What is a patient circuit alarm?
A patient circuit alarm measures pressure in the breathing circuit (Bain or rebreathing) and will alert the team when the pressure goes over a preset threshold. The alarm is connected between the common gas outlet and the breathing circuit. Except in rare cases (see below: limits of the alarm), the pressure in the circuit is the same as the pressure in the airway of the patient.
Usually, the alarm is preset at 12-15 cmH2O. This threshold is relatively safe for the vast majority of patients, and manual ventilation is possible without the alarm going off, while still warning before the pressure gets too high.
This does not mean a pressure below 15 cmH2O is always safe, or that pressure above 15 cmH2O is always dangerous. For example, the pressure of 30 to 40 cmH2O is sometimes used during recruitment maneuvers, to correct hypoxemia, and it is also possible to cause volutrauma (by insufflating too large a volume) in patients whose thorax is too elastic, such as in pediatric patients. Therefore, it does not replace clinical judgment.
Finally, the objective is not for the alarm to sound at a value that is dangerous but at a value that gives us time to react before it becomes a danger for the patient.
If the air inside the circuit could not escape (for example, if the pop-off valve (or APL valve) is closed), once the breathing bag is full, the pressure rises very quickly. If the alarm triggers at too high a level, it would be impossible to react in time.
What are the dangers of
overpressure in the breathing circuit ?
As pressure rises in the circuit, if a patient is connected to the circuit, pressure also rises in the patient’s airways. This pressure can compress blood vessels, decreasing venous return to the heart, and causing cardiac arrest. Overextension of the lung parenchyma could also cause a pneumothorax (rupture of the parenchyma) or pulmonary edema (increased permeability). Neither situation presents a good prognosis, so the alarm helps prevent this situation. It is, therefore, necessary to act quickly when the alarm sounds.
What should you do
when the alarm goes off?
The quickest way is to disconnect part of the circuit to release the pressure in the circuit. It can be any part of the circuit, but generally, it is easier to disconnect the breathing bag, the breathing tube, or the connection to the endotracheal tube. When pulling on the breathing bag, be careful not to press on it, which would very transiently increase the pressure in the circuit.
The other possibility is to open the pop-off valve, but it may take longer. The most important thing is to release the pressure quickly.
Once the pressure is released, it is important to examine the patient for any signs that may suggest cardiac arrest, pneumothorax, or pulmonary edema. Some signs are immediate (cardiac arrest, tension pneumothorax), but others may develop within minutes (edema, subcutaneous emphysema). It is important to remain vigilant.
How to make sure the
alarm remains functional?
Patient circuit alarms require minimal maintenance. The batteries should be changed regularly and the alarm should be tested daily. When you perform a leak test on the anesthesia machine, if the alarm is functional, it should sound when the pressure exceeds 12-15 cmH2O (depending on how it is set). If no sound is heard, it may be unplugged, or no longer have a battery.
What are the limits
of patient circuit alarms?
Despite their usefulness, alarms have some limitations that are important to understand. In the event of one-way valve failure, especially if the exhalation valve remains in the closed position, the pressure may rise in the portion of the circuit connected to the patient, but not in the portion connected to the alarm. This is a rare situation, which should not happen if the machine is properly checked before use. In this situation, the patient would not be able to breathe, and the patient would have an obstructive respiratory pattern or no respiratory movement, and no trace should be visible on the capnograph.
If an increase in airway pressure is suspected, disconnecting the patient circuit helps ensure that airway pressure is normal.
The alarm also requires human intervention to resolve the situation. This is why it is important that a trained person is present during the anesthesia and knows what to do if the alarm goes off.
Finally, the alarm sounds at a fixed threshold, which is not necessarily suitable for all patients as discussed above. Therefore, always use good clinical judgment when using the alarm.
Overpressure in the breathing circuit is a situation that can deteriorate the patient’s condition very quickly, so it is important to be warned very quickly. Patient circuit alarms are therefore an important element of patient safety by preventing possible overpressure in the circuit before it is harmful to the patient. However, the alarm alone does not make operating an anesthesia machine safe. Machines have other safety features that can reduce the risk, such as safety valves. Despite all these elements, the risk is never zero, and it is important that a trained person monitors the patient and is able to react appropriately in the event of a problem. This is the most important element in patient safety.
About the author
Dr. Truchetti graduated in 2009 in veterinary medicine. He completed a Master’s degree in Clinical Sciences and a Specialized Studies Diploma (residency) in Anesthesiology in 2013, at the Université de Montréal. He graduated from the American College of Veterinary Anesthesia and Analgesia (ACVAA).
He has been a consultant in veterinary anesthesia and analgesia since 2013 with several veterinary clinics, companies, and research groups. He has worked with Dispomed on several projects since 2014. He is also an anesthesiologist at Centre Vétérinaire Rive-Sud, Centre Vétérinaire Laval, and Hôpital Vétérinaire Centre-Ville de Montréal, and works jointly with all of the general practices of the Vet-et-Nous group. Dr. Truchetti is passionate about teaching and anesthesia risk management.
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