Question 25

College answer

a)  

Differential diagnosis should include:

• ARDS secondary to sepsis from any source or other inflammatory insult including the following
• Pneumonia (hospital-acquired)
• Aspiration
• Atelectasis/pleural effusions/empyema 
• Fluid overload secondary to resuscitation, renal failure
• Exacerbation of pre-existing condition e.g. heart failure, valvular heart disease, post-op ischaemia/MI, arrhythmia
• Lung diseases e.g. lymphangitis carcinomatosis

b)  

• Use a mode with which one is familiar and aim to limit ventilator-associated lung injury, i.e. oxygen toxicity, barotrauma, volutrauma, shear stress and biotrauma

• Choice of mode (any appropriate answer acceptable e.g. APRV for recruitment benefit, or volume assist control as staff familiarity and no one mode shown to have benefit over another)
• Avoid over-distention of alveoli by keeping tidal volumes at 6-8 ml/kg (predicted body weight which in the ARDSnet studies was ~20% below actual body weight and calculated by a formula linking height and sex)
• Use PEEP to minimise alveolar collapse and derecruitment. 
• Titrate PEEP to achieve a PaO2 of 60 mmHg with lowest FiO2 that is needed 
• Permissive hypercapnea to avoid large minute volumes and alveolar injury through collapse and expansion of lung units

c)

• High FiO2 (titrated to lowest possible level to limit toxicity)
• Confirm ETT pos

   

  ition and patency
• Exclude readily reversible cause of hypoxia e.g. PTX, mucus plug, large effusion
• Increased inspiratory time
• Increased PEEP
• Prone positioning for at least 16/24 hours per day
• Ensure adequate cardiac output

Examiners Comments:

Answered well overall. Lack of detail and structure in some answers.

Discussion

a) List the possible causes for his respiratory failure.    

Tandon et al (2001) mention that this happened to over 14% of their oesophagectomy patients, with a 50% mortality. Why is this post-oesophagectomy patient so hypoxic, and what are those infiltrates? One might classify this into two broad categories:

• Related to the oesophagectomy
  • ARDS due to sepsis
  • Aspiration, eg. even some sort of tracheo-oesophageal fistula
  • Atelectasis/pleural effusions/empyema 
  • Massive blood transfusion
• Unrelated to the surgery
  • Pulmonary haemorrhage
  • Hospital-acquired pneumonia
  • Lymphangitis carcinomatosis
  • Drug-related (eg. eosinophilic) pneumonitis
  • Autoimmune pulmonary vasculitis
  • Pulmonary oedema due to MI, valve disease or fluid overload
  • Pancreatitis

b) Describe the ventilator settings you will prescribe, giving the rationale for your decision.

• Use a Pressure Control mode (it may be safer, though the evidence is not strong). The college recommend to "use a mode with which one is familiar", which is reasonable because none have been shown to be superior in terms of mortality, but which opens the possibility of using a familiar and truly useless mode.
• Lung-protective ventilation: use low tidal volumes (6-8ml/kg)
• Open-lung ventilation: avoid derecruitment by using a higher PEEP
  • Determine the optimal PEEP using any number of strategies, including:
    • ARDSNet tables
    • Lower inflection point or the pressure-volume curve o
    • Stepwise derecruitment manoeuvre
  • As the ARDS severity increases, consider using a higher PEEP. 
• Optimise the driving pressure (ΔP) -that means, using a higher PEEP and aiming for a lower plateau pressure (Amato et al, 2015.) 
• Accept a level of "permissive hypercapnea"

c) List the initial strategies that may be used to improve oxygenation. 

Let's assume you've excluded sophomoric errors like right bronchial intubation and sputum plugging. Other strategies may include:

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 haemodynamics permit. Again, these offer a transient improvement in oxygenation, but do not influence survival.

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.
• ECMO may improve survival (CESAR, 2009) but again there were problems with methodology, and in any case the upper GI surgeon may have a big problem with heparinising the circuit.