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:

• Use a Pressure Control mode (it may be safer, though the evidence is not strong)
• Lung-protective ventilation: use low tidal volumes (6ml/kg)
• Open-lung ventilation: avoid derecruitment by using a high PEEP
  • The ideal PEEP can be found either by finding the lower inflection point or the pressure-volume curve or by observing a stepwise decrease in PEEP after a recruitment manoeuvre.
  • As the ARDS severity increases, consider using a higher PEEP. 
• Use a lower driving pressure (ΔP) -Amato et al, 2015. That means, using a higher PEEP and aiming for a lower plateau pressure
• Accept a level of "permissive hypercapnea"

Additional ventilator manoeuvres to improve oxygenation:

• Use an I:E ratio of 1:1, even though manipulating the I:E ratio does not seem to improve survival, even though it may improve oxygenation.
• One might attempt some recruitment manoeuvres if hemodynamics permit. Again, these offer a transient improvement in oxygenation, but do not influence survival.

Non-ventilator adjunctive therapies for ARDS:

• Minimization of dead space ventilation - Remove as much tubing as you can.
• Low-carbohydrate high-fat nutrition - Keep them off the carbs, and don't overfeed.
• Neuromuscular blockade improves survival, not just gas exchange.
• Sedation decreases energy expenditure and improves ventilator synchrony
• Fluid management should have a goal of neutral balance (keep em dry)

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.