To the Editor:
Norman et al.  reported a critical incident arising from an inability to detect occlusion of the anesthesia circuit (obstructed by a plastic wrapper retained on a CO2 absorber canister) despite adherence to current Food and Drug Administration (FDA) guidelines for preoperative checkout of anesthesia apparatus. Thus, the authors suggest a restoration of a provision of the 1986 FDA guidelines wherein the circuit is tested by breathing through it prior to induction of anesthesia. To us, this practice seems unsafe because of the potential for contamination of either the circuit or anesthesiologist. Our recommendation is that the final evaluation of anesthesia equipment should include observations of reservoir bag excursions and CO2 waveforms during denitrogenation of the lungs . This approach ensures the integrity of the circuit and the functional status of the capnograph and should readily reveal potentially severe problems such as those encountered by Norman et al. and others .
Apparently, many anesthesiologists are reluctant to place a face mask snugly on awake patients, perhaps because they believe that this causes discomfort or provokes anxiety. The vast majority of our patients, however, are willing participants in this final equipment check after we explain our reasons for obtaining a tight mask fit. Moreover, an adequate mask seal is important in those cases in which denitrogenation of the lungs is indicated prior to induction of anesthesia. Nonetheless, we often hear of prolonged attempts at pulmonary denitrogenation via only a loosely fitting face mask (e.g., the 5 min reported by Norman et al.). This practice is puzzling to us because a typical patient has inspiratory flow rates of approximately 30 L/min during normal breathing . If the mask is applied loosely with typical fresh gas flows of 8-10 L, the patient will entrain substantial quantities of room air. Indeed, both a tight mask seal and 100% O2, either via four vital capacity breaths or normal ventilation for 90 s, are required to decrease the end-tidal nitrogen concentration to less than 10% .
The continued occurrence of equipment problems such as those encountered by Norman et al. strengthens our belief that the above-stated recommendation should be included in FDA guidelines.
Steven M. Dunn, MD
Department of Anesthesiology; Baystate Medical Center; Tufts University School of Medicine; Springfield, MA 01199
Phillip S. Mushlin, MD, PhD
Department of Anesthesiology; Brigham and Women's Hospital; Harvard Medical School; Boston, MA 02215
1. Norman PH, Daley D, Walker JR, Fusetti S. Obstruction due to retained carbon dioxide absorber canister wrapping. Anesth Analg 1996;83:425-6.
2. Dunn SM, Mushlin PS, Lind LJ, Raemer D. Tracheal intubation is not invariably detected by capnometry. Anesthesiology 1990;73:1285-7.
3. Murray JF, Nadel JA. Textbook of respiratory medicine. 2nd ed. Philadelphia: WB Saunders, 1994:799.
4. Carmichael FJ, Cruise CJE, Crago RR, Paluck S. Preoxygenation: a study of denitrogenation. Anesth Analg 1989;68:406-9.