Uptake of volatile anaesthetics in children

Pharmacokinetic/pharmacodynamic modeling of anesthetics in children: therapeutic implications

Modeling the pharmacokinetics and pharmacodynamics of anesthetics in children is performed as a response to the clinical need for safe and efficacious administration of drugs with a low therapeutic index. Rates and concentrations of these drugs, which are the primary parameters used by anesthesiologists, depend on physiologic parameters that are markedly affected by development. Volatile anesthetics have been used for >50 years in pediatric patients. The pharmacokinetics of inhalation agents are context sensitive, but little difference between age groups has been described. These agents are not only eliminated unchanged by the lung but they are also metabolized by the liver. Halothane has Michaelis-Menten kinetics, with up to 40% of the administered dose metabolized by the liver. For volatile anesthetics, the effect measured is the minimum alveolar concentration (MAC) that leads to movement of the limb in response to skin incision in 50% of the patients studied. The MAC is higher in infants than in children and adults. Infants aged 6 months have a MAC 1.5-1.8 times the MAC observed in adults aged 40 years. Children have a greater clearance and volume of distribution of propofol than adults. In order to achieve similar plasma concentrations, children require three times the initial dose used in adults. In adults, an increased sensitivity to propofol has been demonstrated with aging, but nothing is known about the effects in children. However, it is clear that equipotent doses of propofol induce marked deleterious hemodynamic effects in infants compared with children. Regional anesthesia is used in pediatrics, both in combination with general anesthesia during surgery or alone for postoperative analgesia. A marked decrease in protein binding has been described in infants. In the postoperative period, a rapid increase in binding because of inflammation decreases the free fraction, but the free drug concentration remains constant because of the resulting decrease in total clearance. A low clearance because of liver function immaturity has been observed during the first year(s) of life for bupivacaine and ropivacaine. Pharmacodynamic interactions between general anesthesia and regional anesthesia need to be modeled. This is one of the future tasks for pharmacokineticists. Methods such as the Dixon up-and-down allocation and the isobolographic technique are promising in this field.

Is intravenous lidocaine an effective adjuvant for endotracheal intubation in children undergoing induction of anesthesia with halothane-nitrous oxide?

STUDY OBJECTIVE

To evaluate the efficacy of intravenous (i.v.) lidocaine in suppressing the cough reflex and increases in intraocular pressure (IOP), heart rate (HR), and mean arterial pressure (MAP) elicited by endotracheal intubation.

DESIGN

Prospective, randomized, placebo-controlled, blinded study.

PATIENTS

60 ASA physical status 1 premedicated children aged 2 to 6 years undergoing induction of anesthesia with halothane-nitrous oxide (N2O) for surgery to correct strabismus.

INTERVENTIONS

Patients were randomly divided into two groups of 30 each. The control group (C) received saline and the treatment group (L) received 2 mg/kg i.v. lidocaine 90 seconds prior to endotracheal intubation.

MEASUREMENTS AND MAIN RESULTS

Awake HR and MAP; IOP, HR, and MAP 45 seconds prior to endotracheal intubation, immediately after endotracheal intubation, and 1 minute later, were recorded. Coughing was noted at endotracheal intubation. Lidocaine prevented coughing and a significant increase in IOP. Although significant increases in HR and MAP were observed in both groups (comparing preintubation and postintubation values), these increases were significantly less in the L group compared with the C group.

CONCLUSIONS

In healthy premedicated children, aged 2 to 6 years, who are undergoing induction of anesthesia with halothane-N2O, 2 mg/kg of lidocaine given 90 seconds prior to laryngoscopy effectively suppresses the cough reflex and increase in IOP secondary to endotracheal intubation and attenuates increases in HR and MAP.

