Brian R. Jones

Fetal tracheal intubation with intact uteroplacental circulation

ongenital anomalies of the head and neck frequently compromise the airway. If sufficiently C large or strategically located, these anomalies can cause asphyxia after delivery of the infant. Untreated, the mortality of patients with these masses is 80-100% (1). Fetal ultrasound was first used to aid in the prenatal diagnosis of these defects in the late 1970s. Antenatal diagnosis is important for two reasons. First, elective cesarean section should be planned to avoid dystocia and fetal trauma. Second, because immediate establishment of a patent airway is essential for survival, a team of pediatric airway experts must be available. The authors describe two fetuses with airway anomalies whose tracheas were intubated successfully while they were only partially delivered. The placenta was used as the organ of gas exchange while the supraglottic airway was secured.

The onset of disuse-related potassium efflux to succinylcholine.

Disuse atrophy of skeletal muscle produces resistance to nondepolarizing relaxants and increased potassium efflux after the administration of succinylcholine. These changes appear to be due to development of perijunctional and/or extrajunctional receptors (up-regulation). In this study, the authors searched for the earliest detectable appearance of increased potassium efflux in beagles in whom disuse atrophy was simulated. Seven beagles underwent unilateral cast immobilization of a hind limb. Between 4 and 42 days, they periodically received succinylcholine 0.25 mg/kg while anesthetized with thiamylal and nitrous oxide. Sequential bilateral femoral venous samples showed that the casted limb did not manifest potassium release greater than the upper limit of normal (1 mEq/l) until cast immobilization periods of 14 days or longer. When this occurred, the increase in the potassium concentration in the femoral venous blood of the casted limb exceeded that from the noncasted limb by at least 0.7 mEq/l (P less than 0.01). The range for the onset of this response after casting was 14-42 days, the mean 27.2 days, and the standard deviation 9.8 days. These findings imply that up-regulation of skeletal muscle receptors, associated with exaggerated potassium efflux after administration of succinylcholine, is dependent on progressive development of extrajunctional receptors over surface membrane areas beyond the endplate.

Limitations of indirect methods of estimating small bowel transit in man

Experiments were carried out in healthy volunteers to explore the utility of a new [14C]lactulose breath test for measuring small intestinal transit time in man and to use this procedure to test whether two antidiarrheal agents, codeine and clonidine, alter small intestinal transit time during digestion of a liquid meal. In an initial validation study performed in 12 subjects (three studies in each subject), a liquid test meal containing 10 g [14C]lactulose was administered and the colonic entry time estimated from the time course of14CO2 excretion in breath compared with that of H2 excretion. There was a fair correlation (r=0.77;P<0.001) between results obtained by the two methods; both methods gave similar results, but14CO2 output was delayed when compared to H2 output and was incomplete. The meal also contained xylose and [13C]glycine, permitting the duodenal entry time of the meal to be estimated by the appearance of xylose in blood and13CO2 in breath, respectively. The same liquid meal was then used to examine the effect on small intestinal transit time (colonic entry time minus duodenal entry time) of codeine or clonidine.99Tc-sulphur colloid was also added to the meal to permit a comparison of small intestinal transit estimated by imaging with that estimated by the14CO2-lactulose breath test.99Tc radioactivity appeared in the cecum (as assessed using gamma scintigraphy) about 2 hr before14CO2 radioactivity appeared in breath; the correlation between transit time estimated by the two methods was moderate (r=0.61;P<0.05). Based on the [14C]lactulose data, small intestinal transit time ranged from<1 to 3 hr for a liquid meal containing 10 g lactulose; within-subject variation (coefficient of variation 17%) was considerably less than between-subject variation (coefficient of variation 56%). Codeine increased the small intestinal transit time significantly (from 2.7±0.3 hr to 5.0 ±0.9 hr; mean±SE), whereas clonidine did not alter small intestinal transit time, as estimated by the colonic entry time minus duodenal entry time. Neither drug influenced duodenal entry time. These results suggest that the [14C]lactulose breath test, which has only moderate accuracy, may have occasional utility as a convenient, noninvasive method for estimating small intestinal transit time in man. However, this study also suggests that indirect methods of estimating small bowel transit in man have limitations, variability, and possibly may lack the desired sensitivity.

Strength of continuous spinal catheters

Several commercially available catheters are currently marketed for continuous intrathecal use. Initial studies using continuous spinal catheters have reported several occurrences of retained fragments after removal of the catheter. Accordingly, we measured the break strength of five commercially available catheters. The TFX/Rusch 28- and 32-gauge continuous spinal catheters required 3.18 and 1.92 lb to break, respectively. The Kendall 28-gauge, the Preferred Medical Products 24-gauge, and the 24-gauge Burron catheters averaged 1.22, 1.97, and 3.55 lb to break, respectively. We also tested a commonly used Burron 20-gauge catheter, which is marketed for epidural use, and found it had an average break strength of 6.35 lb. The tested values obtained for the TFX/Rusch catheters were lower than the break strength values supplied by the manufacturers. The authors conclude that the break strength of spinal catheters is one-third to one-half that found for a typical epidural catheter.

