This Month in ANESTHESIOLOGY 

Internal jugular venous cannulation is routinely performed in infants undergoing open heart surgery to monitor central venous pressure and to infuse vasoactive drugs. The incidence of carotid artery (CA) puncture varies from 8.5–23% and is higher in infants than in adults because of the proximity of the CA to the internal jugular vein and to infants' smaller caliber vessels. In 95 infants (all aged < 12 months), Verghese et al. prospectively compared two-dimensional real-time ultrasound with the traditional method of palpating the CA and identifying anatomic landmarks as guides for cannulation.

After induction of general anesthesia and placement of routine monitors and peripheral intravascular catheters, the infants were randomized to one of two approaches for guiding internal jugular venous cannulation: landmarks or ultrasound. All procedures were performed by pediatric anesthesia fellows who had been trained specifically in both methods, and who were supervised by attending specialists in cardiac anesthesia. The cannulation time, number of attempts, success rate, and incidence of complications were compared between the two groups. In the ultrasound group, the success rate was 100%, with no CA punctures, whereas in the landmarks group, the success rate was 77%, with a 25% incidence of punctures. The larger number of CA punctures in the landmarks group may have been partially attributable to the high approach that was chosen by the researchers to avoid the increased incidence of pneumothorax associated with the low approach. In addition, all infants in the study weighed less than 10 kg. Using real-time two-dimensional ultrasound guidance, the impact of anatomic variations of the internal jugular vein and CA in the infant neck was minimized.

Does the use of epidural analgesia affect the progress and outcome of labor and increase the incidence of cesarean section for dystocia? In an attempt to address the ongoing controversy and shed light on the possible relationship between type of analgesia and delivery outcome, Segal et al. analyzed data from 110 obstetricians caring for low-risk patients who were admitted for intended vaginal deliveries. Providers with 50 or more deliveries were included in the study, which spanned a 5-yr period. Type of delivery (spontaneous vaginal, vacuum extraction, forceps, or cesarean section), reasons for cesarean section, type of anesthesia, and patient data (e.g., maternal age, frequency of nulliparity, preterm or postdate infants) were included in the data summaries.

For analysis, the researchers listed the number of each type of delivery and number of patients that received each type of anesthesia for every obstetrician in the study. The total number of patients who received an epidural block divided by the total included patient volume yielded the epidural use rate. Cesarean sections rates for dystocia and for all indications were also calculated. Stepwise linear regression modeling was used in an attempt to predict a given obstetrician's cesarean section rate for dystocia from the characteristics of the practitioner's patient population. Associations between mode of delivery and epidural analgesia, maternal age, nulliparity, birth weight, induction of labor, nonterm gestational age, insurance type, and low Apgar scores were also analyzed.

More than 50% of the patients included in the investigation received epidural analgesia for intended labor and vaginal delivery. However, the researchers found no relationship between the frequency of epidural analgesia and rate of cesarean section for dystocia across practitioners. The only four factors significantly related to a practitioner's cesarean section rate for dystocia were nonterm gestation, frequency of low or high birth weight, frequency of induced labor, and low 1-min Apgar scores for the infant. There are individual practitioners with low cesarean section rates and high epidural analgesia rates, and those who have opposite or intermediate patterns. More studies are necessary to illuminate the mechanisms of the former pattern, which might foster reductions in cesarean section rates while retaining the ability to offer epidural analgesia to women who desire it.

Reducing the dose of intravenous anesthetic agents in patients suffering from hemorrhagic shock is common in clinical practice. Although clinicians intuitively accept the notion that hemorrhagic shock alters pharmacokinetics, there is scant scientific foundation for this assumption. Accordingly, Egan et al. fasted 18 Hampshire-Yorkshire crossbred pigs and then randomly assigned them to one of two groups to investigate whether the distribution and clearance of fentanyl would be decreased in a porcine hemorrhage model. After induction of anesthesia and appropriate instrumentation, the researchers recorded baseline values of heart rate, mean arterial pressure, central venous pressure, pulmonary capillary wedge pressure, cardiac output, pHi, temperature, hematocrit, lactate, and arterial and venous blood gases. Pigs assigned to the "shock" group were subjected to hemorrhagic shock after intravenous administration of 5,000–6,000 U of heparin. The animals were bled until the mean arterial pressure was reduced to 40–45 mmHg; it was maintained at that level throughout the study. Fentanyl (50 [micro sign]g/kg) was infused intravenously over a 5-min period in both groups of animals. Those in the shock group received fentanyl after the target mean arterial pressure of 40 mmHg had been maintained for 1 hr. Blood samples were collected from aortic catheters before drug administration and at regular intervals up to 370 min after drug infusion was begun.

Each animal's pharmacokinetic parameters were estimated by fitting a three-compartment model to the concentration versus time data. Nonlinear mixed-effects population PK models examining the influence of mean arterial pressure and cardiac index were also constructed. A computer simulation examined the concentration versus time profiles that result from a typical dosage scheme in shock versus control subjects. Results of that simulation suggested that, compared with controls, shock subjects receive a relative overdose. The 50% and 80% decrement time simulations demonstrated that fentanyl was longer-acting in the shock animals, even when dosing adjustments were made.

Pigs in the shock cohort had higher fentanyl concentrations than those in the control group. Central clearance and central and second compartment distribution volumes were significantly reduced for those in the shock group. However, the study did not address the impact of pharmacodynamic changes on the overall pharmacologic behavior of fentanyl, and extrapolating these results to a human population may be difficult.

In a four-part study in rabbits, Nishina et al. determined the effects of exogenous surfactant administered after acute lung injury induced by intratracheal instillation of acidified milk products. After induction of anesthesia, intubation, and muscle relaxation, all animals were mechanically ventilated using an infant ventilator. Baseline values of lung compliance and hemodynamics were measured, and arterial blood samples were obtained for determination of Pa^O(2), Pa^CO(2), and peripheral leukocyte counts. In study 1, 28 rabbits were assigned randomly to one of four groups: three received acidified infant formula (titrated to a pH of 1.8 by adding 6 N HCl), and the fourth received acidified saline. Two of the first three groups received intratracheal surfactant (100 and 200 mg/kg, respectively) 30 min after instillation of the acidified infant formula. In study 2, three groups of rabbits received acidified breast milk, and the control group received acidified saline. Two groups received surfactant in doses of 100 mg/kg or 200 mg/kg 30 min after instillation of the acidified breast milk. The third group received no postinjury surfactant.

The animals were observed for 4 hr in studies 1 and 2. In studies 3 and 4, experiments were conducted in the same manner, but the animals were observed for 12 hr. After each study was completed, the animals were killed, and their lungs were excised for bronchoalveolar lavage, lung wet- to dry-weight ratios, and histologic examinations.

Both types of acidified milk products caused physiologic and histologic lung damage, confirming results of other observations of this model of acute lung injury. The alveolar to arterial oxygen gradient increased, as did the lung wet/dry ratio, and the percentage of neutrophils in the bronchoalveolar lavage fluid. The exogenous surfactant improved these variables in a dose-dependent manner, with animals that received 200 mg/kg of surfactant improving more than those that received 100 mg/kg. Although further study is needed, early postinjury treatment (within 30 min after gastric aspiration) with exogenous surfactant might be a promising approach in pediatric patients.

Gretchen Henkel