[Anesthesia in the management of pyloric stenosis. Evaluation of the combination of propofol-halogenated anesthetics]

This study was aimed at evaluating haemodynamic changes during an anaesthetic sequence for full stomach, using propofol as induction agent and volatile anaesthetics for maintenance of anaesthesia in infants scheduled for surgical cure of hypertrophic pyloric stenosis. After correction of preoperative blood electrolyte and metabolic disturbances with appropriate i.v. hydrating solutions, anaesthesia was induced with propofol and suxamethonium. Infants were divided in two groups according to the volatile anaesthetic agent used for maintenance of anaesthesia after tracheal intubation: halothane (n = 16) or isoflurane (n = 15). The two groups were identical regarding weight (4.28 +/- 0.6 vs 4.14 +/- 0.76 kg), age (1.6 +/- 0.9 vs 1.5 +/- 0.6 months), preinduction heart rate (155 +/- 22 vs 151 +/- 22 b.min-1) and systolic-diastolic arterial pressure (96 +/- 18/58 +/- 12 vs 105 +/- 16/67 +/- 15 mmHg). Propofol and suxamethonium doses were identical, 3.9 +/- 1 mg.kg-1 and 1.3 +/- 0.6 mg.kg-1 respectively in halothane group, vs 4.3 +/- 0.8 mg.kg-1 and 1.3 +/- 0.4 mg.kg-1 in isoflurane group. Heart rate did not change after induction of anaesthesia, while arterial blood pressure decreased significantly (p < 0.001). However, blood pressure remained within the normal range for age throughout the procedure. Mean duration of surgery was shorter in halothane group (64 +/- 16 vs 79 +/- 17 min, p < 0.05), however time-interval from the end of surgery to tracheal extubation (12 +/- 6 vs 15 +/- 8 min) was short and identical in the two groups.(ABSTRACT TRUNCATED AT 250 WORDS)

[Comparison of the pharmacokinetics of etomidate in children and in adults]

Etomidate pharmacokinetics were compared in 12 children (P group) (age 7 to 13 years, weight 22 to 48 kg) and in 4 adult women (A group) (age 28 to 52 years, weight 46 to 72 kg), A.S.A. 1, undergoing minor non abdominal surgery. They were unpremedicated and anaesthetized with alfentanil 100 micrograms.kg-1, and isoflurane 2 vol % in N2O/O2 (1/1). Etomidate was administered as a bolus: 0.3 mg.kg-1 in adults and 0.4 mg.kg-1 in children. Venous plasma samples were frozen until further etomidate assay with a HPLC technique. In all patients but two children, data were fitted to a three rather than a two compartment model. Differences between groups (mean +/- SD values) included Vdc (P: 0.66 +/- 0.31 l.kg-1; A: 0.27 +/- 0.15 l.kg-1; p less than 0.01), t1/2 pi (P: 5.4 +/- 2.9 min; A: 2.7 +/- 5.7 min; p less than 0.05) and plasma clearance (P: 17.2 +/- 4.6 ml.kg-1.min-1; A: 10.9 +/- 3.3 ml.kg-1.min-1; p less than 0.05). No statistical difference was found between A and P groups for the following parameters: t1/2 alpha (37.1 +/- 12.0 min vs 26.8 + 15.1 min), t1/2 beta (260 +/- 99 min vs 175 +/- 99 min), Vdss (2.5 +/- 1.11.kg-1 vs 2.8 +/- 1.61.kg-1), Vd beta (4.1 +/- 2.41.kg-1 vs 4.0 +/- 2.21.kg-1), and MRT (228 +/- 80 min vs 172 +/- 101 min). No age-related difference was found inside P group with regard to pharmacokinetic parameters. In conclusion, a 30% higher etomidate bolus dosage is required in children than in adults to achieve similar plasma concentrations, due to a higher volume of the initial compartment. In comparison to adults the higher clearance suggests higher maintenance dose requirements in children.

