Vent settings in metabolic acidosis

[leviathan]

I recently saw a patient in emerg with acute kidney injury and lithium toxicity who was in resp failure with a bilateral pneumonia/possibly ARDS. She had a pretty significant metabolic acidosis and a resp rate in the 40s, with a pretty low pco2 on blood gas to compensate for her acidosis.

ICU came down and intubated and put the patient on a Vt of 500, RR of 12. I let the fellow know that she had a pretty significant metabolic acidosis prior to the tube, and suggested she might want to increase the vent settings. She said it didn't matter and gave some vague explanation as to why.

Is compensating for the acidosis a legitimate concern or is it just dogma that we don't need to worry about?

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[ORL10]

I recently saw a patient in emerg with acute kidney injury and lithium toxicity who was in resp failure with a bilateral pneumonia/possibly ARDS. She had a pretty significant metabolic acidosis and a resp rate in the 40s, with a pretty low pco2 on blood gas to compensate for her acidosis.

ICU came down and intubated and put the patient on a Vt of 500, RR of 12. I let the fellow know that she had a pretty significant metabolic acidosis prior to the tube, and suggested she might want to increase the vent settings. She said it didn't matter and gave some vague explanation as to why.

Is compensating for the acidosis a legitimate concern or is it just dogma that we don't need to worry about?

You need to make the minute ventilation appropriate for the patients exsisting metabolic acidosis. So if they are hyperventilating before you should probably match their minute ventilation assuming they were not alkalemic. This patient was getting 6L of minute ventilation, which is not going to keep her PCO2 low enough to compensate for their MA. Again endpoint is pH so don't get fixated on PCO2 as a lot of residents and RTs initially fixate on that number.

Just remeber that sometimes it is hard to match a patients spontaneous minute ventilation with positive pressure ventilation especially peri-induction and immediately post-induction.

 

[leviathan]

I don't remember the exact numbers but the pH was around 7.25-7.30 pre-intubation, non-anion gap metabolic acidosis.

 

[Hernandez]

I don't remember the exact numbers but the pH was around 7.25-7.30 pre-intubation, non-anion gap metabolic acidosis.

By chance was the pt on bipap prior to being tubed? Did they use a long acting paralytic when tubing?

On one hand, if they didn't use a paralytic or used sux, the pt will likely take care of the respiratory rate themselves, on the other, they have a high likelihood of bucking the vent and I would not just set a MV of 6L/min if there is a bad acidosis. That's the kind of stuff you see surgeons or ER docs do

How low were the PaCO2 and HCO3?

 

[leviathan]

That's a good point about the resp demand. They weren't on bipap prior to tube. They used roc to intubate. Don't remember the exact pre-intubation gases but I know the PCO2 was pretty low (appropriately) and the bicarb was in the low teens. I do remember checking their blood gases the next day and their PCO2 was a 'nice' 40 but their pH had dropped to 7.1. Oops.

 

[jdh71]

In spontaneously breathing patients there is no need to "match" the minute ventilation as the patient will still be setting their own MV.

Something I do like to do though is set the ventilator rate to about 75-80% the rate being used by the patient. This is just incase someone ends up paralyzing the patient or over sedates them you don't go from a super high MV to a basic vent set MV with consequences. No way to prove it but i think Ive seen a couple cerebral edemas that happened this way in patients who were susceptible to the big swings.

 

[Hernandez]

They used roc to intubate.

I ask about bipap as you can cheat and get the MV

 

[sluggs]

TV 500? Somebody? Anybody? ARDS net? Better ways to inc her MV???

 

[sluggs]

WNBA player?

 

[Hernandez]

TV 500? Somebody? Anybody? ARDS net? Better ways to inc her MV???

Well to be fair.....the protocol should actually start at 8mL/kg then decrease to 6 or less..so a 5'7" woman would be ~500 for 8mL/kg.... But I doubt that was what they were thinking. Too many people turn off brain...turn on VACV 12x500 100% +5cm.....otherwise...yes, she'd need to be 6'5" for 500 to be 6mL/kg

 

[ORL10]

95% of the time I'm intubating in the ICU I use paralytics, so I match minute ventilation, and drop the rate as the patient wakes up. Personally I'd rather set the rate higher and titrate down than have them accidentally set low, especially if their pH is <7.2. I see this occasionally (i.e; DKA, bad sepsis, asa OD...) where the patient is breathing vital capacity breaths at a rate > 20 and then gets tubed and initial vent settings are 12/500/100%/+5 as the decerebrate vent settings placed.

 

[Idiopathic]

Is a ph of 7.1 inherently bad in the short term? Which is worse, tolerating that number, a RR if 24, or adding bicarb?


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[Hernandez]

Is a ph of 7.1 inherently bad in the short term? Which is worse, tolerating that number, a RR if 24, or adding bicarb?


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Short term? No, but the it reflects poorly of an advanced level fellow when a minimal amount of forethought would realize that the set minute ventilation is horribly inadequate....especially in a clinical condition that is known to cause metabolic acidosis.

 

[jdh71]

Is a ph of 7.1 inherently bad in the short term? Which is worse, tolerating that number, a RR if 24, or adding bicarb?


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I'm nor sure what the context of the question is. I won't tolerate a pH of 7.1 unless I HAVE TO because the clinical scenario dictates that I have to live with it. Bicarb never seems to have any kind meaningful benefit when it's been studied (and may even make things worse when given to folks with pH's above 7.0), so I personally don't reach for bicarb unless I have some kind of compelling reason to do so. RR of 24, especially in the spontaneously breathing patient doesn't bother me a bit.

 

[souljah1]

I absolutely cannot stand vent settings of RR 12 Vt 500. They are a reflection of thoughtlessness.

If someone is breathing > 40 times per minute and has increased work of breathing due to ARDS/severe hypoxemia you have to consider their minute ventilation in your initial ventilator settings. With an additional metabolic acidosis from renal failure, you have to be extra careful to set your minute ventilation appropriately.

For a patient like this I'd give feedback to that fellow and say:

Immediately following paralysis is THE best time to get your mechanics. Put the patient on 6cc/kg and a decent PEEP and see what your plateau pressures are. That Vt is too high.

Set a respiratory rate that provides a minute ventilation that is at least 10 and check a gas in 15 minutes to make sure that you aren't at the extremes of hypercapnea. Walking away after setting a minute ventilation of 6 in a paralyzed patient who will only breath at the set rate is a practice that will eventually kill someone. Setting a minute ventilation of 6 in an air-hungry awake patient breathing in the 30-40s on the vent is still not smart b/c if they tire or get too much sedation and substantially lower their minute ventilation they are get hypercapneic => acidemic => bradycardic/hypotensive => die.

Overall if I'm staring at a patient huffing and puffing with a bad PNA/possible ARDS and AKI w/ an acidosis I'm going to make sure I provide enough ventilation while applying low-stretch and target a pH > 7.2. If they are in septic shock and need a large volume resuscitation I would not use normal saline here, but would reach for something that won't further lower the pH.