This topic came up in the first 2001 paper, as Question 13: "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?"

Device factors

• 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

Patient factors

• 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)

Relative reliability of blood pressure measurements

• Invasive measurement is the "gold standard" of BP measument overall
• Mercury sphygmomanometer is the gold standard of non-invasive measurement
• Peripheral and central invasive measurements of arterial pressure tend to show good agreement, but in context of severe shock the peripheral lines tend to overestimate the blood pressure.

Selecting the most believable arterial blood pressure measurement

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.

The college, in their answer to Question 15 from the first paper of 2001, suggest that "if there is doubt ... a more proximal recording (eg femoral catheter or long brachial catheter or implantable transducer) may be necessary". They also remark that in a patient with severe peripheral vascular disease the higher pressure should be trusted.