Respiratory depression in children at different end tidal halothane concentrations

Respiratory motor function and timing were investigated at end tidal halothane concentrations of 1.5%, 1.0% and 0.5% before and during 4% carbon dioxide stimulation in 10 spontaneously breathing children who weighed between 10.2 and 25.2 kg, during hypospadias repair under halothane anaesthesia. Their tracheas were intubated and all received a caudal block to eliminate surgical stimulation. Pneumotachography and capnography were used and in three cases movements of ribcage and abdomen were also studied by magnetometers. Respiratory drive was evaluated by occlusion tests. Ventilation was depressed at an end tidal halothane concentration of 1.5%, with smaller tidal volumes, higher respiratory rates, higher end tidal carbon dioxide tensions and a weaker respiratory drive compared with 1.0% and 0.5% halothane. Paradoxical breathing was noted at 1.5% as well as at 1.0% but not at 0.5% halothane anaesthesia; the ribcage moved inwards during inspiration. Respiratory compensation during periods of 4% carbon dioxide stimulation was inadequate at 1.5% halothane, as indicated by higher end tidal carbon dioxide tensions, less negative occlusion pressures and movements of ribcage and abdomen that were unresponsive to 4% carbon dioxide, when compared with 1.0% and 0.5% halothane. Respiratory rates were higher and duration of inspiration longer at 1.5% than at 1.0% and 0.5% halothane. Respiratory timing was unaltered by carbon dioxide stimulation. It is concluded that the ventilatory motor response to carbon dioxide is dose dependent and improves at more superficial anaesthetic levels, while respiratory timing is unresponsive to carbon dioxide stimulation irrespective of the halothane concentration used. Paradoxical breathing existed at end tidal halothane concentrations higher than 1%.

Pulmonary ventilation and gas exchange in children anaesthetized with halothane, enflurane and isoflurane

The effects of similar anaesthetic levels of halothane (1.3 MAC), enflurane (1.2 MAC) and isoflurane (1.1 MAC) on pulmonary ventilation and gas exchange were investigated in 24 children subjected to minor and intermediate paediatric surgical procedures. Eight children were anaesthetized with each agent, pneumotachography and capnography were used, and airway as well as oesophageal pressures were measured. Minute ventilation (VE) was lower with enflurane than with halothane (P less than 0.001) and isoflurane (ns). Tidal volumes were, however, similar and variations in VE were thus caused by lower respiratory rates with enflurane than with the two other agents. Alveolar ventilation (VABohr) and carbon dioxide elimination (VCO2) were smaller and end-tidal CO2 tension higher with enflurane. Ventilatory efficiency was, however, somewhat better with enflurane as indicated by lower VDBohr/VT (ns) and VE/VCO2 (P less than 0.05) ratios compared with the two other agents. The effects of all three agents on dynamic compliance were similar, while total pulmonary resistance was less with isoflurane than with halothane and enflurane. It is concluded that although minute ventilation was smaller with enflurane than with halothane and isoflurane, ventilatory efficiency was similar due to a smaller dead space ventilation as a result of the lower respiratory rates in children anaesthetized with enflurane.

Halothane, enflurane and isoflurane anaesthesia for adenoidectomy in children, using two different premedications

In 48 children subjected to adenoidectomy, comparisons of airway problems, heart rates, cardiac arrhythmias, ventilation and stress hormone reactions were studied during halothane, enflurane and isoflurane anaesthesia. Sixteen children were anaesthetized with either of the three agents and eight patients in each group received diazepam 0.25 mg kg-1 and atropine 0.015 mg kg-1 rectally (DA) as premedication and the remainder diazepam 0.5 mg kg-1, morphine 0.15 mg kg-1 and scopolamine 0.01 mg kg-1 (DMS) rectally. All children were intubated and breathing spontaneously. Equianaesthetic inspired concentrations of halothane, enflurane and isoflurane were used. Airway problems were of the same magnitude during halothane and isoflurane anaesthesia but were less frequent with both agents compared with enflurane anaesthesia. DMS reduced the number of airway reactions in all groups. Respiratory rates were uninfluenced by anaesthesia, intubation and surgery during enflurane anaesthesia. Cardiac arrhythmias were less frequent with enflurane and isoflurane than with halothane. Plasma ACTH and cortisol were similar with all three agents. During induction of anaesthesia in the DA-premedicated halothane group, however, plasma catecholamines were higher than in the group which received DMS, in contrast to the findings during enflurane and isoflurane anaesthesia. The DMS premedication decreased the response of plasma ACTH, cortisol and plasma catecholamines to surgery.