Oscillometric vs Invasive Arterial Blood Pressure Monitoring

NIBP

Invasive BP

Overview

Cuff applied on arm, connected to an inflating device, and Koratkoff sounds are heard to determine BP

arterial catheter connected to a pressure transducer

Uses

BP measurement (SBP,DBP)

• blood pressure (systolic, diastolic, mean and pulse pressure)
• arterial blood sampling

Specific Indications

BP monitoring

• Labile blood pressure
• Anticipation of haemodynamic instability
• Titration of vasoactive drugs
• Frequent blood sampling
• Morbid obesity (unable to fit an appropriately sized NIBP cuff)

Description / Components

• Two cuffs
  • Occlusive cuff
  • Measurement cuff
• Tubing
• Device for inflating the occlusive cuff and gradually deflating it
• Aneroid barometer for transducing pressure
• Display

• arterial line
• 48 inches of non-compressible rigid-walled, fluid filled tubing
• pressure transducer and automatic flushing system
• pressure bag and automated slow infusion (1-3mL/h) of pressurised saline
• electronic transducer amplifier display

Method of Insertion / Use

• Cuff is inflated until the radial pressure is no longer palpable
  This is approximates SBP.
• Cuff is deflated, and reinflated to 20mmHg above the estimated SBP
• Cuff is deflated at a rate of 2-3mmHg.s^-1 whilst auscultating the brachial artery
  When cuff pressure equals:
  • SBP
    Turbulent flow occurs past the cuff, turbulent flow causes the first of the Korotkoff sounds (clear tapping pulsations) to be heard.
  • DBP
    The cuff no longer compresses the vessel at all, so no turbulent flow occurs and nothing is auscultated.

• fluctuations of vascular pressure cause a pulsation of the saline column
• displaces electromanometer’s diaphragm which has a built in strain gauge (Wheatstone bridge principle)
• deformation leads to a change in resistance of the strain gauge which is sensed electronically
• wave form built up by Fourier analysis from sinusoids or simple wave forms
• wave forms differ depending on where the cannula is inserted

Needs Calibrating (‘zeroing’) and Square wave test (aka fast flush test) to check for damping of system

Accuracy and Errors

• Requires an appropriately sized cuff
  Cuff should be ~20% greater than arm diameter.
  • Cuffs that are too small will over-read
  • Cuffs that are too wide will under-read
• Requires a regular rhythm
• Inaccurate at extremes of blood pressure
• Inaccurate when used more frequently than once per minute
• Inaccurate when the vessel is incompressible
  • Heavily calcified vessels
  • When applied to forearm/foreleg
• Time consuming, cannot provide continuous monitoring
• Does not require calibration

Common sources of error

• Cannula insertion/position error
• bubbles in catheter-transducer system → decreased resonant frequency
• clotting in arterial catheter
• elastic walls causes increased damping
• Transducer not levelled (Not calibrated properly)
• cannula won’t flush – kinked, clotted, tissued

Complications

• Pain
• Local pressure injury
• May cause neuropraxia

• Pain
• thrombosis and distal ischaemia
• infection
• increased diagnostic blood loss and anemia
• retrograde air embolism
• inadvertent drug/air injection
• haematoma (+/- nerve compression)
• retroperitoneal haematoma (femoral)
• bowel perforation (femoral)
• vessel damage may lead to stricture and prevent future AV fistula formation for haemodialysis
• pseudo-aneurysm
• arterial dissection
• arteriovenous fistula

Other Information

• MAP can be calculated
• Variation seen at different limbs

Information other than blood pressure can be obtained:

• pulse rate and rhythm
• effects of dysrhythmia on perfusion
• ECG lead disconnection
• continuous cardiac output using pulse contour analysis
• specific wave form morphologies might be diagnostic
  — e.g. slow rising = AS, pulsus alternans = tamponade
• pulse pressure variation (suggests fluid responsiveness)
• steeper upstroke of pulse pressure = increased contractility
• area under upstroke = SV
• steep downstroke = low SVR

Variation in arterial waveform at different sites