2020B13 Describe the principles (50%) and sources of error (50%) in the measurement of arterial blood pressure using an invasive arterial line and transducer.

· Diagramme

· Components

· Function

· Calibration

· Sources of error

A	Catheter	20 or 22 gauge Short, wide (20g or 22g), stiff Clot-resistant, kink resistant material e.g. FEP polymer
B	Tubing	Short (<1.2m), wide (lumen >1.5mm), stiff tubing Low density fluid (saline)
C	Sampling port	Sampling port and three-way tap
D	Transducer	Infusion at 3mL/h Rapid flush lever for a) clearing the catheter b) test for damping
E	Reservoir	0.9% NaCl at 300mmHg
F	Electrical cable
G	Processor Display	May include other functions including pulse pressure variation, pulse contour analysis

Oscillation	· Oscillations in arterial pressure transmitted to saline column · Column displaces transducer’s diaphragm and strain gauge
Transduction	· Stretch of strain gauge increases electrical resistance · ±Multiple strain gauges in Wheatstone bridge for accuracy · Electrical signal transmitted to processing unit
Processing	· Signal filtered + amplified · Signal broken down into component sine waves (Fourier analysis) · Waveform constructed using fundamental freq + several harmonics · Read-outs calculated
Display	· SBP, DBP, mAP and waveform displayed on the monitor · +/- Pulse pressure variation, pulse contour analysis

i.e. static accuracy

Zero point

· Relative to atmospheric pressure

· “Off to patient, open to air”

Height

· Raise transducer against a standard

· 7.4mmHg per 10cm

Time

· Observe steadiness at zero across time

Static inaccuracy:
- Zero	Failure to calibrate: · Unpredictable. False ↑ or ↓
- Height	Failure of target selection: · If supine: phlebostatic axis (4th intercostal space, mid-axillary line) · If beach chair: brainstem (external acoustic meatus) Failure to adjust with patient movement: · Transducer too high: BP falsely low · Transducer too low: BP falsely high Risk: overtreat ↑BP or undertreat ↓BP -> organ ischaemia
- Time	· Natural drift of strain gauge. False ↑ or ↓ · Equipment dysfunction. False ↑ or ↓
Dynamic inaccuracy:
- Resonance	= exaggeration of oscillatory amplitude if system stimulated at a close multiple of natural frequency (F_N) ->>False ↑SBP, ↓DBP, ↔ mAP Avoided if F_N > 10x F₀ (heart rate): · ↑potential energy: stiff tubing and diaphragm · ↓kinetic energy: short, wide, stiff tubing and cannula; and low density fluid
- Damping	= minimisation of oscillatory amplitude through viscosity and friction ->> Overdamped: False ↓SBP, ↑DBP, ↔ mAP ->> Underdamped: opposite Optimised if damping coefficient 0.64 · Intact cannula, no kink/blood/clot/bubble (N.B. occlusion -> zero pressure) · Optimal cannula and tubing design

N.B. modern systems are underdamped, but natural frequency is sufficiently high to prevent resonance