Letty M. P. Liu

Life-threatening Apnea in Infants Recovering from Anesthesia

To determine whether prematurely born infants with a history of idiopathic apneic episodes are more prone than other infants to life-threatening apnea during recovery from anesthesia, the authors prospectively studied 214 infants (173 full term, 41 premature) who received anesthesia. Fifteen premature infants had a preanesthetic history of idiopathic apnea. Six of these required mechanical ventilation because of idiopathic apneic episodes during emergence from anesthesia. Two were ventilated for other reasons, and seven recovered normally. Infants ventilated for apnea were younger (postnatal age 1.6 +/- 1.2 months, mean +/- SD; conceptual age 38.6 +/- 3.0 weeks) than those who recovered normally (postnatal age 5.6 +/- 2.7 months; conceptual age 55.1 +/- 11.3 weeks) (P less than 0.01). No other premature or full-term infant was ventilated because of postoperative apneic episodes. The authors conclude that anesthetics may unmask a defect in ventilatory control of prematurely born infants younger than 41-46 weeks conceptual age who have a preanesthetic history of idiopathic apnea.

Induction and maintenance characteristics of anesthesia with desflurane and nitrous oxide in infants and children.

To determine the induction and maintenance characteristics of desflurane in pediatric patients, the authors anesthetized 206 infants and children aged 1 month to 12 yr with nitrous oxide plus desflurane and/or halothane in oxygen. Patients were assigned to one of four groups: anesthesia was 1) induced and maintained with desflurane after premedication with an oral combination of meperidine, diazepam, and atropine; 2) induced and maintained with desflurane; 3) induced with halothane and maintained with desflurane; or 4) induced and maintained with halothane. An unblinded observer recorded time to loss of consciousness (lid reflex), time to intubation, and clinical characteristics of the induction and maintenance of anesthesia. Moderate-to-severe laryngospasm (49%) and moderate-to-severe coughing (58%) occurred frequently during induction of anesthesia with desflurane; the incidence of these was not altered by premedication. In contrast, laryngospasm and coughing were rare during induction of anesthesia with halothane. In unpremedicated patients, time to loss of lid reflex (mean +/- SD) was similar for desflurane (2.4 +/- 1.2 min) and halothane (2.1 +/- 0.8 min). During induction of anesthesia, before laryngoscopy and intubation, mean arterial pressure less than 80% of baseline was more common with halothane; heart rate and mean arterial pressure greater than 120% of baseline were more common with desflurane. Intraoperatively, heart rate greater than 120% of baseline was more common with desflurane; blood pressures were similar for the two anesthetics. The authors conclude that the high incidence of airway complications during induction of anesthesia with desflurane limits its utility for inhalation induction in pediatric patients. Anesthesia can be safely maintained with desflurane if induced with a different anesthetic.

Intraoperative events diagnosed by expired carbon dioxide monitoring in children

Expired carbon dioxide measurements (PECO2) were used (1) to assess the adequacy of initial alveolar ventilation, and (2) to document intraoperative airway events and metabolic trends. Three hundred and thirty-one children were studied. Thirty-five intraoperative events were diagnosed by continuous PeCO2 monitoring; 20 were potentially life-threatening problems (malignant hyperthermia, circuit disconnection or leak, equipment failure, accidental extubation, endobronchial intubation, or kinked tube); only two of these were also diagnosed clinically. The duration of anaesthesia may be a factor: 3.9 hours for cases with events vs. 2.5 hours for cases without events (p < 0.002). There was a higher incidence of hypercarbia (peak expired PeCO2≥ 50) in children who were not intubated (29 per cent) compared to those who had an endotracheal tube in place (12 per cent) (p = 0.0001). Hypocarbia (peak expired PeCO2≤30) was more frequent in intubated cases (11 per cent) than in unintubated cases (three per cent) (p = 0.03). There was a high incidence of hypocarbia in infants less than one year of age (p = 0.02). We conclude: (1) lifethreatening airway problems are common during anaesthesia in paediatric patients; (2) quantitative measurement of PECO2 provides an early warning of potentially catastrophic anaesthetic mishaps; (3) the incidence of events increases with duration of anaesthesia.RésuméL’6tude du CO2 en fin dexpiration (PeCO2) a été utilisée afin d’évaluer (1) la fonction respiratoire initiate et (2) pour documenter les événemenls per-opératoires touchant les voies aériennes ainsi que les changements métaboliques. 331 enfants ont été étudiés. 35 événements per-opératoires ont été diagnostiqués par une surveillance constante de la PeCO2; 20 représentaient des problèmes mettant en danger la vie (hyperthermie maligne, disconnection de circuit, fuite, bris d’équipement, extubation accidentelle, intubation endobronchique, ou tube endotrachéal coudd); seulement deux de ces événements ont été aussi diagnostiqués cliniquement. La durée de l’anesthésie pouvait aussi être un facteur: les présentants les accidents ont duré en moyenne 3.9 heurs contre 2.5 heures pour les cas n’ayant pas présenté de problèmes (p < 0.002). Il y avait un incidence plus élevée d’hypercarbie (PECO2 ≥ 50,) chez les enfants qui n’étaient pas intubés (29 pour cent) a comparé à ceux dont le tube endotrachéal élail en place (12 pour cent) (p = 0.0001). L’hypocarbie (PeCO2 ≤ 30) était plus fréquente chez les patients intubés (11 pour cent) que chez ceux qui n’étaient pas intubés (trois pour cent) (p = 0.03). It y avait une incidence plus grande d’hypocarbie chez les enfants âgés de moins qu’un an (p = 0.02). On colclut: 1) les problemes de voies aeriennes pouvant mettre en danger la vie sont fréquents lors de I’anesthésie pédiatrique; 2) la mesure quantitative de la PeCO2 fournie un signal d’alarme précoce pour les accidents anesthésiques potentiellement catastrophiques; 3) l’incidence des accidents augmente avec le temps.

