Carrying out the experiment
Connect the output of the amplifier to the input of the interface and run PicoLog set to record 500 samples at 20 ms intervals. Switch on and clip the peg to an earlobe or index finger; wait for about 10 seconds to allow the circuit to settle and then start recording. Adjust the potentiometer so that the output is about 2 Volts peak, the trace should then look like that in figure 4, with each pulse wave clearly visible. This was acquired using an ADC200 but with its capability of 100Ms/s it is overkill for this circuit, which should work with DrDAQ, all of the PC oscilloscopes and all but the slowest voltage-input data loggers.
Two problems are common when using a pulse plethysmograph: movement will cause the trace to swing around wildly, so persuade the subject not to move as much and if the subject is very cold (pale, pinched looking) the circulation at the extremities may be reduced to the point where there is very little signal.
A useful feature of Picolog is that, if the alarm is set to trigger at about 1 Volt, there will be a beep in time with the heart beat. Having proved that everything is working, the device can be used for such investigations as comparing individual resting heart rates, following heart rate changes during and after exercise and looking at the changes in heart rate that occur as the subject breaths in and out. For example, figure 5 shows 100 seconds of deep, slow breathing. Two effects, synchronised with ventilation, are visible: variations in heart rate can be seen as changes in the spacing between the pulse waves and variations in the stroke volume of the heart can be seen as changes in the amplitudes of the pulse waves.
Calculating Heart Rate
Unlike many heart rate monitors, this set up will allow the calculation of beat–by–beat (instantaneous) heart rate. Using the cursor, note the times of two adjacent peaks, subtract the first from the second to give the time between the two beats (beat to beat interval) and express this in seconds. Divide this into 60 to obtain the instantaneous heart rate in beats per minute. Thus, for the trace in figure 4, the first peak is at 1052 ms and the second is at 2190 ms. The difference is 1138 ms or 1.138 s, which gives a rate of 52.7 beats per minute. The last peak is at 9206 ms, or 8.154 s after the first and as there are seven intervals between, so the average interval is 1.165 s and the average rate is 51.5 beats per minute.