Episode #51 The Low Pressure Alarm Response Algorithm: Part 3

June 29, 2021

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Welcome to the next installment of the Anesthesia Patient Safety podcast hosted by Alli Bechtel.  This podcast will be an exciting journey towards improved anesthesia patient safety.

This is an Articles From the Archives show. Our featured article is from the Winter 1998 ASPF Newsletter called, “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. In addition, this is one of the top APSF articles of all time. You can find the article here. https://www.apsf.org/article/the-low-pressure-alarm-condition-safety-considerations-and-the-anesthesiologists-response/

This is the third and final installment of our series on the Low Pressure Alarm and the Safety Considerations. The response algorithm incorporates a quick visual scanning step followed by more detailed hands-on testing of the gas source, breathing circuit, ventilator, and waste gas scavenging system.

Make sure you have a back-up method for manual ventilation with a self-inflating resuscitation bag with an independent source of oxygen available in the OR anytime you provide anesthesia care.

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© 2021, The Anesthesia Patient Safety Foundation

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 moment you have all been waiting for…the exciting conclusion to our review of the low pressure alarm and the response algorithm in order to address the alarm and keep patient safe during anesthesia care. If you haven’t listened to Parts 1 and 2 which were episodes 49 and 50 respectively, then what are you waiting for?! Also, you heard that right when I said Episode 50 which was published last week!  That’s 50 episodes of the anesthesia patient safety podcast. Thanks for listening and we are so excited for so many great shows in the future.

Before we dive into today’s episode, we’d like to recognize GE Healthcare, a major corporate supporter of APSF. GE Healthcare 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, GE Healthcare- we wouldn’t be able to do all that we do without you!”

Now, it’s time to head into the APSF archives. Our featured article today can be found by clicking on the newsletter heading. 4th 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 4th one down which is our featured article today, “The Low-Pressure Alarm Condition: Safety Considerations and the Anesthesiologist’s Response” by David Raphael. 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.

We left off with a cliffhanger last week. We had already moved through the response algorithm to rule out problems in the breathing circuit and fresh gas supply limb. So, today, we are turning our attention to the ventilator limb for anesthesia machines with ascending bellows. So, take a deep breath…because here we go!

While moving through the response algorithm, make sure to provide manual ventilation to ensure adequate oxygenation and ventilation for the patient. You may need to call for help in order to have an extra set of hands and eyes as well.

The first step is to check the bag-ventilator selector valve to ensure that it is set in the ventilator position to provide mechanical ventilation and bag position for manual ventilation. The next step is to check the ventilator settings to evaluate the tidal volume, ventilator rate as well as the delivered tidal volume and adjust the inspiratory flow rate if needed. This is the time to change from manual reservoir bag ventilation back to mechanical ventilation to see if the alarm has been resolved.

Make sure that the pressure limiter setting is not set below the low-pressure alarm threshold since the result will be pressures in the breathing system below the threshold for the low pressure monitor, and the alarm will sound. Next up is to evaluate the ventilator hose which serves as a connection between the ventilator and the breathing system for a leak, obstruction, or disconnection. The 4th step is to check the electrical power or driving gas supply. Once the ventilator is turned on, if it fails to cycle then electrical power is not being supplied to the machine or the driving gas supply to the vent has been disconnected, turned off, or is being delivered at a pressure that it too low. If the anesthesia machine does not pass this step, it will need to be replaced.

The fifth step is to evaluate the bellows for the following situations:

#1: If there is little or no movement with ventilator cycling this may be due to a significant obstruction somewhere between the bellows and the breathing circuit or a there is a decreased supply or excess venting of the driving gas or there is a ventilator canister leak. In order to locate the source of the problem, first open the oxygen supply flush valve which should lead to movement of the bellows. If this does not occur, then there is an obstruction in the ventilator patient circuit. An example of this type of obstruction is when the selector switch is in the “bag” position. Once you have determined that there is an obstruction, you will need to provide manual ventilation and you will need to replace the ventilator. The next step is to check the inspiratory flow rate and turn up the flows. If the bellows compression resumes, then you know that your inspiratory flow rate was too low. The last option on the differential for this step is too low ventilator driving gas pressure to compress the bellows. To check for this, look at the bellows housing and listen for evidence of a leak. If no leak is found, then the problem is with a low driving gas supply pressure which should have its own specific alarm or there is venting of the driving gas during the compression cycle. In this case, the ventilator and anesthesia machine may need to be replaced.

