ABGs in COPD, Asthma

[mosquitoman]

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Just wanted to make sure I'm getting this right..

Asthma exacerbation start of hyperventilating

pH High (Alkalotic) CO2 LOW O2 LOW --> as progresses ---> pH LOW (norm-Acidotic) CO2 norm-high O2 LOW

COPD exac already live being hypercarbic?

pH Low (Acidotic) CO2 high O2 LOW


is that right?

and on a similar note, when to intubate?

do you guys follow that 25/35/45 rule? RR>25,pH<7.35,paCO2>35 ?

thanks

 

[AndyRSC]

Sounds like you got it (including the fine points of pH being normal or CO2 being normal in an asthma attack both being concerning).

As for intubation guidelines, from Kaplan:

Persistent hypoxemia
Increasing oxygen demands
Presence of respiratory acidosis and hypercapnia in a patient presenting with asthma exacerbation
Upper airway injury (burns, laryngeal edema, trauma)
Neurological depression with loss of protective reflexes, including gag and cough

Seems like a pretty incomplete list, but I guess it'll do for the exam. Mostly just tube them when they can't control their airway or you anticipate they'll lose it in near future, if patient has agonal or absent respirations, or if anything is keeping them hypoxemic and they seem to be losing the battle (lactic acidosis setting in).

There's also the "less than 8, intubate" rule, as far as RR/RTS/GCS, but it's a total-picture judgment in the real world.

 

[OveractiveBrain]

Just wanted to make sure I'm getting this right..

Asthma exacerbation start of hyperventilating

pH High (Alkalotic) CO2 LOW O2 LOW --> as progresses ---> pH LOW (norm-Acidotic) CO2 norm-high O2 LOW

COPD exac already live being hypercarbic?

pH Low (Acidotic) CO2 high O2 LOW


is that right?

Hypoxemia drives hyperventilation, bringing down CO2 from wherever it started. As air trapping worsens, oxygen exchange fails, and CO2 begins to rise, with a persistent hypoxia.

So, O2 always low.

Asthma CO2 goess lower than it started, normal to low, and with it the pH rises. Then, the CO2 rises, and with it, the pH falls.

COPD CO2 goes lower than it started, high to normal or low, and with it the pH rises. Then, the CO2 rises, and with it, the pH falls.

and on a similar note, when to intubate?

do you guys follow that 25/35/45 rule? RR>25,pH<7.35,paCO2>35 ?

thanks

If you intubated everybody who was breathing faster than 25, you would compromise their ability to blow off acid. If you intubated everybody with a pH < 7.35 (even if they were just Asthma/COPD exac) you would need more ICUs. And... every COPDer is going to have a CO2 > 45, even at baseline!

Intubation has no definitive cutoffs. When do you try rescue Bipap? When do you Intubate? THere are no clear guidelines, but "tiring out" is a good one. If you are looking at labs, and not clinical picture, look at persistent hypoxia (intubation controls ventilation, but allows for maximum oxygenation above a nonrebreather) or worsening hypercapnia (CO2 rising or pH falling despite therapy).

 

[Jacob626]

What I don't get is what initiates the hyperventilation in the first place. So in an asthma attack you start with bronchial inflammation which sets up air trapping. I would think that the initial blood gas derangement would be increased CO2 and normal/low O2. Especially since I thought it is the CO2 that is the main driver of respiration in humans (not oxygen). Can someone help explain this sequence of events for me? Thank you!