Flow increases with an enlarging intravenous catheter

STUDY OBJECTIVE To determine the increase in flow of a hydratable enlarging intravenous (IV) catheter in anesthetized patients. DESIGN A randomized, nonblinded study, with standard Teflon IV catheters used as controls. SETTING Operating room at a university medical center. PATIENTS Thiry adult patients receiving general anesthesia for lower extremity surgery. INTERVENTIONS An IV catheter was placed in the upper extremity, and flow measurements were made by measuring the time for infusion of 250 ml of normal saline within 1 minute after placement and at 1 hour after placement. MEASUREMENTS AND MAIN RESULTS The enlarging catheters had a statistically significant average flow increase of 26% after 1 hour indwelling time. The standard Teflon catheters had no statistically significant change in flow after 1 hour. The percentage increase in flow for the enlarging catheters was not as great as previously seen in vitro. CONCLUSIONS Flow through enlarging IV catheters placed in anesthetized patients increases after 1 hour. The percentage increase in flow is not as great as previously seen in vitro and may be due to skin, vein, and subcutaneous tissues preventing complete expansion.

The influence of fresh gas flow and inspiratory/expiratory ratio on tidal volume and arterial CO2 tension in mechanically ventilated surgical patients

The relative importance of fresh gas flow and inspiratory/expiratory ratio in determining delivered tidal volume and PaCO2 was studied in anesthetized adult patients ventilated with a fixed ventilator bellows volume. The fresh gas flows studied were 2, 6, and 10 L/min, and inspiratory/expiratory ratio was either 1:2 or 1:4.5. Bellows volume and respiratory rate were held constant throughout the study. At the lowest fresh gas flow and smallest inspiratory/expiratory ratio, PaCO2 was 43 +/- 2 mm Hg. The PaCO2 decreased progressively and significantly with each increase in fresh gas flow during ventilation with either inspiratory/expiratory ratio setting. PaCO2 averaged 30 +/- 3 during ventilation with the highest fresh gas flow and largest inspiratory/expiratory ratio. As fresh gas flow increased, PaCO2 and tidal volume changed to a significantly greater degree in response to changes in inspiratory/expiratory ratio. These data demonstrate that altering either fresh gas flow or inspiratory/expiratory ratio can produce clinically significant perturbations in PaCO2 and tidal volume during anesthesia. These perturbations occur even if bellows volume is held constant. Furthermore, changes in inspiratory/expiratory ratio will affect these parameters to a greater degree as fresh gas flow is increased.

An inexpensive, disposable adapter for increasing the safety of blind nasotracheal intubations.

Complications that may involve the patient during blind nasal intubation of the trachea include trauma to the nasal passage, sinusitis (l), bacteremia (2), and submucosal dissection of the nasopharynx (3) . Little attention has been paid to possible complications involving the anesthetist performing the intubation. Successful intubation may be heralded by a stream of upper airway secretions, often blood tinged. With the recent isolation of human immunodeficiency virus in saliva as well as blood, this source of health worker risk is real (4). Donenfeld (5) has described an aid to blind nasotracheal intubation that involves the use of the distal end of the anesthesiologist’s ear piece tubing placed loosely inside the endotracheal tube. This procedure decreases the risk of infection to the anesthetist but requires that a piece of specialized equipment be present and, thereby, poses the risk of patient cross-contamination when using the same equipment in different patients. We recently adapted our technique for blind nasal intubation by the addition of a simple, inexpensive heat and moisture exchanger (HME) (Humid-Vent 1, Gibeck Respiration, Sweden) on the proximal end of the endotracheal tube (Fig. 1). This disposable device effectively creates a trap for respiratory secretions if the patient should cough or exhale vigorously. We have used this technique on multiple patients over the last year and found it to be most effective in preventing the anesthetist from receiving an earful of secretions. The small size of the HME makes it an easy addition to our emergency resuscitation kit. The character of the breath sounds is changed somewhat by the device and becomes more whistling in nature. The resistance to flow offered by the heat and moisture exchanger is minimal (6) and has not interfered with recognition of breath sounds emanating from the tube. The efficacy of the device to trap very small particles is unknown and thus careful attention to good technique remains mandatory. Michael J . Dorsey, MD Brian R. Jones, MD Department of Anesthesiology University of California, San Diego San Diego, CA 92203 Figure 1. The disposable heat and moisture exchanger (HumidVent 1) attached to an endotracheal tube during blind nasotracheal intubation.