cardiotachometer

What Is a Cardiotachometer?

A cardiotachometer is a device that measures and displays heart rate in real time, typically showing the instantaneous rate in beats per minute derived from the time interval between successive heartbeats. In the context of polygraph testing, cardiotachometer functionality is incorporated into modern digital polygraph software, providing the examiner with a continuous readout of the examinee's heart rate alongside the raw cardiovascular waveform.

Measurement Principle

The cardiotachometer calculates instantaneous heart rate by measuring the interbeat interval (the time between consecutive pulse peaks or R-waves) and converting it to a rate. For example, if the interval between two consecutive heartbeats is 0.75 seconds, the instantaneous heart rate at that moment is 80 beats per minute (60 ÷ 0.75). By plotting these instantaneous values continuously, the cardiotachometer produces a heart rate trend line that reveals rate changes over time.

Value in Polygraph Testing

Heart rate changes are one of the cardiovascular features evaluated in polygraph scoring. Sympathetic arousal associated with deception typically increases heart rate, while parasympathetic activation may decrease it. The cardiotachometer display allows the examiner to quickly identify heart rate changes associated with specific questions without needing to manually count pulse peaks in the raw waveform.

Integration with Modern Instruments

Contemporary polygraph software from manufacturers such as Lafayette, Limestone, and Stoelting typically includes cardiotachometer functionality as a derived channel — an additional tracing computed from the raw cardiograph data. This derived channel supplements the raw cardiovascular waveform and can be displayed alongside the standard three-channel recording. Computerised scoring algorithms often incorporate heart rate features extracted from the same interbeat interval data.