Now, we are on to the 2nd situation in which the bellows collapse. The first step is to evaluate the fresh gas flow setting which may have been set too low or there may be a small internal leak and increasing the fresh gas flow will prevent the bellows from collapsing. However, if the bellows still collapses, then there is likely a large leak in the ventilator patient circuit somewhere between the opening of the bellows to the breathing circuit including the pathway between the bellows and the ventilator pressure-relief valve. You may be able to diagnosis a major leak by opening the oxygen flush valve with the selector valve set in the ventilator position. If this maneuver allows the bellows to fill, then the leak was overcome with the high oxygen flow rates or the bellows will remain collapsed and you will hear the hissing sound of a large leak.

The next step is to check the ventilator pressure-relief valve. When the machine is functioning, the ventilator pressure-relief valve is closed during the inspiratory phase of mechanical ventilation due to the pressure of the driving gas which stops breathing system gas from entering the scavenging system and directs the breathing system gas to the patient circuit. Then, at the end of expiration, this valve opens so that the waste gas can go into the scavenging system. If this valve malfunctions, breathing system gas may enter the lower-resistance scavenging system during inspiration leaving lower pressures in the breathing circuit and the low pressure alarm will sound. An incompetent ventilator pressure relief valve may be due to ventilator valve rupture, flapper valve malfunction, or a disconnected pilot line.

You may be able to diagnose this problem by disconnecting the ventilator scavenging hose and listening for a leak during inspiration or check for paradoxical distention of the scavenging system reservoir bad during inspiration. I know that we just went through a lot of scenarios, but the quick answer is that if the bellows are collapsed, increase the fresh gas flow and the inspiratory flow rate settings to see if this addresses the low pressure alarm and allows the bellows to fill. If not, the ventilator will likely need to be replace.

The 3rd scenario is when the bellows motion is normal. First, check the low pressure alarm threshold to make sure that the monitor is functioning. If you are unable to adjust the alarm threshold, then the monitor may not be working and will need to be repaired. If the monitor appears to be functioning, then the next step is to evaluate the fresh gas supply system.

The low pressure alarm may sound when the manual reservoir bag doesn’t fill up when the oxygen flush valve is open.  Here are the 4 causes:

  1. Loss or reduction of pipeline pressure
  2. Internal machine leak or obstruction
  3. Fresh gas flow disconnection or obstruction between the machine common gas outlet and the breathing system
  4. Leak or obstruction in the CO2absorber

We are doing great moving through the algorithm, so let’s take the path to evaluate the fresh gas limb. The first stop is to check the fresh gas outlet hose and connections between the anesthesia machine and the breathing circuit looking for any occlusions or disconnections. It may be necessary to reattach the fresh gas outlet connections to rule out a partial disconnection. A hint that there is a disconnect at the common gas outlet is that there may be a decreased value read by the oxygen analyzer. The next stop is to evaluate the CO2 absorber canister. If the CO2 canister is unlocked or the canister fails to seal completely with the CO2 canister housing, then the reservoir bag will not fill up when the O2 flush valve is opened. So, lock in the CO2 canister or get ready to move to the next step. Make sure that the flowmeter bobbins are floating and that you are able to increase the gas flow. If the bobbins are up, this indicates a leak or obstruction in the common gas outlet. If the bobbins are down, open the O2 reserve cylinder and if the bobbins then rise up, this points to a problem with the pipeline pressure and the pipeline system will need to be reviewed carefully including the pressure gauges, gas hoses, and wall supply. On the other hand, if the bobbins do not rise, the points to an obstruction or leak that is within the anesthesia machine involving the O2 flush valve conduit and the O2 supply pathway leading to the oxygen flow control valves. Keep in mind that the reserve O2 cylinder will also need to be evaluated to make sure that it is not empty, it is mounted correctly and there is no obstructed cylinder flow outlet valve. For these cases, the appropriate action is to switch back to the default mode of ventilation and replace the anesthesia machine.

The next branch point on the algorithm involves the scavenging system and what may happen when there is too much negative pressure in an active scavenging system. Passive scavenging systems do not have the same concerns since there is no active vacuum suction in use. One area of concern is an obstruction at the negative-pressure relief valves or port holes leading to excessive negative pressure in the scavenging system. Negative-pressure relief valve obstructions have been reported due to plastic bags, dust, tape, and other objects. Excessive negative pressure within the active scavenging system may be applied to the breathing system. So, how does this happen? During spontaneous ventilation, there may be loss of gas from the breathing system to the scavenging system, with its excessive negative pressure, leading to low or even sub-atmospheric pressure in the breathing system. If we change to mechanical ventilation, the excessive negative pressure in the scavenging system may lead to increased or decreased pressures in the breathing system depending on the ventilator pressure-relief valve. This is why it is vital to have a way to monitor and limit sub-atmospheric pressures in an active disposal system.

