Lab Case 1

Describe and interpret this arterial blood gas:

History: 45y male found in his garage. Brought in by ambulance with altered mental state.  His arterial blood gas on arrival (FiO2 = 0.21%):

pH 6.90                                                                Na 153

pCO2 20                                                               K 4.1

pO2 115                                                                Cl 109

HCO3 4.0                                                              Glucose 5.2

COHb <1.0%                                                         Urea 5.5

Ethanol <0.01%                                                      Osmolality  335

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3 thoughts on “Lab Case 1

  1. It’s been awhile but here’s my thoughts:

    pH – Severe acidaemia – low pCO2 and low HCO3 – primary metabolic acidosis
    pCO2 – hypocarbia but expected pCO2 given HCO3 of 4 is 12-16 (Winter’s formula 1.5xHOC3 + 8 +/- 2) – associated slight respiratory acidosis likely secondary to altered concious level.
    pO2 – on room air – I get A-a gradient to be ~10 – normal
    HCO3 4 – severely low – anion gap is 40 ! Delta ratio of 1.4 (pure high anion gap)- with a normal urea, COHb, and glucose – would want an urgent lactate / ketones.
    COHb & ethanol & Glu – normal – not cause of acidosis or decreased GCS also treatment implication of normal ethanol.
    Na 153 – hypernatraemia – ? renal / GIT loss ? iv fuid Tx
    K 4.1 – adjusting for pH actual K 1.9 severe hypokalaemia
    Cl – ? slight elevation can’t remember upper limit normal – as per Na
    Urea – normal – would want urgent Creat re: potential renal failure but uraemia not cause of acidaemia.
    Calculated osmolarity = 2x Na + BUN + Glu + ETOH = 317
    Osmolar gap = 18 = raised DDx given scenario is toxic alcohol ingestion, lactic acidosis, or ketoacidosis

    Given severe acidaemia and raised osmolar and anion gap coupled with garden shed (which usually means either snake bite, organophosphate or toxic alcohol) I’ll go with toxic alcohol poisoning.

    Need to check lactate remebering potential for spuriously high lactate on point-of-care analyser not present on formal lab lactate measurement due to cross reactivity. Check calcium – hypocalcaemia indicating likely ethylene glycol as oppose to methanol. Also needs other baseline tox investigations – para + ECG.

    Urgent toxicology/ICU liaison – clear indication for haemodialysis with bridging ETOH treatment given undetectable ethanol level + resuscitation / supportive care / monitoring.

    Caution given altered concious level, if intubation needed give bolus sodium bicarb pre-induction and maintain hyperventilation to avoid rapid worsening of acidaemia.

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  2. Alright, I’ll bite…

    1) pH → acidemia
    2a) “Primary” acidosis = metabolic (HCO3 = 4) with respiratory compensation (PaCO2 = 20)
    2b) Is respiratory compensation appropriate for disorder?
    —– Winter’s Formula → 1.5 x 4 + 8 = 14 — actual = 20… partial respiratory acidosis present.
    3) PaO2 → good
    4) Anion gap = 153 – (109 + 4) = 40 → High Anion Gap Metabolic Acidosis (HAGMA)
    5) Delta Ratio (considering normal AG = 12; Norm HCO3 = 25) → (40-12)/(25-4) = 28/21 = 1.3 → Delta Ratio suggests uncomplicated HAGMA
    6) Osmolar Gap: OG = osmoLALity (335) – OsmoLARity (calculated)
    OsmoLARity = (2 x Na+) + glucose + urea
    (assuming your glucose units are mmol/L)….
    OsmoLARity = 317
    OG = 335 – 317 = 19 (abnormal)

    THUS → this patient has a primary high anion gap metabolic acidosis (uncomplicated) with partial respiratory acidosis. Due to high osmolar gap and clinical scenario (also because of your tag of this post as ethylene glycol/methanol) high index of suspicion for ethylene glycol/methanol ingestion. Check patient’s Ca++ levels (hypocalcemia surrogate for ethylene glycol [thanks rob orman]) and due single digit bicarb also highly suspect ethylene glycol (thanks rob orman). This patient bought himself some fomepizole.

    Thanks for the case! Definitely a fun one! 🙂

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    • Good job Derek.
      To the Australasian guys out there, remember that Fomepizole is not available in Australia. Its temporizing alcohol, HCO3 and dialysis for us.

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