20A12: Exam Report
Explain resonance and its significance and the effects of damping on invasive arterial blood pressure measurement.
23% of candidates passed this question.
Many candidates gave detailed answers that involved the set up and components of the arterial line system that was not asked for in the question and did not attract marks.
There was confusion around the correct use of the terms natural frequency, resonance frequency and harmonics – candidates that were able to describe these frequencies correctly went on to achieve a good mark – the graphs and discussion around optimal dampening, over and underdamped traces were often drawn poorly or without sufficient detail, and at times were not used within in the context of the answer.
Descriptions of the clinical effect seen with over / under dampened traces on blood pressure was well described.
17B17: Exam Report
Define and explain damping, resonance, critical damping and optimum damping.
25% of candidates passed this question.
Concise definitions were required with a clear explanation of the underlying physical principles.
The response time of the system, degree of overshoot, effect on amplitude, noise and ability to faithfully reproduce frequencies relative to the natural resonant frequency were important considerations.
Many candidates interpreted the question as relating to arterial lines and a detailed discussion of the components and characteristics of an intra-arterial catheter and transducer system did not attract marks.
15A12: Exam Report
Describe the effects of resonance and damping on an invasive arterial blood pressure tracing.
33% of candidates passed this question.
Many candidates seemed to get some of the basic concepts but few were able to expand on simple concepts.
It was expected that candidates could describe that the arterial pressure waveform is made up of many different sine waves (as determined by Fourier Analysis) with each sine wave having a specific frequency. Every system has its own natural oscillatory frequency, or resonant frequency. If this is less than 40 Hz, it falls within the range of frequencies present in the blood pressure waveform and oscillations may produce a sine wave which is superimposed on the blood pressure wave form.
Some damping is inherent in any system and acts to slow down the rate of change of signal between the patient and pressure transducer. It may be caused by air bubbles or blood clots or occlusion. This reduces the deflection of the transducer diaphragm and hence the size of the waveform. The effect of damping on temporal response was rarely mentioned.
Accurate graphical representations of invasive pressure traces are important. Many candidates provided poor drawings without axis, labels, reference to normal or discussion in text.
G6ii / 20A12 / 17B17 / 15A12: Effects of resonance damping on invasive IABP tracing
NOTE: IABP is the gold standard of BP measurement
- Pressure waves are periodic events occurring at repetitive frequencies
- Any wave can be broken down into a sum of simpler sine waves → this is FOURIER ANALYSIS
- FUNDAMENTAL WAVES = the number of times per second the cycle occurs
- HARMONIC WAVES = a sine wave that is a multiple of the fundamental frequency
→ These waves are added together & resemble the arterial pressure wave
- The arterial wave is broken down by a microprocessor into the sine waves
- It is then reconstructed from the fundamental wave and 8+ harmonic waves to give accurate representation of original wave
- The IABP system must be able to transmit & detect high frequency components of the original wave (at least 24Hz) to be able to accurately represent arterial pressure
→ This is important when considering natural frequency of the system
→ The aim of measurement is to obtain an exact copy of a physiological event
→ If the copy is not an exact record, then there is error in measurement
→ There are 2 types of errors: STATIC & DYNAMIC
1) Dynamic Calibration
- Dynamic calibration = ability to reliably record rapidly changing events
- Catheter transducer systems are ‘Second Order Systems’
- They have 2 easily measurable parameters
- Resonant Frequency
- Damping Coefficient
- Every material has its own natural frequency
- NF = the frequency at which the material oscillates (freely without stimulus)
- RESONANT FREQUENCY = the frequency a system oscillates when disturbed
- In order to accurately record a pressure wave, the NF of the IABP system should be much higher than the primary frequency of the pressure wave
- Otherwise, as IABP NF approaches frequency of any sine wave, the system will resonate excessively
- ∴Over-estimates SBP & under-estimates DBP (widening pulse pressure)
- For accuracy, the NF of IABP system should be at least x 8 the fundamental frequency of the waveform
- Most have NF 200Hz, but this is reduced by bubbles, clots, 3-way taps, additional tubing
- NF of a system is determined by the properties of its components:
→ Directly proportional to lumen radius
→ Inversely proportional to tube length, compliance & fluid density
∴system should be SHORT, FAT, NON-COMPLIANT
- To measure NF we use Fast Flush Test
- Column fluid at 300mmHg
- Flush 1 sec
- Creates undershoot & overshoot of waves
- Resonating the NF of the system
- Check for oscillations
- DAMPING = the absorption of the E. of oscillations which results in reduced amplitude of oscillations
- Any system with elasticity when disturbed will oscillate before settling to a new value
- Damping will ↓ the oscillations before the system comes to rest
- Damping causes:
- ↓amplitude oscillations more quickly
- ↓frequency oscillations
- ↑lag of display
- Degree of Damping can be quantified by DAMPING COEFFICIENT (no units)
- DC is calculated from the rate of the magnitude of successive oscillations
- No Damping (DC = 0) → oscillations go on forever, reaching same height each time
- Under Damped (DC = 0.2) → oscillations eventually ↓ taking a while to come to rest
- Over Damped (DC = 0.8) → minimal overshoot, oscillations come to rest very quickly
- Critical Damping (DC = 1) → no overshoot, but the time taken to reach rest is very long
∴it is not clinically useful to have a critically damped system
- Optimal Damping (DC = 0.64) → small overshoot (6%) with rapid response rate
Causes of Damping
- Friction with fluid pathway (bubbles, clots)
- 3 way taps
- Narrow, long, compliant tubing
- Kinks in cannula/tubing
- Assessing Damping: fast flush test → ratio is the amplitude rate of 2 consecutive waves
Mean Arterial Pressure (Map)
- Achieving dynamic accuracy is hard!
- But it is only required for SBP & DBP
- MAP can be measured v. accurately with only Static Calibration (zeroing)
- MAP is also the most important value for autoregulation & perfusion pressure