Tuesday, November 18, 2014

Expert Commentary: Troubleshooting Hypoxia on the Vent

In emergency medicine education, we tend to focus on establishing the airway but spend less time discussing the aftercare and managing the ventilator.  While we have spent more time on this recently, thanks to Brian Fuller and his research on the importance of low tidal volume ventilation in the ER (see EM Journal club summary and podcast from February 2014),  most of us are still more comfortable putting the ET tube in than managing the vent.

Luckily for us, here at WashU we have some EM-Critical Care wise guys who can teach us a thing or two, and today we share with you Brian Fuller's method for trouble-shooting hypoxia on the vent, forwarded to us by PGY-3 Brendan Fitzpatrick:

>>From: Fitzpatrick, Brendan
>>To: Fuller, Brian
>>Subject: vent desats

Dr. Fuller,

Good working with you last night. I was trying to recall how you broke down desats on the vent last night, but somewhere between little sleep and my kids' halloween parade, I've lost the finer points.

In all your free time, would you mind jotting down what you told me so I can review it?




>From: "Fuller, Brian"
>To: "Fitzpatrick, Brendan"
>Subject: RE: vent desats

For the purposes of acute deterioration (in the form of hypoxia) on the ventilator, we are gonna talk about two airway pressures: peak pressure and plateau pressure. As an aside, mean airway pressure is the average pressure over one cycle of inspiration and expiration. It is largely governed by PEEP and I:E ratio. It really governs oxygenation- higher it is, more you open up stiff alveoli in sick vented patients.

Peak pressure is the summation of pressure generated from: 1) tidal volume and compliance; 2) resistance and peak inspiratory flow; and 3) PEEP

Plateau pressure is a reflection of compliance. Think "how stiff the lungs are"; or "how much transalveolar stretch is occurring".
Compliance is ∆ volume/∆pressure. Specifically, tidal volume/(plateau pressure - PEEP)

So the first thing I do when somebody becomes acutely hypoxic on the ventilator is to look at their peak airway pressure:

1. If decreased: you have an air leak or the patient is hyperventilating/tugging hard and therefore pulling the airway pressures down. Air leak would be something like: bronchopleural fistula, the chest tube you just put in has a leak in the system, your ETT has migrated or cuff has a leak and air is escaping.

2. If increased: see above- this could either be primarily a compliance or resistance problem. So your next step is to look at the plateau pressure to figure out where the problem lies.

If no change in plateau, you therefore have a bigger difference between the peak pressure and the plateau pressure than existed before the hypoxic event. See above for what governs these pressures, so you can tell that this is therefore a resistance problem. Think: airway obstruction from bronchospasm, clogging of the ETT with secretions, kinking of the ETT.

If plateau pressure is also increased, you now have a situation where the peak and plateau pressures both increased. See above for what governs these pressures, so you can tell that this is therefore a compliance problem. Think: pulmonary edema, abdominal distention, pneumothorax, atelectasis, etc.

3. If no change: think "Something made my patient hypoxic but didn't change my airway pressures." Not a lot of stuff does that. Think: pulmonary embolism, PFO.

Sometimes it is difficult to figure out "Is the ventilator and my ventilator settings the problem, or is this a patient problem?" If you remove the patient from the ventilator, and therefore take that out of the equation, bag them and they get better, it is probably a ventilator problem. If you bag them and they stay bad, it is probably a patient problem.

Hope this helps. Hit me back with questions PRN.

Feel free to share with others.

Be good man.

For those of you who like pictures, here is a visual representation of the algorithm:

Want to do a little more reading or watching about this? Life in the Fast Lane had a nice review of Pulmonary Mechanics.  I recommend the second Eric Strong video on pulmonary pressures as a supplement to understanding the above material (and hell, it only takes 9 minutes to watch).

Expert Commentary by Brian Fuller
Visual aids by Maia Dorsett (@maiadorsett), PGY-3 
Expert inquiry, sharing skills and parenting by Brendan Fitzpatrick, PGY-3.

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