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.

 

List:

·      Device

·      Functioning

·      Considerations

 

Device:

Examples

·  DINAMAP (device for indirect non-invasive mean arterial pressure)

·  Von Recklinghausen oscillotonometer

Components

·  Cuff with an inflatable bladder

·  Port for air insufflation

·  Port for pressure transduction

·  Transducer in the module

·  Processor and display

Application and sizing

·  Cuff width: covers 2/3 of arm length

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

·  Ensure cuff at level of right atrium

 

How it works:

Mechanism

·  Cuff is inflated above estimated SBP then slowly released

·  Pulsatile flow through partially compressed artery

·  Oscillating pressure sensed by a strain gauge

·  Change in oscillatory amplitude monitored by processor

Pressures

·  Systolic pressure: max rate of ↑ amplitude

·  Mean pressure: maximum amplitude (most accurate)

·  Diastolic pressure: several methods (least accurate)

o Calculated: DBP = 3 x mAP – 2 x SBP

o Point of most rapid ↓amplitude

o Point at which amplitude is 85% of max

 

Considerations:

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 venipuncture

Cons

·   Intermittent; may miss sudden change in BP

·   Many sources of error

Repeated or prolonged inflation can cause neuropathy or limb ischaemia

 

 

Feedback welcome at ketaminenightmares@gmail.com