Episode #50 Addressing the Low Pressure Alarm: Part 2

Hello and welcome back to the Anesthesia Patient Safety Podcast.  My name is Alli Bechtel and I am your host. Thank you for joining us for another show. This is the exciting Part 2 all about the low pressure alarm. Last week, we talked about anesthesia monitoring standards and the low pressure alarm, but there is still so much more to talk about today. This is an important topic and our featured article is one of the top APSF Newsletters articles of all time.

Before we dive into today’s episode, we’d like to recognize Fresenius Kabi, a major corporate supporter of APSF. Fresenius Kabi has generously provided unrestricted support as well as research and educational grants to further our vision that “no one shall be harmed by anesthesia care”. Thank you, Fresenius Kabi- we wouldn’t be able to do all that we do without you!”

Let’s head into the APSF archives. Our featured article today can be found by clicking on the newsletter heading. 4^th one down is newsletter archives. From here, scroll down to 1998 and click on the Winter 1998 Issue. Looking at the Articles column on the left, click on the 4^th one down which is our featured article today, “The Low-Pressure Alarm Condition: Safety Considerations and the Anesthesiologist’s Response” by David Raphael. As we go through this article, keep in mind that this is an article from our archives episode so things may have changed since the publication of this article. This will just give us more to talk about on future shows, but this article is of interest from our archives.

Have you heard or seen a low pressure alarm in the OR while providing anesthesia care? The anesthesia machines are incredibly complex pieces of advanced medical technology. This was true back in 1998 and it remains true today. When functioning appropriately these machines provide oxygenation, ventilation, and anesthesia. When patients require mechanical ventilation, it is vital that the anesthesia machine and breathing circuit be functioning appropriately and be intact. If a low pressure alarm occurs, this is the time for the anesthesia professional to take action to troubleshoot and address the alarm in order to be able to continue to provide mechanical ventilation to the patient. First, we are going to review the differential diagnosis and then, we’ll take you through a response algorithm with practical steps to follow in order to troubleshoot a low pressure alarm.

In order to generate an accurate differential diagnosis, we will review the 4 components of the mechanical breathing system including

1. Gas source
2. Conducting system or the breathing circuit
3. Pressure generator or the mechanical ventilatory in this case
4. Waste gas scavenging system which only interacts with the ventilator limb and the adjustable pressure-limiting valve or the popoff valve.

These 4 components come together at a common location where the system can be converted from mechanical ventilation to manual ventilation. Check out figure 1 in the article for a visual representation of this. So, what can cause the low pressure alarm in this system? Here is our differential:

1. Inadequate fresh gas inflow
2. Breathing system leak or obstruction
3. Faulty or inappropriately set mechanical ventilation
4. Excess removal of gas from the breathing system

Now, that we have our differential let’s go through the algorithm. Anesthesia professionals need to be ready to move through this response algorithm when there is a low pressure alarm in order to maintain patient safety during anesthesia care. Here we go!!

The first priority is to maintain oxygenation and ventilation for the patient.  One of the important steps for preparing for anesthesia care is to ensure that there is a back-up method for ventilation available which may include a self-inflating resuscitation bag or Ambu-bag and a tank of 100% oxygen.

Now, let’s say that the low pressure alarm has sounded.

The first step is to check the available additional monitors including the pulse oximeter, end-tidal CO2, and the pressure gauge on the ventilator or the pressure trace. You will also want to confirm the alarm is the low pressure alarm with the visual and auditory alerts. The author includes a response algorithm in Figure 2 that you can follow along with for the next steps.

Next, evaluate for a complete or partial circuit disconnection. Start at the y-piece and make your way proximally to the anesthesia machine making sure to check all sites distal to the switchover between mechanical and manual ventilation. This scan only takes a couple of seconds. The most common location is at the Y-piece and may occur while repositioning the patient or during prepping and draping for surgery, or with patient movement as well. If a disconnection is discovered, reconnect the tube and continue to monitor to ensure that the alarm has been addressed and that the patient is being adequately oxygenated and ventilated.

