Over the last five years, the introduction of monitors which alarm unsafe conditions requiring the immediate intervention of the anesthesiologist has been largely responsible for very significant improvements in patient safety. However, along with these advances have come new dangers from an unjustified belief in the security provided by these same monitors. In part, this has been due to some confusion concerning the intrinsic limitation of such automatic devices. The confusion has even led to the misnaming of some monitors. A primary example of such a misnomer is the “disconnert alarm”.
What are commonly called disconnect alarm cannot warn of all disconnects. Moreover, they may alarm in situations of no disconnect. In fact, the term “disconnect alarm” should probably be abandoned. Several different monitors are currently used in an attempt to reduce disconnect accidents. Their limitations are examined in the following discussion.
Pressure monitors: The pressure in the anesthesia machine circuit may be continuously measured and an alarm sounded if the pressure fails to reach a predetermined level periodically. This is the alarm most commonly called a disconnect alarm. There are several practical problems with this approach. First, the monitor is of little or no use during spontaneous breathing where the pressure in the circuit does not rise and fall periodically.
Second, if a disconnect is associated with an obstruction, the pressure may continue to vary periodically and the alarm may not sound. This can happen, for example, if the open end of the disconnected circuit comes to rest against the operating room table. Even the high resistance of a pediatric endotracheal tube connector, disconnected from the endotracheal tube itself, may allow sufficient pressure fluctuations in the circuit to prevent the alarm from sounding.
A problem arises in the case of certain obsolete ventilator designs which incorporate the pressure sensing point in the ventilator itself. If such a ventilator is used in conjunction with a manual selector valve which switches between the ventilator and the rebreathgin bag, no warning will be generated for the case of the selector valve being erroneously placed in the manual position during mechanical ventilation. The pressure sensing point should be as close to the airway as practical, but in any case at least as close as the absorber.
It is true that this pressure alarm, when correctly adjusted, will detect a large percent of disconnects during positive pressure ventilation. For even this to be true, the anesthesiologist must manually adjust the pressure threshold to a level which is just below the peak pressure during inspiration. This pressure will vary not only from patient to patient but even during a given case if tidal volume, inspiratory flow or pulmonary mechanics changes.
Far too often, the alarm threshold pressure is set to its lowest limit in a misguided attempt to prevent false alarms. Such a practice will predictably result in the failure to detect disconnects. Carefully and periodically readjusted, this pressure alarm is quite useful, particularly in conjunction with intelligent interpretation of the airway pressure gauge, but in no sense is it a foolproof disconnect alarm.
Expired flow monitors: A spirometer may be placed in the expiratory limb of a circle system to monitor tidal volume. Recently, units which electronically transduce flow and alarm its lack have become available. However monitors have the advantage of being useful during spontaneous breathing where the circuit pressure does not vary significantly.
However, even with total occlusion of the airway, it is possible that there may be significant flow from the compression of gas within the circuit during inspiration. If the expiratory flow alarm limit is set improperly, it may then fail to sound. Furthermore, during, spontaneous breathing, a disconnect on the machine side of either directional valve will go undetected. Finally, monitoring of flow in the expiratory limb is of limited utility in the many non-rebreathins circuits which were never designed with patient safety monitoring in mind. But most important, flow in the expiratory limb does not guarantee gas exchange First, in the case of esophageal intubation, it is possible to ventilate the stomach and have reasonably normal flow in the circuit. Second, a mechanical ventilator with descending bellows may draw in room air through a disconnect, producing a near normal flow through the spirometer.
Carbon-dioxide monitors: The capnograph is probably the single most useful monitor of ventilation and, thus, of disconnects, for it relies on the presence of carbon dioxide exchange This is particularly true if the trace is continuously displayed.
Capnography has most of the advantages of expiratory flow monitoring and suffers less from the disadvantages. For example, it can be used to help detect an esophageal intubation. Even hem there can be confusion as the stomach can contain some carbon dioxide. In general, however, with esophageal intuba6on, the capnograph will show a marked decrease in the carbon dioxide concentration over the first few breaths.
A more difficult situation arises in profound shock or cardiac arrest where there is little or no pulmonary blood flow. Here there may be no carbon dioxide exchange despite appropriate ventilation of the lungs with an intact anesthetic circuit. Despite an expired carbon dioxide level of zero, or close to zero, it would be a serious error to conclude that the patient required reintubation. Furthermore just as with expiratory flow monitors, various disconnections will so undetected in the spontaneously breathing patient.
Frank disconnects and other technical difficulties with ventilation are among the most important causes of catastrophic injury during anesthesia. Vigilance, supported by a combination of modern monitoring and display techniques, is the primary defense. However, an understanding of the limitation of our monitors is needed in order to use them most effectively.
Dr. Epstein is Professor and Chairman of Anesthesiology at the University of Connecticut and a member of the APSF Newsletter Editorial Board.