Question 16

Highest mark
7.8
Pass rate
90%
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Other SAQs in this paper:

A 65-year-old male with a past history of ischaemic heart disease is admitted to the ICU after a motorcycle crash having sustained long bone fractures of the lower limbs. He has no head, chest or abdominal injuries.

Prior to surgery, his Glasgow Coma Scale (GCS) was 15 and Sp02 was 98% on 4 L/min oxygen via a Hudson mask, and chest X-ray was normal. He required prolonged operative fixation of his fractures and that was complicated by significant blood loss. Intra-operatively, he also developed an increasing oxygen requirement.

On arrival in ICU, his most recent arterial blood gas, taken on a Fi02 of 0.7 shows Pa02 of 55 mmHg (7.3 kPa).

List the differential diagnoses for his respiratory failure.  (30% marks)

Outline the steps in your assessment of this patient to help determine the diagnosis. (70% marks)

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College answer

a) Differential diagnoses

  • Iatrogenic fluid volume overload due to blood product/ resuscitation fluid
  • Atelectasis/Collapse/ sputum plugging
  • Unrecognised pulmonary contusions
  • Unrecognised pneumothorax – Mech vent, line insertion
  • Aspiration at time of MBA or at intubation
  • Endobronchial intubation
  • Transfusion related acute lung injury (TRALI)
  • Cardiogenic pulmonary oedema/myocardial event
  • Fat embolism syndrome
  • Anaphylaxis
  • PE

b) Assessment

  • History
    • Details of accident
  •  PMH
    • Allergies
  • Clinical examination
    • Ensure adequate tertiary survey
    • Detailed respiratory examination
    • Review fluid balance and urine output
    • Evidence of generalised allergic reaction FBE – Hb, WCC, eosinophilia
  • Investigations
    • Coags – ongoing coagulaopathy, 
    • Chest XRay – infiltrates, ETT position, hardware, PTx, pleural effusions
    • Cardiac enzymes – TnI
    • ECG – ischaemic changes, arrhythmia, R heart strain
    • Echocardiogram – if suspect cardiogenic component, assess LVF, or RVF for PE
    • CTPA – early for PE but possible if patient delayed in ED
    •  Bronchoscopy – if evidence of localised collapse or unexplained infiltrates

Discussion

The possible differentials must be broad. Why?

  • Old guy with ischaemic heart disease
  • Long bone injuries
  • Extensive blood loss (thus, likely a massive transfusion)

Thus, perioperative hypoxia could have resulted from any combination of the following differentials:

  • Fat embolism
  • Acute MI with pulmonary oedema
  • Transfusion-associated circulatory overload
  • Trasfusion-associated lung injury

To discriminate among them, the following investigative steps might be taken:

History from the anaesthetist:

  • Exact timing of onset of hypoxia, as related tos urgical manipulation
  • Any changes in ST segments or arrhythmias intraoperatively
  • Exact volume of blood products and fluid resuscitation
  • Any perioperative TOE findings (if they did one)
  • Ventilation mechanics and EtCO2 trends - anaesthetists often note a sudden drop in end-tidal CO2 concentration during a stable steady state

Examination of the patient, looking for

  • Respiratory features: moist crepitations over all lung fields, hypoxia, cyanosis
  • Characteristic petechial rash, usually over the anterior axillary fold and at the root of the neck, as well as on the buccal mucosa and the conjunctiva. This distribution can be explained by fat droplets accumulating in the aortic arch prior to embolisation to nondependent skin via the subclavian and carotid vessels.
  • Fever
  • Tachycarda
  • Retinal haemorrhages
  • Visible fat droplets on ophthalmoscopy
  • Jaundice
  • Renal impairment

Laboratory tests,  looking for:

  • Thrombocytopenia
  • Anaemia (sudden decrease)
  • High ESR
  • Fat macroglobulinaemia
  • Troponin

Imaging

  • ECG looking for ischaemic changes
  • TTE looking for right sided strain and LV function 
  • CXR looking for pulmonary oedema

References

Mellor, A., and N. Soni. "Fat embolism." Anaesthesia 56.2 (2001): 145-154.

Gurd, Alan R., and R. I. Wilson. "The fat embolism syndrome." Journal of Bone & Joint Surgery, British Volume 56.3 (1974): 408-416.

Myers, R., and J. J. Taljaard. "Blood alcohol and fat embolism syndrome." J Bone Joint Surg Am 59.7 (1977): 878-880.

Hofmann, S., G. Huemer, and M. Salzer. "Pathophysiology and management of the fat embolism syndrome." Anaesthesia 53.S2 (1998): 35-37.

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