A 73-year-old female presents to the Emergency Department with breathlessness, after a minor car accident two days earlier. Since the accident, she has had pain in her left knee and chest pain on breathing and coughing. She has previously had bilateral knee replacements. She deteriorates over 3 hours in the Emergency Department and now looks unwell. Her vital signs are as follows:

  • Heart rate 105 beats/minute
  • Blood Pressure 80/45 mmHg
  • Temperature 38.2°C

An arterial blood gas analysis is performed along with other blood tests as shown below:

Parameter

Patient Value

Adult Normal Range

FiO2

0.5

pH

7.18*

7.35 – 7.45

pO2

68 mmHg (9.1 kPa)

pCO2

42.0 mmHg (5.6 kPa)

35.0 – 45.0 (4.6 – 6.0)

SpO2

91%

Bicarbonate

15.0 mmol/L*

22.0 – 26.0

Base Excess

-9.2 mmol/L*

-2.0 – +2.0

Lactate

3.4 mmol/L*

0.5 – 1.6

Sodium

135 mmol/L

135 – 145

Potassium

5.0 mmol/L

3.5 – 5.0

Chloride

105 mmol/L

95 – 105

Glucose

10.1 mmol/L*

3.5 – 6.0

Urea

12.0 mmol/L*

3.0 – 8.0

Creatinine

150 µmol/L*

45 – 90

Albumin

30 g/L*

35 – 50

Total bilirubin

36 µmol/L*

< 26

Aspartate transferase

405 U/L*

< 35

Alanine transferase

336 U/L*

< 35

Alkaline phosphatase

168 U/L*

30 – 110

g-Glutamyl transferase

198 U/L*

< 40

Ionised calcium

1.04 mmol/L*

1.10 – 1.35

CRP

186 mg/L*

< 5

Haemoglobin

114 g/L*

120 – 160

White Cell Count

1.7 x 109/L*

4.0 – 11.0

Platelet count

87 x 109/L*

150 – 350

International normalised ratio (INR)

1.4*

0.9 – 1.3

Activated partial thromboplastin ratio (APTT )

41.0 sec*

27.0 – 38.5

Fibrinogen

4.0 g/L

2.0 – 4.0

a)    Interpret the arterial blood gas analysis provided on page 7. (20% marks)

b)    List six differential diagnoses for her presentation. (30% marks)

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College answer

  1. Metabolic acidosis. Anion gap is mildly elevated (15) with base excess of -9, and delta ratio of
    1. suggesting both an elevated anion gap component and a normal anion gap component.
      1. Elevated anion gap component is accounted for by increased lactate
      2. Normal anion gap component (possibly due to renal impairment, sepsis)

Lack of respiratory compensation for acidosis (or alternatively, a co-existing respiratory acidosis).

A-a gradient of about 240, P:F ratio 136; implies severe hypoxia.

  1. Differential diagnosis for presentation:
    1. Pulmonary embolism
    2. Fat embolism syndrome
    3. Chest trauma with haemothorax
    4. Pneumothorax
    5. Chest infection due to inadequate respiratory secretion clearance in setting of chest wall pain
    6. Septic arthritis at site of knee prosthesis with sepsis and ARDS
    7. Cardiac tamponade
    8. Pulmonary contusions
    9. Cardiac contusion

Discussion

"Interpret the arterial blood gas analysis" is what they specifically asked for. Thus:

  • The A-a gradient is elevated: (713 × 0.5) - (42  / 0.8) - 68 = 236
  • There is acidaemia
  • There is a metabolic acidosis; the SBE is -9.2
  • The CO2 is unhelpful. The expected CO2 would be 31 by the SBE method, or (1.5×15)+8 = 30.5 using the Boston rules. Either way, there's a respiratory acidosis.
  • The anion gap is slightly elevated (135-105-15 = 15)
  • The delta ratio is therefore (15-12)/(24-15) = 0.33, i.e. this is entirely a normal anion gap metabolic acidosis
  • The lactate is trivially elevated

So: what are six differentials for this? The other bloods show a bunch of other abnormalities which we were not asked to report on:

  • Renal impairment
  • Hypoalbuminaemia
  • Raised bilirubin
  • Deranged LFTs, mainly in a hepatotoxic pattern
  • Trivial hyperglycaemia
  • Anaemia and thrombocytopenia
  • Hypocalcemia
  • A low white cell count and raised inflammatory markers
  • Essentially normal coags, within reason

So; integrating this with the salient features of history, you'd have to list the following differentials:

  • Massive PE: hypoxia, recent trauma, knee pain (was it popliteal?); the LFT derangement is the consequence of hepatic congestion from right heart failure
  • Fat embolism due to periprosthetic fracture at the knee
  • Rib fractures followed by pneumonia: the chest pain with breathing and coughing, with hypoxia and raised inflammatory markers. She is febrile, and a low WCC is consistent with this (they are all sequestered in the lung). LFTs may reflect some collateral damage to the liver, eg. a subcapsular haematoma. 
  • Aortic dissection (chest pain);  and biochemical phenomena are due to organ hypoperfusion by the false lumen. The knee pain could represent limb ischaemia
  • Pneumothorax and/or haemothorax could account for the hypoxia
  • Sepsis could account for everything,  including the car accident. Does this patient have some sort of grumbling orthopaedic infection which occasionally causes her to become vague and collide with parked cars? Could this just be a UTI?

References