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Because I Was Inverted: What You Need to Know About APRV

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The nurse approaches you as you're taking off your helmet and wheeling the stretcher into the ED from the helipad. 

"We can't get his pulse ox up! We have tried EVERYTHING!"

 This is a small outlying community access hospital and they rarely meet you at the door. Original page was a 43 year old male who is being transferred back to a tertiary care center to receive higher level pulmonary and critical care (they are even using the E word).

The patient presented to the community hospital by EMS with a five day complaint of progressive shortness of breath. 

Nurse Report: 

  • Presented profoundly dyspneic and hypoxic and failed a trial of BiPAP (yah, I know its a trademark name..psh).
  • Intubated and placed on a ventilator.
  • Remains hypoxic with dangerously elevated airway pressures, despite multiple ventilator adjustments.
  • Chest X-ray reveals widespread “ground-glass” opacities. 

Upon your arrival, you find the patient laying on ED stretcher with high pressure alarms sounding.

His vital signs are a heart rate of 154, blood pressure of 170/100 and a pulse ox of 74%.

Current Ventilator Setting:

  • Volume – Assist Control
  • Rate: 24
  • Tidal Volume: 450ml's
  • PEEP: 22 cmh2O
  • FiO2 of 100%
  • Pplat 48 cmh2O

An ABG was obtained just as you arrived with revealed a pH of 7.15, CO2 of 55.3, HCO3 of 21 and PaO2 of 53. 

Given the patient’s hypoxia in spite of increased PEEP, elevated plateau pressures and ventilator desynchrony, you discuss transitioning the patient to airway pressure release ventilation (APRV) with medical control - medical control agrees. 

Anyone who has been in critical care transport for any length of time has had to transition a patient from one mode of ventilation to another, usually from pressure to volume or vice versa. But with APRV being available on some transport ventilators, it is important to have a sound understanding of this mode, as well as its utility for when it may be an option. 

APRV has a reputation of being a somewhat "new form" of ventilation when in actuality it has been utilizing for over three decades, however it has become more prominent as of late, especially in Acute Respiratory Distress Syndrome (ARDS) and Acute Lung Injury (ALI). APRV has been found to be safe and effective in both of these disease states. However, it is important to note, that APRV has not been shown to have an improved mortality benefit. Though it has shown to improve oxygenation, shorten mechanical ventilator duration and ICU length of stay.

APRV is a pressure-limited, time triggered, time cycled, inverse ratio ventilation which allows for spontaneous breathing.It is a form of open lung ventilation, which alternates  between high and low pressures for a predetermined length of time while allowing spontaneous breathing similar to Continuous Positive Airway Pressure (CPAP).

The beauty of APRV is that it allows spontaneous breathing between both inspiratory and expiratory phases. Should the patient have no respiratory effort, APRV becomes a form of inverse ratio, pressure limited ventilation.

So what does all this mean exactly? It is equivalent  to sticking your head out the window going 50mph and being pulled back in for a half a second and repeating this process over and over again.  

UNDERSTANING THE VERNACULAR 

APRV comes with its own vernacular, which may cause some  apprehension to clinicians that are not familiar with the terminology and their meaning. 

With conventional ventilation we are all familiar with the standard settings of rate, tidal volume or pressure, PEEP and FiO2. With APRV there are four main settings; Pressure High, Pressure Low, Time High and Time Low, in addition to FiO2. These four settings determine how much pressure will be given over a defined period of time. 

 To even further complicate matters different manufactures use different names for APRV. It is also important to note that with the different names there may be subtitle differences in their specific mode.

The Highs & Lows: 

With APRV there are two set pressure: Pressure High (P high) and Pressure Low (P low) which occur over two defined periods of time: Time High (T high) and Time Low (T Low). Pressure high and Time high as well as Pressure low and Time low occur in conjunction with one another. 

Pressure high is the upper pressure level most commonly thought of as the CPAP pressure or inspiratory pressure. Time high is the length of time of which pressure high is spent and will be significantly longer than that of time low.

During Pressure High, which can occurs up to 80-90% of the ventilator cycle based on the Time High, alveolar recruitment in addition to the maintenance of lung volumes, occur through an open lung ventilation approach creating a prolonged increase in mean airway pressure resulting in near constant lung recruitment.

Pressure low, is the lower pressure level or the expiratory pressure. Time low is the shorter of the two times and is the expiratory time or release phase. 

During the release phase, it allows for ventilation and CO2 removal. Given the short time that this release occurs, it prevents alveolar decruitment but is long enough to create an acceptable tidal volume.

