This chapter is relevant to Section G7(iii) of the 2017 CICM Primary Syllabus, which asks the exam candidate to "describe the invasive and non-invasive measurement of blood pressure, including limitations and potential sources of error". It deals with the practical aspects of measuring the performance characteristics of the arterial pressure transducer system. The theoretical aspects of frequency response and damping coefficient are fascinating but likely not essential to the exam-going candidate; as such they have been dismissed to the largely apocryphal Principles of Pressure Measurement section.
This topic has not appeared in the CICM Part I, but in the Fellowship exam Question 11.2 from the first paper of 2010 asked the trainees to comment on an obviously underdamped fast flush test result.
- Dynamic Response is a function of Natural Frequency and Damping Coefficient
- The Natural Frequency: the frequency at which the system will oscillate in the absence of a driving or damping force, i.e. how fast the system vibrates in response to a single disturbance.
- The Damping coefficient: How quickly those vibrations come to rest in the system
- The dynamic response of an arterial line system is tested using the "fast flush" test, where the transducer is briefly exposed to pressure straight from the counterpressure bag.
- When the fast flush abruptly ends, the transducer system oscillates at its natural frequency.
- This can be measured and assessed for adequacy. The time between oscillation "peaks" gives you the natural frequency of the system; i.e. a system with 50 msec between peaks has a natural frequency of 20Hz.
- The transducer system needs to have a natural frequency in excess of 24 Hz in order to resolve fine features of the arterial line trace (eg. dicrotic notch)
- Excessive damping leads to underestimated systolic and overestimated diastolic
- Underdamping leads to overestimated systolic and underestimated diastolic
- MAP remains largely unchanged, as it is a mean pressure over the entire pulse cycle.
The Square Wave Test
When you squeeze the fast flush valve, you let the transducer taste some of the 300mmHg in the pressurized saline bag. This produces a waveform that rises sharply, plateaus, and drops off sharply when the flush valve is released again.
This is the "square wave".
After the fast flush has ended, the transducer system returns to baseline. It does so as a harmonic oscillator, "bouncing" a couple of times before coming to rest. This "bounce" can be used to determine the resonance characteristics of the system. The accurate, responsive, adequately damped arterial line waveform will have the following features:
- The time between oscillations will be short. This is the natural frequency of the system, and it should be less than 20-30 msec in order to resolve the details in the arterial pulse waveform.
- There should be at least one "bounce" oscillation. If the system does not oscillate, there is too much damping.
- There should be no more than two oscillations; a system which oscillates too much is underdamped.
- There should be a distinct dicrotic notch. The dicrotic notch is resolved from high-frequency waveforms, which are usually of low amplitude and therefore more susceptible to damping. If the arterial line is progressively becoming more and more damped, the dicrotic notch is the first feature to disappear.
The over-damped arterial line waveform
The over-damped trace will lose its dicrotic notch, and there won't be more than one oscillation.
This happens when there is clot in the catheter tip, or an air bubble in the tubing. The higher frequency components of the complex wave which forms the pulse are damped to the point where they no longer contribute to the shape of the pulse waveform.
The under-damped arterial line waveform
The under-damped trace will overestimate the systolic, and there will be many post-flush oscillations.
The MAP remains the same in spite of damping.