The Dose-Response Effects of Oral Cimetidine on Gastric pH and Volume in Children

The effects of preanesthetic oral cimetidine on gastric fluid pH and volume were studied in 97 infants and children. A dose-response curve was constructed using doses of 2.5, 5.0, 7.5, and 10 mg/kg. The ED50 of cimetidine that produces pH values higher than 2.5 was 3.0 mg/kg, and the ED95 was 7.5 mg/kg. Cimetidine exponentially reduced the volume of gastric fluid. Cimetidine was most effective between one and four hours after oral administration. In children who are at high risk of pulmonary aspiration, we recommend that oral cimetidine, 7.5 mg/kg, be given 1-3 hours preoperatively in order to protect the lungs from the accidental aspiration of acidic gastric secretions.

A Comparison of the Cardiovascular Effects of Sodium Nitroprusside and Trimethaphan

In dogs anesthetized with pentobarbital–chloralose, cardiac output and blood flows of four regional vascular beds (superior mesenteric, left renal, left circumflex coronary and left femoral) were continuously monitered with electromagnetic flowmeters. Arterial blood pressure and heart rate were also measured. Hypotension was induced with intravenous infusions of sodium nitroprusside and trimethaphan for 5–16 min to produce comparable reductions of mean arterial pressure (32 mm Hg or 26 per cent with nitroprusside and 37 mm Hg or 31 per cent with trimethaphan). Cardiac output also decreased, but to a lesser extent (11.5 per cent with nitroprusside and 12.5 per cent with trimethaphan). Thus, total peripheral resistance was consistently decreased. Nitroprusside caused slight tachycardia, while trimethaphan produced bradycardia. Both drugs decreased mesenteric blood flow and increased mesenteric vascular resistance. Renal blood flow was maintained or increased with nitroprusside; thus, renal vascular resistance decreased; with trimethaphan, renal blood flow decreased and renal vascular resistance did not change. Both nitroprusside and trimethaphan reduced coronary blood flow; the reduction was more pronounced with the latter. Nitroprusside affected femoral blood flow minimally, with a slight reduction of femoral vascular resistance. In contrast, trimethaphan increased femoral blood flow and markedly decreased femoral vascular resistance. Redistribution of cardiac output favoring the dilated skin and muscle vascular beds appears to be an important undesirable effect of trimethaphan.

Methohexital Plasma Concentrations in Children Following Rectal Administration

Despite the increasing use of rectal methohexital as a premedicant-induction agent in pediatric anesthesia, there are no data to confirm the assumption that low plasma methohexital concentrations are the cause of inadequate sedation of children and that high concentrations are associated with the loss of consciousness. Plasma methohexital concentrations were determined in 20 ASA Class I children, ages 2–7 yr, after the rectal administration of methohexital (25 mg/kg). Seventeen of the 20 children in this study fell asleep after receiving the drug and achieved peak plasma concentrations greater than 2 μg/ml. The maximum plasma methohexital concentration in children that did not fall asleep was less than 2 μg/ml. The mean time to the onset of sleep after drug administration was 8.3 min (at which time the mean plasma concentration was 4.4 μg/ml). The mean peak plasma concentration and the mean time to peak plasma concentration were 4.7 μg/ml and 13.9 min, respectively. Loss of consciousness after rectal administration of methohexital correlates well with the plasma concentration of the drug.

Safety and efficacy of vecuronium in adolescents and children.

