUSIONS

On the basis of this small pilot study of MF use in children in the ED, this agent appears to be a powerful analgesic. MF seems most useful as a self-titrated bridging analgesic agent in patients after extremity trauma. It appears less useful as a procedural agent when patients are unable to anticipate and achieve a sufficient level of analgesia before painful stimulus infliction. Pre- and intraprocedure coaching is an important aspect of its use especially if initial pain scores are low.

Inhaled methoxyflurane as a prehospital analgesic in children

OBJECTIVE

Despite widespread use of methoxyflurane as an inhaled analgesic by ambulance services in Australia there are no published data as to its use pattern, efficacy and safety in the prehospital setting. We set out to characterize methoxyflurane use in children in the prehospital setting.

METHODS

An observational case series was conducted over an 8 month period. Children who received methoxyflurane while being transported to a tertiary children's hospital by ambulance were enrolled. We analysed indications for use, verbal numerical pain scores, adverse events and depth of sedation based on paramedic, patient, parent and ED staff surveys and review of ambulance care records.

RESULTS

During the study period 105 patients were enrolled with an age range of 15 months to 17 years (median age 11 years). Methoxyflurane was mainly used for extremity injuries (82%). Paramedic pain scores dropped from a mean of 7.9 (95% confidence interval [CI] 7.5-8.3) prior to methoxyflurane use to 4.5 (95% CI 3.9-5.0) at 2-5 min and to 3.2 (95% CI 2.8-3.7) at 10 min. There were no serious adverse events (one-sided 97.5% CI 0-3%). Mild adverse events occurred in 38 patients (36.2%; 95% CI 27.0-46.1%). Five of 15 (33.3%) patients under 5 years of age were deeply sedated.

CONCLUSION

In the present paediatric case series methoxyflurane appears to be an efficacious analgesic with a low adverse events profile. In young children in particular it can briefly lead to deep sedation.

A methoxyflurane delivery system for stereotaxic surgery

Methoxyflurane (2,2-dichloro-1,1-difluro-ethyl methyl ether; Metofane) is a potent general inhalation anesthetic that is well-suited for small animal surgery. Methoxyflurane is particularly attractive as an anesthetic agent in neurological stereotaxic surgery, because methoxyflurane does not markedly attenuate the rate of anterograde or retrograde transport of horseradish peroxidase, or reduce the consistency and/or extent of excitatory neurotoxin damage. Methoxyflurane also is non-flammable when mixed with O2 or air at anesthetic concentrations. The use of methoxyflurane anesthesia in stereotaxic surgery has been limited because methoxyflurane must be delivered via a vaporizer system that will easily interface with standard stereotaxic headholders. The present report describes a simple, reliable and inexpensive methoxyflurane delivery system for stereotaxic surgery.

Anaesthesia and the kidney

Applied anatomy and physiology of the kidney are briefly reviewed. This includes an account of renal blood flow, glomerular filtration rate, juxtaglomerular apparatus, renal autoregulation and intra-renal blood flow distribution, tubular transport mechanisms, solute handling in proximal tubule, function of loop of Henle and distal tubule system. This section concludes with a summary of changes in tubule fluid along the length of the nephron. Acute effects of anaesthesia are reviewed in detail. Indirect effects include those on circulatory and sympathetic nervous systems, autoregulation, endocrine systems such as those involving anti-diuretic hormone, adrenaline and noradrenaline, renin-angiotensin and aldosterone. Direct effects of anaesthesia on renal function have now been confirmed both in vitro and in vivo. Delayed direct nephrotoxicity of anaesthetics relates predominantly to methoxyflurane (MOF) and its metabolism to inorganic fluoride. Other factors are MOF dose, genetics, age, enzyme induction, obesity, other nephrotoxic drugs. Clinical implications are presented. Enflurane nephrotoxicity is rare but aetiologic factors are similar to the foregoing. Isoflurane and halothane are not nephrotoxic. A consideration of the influence of anaesthetic management on the incidence and severity of postoperative acute renal failure concludes the review.

