CARDIOTACHOMETER

Typical Number in Hospital:

Cost Bands:

References: 9

This is a device for determining heart rate. The input signal is usually the ECG; however, the arterial blood pressure wave, pulse, heart sounds or ultrasonic doppler blood flow signals may be used. There are two basic types of cardiotachometer, the averaging types which calculate the rate over a known period of time, and the beat-to-beat types which calculate the rate from the interval between each successive beat.

The averaging cardiotachometer converts the input signal to an electronic pulse corresponding to each heart beat, and these charge up a capacitor in parallel with a leak resistor. A constant rate input will cause the capacitor to charge to a steady voltage which can be applied to an indicating instrument showing heart rate. When the rate changes, the charge on the capacitor will gradually adjust to a different level. The disadvantage of this type of instrument is the insensitivity to short-term changes in rate and the long time it takes to achieve a steady state. Its advantage is its simplicity.

Beat-to-beat ratemeters work by measuring the time between each beat and then calculating the reciprocal of this time to indicate the rate. These are useful in demonstrating the erratic behaviour of the heart beat or the presence of extra or missed beats. Many cardioscopes include a cardiotachometer, presenting the rate on a meter, or as a numeric display on the cardioscope screen. Beat-to-beat ratemeters are commonly found in foetal heart monitors where changes in rate are compared with the occurrence of uterine contractions. The delays in displaying an accurate figure associated with the averaging type of ratemeter would be unacceptable in this application since the exact timing of the changes in rate must be displayed alongside the uterine contraction record.

Cardiotachometers often include alarm circuits to indicate when the rate has exceeded or gone below preset levels. Beat-to -beat ratemeters may indicate very large short-term changes in heart rate due either to extra or missed beats, or due to artefacts. Special circuits can be included to identify these three possibilities and count the number of extra beats, or suppress the rate display during artefacts.