Discrepancies between NIBP and Arterial Lines Pressures
- many reasons
Error in Intra-arterial measurement
- zero error (poor calibration, drift, wrong height)
- poor system (long tubing, soft wall, narrow bore)
- catheter kinked
- no pressure in the pressure bag
- local arterial stenosis
- intense vasoconstriction
- subclavian stenosis
Error in the NIBP measurement
- wrong cuff size
- irregular pulse (AF)
- subclavian stenosis
Lack of correlation because measures are from different sites and use different principles
- fluctuations of vascular pressure cause a pulsation of the saline column
- displaces electomanometer’s diaphragm
- sensed electrically
- wave form built up by Fourier analysis from sinusoids or simple wave forms.
- wave forms differ depending on where the cannula is inserted.
- single cuff inflated above systolic and then incrementally deflated while the amplitudes of cuff pressure oscillations measured by pressure transducer.
- cuff should be near level of heart.
- cuff pressure at which the amplitudes start to increase to 25-50% of maximum = systolic.
- the point of maximum oscillation = MAP (most reliable measurement)
- cuff pressure at which the amplitudes decrease by 80% or disappear = diastolic.
- time vs pressure graph constructed with above variables derived
Which reading do you believe?
- depends on the above factors and which recording you determine to be the most accurate
- MAP from the intra-arterial line is the most reliable
- if there is doubt may require a more proximal reading (femoral or long brachial catheter)
- in the vasculopath it would seem wise to trust the higher pressure
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.