Question 5

College answer

Not available.

Discussion

This question resembles questions that ask the candidates to critically evaluate NIV, insofar as the answer to both types of question are very similar. However, in an a glorious outburst of good SAQ design, the examiners have wrapped the discussion around a clinical case scenario, and reframed the otherwise very nebulous "critically evaluate" stem into a laudably discrete series of practical questions. This sort of assessment has face validity: you can look at it, and confidently say to yourself that a person who can readily answer this question is probably a good intensivist.

So: this patient has COPD and CCF, which tend to respond well to NIV, and community-acquired pneumonia, which does not tend to respond very well.

How can NIV can improve the underlying pathophysiology in this patient?

• For respiratory failure in general:
  • Positive pressure ventilation in general has benefits which are common to both NIV and IPPV.
  • Increased inspiratory positive pressure decreases work of breathing by improving lung compliance by recruiting collapsed alveoli
  • Increased FRC due to increased closing capacity improves oxygenation
  • Tight-fitting mask ensures accurate delivery of prescribed FiO₂
• CCF:
  • Increased alveolar pressure decreases the effects of pulmonary oedema on gas exchange
  • Increased intrathoracic pressure improves LV performance:
    • Decreased LV transmural pressure = decreased afterload
    • Decreased venous return = decreased preload
  • Improvement in pulmonary oedema and respiratory distress also indirectly decrease myocardial oxygen demand by moderating the sympathetic response and decreasing the effort of breathing
• COPD:
  • Increased end-expiratory positive pressure decreases work of breathing vs. intrinsic PEEP
  • Increased expiratory air flow can improve CO2 clearance
  • Increased minute volume can better compensate for the increased dead space of emphysema
  • Splinting open of smaller airways allows better bronchodilator delivery
• Community-acquired pneumonia:
  • NIV does not tend to benefit this group of patients, because:
    • It can impair secretion clearance
    • It can increase shunt
  • However, the delivery of humidified oxygen by mask can improve secretion clearance
  • In patients with CCF or COPD and pneumonia, NIV can improve mortality ( Carrillo et al, 2012) 

How you could assess efficacy of NIV in this patient:

• NIV should be viewed as effective if it achieves the desired clinical endpoints over a specified timeframe, and if the patient does not manifest any features suggestive of impending NIV failure during the trial. 
• To assess the efficacy of NIV, one should:
  • Establish endpoints for this therapy
  • Establish a timeframe for the trial of NIV
  • Monitor for features suggestive of NIV failure throughout this trial
• Endpoints:
  • Clinical endpoints:
    • Achievement of the SpO₂ goal of 88-92%
    • The FiO₂ should decrease with NIV (compared to what it was pre-NIV), i.e. the PaO₂/FiO₂ ratio should improve
    • Improved tachycardia
    • Improvement in the respiratory rate (if the patient was tachypnoeic)
    • Subjective improvement in patient respiratory distress and reported comfort
    • Satisfactory mask tolerance, without the requirement for sedation
    • Improved level of consciousness
  • Gas exchange and pulmonary function:
    • PaCO₂ decrease (if the patient was hypercapneic)
    • pH increase (if the patient was acidaemic due to hypercapnia)
    • Improvement of CXR appearance, specifically looking for radiological features of pulmonary oedema
• Timeframe:
  • Within 1 hour,
    • Resp rate should improve
    • PaO₂/FiO₂ ratio should improve
    • Heart rate should improve
    • PaCO₂ and pH should demonstrate an encouraging trend
      (Nicolini et al, 2014)
  • Within 4 hours,
    • Level of consciousness should improve
  • Within 24 hours,
    • Radiological appearance should improve (Carillo et al, 2012)
• Features suggestive of NIV "failure":
  • Persistent metabolic acidosis
  • Haemodynamic instability
  • Persistently decreased level of consciousness
  • Poor mask tolerance and ventilator dyssynchrony