Question 7.1

Highest mark

9.3

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

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.

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

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