time required for blood propagation through the patient 39 s vasculature for example from the left ventricle to the left index finger this temporal delay combined with delays associated with internal processing of the oximeter itself offsets the plethysmogram and the digital trigger from the true motion of the heart the pulse oximeter can be calibrated to predict heart motion by monitoring a patient 39 s heart rate simultaneously using an ecg and a pulse oximeter calibration of the pulse oximeter will now be described with reference to fig2 fig2 depicts a sample graph displaying the signal results for a patient monitored simultaneously using an ecg and a pulse oximeter in the ecg trace 200 the qt interval 210 is defined as systole the qrs complex 220 can be used to predict the start of systole the delay between the qrs complex 220 and various temporal landmarks in the plethysmogram 230 can be determined the delay between the qrs complex 220 and the digital trigger 240 can also be determined these delays have been found to be stable across both heart rate and age the delay between the ecg signal and the mechanical event of the start of systole must also be taken into account to determine the true delay between the start of systole and the digital trigger.