With respect to positive end-expiratory pressure (PEEP) in a ventilated patient with acute respiratory distress syndrome (ARDS):
a) Describe the possible approaches to setting PEEP. (80% marks)
b) List the disadvantages of excessive PEEP in this situation. (20% marks)
• PEEP setting adjusted to FiO2, increasing with increasing FiO2 according to the ARDSNet studies (low Vt study NEJM 2000, PEEP 5 – 20+ with FiO2 0.3 – 1.0 and ALVEOLI high PEEP study NEJM 2004 PEEP 5 – 24 with FiO2 0.3 – 1.0) or clinical assessment
• Use of lung mechanics to set PEEP – requires the static measurement of P-V curve e.g. using super-syringe. Patient sedated and paralysed and ventilated at FiO2 1.0 and zero PEEP with lung inflation in 50-100ml increments from FRC using a super-syringe followed by deflation in similar steps. Pressure and volume are recorded simultaneously and the P-V curve is constructed from the data.
Typically requires identification of lower infection point on P-V curve (on VC mode) and setting of PEEP 2 cm H2O above the LIP.
• PEEP adjusted to maximise static compliance
(C = Vt / (Pplateau – PEEP)
• Optimal (or best) PEEP – a level of PEEP that optimizes PaO2 and compliance without interfering with tissue oxygen delivery – ideally achieved during or immediately after recruitment manoeuvre, e.g. in Staircase Recruitment Manouevre best PEEP is 2.5 cmH2O above derecruitment point
• Transpulmonary pressures (TPP) to guide setting of PEEP – this requires real time measurement of oesophageal pressures (by placement of an oesophageal balloon) to keep the TPP (Paw-Pes) < 25 cm H2O at end inspiration and between 0 – 10 cm H2O at end expiration, while applying the low tidal volume ARDSNet ventilation strategy.
b) Disadvantages of excessive PEEP in patients with ARDS
• Overdistention of non-diseased alveoli resulting in further injury (VILI)
• Increased risk of barotrauma
• Increased dead space effect due to over-distension and also due to reduction in blood flow to alveoli • CO2 retention
• Reduced venous return to the heart, decreased cardiac output and a fall in blood pressure, vital organ perfusion.
• May decrease venous return from the abdomen, increasing renal/portal vein pressure and decreasing perfusion of kidneys/gut and increasing IAP
• Increased ICP
• May increase right to left shunt (increased pulmonary vascular resistance)
Additional Examiners' Comments:
Overall there was poor understanding of this topic and some candidates were unable to provide basic details. In the responses to part (a) there was generally good breadth in regard to the options of setting best PEEP, however there was often little depth in the options given. In part (b) most answers focused on the cardiorespiratory complications. There were very few candidates who mentioned increased intra-abdominal and intra-cranial pressures as potential complications.
These issues undergo a thorough exploration in the chapter on how to determine the optimal PEEP for open lung ventilation in ARDS.
Disadvantages of excessive PEEP are discussed in the chapter on ventilator-associated lung injury, as excessive pressure at end-expiration is pathologically indistinguishable from excessive pressure at inspiration, except in terms of magnitude. The short statement about each issue borows heavily from the college answer to Question 10 from the first paper of 2012 . In summary, the problems are:
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