If there is no disconnect, let’s move on to the next step. Make the switch from mechanical ventilation to manual ventilation. Changing to manual reservoir bag ventilation excludes any ventilator-related malfunctions and narrows the differential to causes related to the breathing circuit of the fresh gas limb. Now, push the O2 Flush Valve Button to fill the reservoir bag with 100% oxygen. This step allows for evaluation of the following 3 situations:

1. The reservoir bag fills with 100% O2 which tells us that the pipeline to oxygen flush valve system is intact and functioning properly and we can turn our attention to the brahting circuit and fresh gas and ventilator limbs.
2. The reservoir bag does not fill with oxygen which tells us that there is a problem with the fresh gas supply or the oxygen flush valve itself.
3. The reservoir bag fills at first, but then collapses which tells us that there is a problem with an active suction device in the breathing circuit, a misconnection in the circuit, or a problem with the scavenging system.

Making our way through this algorithm, we will review each of the above 3 scenarios separately. Let’s start with the first one when the reservoir bag does fill up appropriately. Now, that we know the pipeline is intact, we can turn our attention to the breathing circuit. Here are the steps to evaluate the breathing circuit.

1. First, attempt to provide manual ventilation by partially closing the adjustable pressure relief valve and squeezing the now filled up reservoir bag. Look for adequate chest rise and listen for breath sounds. Evaluate the capnograph and respirometer to confirm exhalation as well. If you do not hear breath sounds or see chest rise or evidence of exhalation, then ventilation is not occurring. During attempted manual ventilation, the circuit pressure gauge or the pressure trace monitor on the anesthesia machine may demonstrate failure to generate pressure within the breathing system.
2. Step 2: Increase pressure within the reservoir bag. You may need pressures as high as 40cm H2O. Then, apply pressure to evaluate for an audible leak which may provide a clue as to where the partial disconnection is located. Examine the breathing circuit carefully to make sure that all of the components are connected securely.
3. The third step is to discontinue or remove any suction devices. Remember, the nasogastric tube may inadvertently be placed into the tracheal and when suction is applied the low pressure alarm may sound. In this situation, the applied suction within the tracheal may even lead to a sub atmospheric pressure alarm as well.
4. The fourth step is that if we made it this far in the algorithm, then we need to make sure that the problem is not with the location of the endotracheal tube or a faulty cuff. The next step is to evaluate the cuff and if it does not hold pressure then you may need to replace the endotracheal tube. This may be difficult to do during surgery, so consider using a GlideScope and airway exchange catheter to do this safely. If the cuff can hold pressure and is intact, then this may be due to an inadvertent extubation and it is time to act quickly to remove the endotracheal tube and provide mask ventilation while preparing to reintubate if necessary.

Now, what if we return to step 1 and we are able to provide adequate oxygenation and ventilation manually with the reservoir bag…this means that we see good chest rise and can hear bilateral breath sounds and have positive end-tidal CO2. This means that the problem must be somewhere else in the breathing circuit. Remember, we have already ruled out a clearly visible disconnection, a cuff tear or deflation, and an accidental extubation. Now, we need to examine the entire circuit for evidence of a leak or obstruction starting with the endotracheal tube-to-adapter and moving to the endotracheal tube adapter-to-breathing circuit followed by the sidestream sampling gas monitors (for example, the mass spectrometer and capnograph), condenser/ humidifier, pressure sensor, O₂ analyzer, PEEP valve, or other “add-on” connections. Next, we will need to evaluate the inspiratory and expiratory limb connections until we reach the reservoir bag mount-to-bag connection.

Another important consideration at this time is anything that may be providing high-flow rates to the gas analyzer or bronchoscopic suctioning. In order to troubleshoot, remove the bronchoscope in the breathing system if bronchoscopy is being performed and disconnect the high flow rate sidestream sampling gas analyzer from the system. Did this resolve the alarm?

We have now reached the end of our evaluation of the breathing circuit. If the problem has not been identified yet and manual ventilation is possible with the reservoir bag, then we will need to evaluate the fresh gas limb.

