EKG and Heart Rate are measured with our EKG device, which is often combined with Skin Conductance as in the PsychLab SC-EKG shown here. Standard dot snapper electrodes are used to pick up the heart wave-form. They may be positioned at any convenient location either side of the heart, including convenient locations on the right collar bone and left stomach, on the back or on the wrists. The further electrodes are from the heart, the greater the chance of muscle activity interposing on the EKG wave, however EKG is a very strong signal and reliable measurement is usually easy to obtain.
The PsychLab EKG device has an inbuilt IBI detector, however more reliable IBI is obtained by analyzing stored raw EKG wave off-line. Heart rate is directly inversely proportional to IBI. To extract Heart Rate (HR) from raw EKG, each R wave is detected using the PsychLab Wave algorithm.
PsychLab can reliably process the entire recording, or just required trial sections, fully automatically.
PsychLab analysis software allows Results Format Processing (RFP) using intelligent user script processing to work on data before it is expressed as result text. By loading a standard script designed for Heart Rate cleaning, which can easily be modified if required, the following process is performed to check each new detected R wave:
The average of 20 beats is obtained, and then each new beat is checked such that its interval must fall within a given percentage of the average. If the next R wave is outside this window, it is considered as artefact and the next R wave is used. The above example shows a noisy section of EKG, with resulting disruption caused to the green IBI trace produced by the hardware. Below that is a plot of IBI produced by off line R wave detection using the cleaning script, demonstrating how it can improve a poor recording without risking inventing data.
The RFP may also be configured to manipulate IBI data produced by applying the Wave algorithm for R wave detection to produce average heart rate over any given period, in blocks, trials, grand average, as required.
The RFP has also been configured to compare beats following a stimulus, such that tabulated result data can include such difference readout.
The ´User Process´facility in PsychLab software makes it possible to first obtain beat by beat information using the methods discussed above, and then re-arrange the information into one second intervals. The advantage of Second by Second IBI or Heart Rate is that for a given trial period, a standard number of fields will be produced, whereas with beat by beat IBI, the number of beats per given trial period will vary depending on HR.
The same way, data can be expressed in half second, quarter second or any other such desired format.
Using the R wave detection and data cleaning methods discussed above, high quality IBI data series may be exported directly into the wide range of free programs available for HRV processing, for example Kubios HRV, as shown left at http://kubios.uef.fi/