Indications

Beat-to-beat monitoring required in patients:

• on vasoactive drugs (e.g. inotropes)
• blood pressure control
• prone to rapid change in condition
• requiring frequent blood sampling
• extremely hypotensive or arrythmias
• pulse contour analysis

Method

1. Hydraulic coupling – blood vessel connected with incompressible column of fluid that oscillates with changes in pressure (i.e. hydraulic coupling).
2. Tranduction – converting pressure changes into electrical signals
3. Signal processing – amplification, visualising on monitor as a waveform

Components

• Blood vessel – arteries with good collateral circulation: radial artery, brachial, dorsalis pedis, axillary.
• Cannula – situated in blood vessel. Made of PTFE, Teflon which reduce thrombus. Smaller calibre, less thrombus risk. But smaller calibre, greater damping.
• Tubing – short + wide (reduces damping) stiff (reduces resonance); fluid-filled without bubbles (bubbles cause damping)
• Infusion fluid – heparinised saline pressured to well above systolic (e.g. 300mmHg). Slowly infused to avoid thrombus.
• Transducer – Wheatstone bridge: converts strain on wire/silicone in ciruit to change in its resistance, and hence the electrical signal.  Must be able to detect high frequency components of the pressure changes (e.g. >20Hz)
• Signal processor – Fourier Transform applied to reconstruct electrical signal

Resonance – oscillation occurring at natural frequency. Energy added to system outside the cannula, falsely elevating pressure readings.

Damping – energy removed from the column of fluid, falsely reducing pressure readings.