Tracheal Mucus Velocity Remains Normal in Healthy Sheep Intubated with a New Endotracheal Tube with a Novel Laryngeal Seal 

All animal studies were approved by the Animal Care and Use Committee of the National Heart, Lung, and Blood Institute of the National Institutes of Health. Seven adult female sheep weighing 28–38 kg were sedated with ketamine chloride (7 mg/kg) infused into the right external jugular vein through a 16-gauge catheter. The right femoral artery was cannulated through a 16-gauge catheter for continuous blood pressure monitoring and for blood gas analysis. Sheep were positioned prone with their heads immobilized. Sedation was maintained by continuous infusion of ketamine chloride (1 mg [center dot] kg sup -1 [center dot] h sup -1). Sheep tolerated the procedures well and were breathing spontaneously, with blood gases in the normal range (partial pressure of oxygen > 90 mmHg, partial pressure of carbon dioxide < 50 mmHg). Arterial blood gases, electrolytes, and blood glucose were measured hourly (Stat Profile plus 9; Nova Biomedical, Waltham, MA). Arterial blood pressure was recorded continuously using a strain-gauge transducer (Gould Statham P23 ID, Gould, Inc., Cleveland, OH).

Each sheep was studied twice, with an interval of 4–6 weeks between studies. Before each study, the absence of respiratory and systemic infection was demonstrated by the results of physical examination, chest radiographs, and blood counts.

We used a modification of the methods described by Friedman et al. [13] Ten tantalum discs, 1 mm in diameter, 0.12 mm thick, and weighing 1.8 mg, were insufflated into the trachea with compressed air through the operating channel of a 6-mm flexible bronchoscope (model BF 5B2; Olympus Corp., Columbia, MD). We took great care not to touch the tracheal mucosa with the bronchoscope. We measured the movement of the tantalum discs during a 1-h period from serial radiographs taken in the lateral plane at 2-min intervals during expiration, as previously described. [14] A collar containing radiopaque markers of known length was applied to the neck of the sheep, as references to correct for the magnification error inherent in the radiographs. We tracked discs within the distal trachea moving toward the glottis over a distance of 14–16 cm, starting from the level of the right upper lobe bronchus.

After obtaining baseline data for TMV, sheep were randomly intubated with an 8-mm Mallinckrodt Hi-Lo ETT (Mallinckrodt Critical Care, Argyle, NY)(control group), or with a new ultrathin-wall ETT of the same outside diameter (study group). The hi-lo ETT was positioned with the proximal end of the cuff approximately 2 cm below the glottis. The position of the ETT was checked radiographically. The cuff was inflated to a pressure of 15–20 cm H²O. The new ETT was positioned with several gills visible above the glottis. [12] The ETTs were secured with a bite-block and connected to a Siemens Servo 900C ventilator (Siemens Elema, Solna, Sweden) in the continuous positive pressure mode, with positive end-expiratory pressure at 4 cm H²O. The inspiratory gas was fully humidified and warmed to 38 degrees Celsius with a Conchatherm III humidifier (Hudson Respiratory Care, Tennecula, CA). After 2 h of intubation, we introduced a second set of tantalum discs through the bronchoscope and determined TMV as before.

We averaged the linear velocity of five discrete discs that could be tracked readily. Measurements were taken by two independent observers, and measurements that differed by more than 10% were rejected. Four to six weeks later all sheep underwent a repeated study. After establishing baseline values of TMV, sheep that had served as controls were intubated with a new ETT, and those that had been in the study group were intubated with a hi-lo ETT.

The baseline values of TMV for the hi-lo ETTs, and the new ETTs with gills, as well as the changes from baseline TMV after intubation with a cuffed hi-lo ETT and after intubation with a new ETT with gills, were analyzed using analysis of variance for repeated measurements. A probability value of 0.05 or less was considered significant.

The baseline TMV (before intubation) for the cuffed hi-lo ETT was 8.2 +/- 1.9 (SD) mm/min, and 7.9 +/- 1.5 mm/min for the new ETT with gills (P = 0.801). Figure 1shows the TMV after intubation with a cuffed ETT and a new ETT with gills. The TMV after intubation with a cuffed ETT was significantly lower than that after intubation with a new ETT with gills (2.7 +/- 1.6 mm/min, vs. 7 mm +/- 1.8 mm/min; P < 0.001). Figure 2and Figure 3show the changes in TMV from baseline for the cuffed ETTs and for the new ETTs with gills for each individual study. There was a significant decrease in TMV from baseline after intubation with a cuffed ETT (2.7 +/- 1.6 mm/min vs. 8.2 +/- 1.9 mm/min vs. 7.9 +/- 2.6 mm/min; P < 0.001). For the study group, TMV was not significantly different from baseline (7.0 +/- 1.5 mm/min vs. 7.9 +/- 2.6 mm/min; P = 0.243). As seen on radiographs, the tantalum discs traveled well beyond the tip of the new ETT. After extubation, several tantalum discs were seen within the folds of the gills in the tracheal portion of the ETT in three sheep.

