Lab Case 2 – Interpretation

50 year old with MND:

pH 7.122  severe acidaemia

pCO2 108  severe respiratory acidosis

compensation –

if acute expected HCO3 = 31

if chronic expected HCO3 = 52

actual HCO3 = 38 => acute on chronic respiratory acidosis

pO2 171 – high –??supplemental oxygen

O2 sats 98% within normal limits

unable to calculate A-a gradient because O2 not mentioned

Hb 183 – Polycythaemic – confirms long standing CO2 retention

Normal Na, K, Cl

anion gap 6

Glucose 9.5 – stress response, diabetes, steroids

normal lactate


50 year old male with MND presents with life threatening acute on chronic respiratory acidosis

Causes – chronic CO2 retention (due to end stage MND, drugs, other eg. COPD) exacerbated by supplemental Oxygen, drugs eg. opiods for pain

Immediate Implications – stop O2, BiPaP, art line (monitor ABG), consider naloxone (if drug induced), ICU consult, avoid intubation (weaning issues)

Vitals – massive sympathetic surge and thorarcic back pain –> consider hypertensive emergency ( eg. aortic dissection)

Further implications – CT Aortogram in unstable patient with respiratory compromise, operative vs conservative management if dissection, management of tachycardia and BP, discussion between family, patient and cardiothoracic surgery regarding prognosis and expected course if diagnosis confirmed, ICU admission.

As it turns out this patient had a negative CT scan and other investigations failed to reveal a cause. Possible autonomic dysfunction which can occur in motor neurone disease.

3 thoughts on “Lab Case 2 – Interpretation

  1. Thanks Yusuf, great case

    As John mentioned in his comment on the case, the total body K is in fact low (excess H+ ions in plasma/interstitial fluid push K+ into cells), so Potassium replacement may be needed as acidosis is corrected.

    What was the formula for correction, that you mentioned during the tutorial?

    • Colin,
      Corrected K+: The actual K+ is reduced by 0.5 for every 0.1 reduction in the pH. (0.3-0.7)

      • Acidosis causes K+ shift out of he cells into the ECF. K+ can then be lost in urine, GI tract causing a total body depletion. As you mentioned, correction of acidosis (ie. the pH is now moving in an alkalotic direction) will move K+ intracellularly (risk of further decrease in K+).

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