4.2. Measurements
            4.2.4. Manometers
                4.2.4.1. Blood pressure measurement
4.2.4.1.2. Measurement accuracy

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

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 (Fn)

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

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

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

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)

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

Elastic catheter walls

Clotting in the arterial cannula

Keywords:  Damping coefficient ; Dynamic accuracy ; Harmonics ; Natural frequency ; Noise ; Resonant frequency ; Static accuracy
Bibliography
About
Created20050601
Reviewed20050602


Table of contents  | Bibliography  | Index