Laringeal Mask Airway (LMA) and air leakage: Is expired volume the whole truth?

Roberto García-Aguado MD, Ph.D, Section Chief Ambulatory Anesthesia
Dept of Anesthesiology
Consorcio Hospital General Universitario,Valencia, Spain.Address for correspondence
Dr. R. García-Aguado, Pianista Amparo Iturbi nº 73-1ª
46006 Valencia, España
E-mail: aguador@saludalia.com
Fax +34963862946.
Phone: +34656556604

The information provided by monitors used during artificial ventilation - such as ETCO2, SPO2, anaesthesia gas concentration and airway pressures and volumes - never seems to be questioned. Such information is often used for diagnostic purposes and clinical decision making with little or no consideration for the actual technology used to obtain such data.
This letter aims to concentrate on spirometry and draw attention to the use made of determining the expired volume in certain scientific publications dealing with the laryngeal mask airway (LMA) in determining air leaks. For patients undergoing mechanical ventilation, in some cases1 the expired volume is subtracted from the inspired volume to calculate air leak percentages. In other cases, a similar method is used, but it assumed that a expired volume of 8ml.Kg-1 indicates appropriate ventilation, with some authors specifying that the inspired volume selected is the same2 (8 ml.Kg-1) and others accepting this as being understood.3,4
At present, however, there are various manufacturers of anesthetic devices including monitoring, and although they all provide spirometric data (current volume and minute volume), they do not necessarily use the same technology, since other normal turbine spirometers, hot-wire spirometers, or pressure differentials measured at two points using flow resistance elements can all be used. The latter are more frequent used in critical care patient. Moreover, there are various different systems to restrict flow: mainly membranes, channels, and "cells".
It is the manufacturer's decision to incorporate one type of technology or another, but do we really know to what degree these decisions affect performance, precision, servicing, economy and patient safety? The requirement for frequent routine tests/checks make these systems difficult to use, with such difficulties being conducive to omission or non-fulfilment of proper maintenance as required by the manufacturer. Also of relevance are the number of connections to be made and tubings to be used. The site chosen for placing the spirometer (proximal or distal from the patient) and the effects these may have on the results obtained. These factors encouraged us to reconsider the precision and reliability of such measurements, which show differences when volume sensors are placed at different points along a circuit in one single process. Additionally, the discrepancies observed between results for inspired and expired volumes, often above 20%, can be attributed to physical phenomena such as the law of gases, patient condition (obstructive/restrictive) and conditions under which corrective algorithms are applied. In this respect, modifications in pressure, temperature, humidity and flow type (laminar or turbulent) of expired gases are the prime causes.5 Because of all this, permitted tolerances for this type of equipment under relevant standards (UNE 740) accept errors up to 20%.
For these reasons, we suggest that to obtain a clinical evaluation of the air-tightness of airways using the LMA, it would be more efficient to test for leakage based on pressure factors.6,7 At present, airway pressure curves can be visualised optimally in the monitors incorporated in certain respirators available on the market today.

References

1. Brimacombe J, Keller C, Boehler M, Puhringer F. Positive pressure ventilation with the ProSeal versus classic laryngeal mask airway: a randomized, crossover study of healthy female patients. Anesth Analg 2001; 93: 1351-3.
2. Brain AI, Verghese C, Strube PJ. The LMA 'ProSeal'--a laryngeal mask with an oesophageal vent. Br J Anaesth 2000; 84: 650-4.
3. Brimacombe J, Keller C, Judd DV: Gum elastic bougie-guided insertion of the ProSeal laryngeal mask airway is superior to the digital and introducer tool techniques. Anesthesiology 2004; 100: 25-9
4. Evans NR, Gardner SV, James MFM, King JA, Roux P, Bennett P, et al. The Proseal laringeal mask: results of a descriptive trial with experience of 300 cases. Br J Anaesth 2002; 88: 534-9.
5. Dorsch JA, Dorsch SE - Gas Monitoring, em: Dorsch JA, Dorsch SE - Understanding Anesthesia Equipment, 4th Ed, Philadelphia, Lippincott Williams & Wilkins, 1999; 679-754.
6. Keller C, Brimacombe J, Keller K, Morris R. A comparison of four methods for assesing airway pressure with the laringeal mask in adult patients. Br J Anaesth 1999; 82: 286-7.
7. Cook TM, Nolan JP, Verghese C, Strube PJ, Lees M, Millar JM, et al. Randomized crossover comparison of the proseal with the classic laringeal mask airway in unparalysed anaesthetized patients. BJ Anesth 2002; 88: 527-33.

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