Avoiding Barotrauma

Unless you’ve had a very short career in veterinary anesthesia or have been incredibly lucky, all of us have had that sickening feeling or looking over at the rebreathing bag on our veterinary anesthesia machine, to find it hugely distended and see the pressure climbing on the manometer. Fortunately, we often intervene in the “nick” of time, but whether we do or we don’t, those type of events will often haunt us for a long time to come. This post is dedicated to simple, reasonably priced things we can use to avoid barotrauma in our patients.  I hope the following blog will help some of our veterinary community avoid the trauma that comes from potential pulmonary barotrauma.

What is pulmonary barotrauma?

Let’s start at the beginning . . . what is pulmonary barotrauma? Pulmonary, or lung, barotrauma is damage to the lungs from excessive pressure buildup, usually in a short span of time. Because of that short time span, the lungs can’t accommodate this pressure increase. This excessive pressure buildup is almost exclusively the result of a mechanical error. Why? Well, most mammals’ breathe using negative pressure (i.e. they inhale air from their environment by moving musculature and entraining that air down to the alveoli, where gases are exchanged). Negative pressure means that the diaphragm and the intercostal muscles control the influx of air.

General anesthesia might wreak havoc on this beautiful system. One of the most common anesthesia complications, hypoventilation, occurring anywhere from 13.5% to 63.9% of cases (1, 2). So positive pressure is used to artificially introduce gases to a patient’s lungs. This is done by hand, via occluding the adjustable pressure-limiting valve, or via a machine, called a mechanical ventilator. Depending on the species in question, the alveoli can endure up to 36 cmH₂O before there is possible barotrauma.

How does pulmonary barotrauma occur when a patient is not being ventilated by a machine or by hand?

There is a second reason we can see barotrauma. In an effort to ensure a system does not have a leak (preventing effective gas containment), this system requires a pressure check. In order to effectively pressure check a system, this system must be fully closed off to possible means of venting the gas to outside system. To err is human, and so in circumstances where the adjustable pressure-limiting valve is not re-opened, pressure can build in the system and result in pulmonary barotrauma.

What are some methods to alert or to prevent pulmonary barotrauma?

Alerting the anesthetist to pressure building in the system relies on the anesthetist making the appropriate intervention to reduce the likelihood of barotrauma. These features make excellent adjuncts to equipment that prevents pulmonary barotrauma, but should not be relied on as a sole means for addressing the possibility of barotrauma. Two methods for this feature include:

1. Dispomed (not MRI compatible) patient circuit alarm
   • After machine manufacture add on: https://www.dispomed.com/products/patient-circuit-alarm/
   • Built into machine: https://www.dispomed.com/products/moduflex-elite/
2. Smiths medical (not MRI compatible) patient pressure alarm
   • After machine manufacture add on: https://www.smiths-medical.com/products/veterinary/anesthesia/anesthesia-accessories/high-patient-pressure-alarm
   • Note: Patterson also distributes this model.

In order to prevent pulmonary barotrauma, there are two broad strategies:

1. Prior to machine manufacture: Pulmonary barotrauma is prevented by integrating features, which preclude the necessity of a human performing a leak test. These machines are often quite expensive (>$20,000); feel free to contact me if you’re in the market for such a machine!
2. After machine manufacture:
   1. Step 1: Temporary occluder valves help to reduce the possibility of barotrauma secondary from positive pressure ventilation during manual ventilation.
      • These valves are reasonably priced and available from multiple manufactures, such as Smith’s medical. (https://www.smiths-medical.com/products/veterinary/anesthesia/anesthesia-accessories/pop-off-occlusion-valve)
   2. Step 2: Add on relief valves are important to prevent barotrauma due to closed adjustable pressure limiting valves, from both giving a manual breath, and from pressure checking a system. These valves are retrofitted to most present day anesthesia machines, and will automatically vent pressures in the system, as that pressure becomes supramaximal. The additional component of scavenging this gas must be accommodated to prevent waste gas in the operating room.  
      • Relief valve
        1. Essential Medical Devices (EMD) valve https://www.essentialmedicaldevices.com/products/emd-safety-valves
           • Benefits:
             • Valves open at approximately 26 cmH₂O and allows for flow up to 4 L/min when open.
             • The system comes with a simple means to scavenge waste anesthetic gases.
           • Drawbacks are that training is required to appropriately utilize this device.
        2. JD Medical Safety pressure relief valve (http://jdmedical.com/veterinary-anesthesia-machines-products/anesthesia-accessories/safety-pressure-relief-valve/)
           • Benefits:
             1. Two available valves which vent at either 20 or 30 cmH₂0, so they are ventilator compatible.
             2. Vents directly into scavenger system.
           • Drawbacks are that training is required for proper use.
        3. Supera valve (http://www.superavet.com/safety)
           • Benefits:
             • Even when the valve is in the closed position, a pressure release mechanism exists that prevents build up of excessive pressure and will thus still open in the event that excess pressure builds in the system.
             • This is a replacement for the adjustable pressure-limiting valve so it is incorporated into the system.
           • Drawbacks:
             • Pressure at which this valve automatically opens is not noted.
             • Training is required for use.
        4. Hallowell relief valve (combination of a tee piece and a PEEP valve) (http://www.hallowell.com/index.php?pr=z200A3707)
           • Benefits:
             • Easy to use and adapt to existing systems
             • Inexpensive
           • Drawbacks:
             • Requires an F-air to scavenge
             • Fatigues over time; PEEP valve must be replaced every 6 months or so.
             • Training required for use.

So what are the overall recommendations?

1. Some form of a temporary occluder device is recommended.
2. Audio patient circuit alarm systems are highly recommended.
3. Training is required prior to use of any of the previous modifications. Contact me if I can help you with this!
4. I cannot underscore the importance of utilizing a veterinary anesthesiologist for improvement in patient safety initiatives related to equipment utilized for anesthesia. Approximately one third of our training is handling equipment. Contact me if you need help.

Sources:

1. McMillan M, Darcy H. Adverse event surveillance in small animal anaesthesia: an intervention-based, voluntary reporting audit. Vet Anaesth Analg. 2016;43(2):128-35.
2. Redondo JI. Perianaesthetic mortality in dogs: preliminary data of a worldwide multicentric study. [Abstract]. In press 2018.