Our studies show that TMV in sheep intubated with a new thin-wall ETT fitted with a new laryngeal seal (study group) showed little, if any, decline in TMV velocity during 3 h of intubation, whereas the TMV during intubation with a conventional hi-lo ETT decreased significantly.

Inhaled particles are transported cephalad by the mucociliary escalator, frequently in a helical pattern. [15] Measuring and averaging the linear velocity of several individual insufflated markers over several minutes provides adequate information about TMV and the immediate response to diverse stimuli. [16] Several investigators have tracked insufflated Teflon discs by bronchoscopy [7,17] or have visualized radiopaque Teflon discs by fluoroscopy [10,17,18] or serial radiographs. [14] We used discs made of tantalum because they are biologically inert and have a history of use in bronchography. [19] They are easily manufactured and the tantalum discs were similar to the radiopaque Teflon discs in diameter and weight but were markedly thinner and provided good contrast on radiographs.

Our baseline TMV correlated well with studies using radiopaque Teflon discs as markers. Sackner et al. [10] reported a mean TMV of 9 mm/min in nonintubated, anesthetized dog. Landa et al. [4] reported a mean TMV of 17.3 +/- 6.2 mm/min in awake, nonintubated sheep and 11.1 +/- 3.6 mm/min in anesthetized, nonintubated sheep; and Allegra et al. [20] reported TMVs of 10.0–11.4 mm/min in awake nonintubated sheep. Thus the slightly lower TMV values in our study probably can be attributed to the effects of anesthesia. In animals intubated with uncuffed tracheal tubes, Sackner et al. [10](dogs, anesthetized) and Sabater et al. [18](sheep, awake) found TMVs ranging from 8.3 to 9.3 mm/min and 6.1 to 11.5 mm/min, respectively. These values correlate well with the TMV in this study using the new ETT with gills.

Our results correspond well with those of Sackner et al., [10] who found a significant decrease of TMV after inflation of the tracheal cuff (inflation pressures not specified; 37% after 1 h; 52% after 4 h of intubation). The TMV was significantly reduced when tracheal cuffs were inflated to pressures commonly encountered in patients in intensive care units who require mechanical ventilation and was commonly 0 mm/min in patients with nosocomial pneumonia. [1] Although we kept the inflation pressures of the cuffs low, we found a significant decline in TMV. However, the mean TMVs in sheep intubated with the new ETT with gills (study group) were not significantly different from baseline values; that is, TMV did not decrease during the 3 h of intubation.

The mechanism by which TMV is suppressed after the cuffs of standard ETTs are inflated is not clear. Damming of mucus by the inflated cuff is unlikely to explain it, because measurements were made in the distal trachea, whereas the cuff was inflated in the proximal trachea. [10] Other proposed mechanism are a critical increase in the thickness of the mucus layer proceeding as a propagated wave, a neurogenic reflex secondary to mechanical distension of the trachea that affects mucus secretions, or decreased ciliary activity. [10]

It is unlikely that mucus was effectively dammed by the gills, because TMV did not decline during the study. We also saw discs moving cephalad, alongside the endotracheal tube. On extubation we found tantalum discs trapped within the gills in some sheep. The design of our tracheal tube imparted no pressure on the tracheal wall, and thus tracheal injury was reduced, or eliminated, and mucociliary transport was preserved.

Some hospital practices require low cuff inflation pressures and deflation of the cuff every hour, or that the cuffs be deflated during every expiration. [21] There are no data that show whether those measures affect TMV. It has been found that tracheal injury could be prevented only when there was an audible leak during inspiration. [22] It follows that continuous protection from fluid aspiration, using a standard inflatable hi-lo tracheal cuff, is achieved only at the cost of impaired TMV and the risk of tracheal epithelial damage.

Previously we showed that the gills of this new ETT provide an excellent barrier against aspiration, with no damage to the tracheal epithelium. [12] We now show that the TMV in this tracheal tube design also remains normal. Although we used healthy sheep in our studies, our findings may enhance our understanding of issues that may lead to improved patient care.

The authors thank the personnel of the Laboratory of Animal Medicine and Surgery, NHLBI, and David A. Wurfel for help in conducting these studies, and Carol Kosh for secretarial assistance.