To the Editor:
The low pressure alarm (LPA) is an integral monitor of modern anesthesia machines. The Ohmeda Model 7000 ventilator (Ohmeda, Madison, WI) will alert the user through visual and audible alarms if after two or three consecutive ventilation cycles the pressure within the patient circuit is not at least 6 cm H2 O. However, small endotracheal tubes (ETTs), inside diameter size 2.5 through 3.5 mm, introduce an appreciable amount of resistance in the anesthesia circuit. Consequently, the LPA of anesthesia ventilators equipped with "adult size" bellows may not get activated if the patient becomes inadvertently extubated.
Because we recently experienced such an incident, we studied the LPA of three Ohmeda series 7000 ventilators set at similar settings: minute ventilation = 4 L/min, rate = 20/min, I:E ratio = 1:2, ventilating to atmosphere through size 2.5, 3.0, and 3.5 mm inside diameter ETTs. The Modulus II Registered Trademark anesthesia machines were fitted with pediatric circuits 36 times 5/8 inches, and the fresh gas flows of oxygen were set at 2, 3, 4, and 5 L/min. Time from start of ventilation with full bellows to LPA was noted; if it exceeded 3 min, it was entered as infinite (infinity).
The results of this study are as shown in Table 1. At 2 L/min, only the 3.5 mm ETT was associated with a LPA, seen when the bellow was completely empty. At higher flow rates, no LPA was noted.
We conclude that monitoring systems other than the LPA, such as capnography or volume monitors, must be used to alert users of an accidental tracheal extubation in infants ventilated with "adult size" anesthesia ventilators.
Farid J. Azzam, MD
Department of Anesthesiology
St. Louis University Hospital
St. Louis, MO 63110