Case-Based Modules > Case 28

A 42 year-old man with a history of an autoimmune necrotizing myopathy (anti-HMGCoAR Ab) presents to the ED for persistent gait difficulties. He had been treated with steroids and IVIg when he was diagnosed with his myopathy six months ago, which led to clinical improvement. He's been maintained on azathioprine since then. The ED has been very full, so he's been waiting in triage for the past ten hours. At this point, preliminary workup can be obtained, and is notable for:

Labs:

  • CBC: WBC 3.5, Hb 12.1, MCV 89, Plt 285
  • BMP: Na 140, K 3.8, Cl 108, HCO3 33, BUN 10, Cr 0.98, gluc 178
  • ABG: 7.35/59/95/0.8
Chest X-ray

Chest X-ray

It's the start of your night shift, and you get consulted to see the patient. On exam, the patient is mildly sleepy, but awakens to voice. Language is fluent. He has no dysarthria. He has 4/5 shoulder abduction and hip flexion strength bilaterally; full strength elsewhere. He has numbness to pinprick up to his mid-shin bilaterally. Romberg test is positive. Single-breath count is 20.

The ED intern arrives just as you finish up your exam. They're worried about the patient's somnolence and tell you think they need to intubate the patient considering that he has the autoimmune necrotizing myopathy that could be acutely getting worse.

What do you think?

On exam, the patient clearly has proximal weakness that you'd expect with his known diagnosis. He is a little sleepy and has a slightly diminished single breath count. To the ED intern's point, has he developed acute respiratory failure secondary to the myopathy?

Let's look at his ABG first. He is marginally acidemic, with a PaCO2 of 59. This indicates that he has a primary respiratory acidosis. But is this acute? We can tell that it's not, based on the minimal decrease in pH despite a large increase in PaCO2 (normal is 40mmHg). (A helpful shortcut is that the pH changes in increments of 0.08 for every rise or fall of 10mmHg in the PaCO2. So for a PaCO2 of 59 that represents an acute change, pH would be around 7.24.) If we look back at his BMP, we can see that his HCO3 is 33. This makes sense. He's had this chronic respiratory acidosis, for which he's had renal compensation. For every 10mmHg increase in PaCO2, the kidneys can increase HCO3 by 3-4mEq/L. His labs are consistent with this.

So, putting this all together, this man has proximal weakness consistent with his known myopathy, with a chronic respiratory acidosis. It's unlikely that this CO2 retention is responsible for his current mental status. Still, the patient would benefit from NIPPV initiation in the near-term given his chronic respiratory failure.

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