The next part of the algorithm involves an evaluation of the fresh gas limb. We have already made sure that there is no leak or obstruction in the O2 flush valve pathway. We need to take a closer look at the high pressure system located upstream of the O2 flowmeter control valve and the low pressure system which is downstream. If there is decreased oxygen supply pressure in the high-pressure system, this is when the “fail-safe” valve shuts off or reduces the flow of N2O and other gases to the respective flowmeter control valve. The fail-safe safety feature is unique to the high pressure system and will not respond to decreased oxygen supply pressure in the low pressure system. This is an important clinical scenario as well as one that is tested on the boards. If N2O and O2 are bring used and there is a leak or obstruction in the oxygen flowmeter, the flow of N2O will not automatically decrease and the patient may receive a hypoxic gas mixture. This is the type of problem that requires an oxygen analyzer to identify the hypoxic gas mixture since the low pressure alarm will not signal that there is a problem. Check out Figure 3 in the article for a visual representation of the fresh gas system.

The next step is to evaluate the flowmeter bobbins. Check their position up or down and make sure that they are moving freely and not stuck. First, if the bobbin is in the UP position and does not move with changes in the control knob, then the bobbin is stuck. This may occur due to dirt, non-vertical alignment of the flow meters, and static electricity. If this is the case, the anesthesia machine cannot be used for patient care and must be replaced. If the bobbins are in the UP position, but are able to move freely with control knob changes, then there may be an obstruction or leak distal to the bobbins in the low-pressure system. If it is a big leak upstream of the common gas outlet, then very little flow may leave the common gas outlet and the low pressure alarm will sound. One of the ways to temporarily troubleshoot this is to turn up the fresh gas flows. If the problem is an obstruction downstream of the flowmeters, the transmitted pressure to the bobbins compresses the gas and the bobbins float at a lower position. As a result, there will not be adequate fresh gas flow leaving the common gas outlet and the low pressure alarm will sound. We can now move from the flowmeters to the vaporizers to ensure that there is no leak or obstruction. If no problem is identified here, it is important to evaluate the common gas outlet for the presence or absence of gas flow. If there is no flow here then there must be a leak or obstruction in the fresh gas pathway between the bobbins and the common duct outlet. To troubleshoot, use your back-up method for ventilation, like the Ambu-bag and 100% O2 from a tank with a separate flowmeter and the anesthesia machine will need to be replaced.

Keep in mind the Ohmeda anesthesia machines have a common gas outlet check valve that is downstream of the bobbins. This serves to stop retrograde gas flow from the breathing circuit into the anesthesia machine. For anesthesia machines without this check valve, positive pressure during manual or controlled ventilation or transmitted from the oxygen flush valve may transmit to the low-pressure system which would reveal a leak in the flowmeters or distal to the flowmeters.

What happens if the bobbin remains in the down position? This occurs when there is an internal machine leak or obstruction. So, if the bobbin stay in the down position after adjusting the control knob, open the oxygen reserve cylinder. If the bobbins remain down then the leak or obstruction is proximal to the flowmeter control valve and within the high pressure system. Once again, you will need to use your backup ventilation and replace the anesthesia machine.

We have reached another milestone in the response algorithm. If we have not discovered the problem in the breathing circuit or in the fresh gas supply limb, then we will need to continue our investigation with the ventilator limb.

And this seems like the perfect time to leave you with a cliff hanger because we still have a lot to talk about when it comes to the low pressure alarm. Since that is the case, I hope that you will join us for part 3 next week!

In the meantime, if you have any questions or comments from today’s show, please email us at [email protected] or connect with us on Twitter, Instagram, Facebook, or LinkedIn.

Visit APSF.org for detailed information and check out the show notes for links to all the topics we discussed today. Please keep in mind that the information in this show is provided for informational purposes only and does not constitute medical or legal advice. Have you check out the June 2021 APSF Newsletter yet?! It was published online on June 1^st! Go check it out and don’t worry there is some great content that we will be discussing in upcoming shows!!

Until next time, stay vigilant so that no one shall be harmed by anesthesia care.

© 2021, The Anesthesia Patient Safety Foundation