The neuromuscular and cardiovascular effects of vecuronium (Norcuron, ORG NC45) were studied in 40 adolescents (10–17 yr) and children (2–9 yr) anesthetized with 1.5% inspired halothane. Ten adolescents and ten children were given 20 μg/kg incremental doses of vecuronium to establish a cumulative dose-response curve during train-of-four stimulation. The ED95 dose was 56 μ/kg in children and 40 μg/kg in adolescents, children being significantly (P < 0.01) more resistant to the neuromuscular effects of vecuronium than adolescents. Another group of 10 children and 10 adolescents received a bolus dose of 80 μg/kg. This dose provided satisfactory conditions for endotracheal intubation with complete suppression of train-of-four response in all adolescents and children within 2 min. Thereafter, the twitch tension recovered to 5% of control twitch height in 18.5 ± 1.5 min, to 25% in 24.4 ± 1.6 min, and to 95% in 43.3 ± 2.1 min. Vecuronium (20–80 μg/kg) did not significantly alter the heart rate or blood pressure nor did it affect kidney or liver function as assessed by routine clinical laboratory tests. Vecuronium is a useful nondepolarizing neuromuscular blocking agent with a short to intermediate duration of action, which can be used safely in children and adolescents.

Continuous infusion of atracurium in children

Atracurium infusion requirements were determined in 28 children anesthetized with N2O2, narcotic, N2O2 halothane (1% inspired), and N2O:O2 enflurane (2% inspired). When the patient was recovering from a bolus dose of 0.4 mg/kg atracurium, a continuous infusion of atracurium was started and the rate was adjusted to maintain 90–99% muscle twitch depression. Patients receiving enflurane anesthesia required atracurium at an infusion rate of 4.9 ‡ 0.3 μg. kg−1. min−1 which was a significantly lower rate (P = 0.0001) than those anesthetized with halothane (8.3 ‡ 0.4 μg.kg−1 · min−1) or with N2O:O2 and narcotic (9.3 ‡ 0.5μg.kg−1 · min−1). At the onset of neuromuscular blockade, the twitch response disappeared faster after train-of-four stimulation repeated every 10 s than after single twitch rates of stimulation at 0.1 Hz. In children, during halothane anesthesia after 0.4 mg/kg atracurium, the response of the adductor of the thumb was ablated in 2.0 ‡ 0.3 min with train-of-four stimulation, and in 3.7 ‡ 0.4 min with single twitch stimulation. The authors recommend the use of a nerve stimulator during continuous infusion of atracurium because of the marked interpatient differences in infusion-rate requirements.

Comparison of equipotent doses of non-depolarizing muscle relaxants in children.

The neuromuscular effects of equipotent doses of non-depolarizing muscle relaxants used for endotracheal intubation were studied in 27 children anesthetized with thiopental, nitrous oxide/oxygen, and narcotic. Equipotent doses of d-tubocurarine (0.8 mg/kg), metocurine (0.5 mg/kg), and pancuronium (0.13 mg/kg) were used. At these doses conditions for intubation were satisfactory in all children and the twitch was completely abolished in 26 of the 27 patients. The twitch height recovered to 5% of control values in 54 ± 6 minutes, and 29 ± 2 minutes later recovered to 25% of control values. The time from injection of the drug to maximum effect, the conditions for intubation, and recovery times among the three drugs were not significantly different. Train-of-four values correlated (p < 0.001) with the twitch heights. When the twitch height at 0.1 Hz was 21 % of control values, only three contractions were detected following train-of-four stimulation. At 14% of control twitch height, two contractions were detected; at 7% of control twitch height, one contraction was detected. The most frequent reason for administering an incremental dose of relaxant was the beginning of respiratory movements, corresponding to a twitch height of 24% of control values (train-of-four 3%). The second most common reason was unsatisfactory abdominal wall relaxation (twitch height 31 % of control values, train-of-four 11%).

The Neuromuscular Response of Infants to a Continuous Infusion of Succinylcholine

The response of the adductor of the thumb to ulnar nerve stimulation (0.1 Hz) was evaluated during continuous infusion of succinylcholine (SCh) in 20 infants anesthetized with halothane (1%) and N2O/O2. Train-of-four stimulation (2 Hz for 2 s) was used to differentiate between phase I and phase II block. Some infants were very resistant to the neuromuscular effects of SCh. These infants (Group 1) were younger in age, 57 ± 15 days (mean ± SE) and required 24.6 ± 3.3 mg · kg-1h-1 SCh to achieve more than 90% depression of the twitch. Older infants (Group 2), 188 ± 33 days, required significantly less (P < 0.001) SCh (8.7 ± 0.5 mg · kg-1h-1) to achieve the same degree of twitch suppression. Group 1 infants recovered from the effects of SCh very rapidly. After 10 mg/kg SCh, the train-of-four ratio in Group 1 infants recovered to 75% in 4.7 ± 0.6 min, whereas it took 34 ± 10 min in Group 2 infants (P < 0.01). Tachyphylaxis developed in infants after 3.6 ± 0.3 mg/kg (mean ± SE) and phase II block after 5.3 ± 0.7 mg/kg (P < 0.05) SCh. Combining the data of infants with that of children from a previous study conducted in a similar fashion1 resulted in significant correlation (P < 0.001) between the log age and SCh requirement. The rate of administration of a continuous infusion of SCh in infants should be based upon the response of infants and not on a fixed dose (mg · kg-1· h-1).