Apr 2013

(By Ku)

1. Blood Pressure Measurement

Shown a picture of a monitor screen with IA line and NIBP. How is the blood pressure monitored in this case? (two ways IA line and NIBP).

Q What are the causes for inaccuracies in each?

• NIBP:

o cuff size (widths ~ 40% of arm circumference)

o tube

o patient movement, dysrhythmias, hypotension.

o intrinsic inaccuracy with DBP as it’s a derived value.

o Too frequent measure = impedes BF; aim >2min

o Intermittent.

o Haematoma, obesity, lymphedema.

• IABP:

o Static: zeroing, equipment calibration

o Dynamic: damping, resonance

o Damping: tendency for oscillation to minimise through frictional/viscous forces.

o Fn: frequency of oscillation when a system is disturbed.

o Fn needs to be at least 10x above natural frequency (ie. Above 10th harmonic) to minimize inaccuracy from resonance. Eg. Fn1-3 Hz, so

Fn = 30Hz.

o Damping optimal at coefficient: 0.63. ie. Minimal overshoot and rapid response of oscillation wave, provides optimal condition for rapid changing IABP.

Q How is intra-arterial BP monitored? (explained where SBP and DBP and MAP was) You have shown me where the lines are, but how does the IA line get these numbers?

• By pressure transducer with zeroing to atmospheric pressure then algorithm to transducer pressure detected by strain gauge to number, based algorithm.

Q. What do the changes in ascending slope mean? Descending slope? Area under the curve?

Q. Could you draw the expected IABP line for aortic stenosis? HOCM?

Q How is NIBP measured?

Talked about oscillotonometry and drew the graph of oscillation vs time and marked out sbp and map and dbp and explained that DBP can be calculated

• MAP = point of max. oscillation.

• SBP = point when oscillation starts

• DBP = point where sudden drop of oscillation amplitude, or when amplitudeplateaus. Or derived from MAP / SBP value.

Q What is the formula for this calculation?

• MAP = DBP + 1/3 PP.