Anaesthetic experience using a standard technique for laryngeal surgery in infants and children

A 12 year review is presented of 1922 anaesthetic experiences on 285 patients presenting for microlaryngeal operations at the Hospital For Sick Children, Toronto. The lesions treated were laryngeal growths, 198 cases (69.5 per cent): subglottic stenosis, 81 cases (28.4 per cent); and posterior laryngeal cleft, 6 cases (2.1 per cent). The anaesthetic technique consisted of topical analgesia with lidocaine spray (3 mg kg-1) and pharyngeal insufflation of halothane and methoxyflurane both 1-1.5 per cent in oxygen through a special channel in the blade of a Jako Pilling microlaryngoscope in spontaneously breathing patients. Arterial blood gases sampled after 15 and 40 minutes of anaesthesia showed Paco2 = 6.29 +/- 0.31 kPa (47.1 +/-2.39 torr) and 6.44 +/- 0.16kPa (278 +/- 36.2 torr) respectively. Surgical exposure of the larynx was good allowing for a shorter operative time with minimal perioperative complications, in only 94 (4.89 per cent) of the 1922 anaesthetics.

Toxicity following methoxyflurane anaesthesia. IV. The role of obesity and the effect of low dose anaesthesia on fluoride metabolism and renal function

Seven obese and five normal weight patients were studied before, during and after one hour of methoxyflurane-nitrous oxide anaesthesia during peripheral surgical operations and compared with eight patients of normal weight anaesthetized with nitrous oxide-meperidine and d-tubocurare. Estimates were made of renal function, including serum and urinary electrolytes, osmolarity, uric acid, urea and creatinine. Renal clearances for the latter three substances were also calculated. Serum and urinary inorganic and organic fluoride concentrations were measured, as were renal clearances. This low dose methoxyflurane anaesthesia resulted only in a decrease in uric acid clearance among all the measures, when compared to the meperidine-nitrous oxide controls. The clearance of uric acid remained depressed for longer in the obese patients, but otherwise they did not differ from the normal weight patients. It is possible but not proven that depressed uric acid clearance may be related to the organic fluoride metabolite and an early indicator of methoxyflurane renal toxicity. The previously documented biotransformation of methoxyflurane was seen in this study. A double peak in serum inorganic fluoride was shown in all patients but one. Rather large differences in peak levels of serum inorganic fluoride occurred. The only significant difference between the obese and normal weight patients as far as fluoride metabolism was concerned was a greater variability in the serum inorganic fluoride levels in the obese patients. It would appear that the obese patient metabolizes methoxyflurane in a quantitatively if not qualitatively different fashion than the normal weight patient, perhaps because of fatty infiltration of the liver. Caution is advised in the use of methoxyflurane for more than 90 minutes of low concentration administration in view of the unpredictability of the biotransformation.

Paediatric anaesthesia: pharmacological considerations

The young infant differs from the adult in his quantitative responses to many anaesthetic drugs and adjuncts. In the neonate, the larger extracellular fluid volume and blood volume, the smaller muscle mass and fat stores, and presumable greater blood flow to the central organs, not only influence the distribution of drugs to their active site but also secondary redistribution. The neonatal hepatic anzyme systems responsible for the metabolism of drugs are incompletely developed or absent. Glomerular filtration, important for drug excretion, is inefficient by adult standards. The neonate has increased toxicity and sensitivity to a variety of sedative-hypnotics, narcotics, and local anaesthetics. On the other hand, the infant requires more suxamethonium (succinylcholine) and ketamine on a weight basis that does the adult. The response of some infants to non-depolarising muscle relaxants resembles that of the myasthenic patients. The rate of uptake of alveolar levels of inhalation anesthetics is more rapid in infants and children than in adults. In addition, the neonate requires more anaesthetic than the adult for a given surgical stimulus. Biotransformation of inhalation anaesthetics is limited in neonates. Awareness of these pharmacological differences and their probable explanations allows one to provide rational, safer anaesthesia to infants.