Pulmonary Airway Resistance with the Endotracheal Tube versus Laryngeal Mask Airway in Paralyzed Anesthetized Adult Patients 

Thirty-six consecutive patients, classified as American Society of Anesthesiologists physical status 1-3 and undergoing surgery in the supine position were randomly allocated to receive anesthesia through a size-4 LMA or an ETT. The local hospital ethics board approved the study, and all participants gave their written informed consent. Patients were excluded if they were younger than 18 yr or older than 80 yr, weighed less than 50 kg, had respiratory tract disease, were at risk for aspiration, or were taking medication that might trigger bronchospasm. In the ETT group, men (n = 9) received a 9-mm ID ETT, and women (n = 9) received an 8-mm ETT (Portex, Hythe, UK). In the LMA group, men (n = 9) and women (n = 9) received a size-4 LMA. Respiratory variables were measured and analyzed using a pulmonary monitor (CP-100; BiCore Monitoring System, Irvine, CA) attached to a variable-orifice pneumotachograph (Var flex; Allied Health Products, Riverside, CA) and an esophageal balloon catheter (Smart Cath, Allied Health Products). [6-8]The catheter is an 8-French, 69-cm medical-grade polyurethane device that is connected directly to the catheter port on the monitor (BiCore). The balloon has a 0.9-cm diameter and is 10 cm long, with a frequency response of 30 Hz. The monitoring system (BiCore) automatically performs a vacuum leak test and fills the esophageal balloon with 0.8 ml of air.

A standard anesthesia protocol was followed and routine monitoring was applied. Anesthesia was induced with 1 [micro sign]g/kg fentanyl and 2.5 mg/kg propofol. Anesthesia was maintained with 1.5% isoflurane and 33% oxygen in N²O until all measurements were complete. Muscle relaxation was accomplished using 0.1 mg/kg vecuronium and 0.03 mg/kg boluses if the train-of-four count was more than one. The esophageal balloon catheter was introduced through the nose and fed into position (mid esophagus) using Magill's forceps. The LMA was inserted and fixed according to the manufacturer's instructions. [9]The ETT (cut to 25 cm) was inserted using a laryngoscope, and the cuff was positioned in the proximal trachea, immediately distal to the vocal cords. The ETT cuff was inflated with 4 - 8 ml air, and the LMA was inflated with 25-30 ml air. Airway function was assessed using gentle hand ventilation, observation of synchronized bilateral expansion of the chest, auscultation, and capnography. The pneumotachograph was connected directly to the proximal end of the airway tube. Airway pressure was measured at the pneumotachograph. The carbon dioxide sampling port was situated above the flow transducer. The same connectors were used for each type of airway. The position of the esophageal balloon catheter was checked and adjusted when necessary by observation of the cardiac artifact on the esophageal waveform, as recommended by the manufacturer. The machine pop-off valve was set at 40 cm of water. The flow transducer was calibrated for the anesthesia gas mixture.

Controlled ventilation with a constant square wave inspiratory flow profile was started. The tidal volumes were set using the CP-100 expiratory tidal volume reading from the flow transducer to avoid error caused by leaks in the breathing system. Respiratory rates were adjusted to achieve constant end-tidal carbon dioxide levels of 30 mmHg. The inspiratory:expiratory ratio was 1:2 and was held constant. Peak inspiratory pressure and mean airway resistance (transpulmonary pressure divided by the difference in flow taken at the same volume during both inspiration and expiration [10]) were recorded during 10 respiratory cycles when the end-tidal carbon dioxide level was stable. This was repeated at three different tidal volumes: 5, 10, and 15 ml/kg. Measurements began 5-10 min after induction of anesthesia and were completed before surgery began. Mean device resistance was measured in vitro at the same ventilator settings used for each patient. Measurements were repeated on 10 occasions for each ventilator setting and were made with the distal end of the ETT or LMA open to the atmosphere. Pulmonary airway resistance was calculated by subtracting device resistance from mean airway resistance. The leak fraction was measured by noting the difference between the inspired and expired tidal volume. Any data collected when the leak was 3% or more were excluded from the analysis.

Data was analyzed using the Student's t test and analysis of variance for repeated measures. Unless otherwise stated, data are presented as the mean +/− SD. Differences were considered significant at P < 0.05.

There were no demographic differences between groups (Table 1). Two patients in the LMA group had a leak of 3% or more at 15 ml/kg, and two patients in the ETT group could not be ventilated at 15 ml/kg, because the peak inspiratory pressure was more than 40 cm of water. These patients were excluded from the analysis at 15 ml/kg. No patient had clinically detectable bronchospasm at auscultation. Peak airway pressure, mean airway resistance, device resistance, and pulmonary airway resistance were less when the LMA was used (Table 2).

Gal and Surratt [5]showed that tracheal intubation caused a doubling of airway resistance in awake healthy patients with topically anesthetized airways. They considered that this represented reflex bronchoconstriction because of the mechanical irritation of the tube. Patients with chronic obstructive pulmonary disease showed an even greater response. We showed that the LMA has lower pulmonary airway resistance than the ETT in paralyzed anesthetized patients, suggesting that the LMA triggers less reflex bronchoconstriction than the ETT. Pulmonary airway resistance for the LMA included glottic resistance (considered to be 33% of upper airway resistance in awake patients [11]), suggesting that the difference in the subglottic component of pulmonary airway resistance is even greater between the devices.

Our results are in contrast with those of Boisson-Bertrand et al., [12]who performed a crossover study of 10 ventilated patients and showed that pulmonary airway resistance was similar between the LMA and ETT groups. [12]These differences in results may be related to the use of both airway devices in each patient, avoidance of muscle relaxants, and inclusion of patients with pulmonary disease or bronchoconstricting drugs in their study. Our data for device resistance is similar to that of a previous study. [13] 

The LMA triggers less bronchoconstriction than does the ETT in paralyzed anesthetized adult patients. This may have implications for maintaining intraoperative pulmonary function and reducing the risk of atelectasis and pulmonary infection.

The authors thank Drs. D. Mecklem, M. Scully, and H. J. Sparr for advice with the manuscript.