The nurse notes a marked difference between blood pressure recorded via an arterial line in one arm and non-invasive pressure recorded from the other arm. What may be causing this difference? Which reading will you use to guide management?
The difference in pressures may be caused by:
(a) error in intra arterial measurement due to
- zero error (poor calibration, drift, wrong height)
-poor system (long tubing, soft wall, narrow bore)
-local arterial stenosis, spasm, hypothermia, intense vasoconstriction, subclavian stenosis etc
(b) error in NIBP measurement
- wrong size cuff
- irregular pulse, AF (consecutive pulses required)
- subclavian stenosis
(c) lack of correlation because measures are from different sites and use different principles.
The candidate might have explained the oscillotonometric and invasive pressure recording principles to elucidate the problem.
The choice of reading for clinical use depends on the above factors. Mean arterial pressure from the arterial lime in the absence of hypothermia, subclavian stenosis etc may be the most reliable. If there is doubt about this reading then a more proximal recording (eg femoral catheter or long brachial catheter or implantable transducer) may be necessary. In a vasculopath it would seem wise to trust the higher pressure.
This is a practical question.
The discrapancy can arise as a result of device factors, or patient factors.
I.e either the measurements are wrong, or the patient genuinely has different blood pressure in different limbs.
One can approach this systematically:
- Non-invasive measurement error
- The cuff is the wrong size
- The oscillometric measurement is confused by an arrhythmia
- The patient is moving around too much
- Invasive measurement error
- The transducer is zeroed incorrectly
- The zero level is incorrectly selected
- The transducer system is incorrectly set up
- The artery being measured is in spasm
- There is peripheral vascular disease, which is unequally distributed
- The patient has subclavian artery stenosis
- There is aortic pathology which influences flow into the limbs (eg. aneurysm)
Which measurement would you choose? This is a judgement call.
One might wish to exclude all device-related problems before making a decision. Ultimately, one may wish to measure the NIBP manually on the same arm as the arterial line, noting the cuff pressure at the point at which the arterial trace goes flat.
Crul, J. F. "Measurement of arterial pressure." Acta Anaesthesiologica Scandinavica 6.s11 (1962): 135-169.
Beevers, Gareth, Gregory YH Lip, and Eoin O'Brien. "Blood pressure measurement." Bmj 322.7293 (2001): 1043-1047.
Ward, Matthew, and Jeremy A. Langton. "Blood pressure measurement."Continuing Education in Anaesthesia, Critical Care & Pain 7.4 (2007): 122-126.
Pickering, Thomas G., et al. "Recommendations for blood pressure measurement in humans and experimental animals part 1: blood pressure measurement in humans: a statement for professionals from the Subcommittee of Professional and Public Education of the American Heart Association Council on High Blood Pressure Research." Hypertension 45.1 (2005): 142-161.