2020B Question 13
Describe the principles (50%) and sources of error (50%) in the measurement of arterial blood pressure using an invasive arterial line and transducer.
Examiner Report
This question had two equally weighted parts.
The first part of the question required candidates to discuss the principles of operation of arterial pressure measurement systems. To achieve a pass in this section candidates were expected to list the components of the system and describe the function of each component. A discussion of the broad principles of transducer operation was expected, and whilst not required, better answers included a discussion of signal amplification and possible mechanisms of setting the zero point. Specifically, a description of the Wheatstone bridge was not required for a pass, but was often included, with accompanying diagram which was, unfortunately, frequently incomplete or incorrect. The inclusion of accurate diagrams to illustrate understanding of the topic is encouraged, however significant errors tend to detract from the answer. Better answers generally included greater detail of design features of each component, for example indicating important aspects of tubing and cannula selection such as length and compliance, transducer design and the properties of complex wave propagation in fluids. Many answers consisted of an incomplete list of components of the system with no further information which was insufficient detail to pass this component.
To achieve a pass in the second part of the question, candidates needed to demonstrate an understanding of the major sources of error in the arterial pressure monitoring system. These are primarily transducer zeroing, the effect of transducer height relative to the patient, and the effects and interplay of resonance and damping. Good answers included a definition of each source of error, a description of the effect of introducing each source of error on the arterial waveform or reading and design features of the system which minimise each source of error. Additional credit was awarded for advanced understanding.
It was very common for candidates to perform well in one section of the answer and poorly in the other, leading to an overall answer not meeting the requirements for a pass. For questions of this nature it is important to pay equal attention to each section as a poor mark in one section is very difficult to compensate for in the other section.
Model Answer
Structure:
- Components
- Function
- Calibration
- Sources of error
Components
| Variable | Detail |
|---|---|
| Component | Key Properties |
| Catheter | - 20 or 22 gauge - Short, wide (20g or 22g), stiff - Clot-resistant, kink resistant material e.g. FEP polymer |
| Tubing | - Short (≤1.2m), wide (lumen >1.5mm), stiff tubing - Low density fluid (saline) |
| Sampling port | Sampling port and three-way tap |
| Transducer | - Infusion at 3mL/h - Rapid flush lever for a) Clearing the catheter b) Testing for damping |
| Reservoir | Saline pressurised at 300mmHg |
| Electrical cable | |
| Processor & Display | May include other functions including pulse pressure variation, pulse contour analysis |
Function
| Variable | Detail |
|---|---|
| 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 |
Calibration
i.e. Static accuracy
| Component | Detail |
|---|---|
| Zero point | - Relative to atmospheric pressure - “Off to patient, open to air” |
| Height | - Raise transducer against a standard - 7.4mmHg per 10cm(H2O) of vertical height |
| Time | - Observe steadiness at zero across time |
Sources of Error
| Source of Static Inaccuracy | Mechanism |
|---|---|
| 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 ↓ |
| Source of Dynamic Inaccuracy | Mechanism |
|---|---|
| Resonance | Exaggeration of oscillatory amplitude if system stimulated at a close multiple of natural frequency (FN) Avoided if FN > 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 Optimised if damping coefficient 0.64 - Intact cannula, no kink/blood/clot/bubble - Optimal cannula and tubing design |
N.B. Modern systems are underdamped, but natural frequency is sufficiently high to prevent resonance.