Spontaneous Breathing 

With APRV allowing the patient to breath spontaneously during all portions of ventilation, it leads to increased patient comfort, decreased sedation, minimized atelectasis and improved gas exchange. It also leads to patient-ventilator synchrony leading to a decreased need for neuro muscular blockade. 

 

The Good & The Bad: 

Advantages: 

  • Lung protective ventilation strategy 
  • Alveolar Recruitment 
  • Improved oxygenation & hemodynamics 
  • Ability to main spontaneous breathing and patient-ventilator synchrony 
  • Decreased sedative & neuromuscular blockade usage 

 

Disadvantages: 

  • Potential Volutrauma 
  • Increased Work of Breathing 
  • Dynamic Hyperinflation

 

PULLING THE APRV TRIGGER 

Unfortunately there is not a well-established recommendation of when a patient should be transitioned from conventional ventilator settings to APRV. In my experience, from a in-hospital perspective some will employ APRV early in the patient’s clinical course where as others will use it a salvage type therapy. So these suggestions my thoughts, which are worth very little....

 

From a transport perspective, I think APRV should be given consideration in the patient with refractory hypoxia despite escalating PEEP, dangerously elevated plateau pressures despite the use of lung protective ventilation strategies  or the patient who is hypoxic and dyssynchronous with the ventilator despite corrective management. 

In both of these scenarios I believe APRV will be beneficial given the maximization of alveolar recruitment during P and T high, creating the prolonged elevation of mean airway pressure as well as the prevention of decruitment during the release phase. I also believe that APRV would be extremely beneficial in the patient with desynchrony as the patient would have the ability to breath spontaneously throughout the entirety of the ventilator cycle, leading to improved patient comfort.

Again, with there being no well-established recommendation of who and when a patient should be transitioned to APRV, it is important for the clinician to analyze all of the information they have at that time and make a decision based on the options that have available, as to if the patient may benefit in transitioning to APRV. 

 

P High: 30

The P High is going to be set to your desired plateau pressure and should be kept below 35 in order to prevent overdistention. When transitioning a patient to APRV from another conventional mode of ventilation, the previously utilized plateau pressure can be utilized as the P High.  

P Low: 0

The P Low is set to 0 to optimize the expiratory flow. 

T High: 4 or greater 

T High should be set greater than 4. 

A T High below 4 will have a negative affect mean airway pressure. The T high should be occur greater than 90% of the ventilation cycle and is the main determinant of the release frequency, with the frequency  being between 10-14. As the number of frequencies increase alveolar decruitment can occur.  

Release Frequency = 60 / (T High + T Low)

T Low: 0.5

T low is the most important of the four parameters. A T Low that is too short may result in inadequate exhalation and hypercapnia. A  prolonged T low can lead to alveolar decruitment. 

Selecting the appropriate release time will result in adequate ventilation while curtailing alveolar decruitment. Minute Volume will be dependent a T low in addition to patient respiratory effort. 

Patient Management:

Hypoxemia

  • Increase P High, T High or both 
  • Increase FiO2 

Hypercapnia:

When utilizing APRV you will likely encounter some degree of hypercapnia which is usually well tolerated.

  • Decrease T High in increments of 0.5 seconds. 
    • By shorting T High, there will be more releases. (T High should generally not be decreased to less than 4 seconds)
  • Increase P High. Increasing 
    • P High will increase delta P (P High – P Low).

Hypocarbia:

  • Increase T High in increments of 5 seconds 
    • By increasing T high, there will be fewer releases. 
  • Decrease P High 
    • By decreasing P High, delta P will lower. 
      • Monitor oxygenation and be aware of possible decruitment. 

 

 

I would be remiss if I didn’t recommend both Ventilator Management: A Pre-Hospital Perspective and Ventilator Management: Advanced Concepts in Critical Care as exceptional texts on ventilator management as supplaments to this acticle. 

Will you consider APRV in refractory hypoxia? 

 

References:

Farkas, J. (2017, November 19). APRV Guideline. Retrieved March 1, 2020, from https://emcrit.org/squirt/aprv/

Frawley, P. M., & Habashi, N. M. (2004). Airway pressure release ventilation and pediatrics: theory and practice. Critical Care Nursing Clinics of North America16(3), 337–348. doi: 10.1016/j.ccell.2004.04.003

Hess, D., & Kacmarek, R. M. (2019). Essentials of mechanical ventilation. New York: McGraw-Hill Education.

Marino, P. L. (2014). Marinos the Icu book. Wolters Kluwer Health.

Nader Habashi: Airway Pressure Release Ventilation (APRV) – A mechanistic and physiologic view. (n.d.). Retrieved from http://marylandccproject.org/core-content/nader-habashi-airway-pressure-release-ventilation-aprv-mechanistic-physiologic-view/

 

 

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