Question 3

College answer

Aim: To examine the candidate’s knowledge of a ventilation mode in current ICU practice.
Key sources include: Published ventilation strategy, CanMEDS Medical Expert.

Discussion: This mode of ventilation is more common since the advent of Covid19 and is mentioned more frequently in the current ICU literature. Most candidates had a good understanding of APRV and demonstrated competency under the headings stipulated. Some candidates answers were excellent and included diagrams and pressure time graphs. These candidates should be commended. The use of graphs and diagrams is a useful t way to clearly display key knowledge concepts and is encouraged.
The less successful answers confused APRV with pressure regulated volume control or were unclear in their attempts to explain the concepts, for example ‘pressure release’. The instruction to include information “not limited to the following headings” was ignored by some candidates which limited the opportunity to allocate marks. Additional headings used by candidates included - ‘ventilation settings’, ‘indications for use’, or ‘statement of my practice’.
 

Discussion

It is both surprising and not at all surprising that APRV has made its way into the exam even though this author never thought it would. One one hand, its not a common strategy, and you won't see it in every ICU; but on the other, the same can be said for ECMO, inhaled nitric oxide, resuscitative thoracotomy or lung transplantation - but the college still asks questions about this because the written exam is all about breadth (whereas the vivas are all about depth) and because a CICM-trained intensivist is supposed to be some kind of an elite Navy Seal-like organism, capable of every trick in the critical care book. This is the kind of thinking that gives us questions on REBOA, for example. Trainees should observe this sort of examiner behaviour and use it to make decisions about their revision.

Anyway: APRV. To celebrate the use of graphs and diagrams, here is one which may represent something that might have aided the candidates:

• Physiological rationale
  • APRV provides high mean airway pressure with intermittent short releases at a lower pressure, which prevent derecruitment
  • This allows "open lung" ventilation and prevents cyclical atelectasis, thus minimising lung injury and biotrauma
  • It also required deep sedation and/or NMJ blockade
  • APRV maintains a high peak airway pressure and prevents cyclical derecruitment by allowing only a short period of expiratory time, without the use of muscle relaxant
• Potential advantages
  • Patients can breathe spontaneously; less risk of ICU-acquired weakness
  • Alveolar recruitment (especially in the bases) is maintained; 
  • Power of ventilation is low, thus lower risk of VILI and biotrauma 
  • High expiratory flow during expiration may aid in secretion clearance
  • Evidence suggests non-inferiority with conventional modes (one RCT, Zhou et al, 2017)
• Disadvantages
  • High constant intrathoracic pressure impedes venous return; patients need more IV fluid to tolerate this
  • Increased risk of barotrauma and pneumothorax
  • Increased RV afterload
  • CO₂ clearance may be poorer
  • For cooperative patients only; sedation minimises the benefits from this mode
  • For cooperative staff only; this mode is unfamiliar and harder to troubleshoot
• Ventilator settings
  • T_low: the time spent at the low pressure (P_low)
  • P_low: the release pressure
  • P_high: the constant high pressure
  • T _high: time spent at the constant high pressure
• Indications for use
  • ARDS
  • Bronchopleural fistula
  • Where prone ventilation or ECMO is contraindicated
  • Obese patients
• Own practice
  • Any reasonable statement, eg. "I always use APRV for patients with ARDS where the FiO₂ requirement increases beyond 80% on conventional ventilation", or "I never use APRV because I am more comfortable with prone ventilation and VV ECMO"