· Arterial line: components, how it works, accuracy
· Physics of SHM: definitions
· Dynamic accuracy
Components 
· Cannula with anticlot · Compressible tubing with saline · Pressure transducer: diaphragm with four strain gauges (conductor) · Saline bag under pressure · 13mL/h infusion and ability to flush · Electronic display 
Function 
· Continuous column between arterial blood and transducer · ↑↓ oscillations in pressure > displacement of diaphragm > stretch/relax strain gauges > ↑/↓ electrical resistance > amplification > arterial trace · Accuracy improved by multiple strain gauges +/ Wheatstone bridge 
Accuracy 
· Static: o Zero relative to atmospheric pressure o Time calibration: observe steadiness at zero o Height calibration: e.g. move transducer against a standard · Dynamic: o Optimise damping o Minimise resonance 
Natural frequency (F_{N}) 
· Definition: tendency of a system once disturbed to oscillate at a particular frequency about its equilibrium position. · Determinants: F_{N} ∝ potential energy (stiffness) / kinetic energy (mass) o Potential energy ∝ tube elastance x diaphragm elastance o Kinetic energy ∝ (length x tube compliance) / (radius x density) 
Fundamental frequency (F_{0}) 
· Lowest frequency oscillation within a system (e.g. heart rate in ABP trace) 
Resonance 
· Definition: where repeated disturbance of a system at its natural frequency (or at a close multiple of it) causes increasing amplitude of oscillation · Effect on ABP trace: ↑SBP, ↓DBP, ↔mAP, contributes to peripheral dicrotic notch 
Damping 
· Definition: minimization of oscillatory amplitude through viscous and frictional forces · Equation: damping ∝ (Length x volume of displacement x viscosity) / (diameter x density) · Damping coefficient: index of rate of decay of oscillations · Optimal damping: balance between maximizing response speed and minimizing overshoot · Effect on ABP trace: ↓SBP, ↑DBP, ↔mAP, delayed waveform, loss of high frequency components e.g. incisura

Minimise resonance 
Aim: · F_{N} ≥ 10x F_{0} hence resonance unlikely to occur · Flat frequency response at or near the measured frequencies · No exaggeration of amplitude of measured frequencies Means: · ↑potential energy (stiffness): noncompliant tubing and diaphragm · ↓ kinetic energy: short, wide, noncompliant tubing and low density fluid 
Optimise damping 
Aim: · Flat frequency response up to 2/3 of F_{N} · Minimal phase distortion i.e. minimal and more homogenous lag · Damping coefficient 0.64 · Exactly two oscillations after fast flush test · <7% overshoot overall · <2% overshoot for harmonics (if Fourier analysis performed) Means: · Intact cannula, no kink, no clot, no bubbles · System design 
Graph 
*Note: modern systems are underdamped, but natural frequency is high enough to obviate resonance* 
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