2018B01 Briefly describe the principles (50%) and sources of error (50%) in the measurement of
systemic arterial blood pressure using an automated oscillometric non-invasive monitor.

· Device

· Functioning

· Considerations

Components

· Cuff with an inflatable bladder

· Port and cable for air insufflation and pressure transduction

· Strain gauge and pressure transducer in the module

· Processor and display

Application and sizing

· Cuff width: should cover 2/3 of arm length

· Bladder length: should encircle 80% of the arm’s circumference

· Should be at level of right atrium

Mechanism	· Cuff is inflated above estimated SBP then released in small decrements · Arterial pressure is transmitted to the module via the cuff (tonometry) · Pulsatile blood flow causes oscillations in cuff pressure (oscillometry)
Measurements	· Amplitude of oscillations · ∆Amplitude / ∆Pressure · Not ∆Amplitude / Time
Calculations	· SBP: max rate of ↑amplitude · mAP: maximum amplitude (most accurate) · Diastolic pressure: several methods (least accurate) o Max rate of ↓amplitude o Calculated: DBP = 1.5 x mAP - 0.5 x SBP o Fixed ratio: 85% of max
Graph

Sources of error

Intrinsic:

· Diastolic pressure less accurate

· Inaccurate in arrhythmia (irregular amplitude of oscillations)

· Inaccurate at extremes of BP (false falsely low if high, falsely high if low)

· Improper calibration

Sizing:

· Poor fit

· Too short or narrow -> falsely high

· Too long or wide -> falsely low

Cuff:

· Movement artifact e.g. shivering

· Cuff not at the level of RA (e.g. upper arm in lateral decubitus position -> ↓hydrostatic pressure)

· Cuff inflation pain (↑SNS output -> ↑ BP)

Pros

· Non-invasive

· Cheap

· Easy to use

· Can use for venepuncture

Cons

· Intermittent; may miss sudden change in BP

· Many sources of error

· Frequent or prolonged use can cause neuropathy