Pressure vs Volume Loop: graphical representation of relationship between pressure and volume during inspiration and expiration. Spontaneous breaths go clockwise and positive pressure go counter clockwise
- in pressure control or PS loop is almost square because of pressure limiting during inspiration
- can be measured as a dynamic or static technique and requires paralysis
- dynamic: requires a square inspiratory wave form to interpret – constant flow and no inspiratory pause
- static: measures of pressure with small incremental in volume
- bottom of loop is either 0 or PEEP level
- top of loop = PIP
- compliance (imaginary line between start of inspiration and expiration)
-> increased compliance: left shift (emphysema)
-> decreased compliance: right shift (ARDS)
-> total compliance = TV/end-inspiratory pause pressure-PEEP
- lower inflection point on inspiratory limb = place where there is a sudden increase in compliance (set PEEP here to maintain FRC)
- upper inflection point = maximum setting for peak airway pressure
- pig tail at the bottom indicates patient triggering (bigger the pig tail, higher the WOB to trigger breath)
- right shifted curve
- decreased volume for pressure change
- line drawn down middle of loop
- on inspiration (area to right side of line)
- on expiration (area to left side of line)
‘Beak’ or ‘duckbill’
- increase in airway pressure without any appreciable increase in volume
Leaks or Air Trapping
- loop won’t meet the bottom
Lower Inflection Point
- change in slope at the lower end of the inspiratory curve -> recruitment of all/most/some of the collapsed and recruitable alveoli -> helps at setting PEEP
- these assumption has been questioned because there are many limitations to this approach: recent ventilation history, variability due to underlying lung disease, presence of decreased compliance of the abdominal and chest wall, the greater importance of the expiratory component of the curve.
Upper Inflection Point
- has been proposed as a way of detecting overdistention of the lung
- this is too simplistic for many reasons: other parts of lung are already distended prior to reaching this point (CT studies)
Chris is an Intensivist and ECMO specialist at the Alfred ICU in Melbourne. He is also a Clinical Adjunct Associate Professor at Monash University. He is a co-founder of the Australia and New Zealand Clinician Educator Network (ANZCEN) and is the Lead for the ANZCEN Clinician Educator Incubator programme. He is on the Board of Directors for the Intensive Care Foundation and is a First Part Examiner for the College of Intensive Care Medicine. He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives.
After finishing his medical degree at the University of Auckland, he continued post-graduate training in New Zealand as well as Australia’s Northern Territory, Perth and Melbourne. He has completed fellowship training in both intensive care medicine and emergency medicine, as well as post-graduate training in biochemistry, clinical toxicology, clinical epidemiology, and health professional education.
He is actively involved in in using translational simulation to improve patient care and the design of processes and systems at Alfred Health. He coordinates the Alfred ICU’s education and simulation programmes and runs the unit’s education website, INTENSIVE. He created the ‘Critically Ill Airway’ course and teaches on numerous courses around the world. He is one of the founders of the FOAM movement (Free Open-Access Medical education) and is co-creator of litfl.com, the RAGE podcast, the Resuscitology course, and the SMACC conference.
His one great achievement is being the father of three amazing children.
On Twitter, he is @precordialthump.