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Sources of error (damping and resonance)

Created: 5/10/2004


Damping is caused by dissipation of stored energy. Anything which takes energy out of the system results in a progressive diminution of amplitude of oscillations. Increased damping lowers the systolic pressure and elevates the diastolic pressure. Mean arterial pressure is unaltered. Damping can result from air bubbles, blood clots, soft diaphragm or soft tubing.

Resonance occurs when the driving force frequency coincides with the resonant frequency of the system. This may occur if the tube or diaphragm is too stiff or non-compliant. The resonant frequency (or natural frequency) is the frequency at which the monitoring system itself resonates and amplifies the signal. It should be at least 10 times the fundamental frequency. If the natural frequency is less than 40 Hz, it falls within the range of the blood pressure and a sine wave will be superimposed on the blood pressure wave. The fundamental frequency of this system is the heart rate. This is also known as the first harmonic. The first 10 harmonics contribute to the waveform seen.

The transducer should be at the level of the right atrium. Raising or lowering the transducer results in errors.

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