41 - 45 Physics SBAs for the Primary
Question 41
You have successfully inserted your first ever arterial line, and the consultant immediately starts asking you about the pressure transducer.
Which of the following is false?
- Frequency response should be higher than 8 harmonics of the fundamental frequency
- Optimal damping occurs at a damping coefficient of 0.64
- The arterial line catheter should ideally be short, wide and stiff
- Damping won't affect the mean arterial pressure reading
- Critical damping occurs at a damping coefficient of 0.8
Answer
- Critical damping occurs at a damping coefficient of 0.8
The reality of pressure transducers is you're going to look at the monitor and mutter 'hmm, it's under damped', and then carry on with your life.
But apparently it's critically important knowledge for the primary FRCA exam, so here we go.
Resonance occurs when something oscillates back and forth in response to an external force. Everything has a resonant frequency which is the frequency at which it likes to oscillate.
You don't want your transducer system to resonate when force is applied to it, because it'll throw all your measurements out of whack, so the general rule is to design a system with a resonant frequency at least 8 - 10 times higher than the frequency of whatever your measuring (the heart rate).
Damping is how well something resists resonating or oscillating when a force is applied to it.
- Playdough - very damped
- Those little spring door stops that go dooooiiiiiiinnnnnnggggg - not damped at all
An underdamped arterial trace will have sharp tall spiky peaks and lots of up and down wobbles. It will overestimate systolic pressure and underestimate diastolic (basically just overshooting in both directions).
An overdamped trace will be rounded, smooth and sluggish to react to changes in pressure. It will do the opposite - underestimating systolic and overestimating diastolic, giving a narrow pulse pressure.
- A long, narrow, stretchy tube will increasing damping, so we want a short, stiff, wide bore cannula and giving set to prevent this
In both scenarios, however, the mean arterial pressure is usually accurate.
Optimal damping gives a rapid response to a change in pressure, but doesn't have too much overshoot, and this occurs at a damping coefficient of around 0.64.
Critical damping occurs at a coefficient of 1, and describes a system with zero overshoot, but a response that is too slow to be useful for clinical measurement, hence we prefer a slightly more reactive setup with a bit more resonance.