A 42-year-old miner has been transferred to the unit after an industrial accident in a remote location. He was entrapped in a fire underground.

He arrives 30 hours after injury, weighs  70 kg  and is 1.75m tall.
He is intubated and ventilated. His admission chest x-ray shows widespread bilateral pulmonary infiltrates.

His initial arterial blood gas shows the following:

Fi02 1.0  
pH 6.93* 7.35 - 7.45
p02 55 mmHg (7.3 kPa)  
pC02 78.0 mmHg (10.4 kPa)* 35.0 - 45.0 (4.6 - 6.0)
Sp02 87%  
Bicarbonate _ 16.0 mmol/L* 22.0 - 26.0
Base Excess -14.0 mmol/L* -2.0 - +2.0

His current ventilator settings are as follows:

Fi02 
Respiratory rate 
Tidal volume
Peak lnspiratory Pressure
PEEP
1.0
12 breaths/min
650 ml
38 cmH20
5 cmH20


a) List six possible causes for his hypoxaemia. (20% marks)

b) Outline your management strategies for the treatment of his hypoxaemia. (80% marks)

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College answer

a)                                                         
•    Aspiration pneumonitis 
•    Blast Injury 
•    Smoke inhalation 
•    Misplaced ETT 
•    Fluid overload 
•    Pulmonary oedema 
•    Restrictive defect from circumferential burn 
 
b) General ventilator strategies based on ARDS net criteria 
•    Check adequate placement of ETT (no R endobronchial intubation) 
•    ARDS net ventilation Vt 6mls/kg = 420mls best PEEP.  
Use of recruitment maneuvers with derecruitment to assess best PEEP 
CPAP 40/40 or step wise recruitment maneuvers 
Use of flow loops 
Aim Plateau <30cm H2O avoid baro trauma 
Increase I:E ratio towards 1:1 and increase rate if tolerated 
Check for autoPEEP and use of broncho dilators  
Treat reversible causes like PTX 
C- rule out cardiomyopathy and improve V:Q match as cardiac function can be depressed in the severe inflammatory state. 
 
General adjunctive measures 
•    Physio. Suctioning. Consider bronchoscopy  
•    Sedation, heavy sedation will be required. In advanced Hypoxia may require paralysis. 
•    Treat factors increasing metabolic demand (removal of eschar, treatment of sepsis, high risk of pneumonia) 
•    Optimize Hb and oxygen carrying capacity. 
•    If patient has been over resuscitated may require diuresis 
 
General Rescue therapies 
•    Prone positioning- may not be practical in a burns patient 
•    Alternative ventilation strategies - prolonged  
•    ECMO in severe cases 
•    Nitric oxide or inhaled prostacyclin 
 
Burns specific measures 
Bronchodilators 
      B2 agonists such as Adrenaline or Salbutamol 
      Adrenaline reduces blood flow to injured/obstructed airways improving V:Q mismatch 
 
Muscarinic receptor antagonists – reduction of cytokines, reduction of mucus secretions 
NAC 
Inhaled fibrinolytics for reduction of fibrin casts in volume and plugging. 
 
Bronchial Toileting. 
•    bronchoscopy for cast removal and prevention and treatment of mechanical obstruction and plugging 
***rule out toxidromes, cyanide, CO, may need or antidote.**** 
•    Ensure chest expansion not impeded by eschar 
•    May require escharotomies for free chest movement
 

Discussion

This is hypoxia in a miner pulled from an underground fire. His ABG result demonstrates a severe metabolic and respiratory acidosis. Specific elements which arouse concern in that setting are:

  • He's a miner, and therefore possibly exposed to exotic substances. Who knows what he was mining in there?
  • He was underground, which as an enclosed space has two major consequences:
    • It amplifies blast waves
    • It concentrates heat
    • It limits the oxygen supply

The differentials therefore are:

  • Inhalational burns injury
  • Corrosive agent inhalation
  • Asphyxiant agent exposure (eg. carbon monoxide or cyanide)
  • "Blast lung" due to primary blast injury.
  • Aspiration due to a decreased level of consciousness
  • Pulmonary oedema due to primary myocardial damage
  • Barotrauma due to profoundly stupid ventilator settings.

The ventilator settings are mildly inappropriate This guy is being ventilated like a elective theatre case. The college clearly wanted the candidates to discuss the standard approach to ARDS ventilation

In brief:

Initial ventilator strategy:

Additional ventilator manoeuvres to improve oxygenation:

Non-ventilator adjunctive therapies for ARDS:

Ventilator strategies to manage refractory hypoxia

  • Prone ventilation, for at least 16 hours a day (PROSEVA, 2013)
  • High frequency oscillatory ventilation may not improve mortality among all-comers (OSCAR,2013) or it may actually increase mortality (OSCILLATE, 2013) but some authors feel that there were problems with methodology.

Non-ventilator adjuncts to manage refractory hypoxia

  • Nitric oxide was a cause for some excitement, but is no longer recommended.
  • Prostacyclin is still a cause for excitement, and is still vaguely recommended.
    • Neither agent improves mortality, but prostacyclin can improve oxygenation.
  • ECMO may improve survival (CESAR, 2009) but again there were problems with methodology.

References

References

Tredget, EDWARD E., et al. "The role of inhalation injury in burn trauma. A Canadian experience." Annals of surgery 212.6 (1990): 720.

Kimmel, Edgar C., and Kenneth R. Still. "Acute lung injury, acute respiratory distress syndrome and inhalation injury: an overview." Drug and chemical toxicology 22.1 (1999): 91-128.

Gorguner, Metin, and Metin Akgun. "Acute inhalation injury." The Eurasian journal of medicine 42.1 (2010): 28.