Scott Robinson

Anesthetic Requirements for Halothane in Young Children 0–1 Month and 1–6 Months of Age

In a previous study, the authors found that infants, in the first 6 months of life, required the highest minimum alveolar concentration (MAC) of any age group (1.09% halothane). Because only two neonates (0-31 days of age) were included in the original study and because profound depression of blood pressure and heart rate have been reported in neonates, the authors determined 1) whether the MAC of halothane in neonates (n = 12) differs from that in infants (1-6 months of age) (n = 12) and 2) whether the blood pressure and heart rate responses in neonates differ from those in infants at approximately 1 MAC. The authors found that the MAC of halothane in neonates, 0.87% +/- 0.03 SEM, was significantly lower (P less than 0.01) than that in infants, 1.20% +/- 0.06 SEM. With induction of anesthesia, the systolic blood pressure decreased 23% in neonates (P less than 0.05) and 34% in infants (P less than 0.005) from awake values. Similarly, the heart rate decreased 12% in neonates and 22% in infants (P less than 0.05). The incidence of hypotension (greater than 30% decrease in systolic blood pressure from awake) in neonates, 33%, was not significantly different from that in infants, 44%. The authors conclude that the MAC of halothane in neonates is 25% less than that in infants and significantly less than was thought previously. The MAC in infants is the highest of any age group. The decrease in blood pressure and the incidence of hypotension in neonates are similar to those in infants at approximately 1 MAC of halothane.

The cardiovascular effects of vecuronium (ORG NC45) and pancuronium in patients undergoing coronary artery bypass grafting

Vacuronium is a new nondepolarizing muscle relaxant which has been shown to cause no significant cardiovascular effects. Utilizing invasive monitoring in patients undergoing coronary artery bypass grafting, the autors compared the cardiovascular effects of vecuronium (0.28 mg/kg) in seven anesthetized patients with those of pancuronium (0.1 mg/kg) in five anesthetized patients. This dose of pancuronium represents three times its ED90 (dose producing a 90% depression of evoked twitch tension), while the vecuronium dose represents twelve times its ED90. This relatively large dose of vecuronium was chosen deliberately in an attempt to manifest any possible cardiovascular effects. Following administration of vecuronium, cardiac output increased 9% and systemic vascular resistance decreased 12%, while pancuronium produced a significantly greater 17% increase in cardiac output without change in systemic vascular resistance. Heart rate and systemic mean arterial pressure did not change following vecuronium, while increasing 22% and 24%, respectively, following pancuronium. The authors conclude that large doses of vecuronium have minimal cardiovascular effects and thus offer an advantage over pancuronium in patients anesthetized for coronary artery surgery.

The minimum anesthetic concentration of isoflurane in children.

The minimum anesthetic concentration (MAC) of isoflurane was determined in five pediatric age groups. MAC was 1.6% in infants 0–1 month of age, 1.87% in those 1–6 months of age, 1.8% in those 6–12 months of age, 1.6% in those 1–3 yr of age, and 1.6% in children 3–5 yr of age. MAC in neonates was less than predicted.

Comparison of Enflurane, Halothane, and Isoflurane for Diagnostic and Therapeutic Procedures in Children with Malignancies

The authors performed a randomized, prospective trial comparing enflurane, halothane, and isoflurane (each administered with nitrous oxide) to establish which inhaled anesthetic produced the fewest complications and the most rapid induction of anesthesia for children undergoing general anesthesia fo

Greater coronary vascular reserve in dogs anesthetized with halothane.

To compare the effects of two dissimilar anesthetic regimens on hemodynamic factors affecting the risk of myocardial ischemia, the authors recorded myocardial blood flow as an indicator of oxygen supply during autoregulation and maximal vasodilatation at various coronary arterial perfusion pressures while myocardial oxygen demand was stable, and used the relationship between pressure and blood flow as an index of coronary vascular reserve. Pressure–flow relations in the left circumflex coronary artery during light halothane–oxygen–relaxant anesthesia and during nitrous oxide–oxygen–relaxant anesthesia were compared in 11 dogs. Changes in coronary arterial pressure were induced by hydraulic constriction and recorded through a small catheter in the circumflex coronary artery for each anesthetic regimen during autoregulated and during maximally vasodilated coronary arterial blood flows. Flow was measured by an electromagnetic flow transducer on the circumflex coronary artery and by radioactive microspheres. There were two major differences between the two anesthetic regimens. First, myocardial oxygen demand was less during halothane anesthesia, as measured by myocardial oxygen consumption, wall tension, or the rate–pressure product. Myocardial blood supply decreased similarly during halothane anesthesia. Second, with halothane anesthesia the pressure–flow relationship during maximal vasodilatation was significantly shifted to the left. This parallel shift to the left reflects the lower coronary arterial perfusion pressure at which flow becomes zero, and is best explained by the vascular waterfall theory. Based on this theory, minimal coronary vascular resistances are similar with the two anesthetic regimens, since the maximal vasodilatation lines are parallel, but diastolic intramyocardial tissue pressure is probably lower with halothane, accounting for the lower coronary arterial diastolic pressure at which flow stops. In addition, this shift also reflects the lower coronary arterial perfusion pressure at which subendocardial ischemia occurs when autoregulation of blood flow is present. Therefore, dogs lightly anesthetized with halothane have greater coronary vascular reserves than do dogs anesthetized with nitrous oxide.

Halothane Anesthesia Reduces Pulmonary Function in the Newborn Lamb

To study the effects of anesthesia on respiratory function of the neonate, the authors investigated the effect of breathing 100% oxygen and of breathing oxygen plus 0.75 MAC halothane on functional residual capacity, lung and airway resistance, expired minute volume, work of breathing, lung compliance, and blood gases and pH in nine 5–8-day-old, 4.6–7.7-kg lambs. Breathing 100% oxygen increased PaO2 but had no effect on PaCO2, minute ventilation, or lung mechanics. Three-fourths MAC halothane depressed minute ventilation 34% ± 13% (P < 0.05) and increased PaCO2 50% ± 5% (P < 0.05). Lung and airway resistance increased 59% ± 26% (P < 0.05); work of breathing decreased (P < 0.05); and lung compliance was unchanged. Functional residual capacity was reduced 32% ± 6% (P < 0.05), which may be due to loss of diaphragm and intercostal muscle function and to an inability to take deep breaths. The authors conclude that 0.75 MAC halothane significantly impairs the pulmonary function of lambs who breathe spontaneously. Similar changes in human infants could account for the hypoxemia and hypercarbia that often are seen during anesthesia.