Question 8

College comments

Syllabus topic/section: 2.1.5 Respiratory Intensive Care. Mechanical Ventilatory Support L1

Discussion: 

Most Candidates had a broad understanding of the P-SILI and attention to specific details could have earned them more marks. Avoid using generic answers, as they may not help earn full marks. Instead, focus on providing clear, specific responses that show your understanding in both parts of the question i.e. the pathophysiology and the strategies to minimise P-SILI. Succinct answers that specifically addresses the question were awarded more marks. Candidates needed to give due attention to glossary of terms.

In Part a), the candidates needed to be more specific in outlining the pathophysiological mechanisms of P-SILI. Scoring marks in this part of the question required the candidates to mention the specific pathophysiological mechanisms and briefly outlining how it leads to over-distension. Candidates who mention 4 to 5 pathophysiological mechanisms and highlighted their clinical relevance scored more marks.

For e.g. “Inappropriate lung stress and strain- High respiratory rate and work of breathing can lead to high transpulmonary pressures leading to over-distension, thus causing excessive strain in areas of the lung that are less compliant “

Part b) was a “discuss” question (explain underlying principles & include controversies or advantages and disadvantages) and so candidates who included advantages and disadvantages of various strategies to minimise P-SILI in the answer were awarded more marks.

For example, “Sedation protects the lung from further aggravation of P-SILI by reducing the respiratory drive however the disadvantages of long-term sedation such as ventilator associated pneumonia, muscle weakness and subsequent delirium needs to be kept in mind “

The rubric is provided to aid in the candidate's future study.

Rubric

Interpretation

a) Pathophysiology of PSILI:

• Increased transpulmonary pressure: patient respiratory effort that uses large amounts of force to generate large tidal volumes
• Additive effect of pressure support on transpulmonary pressure
• Pendelluft, where gas from non-dependent apical regions is redistributed to the bases (this produces regional overdistension of those regions)
• Increased venous return due to increased negative pressure creates conditions of increased capillary hydrostatic pressure, which favours the formation of pulmonary oedema
• Escalating pattern of increasing tissue irritation and gas exchange failure activating the respiratory drive and promoting higher respiratory effort
• Diaphragmatic "load-induced injury", similar to what happens in skeletal muscle undergoing intense exercise. 

b) Strategies to prevent PSILI:

• Reduce respiratory effort
  • Reduce O₂ consumption and CO₂ production by treating fever, pain and agitation.
  • HFNP to improve the efficiency of ventilation
  • Reduce the dead space
  • Opioids to reduce the central medullary sensitivity to hypoxia and hypercapnia
  • Avoid permissive hypercapnia
  • Correct metabolic acidosis
  • ECMO or ECCO₂R 
• Distribute the lung strain more homogeneously:
  • Prone ventilation
  • High PEEP
• Decrease the ventilator contribution to P-SILI
  • Manage dyssynchrony
  • Adjust the cycle-off variable and pressure support to keep the tidal volume in the 6-8 ml/kg range
• Decrease diaphragmatic activity
  • Sedation and paralysis, or partial neuromuscular blockade