Measurement accuracy

[Ref: KB Primary course May 2005]

Static vs dynamic accuracy

A. Static accuracy

... the reliability of the system to record stationary events

Has 2 qualities: stability and uniqueness.

1. Stability

• Stable base line (stability of zero point)
• Stable calibration factor (stability of gain)
• Time stability

2. Uniqueness

System will respond uniquely to any statically applied signal regardless of the way the signal is applied.

Hysteresis is an example of lack of uniqueness.

B. Dynamic accuracy

... the ability of the system to faithfully record rapidly changing events.

Affected by noise and dynamic response of the system

1. Noise

... describes the rapidly time varying signal that arises from various sources that are not directly related to the physiologic event being measured.

Confounding noise - noise that have a fundamental frequency equal to that of the physiological event being measured.
e.g. Swan Ganz catheter in RV

2. Dynamic response of the system

Dynamic response of a second order system (e.g. catheter transducer system) can be characterised by:
* elasticity
* mass
* friction

which are reflected in:

• natural frequency
  * tested using sine wave method
• damping coefficient
  * tested using square wave method

Natural frequency (Fn)

... the inherent frequency of any oscillating system at which the system would tend to oscillate if disturbed.

• Is affected by damping
  --> thus best to consider the undamped resonant/natural frequency
• When the input signal is at the natural frequency, the amplitude of response of the system will peak (resonate).

Damping coefficient (D)

... is calculated from the ratio of the amplitude of 2 successive peaks (of oscillation)

Optimal damping: D=0.64

Overshoot is 6-7% of the original deflection

• Amplitue distortion is minimised (<2% up to 2/3 of Fn)
• Optimal frequency response is secured (up to 2/3 of Fn)
• Phase distortion minimised
  * phase distortion is when harmonic components of pressure waves are not transmitted at an uniform velocity

Critical damping: D=1
--> damping is such that overshoot is just avoided.

• However, when Fn is very high, the dynamic accuracy may still be good even if damping coefficient is low.

Ideal system

Should record accurately up to 10th harmonics

Thus, required Fn
= 15 x fundamental frequency
= (Pulse rate/4) Hz
--> Maximal frequency response and minimal amplitude distortion for up to the 10th harmonics.

Should have D=0.64

Mean arterial blood pressure (MABP)

• MABP is affected by static accuracy alone
• MABP is the most important for determining perfusion pressure, except for LV myocardium

Measurement errors

3 types:

1. Static error
2. Dynamic errors
3. Errors due to physiologic reactance

Physiologic reactance

... the (usually undesirable) effects of the recording system have on the physiologic event.

e.g. presence of cathether causing vessel reaction, or disturbance of the blood flow.

Practical considerations

In catheter-transducer systems...

Bubbles

• decreased resonant frequency
• increased damping

Elastic catheter walls

• increase damping
• ?decreased resonant frequency

Clotting in the arterial cannula

• greatly increases damping