A closer inspection of the waste gas scavenging system may be necessary and the set-up depends on the ventilator. For the Drager and Ohmeda machines scavenging systems, wall suction is applied downstream of the APL valve and the ventilator pressure relief valve at the location of the air intake valve. A problem with a vacuum source in the breathing system, or a misconnection or excessive negative pressure in the scavenging system may be evident by the circuit reservoir bag filling when the o2 flush valve is opened followed by immediate collapse. The low pressure alarm is not the only alarm that will be activated in this case since the sub-atmospheric pressure alarm will be activated when the pressures fall below -10cm H2O.

The next part of the algorithm involves the removal of any suction devices associated with the patient that may be causing the low pressure. This includes any gastric tubes, bronchoscope, or high-flow sidestream sampling gas analyzers. For the sidestream sampling gas analyzer, the gas flow rates may be between 50-800ml/min and if the fresh gas flows are low this could lead to a low or negative pressure event in the breathing circuit. Keep in mind that for excessive negative pressure in the scavenging system to cause a low pressure alarm there would also need to be a defective or stuck ventilator pressure-relief valve that stays open during inspiration and expiration in order to transmit the pressure to the breathing system.

Let’s look at where we are in the algorithm…we have removed all suction devices and all of the components of the breathing systems are intact, so the next step is to consider the scavenging system. There are 3 situations on our differential.

  1. a blockage of the air intake valves
  2. an absent or malfunctioning vacuum flow control valve at the scavenging interface
  3. excessive central vacuum suction pressure

Here is our treatment plan. The first step is to disconnect the scavenging hose between the ventilator and scavenging reservoir and if this addresses the low-pressure alarm then the problem is in the scavenging system. The next step is to check for misconnections because this may be the situation that a full vacuum flow rate is applied to the breathing system.

To further evaluate the scavenging system, look for obstruction of the port-holes in an open system or the pressure-relief valve air entry sites in a closed system. The next step is to adjust the waste gas exhaust flow control knob in a closed system. This may address the alarm if the setting was too high for the waste gas removal exhaust flow and may reveal a partial obstruction at the air intake valves. Unfortunately, a trained anesthesia machine service person will need to be called in to detect a partial obstruction of the air intake valves due to dust or lint. The final step here is to disconnect the vacuum suction from the scavenging system because the problem may be a malfunction at the vacuum flow control valve in the scavenging system.

Another important area for consideration is the default mode of ventilation or manual ventilation with a self-inflating resuscitation bag with an independent source of oxygen. It is vital to make sure that patient can be adequately ventilated with bag ventilation to quickly rule out an inadvertent extubation, esophageal intubation, or cuff leak or any situation where the patient will need to mask ventilation or re-intubation. It is imperative that a manual, self-inflating resuscitation bag with an independent source of oxygen be available in the OR anytime a patient enters for anesthesia care. If the default mode of ventilation is working appropriately, then it is possible to work through the response algorithm to address the low pressure alarm.

We have almost made it to the end of the algorithm. The final part highlights the importance of the pulse oximeter to provide continuous patient monitoring while working through the response algorithm, but keep in mind that once the oxygen saturation starts to decline, this is a late indication of a problem and rapid action must be taken to provide oxygenation and ventilation to the patient to keep them safe. It may not be possible to work through the algorithm and diagnose the problem while providing safe patient care and if this is the case, it is important to return to the default mode of ventilation to continue to provide adequate oxygenation and ventilation.

Wow, we have made it to the end of the algorithm which first incorporates quick visual scanning step that can be performed within about one minute followed by more detailed hands-on testing of the gas source, breathing circuit, ventilator, and waste gas scavenging system. This algorithm was developed to diagnose a single fault within the system with some consideration for double fault events as well. By using the algorithm to evaluate the four limbs of the mechanical ventilator, it is possible to diagnose multiple faults in the system.

A final consideration is that this response algorithm to the low pressure alarm forms the foundation for a response to the low pressure alarm and may be modified for use with different anesthesia breathing systems. These are complex systems that have changed over time, but the ability to understand and troubleshoot the anesthesia machine and mechanical ventilator is such an important aspect of providing safe anesthesia care.

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. We are moving on from our discussion of the low pressure alarm, so check back in next week for more great anesthesia patient safety content!

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

© 2021, The Anesthesia Patient Safety Foundation