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 armís 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

  Processor monitors:

o Amplitude

o Amplitude / Pressure† (not Amplitude / Time)

Pressures

  Systolic pressure: max rate of ↑amplitude

  Mean pressure: 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

 

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