tubes (ETTs) with that of a size 4 laryngeal mask airway (LMA²). ... resistance of the LMA and larynx together was similar (in males: 3.20 (2.71) mbar´s litre±1) to.
British Journal of Anaesthesia 85 (3): 410±16 (2000)
Resistance of laryngeal mask airway and tracheal tube in mechanically ventilated patients H. Reissmann*, W. Pothmann, B. FuÈllekrug, R. Dietz and J. Schulte am Esch Department of Anaesthesiology, University Hospital Eppendorf, Martinistrasse 52, D-20246 Hamburg, Germany *Corresponding author We compared the air¯ow resistance of 7.5 and 8.5 mm internal diameter (i.d.) endotracheal tubes (ETTs) with that of a size 4 laryngeal mask airway (LMA²). We thought that any difference in the resistance of the devices alone might be offset by the resistance of the larynx. Sixteen adult ASA physical status I and II patients (14 males, two females) undergoing general anaesthesia were anaesthetized and paralysed with intravenous propofol, ketamine and vecuronium. After insertion of the LMA, controlled ventilation (tidal volume 10 ml kg±1, frequency 12 min±1) was established with three different settings for inspiratory ¯ow (5.5, 7.5 and 12.5 ml kg±1 s±1). Ventilation with the same settings was used after orotracheal intubation with an ETT of i.d. 7.5 mm (females) or 8.5 mm (males). The position of the LMA mask and the tip of the ETT were checked through a ®brescope. The resistance of the devices and, in case of the LMA, of the larynx, was derived by relating proximal and distal pressures (measured via catheters) to inspiratory ¯ow. Four patientsÐyoung, tall menÐhad to be excluded from further study because of a leak around the LMA. In the remaining 10 males and two females, resistance of the LMA (mean (SD) at high ¯ow, 1.19 (0.22) mbar´s litre±1 in males) was less than that of the 8.5 mm i.d. ETT (3.34 (0.52) mbar´s litre±1) (P130% of norm), coagulation abnormalities, systemic disease, allergies, malformations or diseases of the head, neck, thorax or abdomen or the structures and organs within them. The patients received oral midazolam (5.0±7.5 mg). For the study they were placed supine with the head resting on a 5 cm thick foam cushion. Anaesthesia was induced and maintained with propofol (2±3 mg kg±1 and infusion 6±12 mg kg±1 h±1) and ketamine (1.0±1.5 mg kg±1 initially with repeated doses) after atropine 0.5±1.0 mg for reduction of secretions. Ketamine was used for analgesia in order to avoid the effects of opioids on chest wall mechanics.
Positioning of LMA and ETT For the ®rst part of the study, a size 4 LMA (Intavent (Cyprus) Ltd, Nicosia, Cyprus) was inserted by a standard technique, and its cuff was in¯ated with the recommended amount of air (30 ml). Absence of leaks was veri®ed by ventilation with positive pressure. If leaks were detected by auscultation, the LMA was repositioned up to three times. The study was continued only if an apparently leak-free position was obtained. The position of the LMA relative to the structures of the larynx was inspected with a ®brebronchoscope (type BF P30, Olympus Optical Co. (Europe), Hamburg, Germany). Special attention was given to lateral deviation and to the proportion of the ®eld of vision occupied by the epiglottis when looking from inside the distal opening. In all patients, the same experienced anaesthetist inserted the LMA and did the laryngoscopy. A standard cuffed ETT (Mallinckrodt lopro, Mallinckrodt Medical, Hennef, Germany, internal diameter (i.d.) 7.5 mm for female patients, 8.5 mm for males) was used in the second part of the study, inserted through the mouth by conventional direct laryngoscopy. The position in the trachea was checked ®breoptically. Because of the limited length of the pressure-measuring catheter (see below), the ETTs had to be shortened to 24 cm at the proximal end. Since the length of an airway has only minor impact on its
A pressure-measuring catheter of 2.5 mm outer diameter, with closed tip and side holes (K-31; Baxter Healthcare Corporation, Deer®eld, IL, USA) was placed through the LMA or ETT with the tip guided into the trachea under direct vision using a bronchoscope (`distal' catheter). An identical catheter was placed with its tip in the proximal end of the devices close to the swivel connector (`connector' catheter) and both were attached to individual transducers (Medex, Klein Winternheim, Germany) measuring pressure relative to atmosphere. The catheters were ¯ushed with air frequently throughout the study; this reliably prevented plugging by mucus or condensation. Signals from these pressure transducers and from the sidestream spirometer (¯ow and Pao) were displayed on a monitor (ASIII; Datex) and stored on a personal computer after digitizing at 100 Hz. The monitor was calibrated according to the manufacturer's speci®cations; standard pressures and a standard volume were recorded on the computer before each study for correction of the digitized signals. Earlier tests had shown that the ASIII monitor delays signal output by 50 ms (pressures) or 250 ms (signals from the sidestream spirometer). The tracks of the digital recordings were shifted appropriately, so the remaining phase shift between signals was
