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.
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 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 trace will overestimate the systolic, and there will be many post-flush oscillations.
The MAP remains the same in spite of damping.