Question 26

College answer

Aim: To explore the candidate knowledge of ventilation strategies.
Key sources include: Paper 2016.2 Q 29. TE Oh Ed 8 Chpt 31 Mechanical ventilator support Chpt 33 ARDS. CanMEDS Medical Expert

Discussion: The successful candidate was able to outline four different methods and provide details of each method to guide the ventilatory management strategy for best PEEP. Candidates who listed four methods without explanation scored less marks. f they were familiar with the glossary of terms. For example, outlining methods included explanations of compliance curves, supporting evidence for methods of PEEP titration (for example ARDSnet tables or PHARLAP) or a discussion of oesophageal balloon use in the titration of PEEP. Inclusion of these methods and the rationale improved the depth of answer and scored more marks.

Discussion

The college refer to Question 29 from the second paper of 2016, which asked almost exactly the same sort of questions. For the first section, one may have chosen any four from the following menu of possibilities:

• Use an arbitrarily high PEEP: set to 15-20cmH₂O.
  • Meta-analysis of LOVS, ALVEOLI and PROGRESS has suggested that severe ARDS patients (P/F ratio under 200) benefit from higher PEEP settings.
  • CT data suggests that in most ARDS patients the optimal PEEP is around 16cm H₂O
• Use the ARDSNet PEEP/FiO₂ escalation tables (setting the PEEP according to the severity of the oxygenation failure)
  • The tables were used in the ARMA and LOVS trials, and are therefore associated with improved survival in ARDS
  • However, the main hypothesis of those trials was related more to tidal volumes and not to PEEP selection.
  • Recently, there has been a move away from oxygenation-based PEEP selection, and towards an "open lung" approach with PEEP being selected on the basis of ideal end-expiratory lung unit recruitment
• Titrate PEEP according to maximum compliance, i.e. set the PEEP which achieves the highest static compliance
  • This has the advantage of being tailored to each specific patient
  • The physiological basis is sound (maximum compliance should occur when maximum recruitment but minimal overdistension has occurred).
  • No strong literature evidence exists
• Set the PEEP using the lower inflection point of the pressure volume curve
  • On the pressure volume curve, the lower inflection point indicates the pressure at which alveolar recruitment is maximal (i.e. fewest alveoli are collapsed).
  • It is unclear where this point is on any given real-life curve
  • It is unclear whether we should use the lower (inspiratory) inflection point or the upper (expiratory) inflection point, and there are good theoretical arguments for each.
  • The measurement requires paralysis and - ideally - serial static measurements
• Use a staircase recruitment (or derecruitment) manoeuvre to find the lowest PEEP at which the maximal oxygenation is maintained.
  • This has the advantage of having a very pragmatic endpoint, SpO₂.
  • One recruits the lung, and then decreases PEEP incrementally until SpO₂ begins to drop
  • The minimum PEEP which maintains the highest SpO₂ is then selected as the "ideal" PEEP
  • The problem is, this PEEP may still expose some of the lung regions to cyclic atelectasis, and will not prevent biotrauma. As long as oxygenation is preserved, those lung regions will be ignored by this technique.
• Using a PA catheter, titrate PEEP to achieve the smallest intrapulmonary shunt
  • Shunt will increase with atelectasis or derecruitment
  • Shunt will also increase with overdistension
  • Monitoring the intrapulmonary shunt is possible only by using a PA catheter
  • These days this technique is at least as unpopular as the PA catheter
• Titrate PEEP according to the transpulmonary pressure
  • Oesophageal pressure (P_es) derived from an oesophageal balloon manometer is a satisfactory surrogate for pleural pressure.
  • Transpulmonary pressure = (P_plat - P_es)
  • This variable ca be used to titrate PEEP as well as tidal volume, as it relates to derecruitment and overdistension.
  • The ideal TPP is 0-10 in end-expiration and no more than 25 in inspiration
• Using electrical impedance tomography, titrate PEEP to achieve the highest electrical impedance in the thorax (i.e. the greatest amount of aerated lung)
  • This is promising bu still largely experimental
  • No hard outcomes adata is available, only animal and "feasibility" studies.
• Sequential CT scans to visually determine a PEEP at which the greatest volume of lung is recruited during end-expiration
  • CT volumetric measurements are the gold standard of recruitment research
  • The disadvantages are related to safety and logistics, i.e. transport to and from the CT scanner as well as radiation exposure.

a)

The lazy man's classification of sodium disturbances was basically designed to answer this sort of differential-generating exercise. The stem of the question gives you nausea vomiting and slurred speech, which might either indicate that the examiners wanted you to suspect hypovolemia, or to explain why the hypertonic saline is indicated (symptomatic hyponatremia), or to suggest that some sort of sinister intracranial horror is quietly lurking. so, the list of differentials would therefore incorporate:

• Hypovolemic hyponatremia
• Cerebral salt wasting
• Mineralocorticoid deficiency
• Hypothyroidism
• Renal sodium loss, eg. renal failure

b)

"patient factors that might impact your PEEP setting" is a novel take on this SQ, and called for extra thinking; but not too much extra thinking, as there was only 20% of the mark to be gained. The question is really asking, "where might you not be able to use a PEEP as high as you might want, and where might you need to use a higher PEEP than you might be usually comfortable with?" A list like this was probably in order, where only four options were expected:

• Obesity
• Pulmonary hypertension
• Hypovolemia
• Pregnancy
• Existing pneumothorax
• Unilateral or very heterogeneous lung pathology
• Bronchospasm with intrinsic PEEP
• Restrictive lung disease

Gattinoni, Luciano, Eleonora Carlesso, and Massimo Cressoni. "Selecting the ‘right’positive end-expiratory pressure level." Current opinion in critical care 21.1 (2015): 50-57.

Grasso, Salvatore, et al. "ARDSnet ventilatory protocol and alveolar hyperinflation: role of positive end-expiratory pressure." American journal of respiratory and critical care medicine 176.8 (2007): 761-767.

Suter, Peter M., H. Barrie Fairley, and Michael D. Isenberg. "Optimum end-expiratory airway pressure in patients with acute pulmonary failure." New England Journal of Medicine 292.6 (1975): 284-289.

Pintado, María-Consuelo, et al. "Individualized PEEP setting in subjects with ARDS: a randomized controlled pilot study." Respiratory care 58.9 (2013): 1416-1423.

Chiumello, Davide, and Matteo Brioni. "Severe hypoxemia: which strategy to choose." Critical Care 20.1 (2016): 1.