Thread by @colombocj1

@chungk1031 APRV...Its gonna be bread and butter during for ARDS so lets get to it. First, when would you consider it? Second, how do I set it up (i.e. initial settings ) Next, how the heck do I adjust this craziness? Last, what is actually happening...Let's Go!

@chungk1031 Ok APRV ...When do I do this? Well, it's a rescue mode. follow ARDSNET, Protective Lung Strategy, Low tidal volume ventilation...whatever we're calling it this week:Tidal Vol of 6-8 ml/kg of ideal body weight, use escalating PEEP/FiO2 to maintain sats >90, permissive hypercapnia

@chungk1031 When that's not working, and you get to PEEP >12-15, and still on FiO2 >60% consider initiating APRV. Don't forget all the other pieces to optimize: PULL PEOPLE UP IN BED! When the bend in the bed is at the T-spine and not the hips, you have bed induced restrictive lung disease

@chungk1031 5 things to set in APRV: High and Low pressure (P_High and P_low) and the time that the vent holds those pressures (T_High and T_Low. Measure the plateau pressure on volume cycle module before you change to APRV, that's your starting P_High. P_Low is typically zero.

@chungk1031 T_High is usually about 3-5 secs to start. From the get go, start thinking in terms of "respiratory cycle length". 1/resp cycle length is "releases" (breaths) per minute. So start simple with 1 per 3sec to 1 per 5sec (20-12 breaths per minute) like a usual ARDS pt on Volume mode

@chungk1031 T_Low is usually 0.5 sec. Sounds really short. It is. This is a start point. When the vent "releases", i.e. switches from P_High to P_Low, there will initially be MAX expiratory flow. This flow will degrade during T_Low, and T_Low should end before its less than 50% of the peak

@chungk1031 FiO2 is the last thing...This is gonna start at 100%. Remember, you switched to this because of hypoxia despite high FiO2, and high PEEP. Here's the hard part. This mode won't do much for oxygenation for 20-30 minutes ....put your hands in your pockets. Give it a chance to work.

@chungk1031 Adjust based on ABG. Remember, just like all other vent modes, we are trying to adjust oxygenation (FiO2 or Mean airway Pressure), or ventilation (Minute Ventilation = (Tidal volume - dead space) x rate) or in this mode (Release volume - dead space)x releases per minute. But How?

@chungk1031 Increase ventilation: Raise P_High (bigger releases, i.e. tidal volume), or shorten T_High (shorter resp cycle length, or more releases per minute, more breaths).
Decrease Ventilation: Lengthen T_High (longer resp cycle length, or less releases per minute, less breaths).

@chungk1031 Increase Oxygenation: Raise P_High (more mean airway pressure!) or Lengthen T_High (longer time at higher airway pressure) or Raise FiO2
Decrease Oxygenation: Lower FiO2. Its the easiest thing to undo if you lower it too soon. No physical change to lung, just ventilator's air.

@chungk1031 Look at the last two tweets, you might notice something strange about adjusting this mode...You are potentially affecting Ventilation and Oxygenation with the same adjustments. In conventional modes, we teach that adjusting Ventilation and Oxygenation are essentially independent

@chungk1031 Not so with APRV!
Raising P_High will increase ventilation (bigger release or tidal volume) AND increase oxygenation (raise Mean Airway Pressure).
Lowering P_High, the opposite. That's why I don't suggest dropping P_High to lessen ventilation (it lessens oxygenation as well)

@chungk1031 T_High changes produce opposite effects on Oxygenation & Ventilation.
Longer T_High increases oxygenation (more time at P_High, & alveolar recruitment), but will lessen Ventilation (Less releases per minute).
Shorter T_High lessens oxygenation, But will increase ventilation

@chungk1031 If you look carefully at the last two tweets, you should be able to figure out adjustments for all different combinations of increasing/decreasing oxygenation & ventilation as you have options that move them in opposite and in parallel! Not scary,Just different from Conventional!

@chungk1031 Pearls/Pitfalls. Adjust P_High, about 2cm of H20 at time. Adjust T-High, about 0.5 sec or 10-20% at a time. Wait 30-90 minutes for the next change. This mode involves Recruitment. Big moves can quickly de-recruit lungs, and it takes TIME to recruit it back! Go Slow!

@chungk1031 Home Stretch: What's happening in this mode? Ok, If I say "Pulmonary Hysteresis Curve" the nerds will cheer, but many others will burst into tears. Instead let's say we'll blow up a balloon for your little niece or nephew. Any color will do...What happens when you first start?

@chungk1031 You huff, and puff and fill out your cheeks, and turn red, and your eyes bulge, and that freakin ballon barely budges. It has almost no change in volume, for a tremendous amount of pressure applied (a very NON-COMPLIANT state)...just like a collapsed, atelectatic alveolus.

@chungk1031 Then, magically the balloon snaps open and starts to inflate. Whatever reasonable effort we apply to blowing it up, it inflates, increasing in volume as we would expect for a given applied pressure (i.e. a much more compliant state) like an open, but not over distended alveolus..

@chungk1031 If you keep trying to inflate the balloon, eventually it is filled/expanded to its max volume, and no matter how much more we blow into it, it doesn't inflate any more (i.e its volume doesn't increase)...it just gets harder and harder to try (Non Compliant), and eventually POP!

@chungk1031 We would be happy if the balloon were always open and never over distended. It would be easy to get volume in and out, without much pressure, and it would never collapse and never pop. That's what we want out of our Alveoli as well! We must prevent collapse, and overdistention.

@chungk1031 In conventional modes we do this by applying PEEP (to prevent collapse) and keep a limit on Plateau Pressure (to prevent over distention). APRV doesn't let exhaling finish (T_Low is short for this reason) to prevent collapse. APRV uses dynamic hyperinflation intentionally!

@chungk1031 To prevent over distention, APRV uses pressure applied OVER TIME (That's why T_High can be upwards of 6-7 seconds!) Over time, stubborn alveoli may be recruited and opened up. Sure we could open them right now with higher pressures, but we risk popping ones that are already open!

@chungk1031 The key is that ARDS is heterogenous. Some alveoli are snapped shut, and some are open. We must recruit the ones that are shut, avoid collapse of the ones that are open, and don't over distend them. How? It takes less pressure to KEEP an alveolus open, than to pop it open.

@chungk1031 If you want to read more, here's an oldie but goodie from the guy who taught many of us. Thanks!

Other approaches to open-lung ventilation: Airway pressure release ventilation
Habashi, Nader M. MD, FACP, FCCP
Critical Care Medicine: March 2005 - Volume 33 - Issue 3 - p S228-S240

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