SCG is a low frequency sternum vibrations, recorded by accelerometers 11. Certain points on SCG morphology correspond to aortic valve opening and closure, and from the time locations of these points, it is possible to derive systolic time intervals of pre-ejection period and left ventricular ejection time 12. PPG sensor on the other hand, is an infrared sensor placed on the finger/forehead/ear to measure the pulse and certain features of the recorded PPGs have been corresponded to central hypovolemia 9.
An algorithm will be developed to extract features relevant to blood loss from every individual signal (PPG and SCG) and also measures pulse transit time (PTT) from the combination of the two signals. PTT is the time that it takes a pulse wave to travel between arterial sites or to get from the proximal side of the artery to its distal site. PTT is inversely proportional to blood pressure and being simple to measure, PTT is proposed as a method for continuous, cuff-less blood pressure estimation 13. In this research, SCG will be used as the proximal pulse and a finger PPG will be used as the distal pulse for calculating PTT. The aortic valve opening point (AO) on SCG is used as the starting moment of PTT. The idea behind estimating PTT using SCG is similar to the one discussed in14 which was further explored in another paper, showingits relationship with pulse pressure 15. Fig. 1 shows the placement of sensors on the body andalso a sample simultaneous recording of SCG and PPG that are used to calculate PTT.
In the algorithm, the PPG data is used as a reference to locate the AO point on SCG, which always occurs before the peak of PPG. The pulse transition time is then calculated from AO point to the foot of the up-going slope of PPG which is shown in Fig. 1 for couple of heartbeats. The first step towards PTT computation is to detect a peak point in the finger PPG signal. An algorithm was developed for this which was similar to the one proposed in 15. After detecting PPG peaks a backward window of 500 milli-seconds is used to detect the AO point in the SCG signal. PTT is computed as time difference between AO point, in the SCG signal, and a rise point, in the PPG signal. Rise point is detected by subtracting the slope value from the PPG peaks; it was found this value to be 140 ms on an average.