Table 4.

Sixteen features derived from the first and second derivative.

Feature	Definition
42. a1	The first maximum peak from the first derivative of the PPG waveform
43. ta1	The time interval from the foot of the PPG waveform to the time at which a1 occurred
44. a2	The first maximum peak from the second derivative of the PPG waveform after a1
45. ta2	The time interval from the foot of the PPG waveform to the time at which a2 occurred
46. b1	The first minimum peak from the first derivative of the PPG waveform after a1
47. tb1	The time interval from the foot of the PPG waveform to the time at which b1 occurred
48. b2	The first minimum peak from the second derivative of the PPG waveform after a2
49. tb2	The time interval from the foot of the PPG waveform to the time at which b2 occurred
50. b2/a2	The ratio of b2 to a2
51. b1/a1	The ratio of first minimum peak of the first derivative after a1 to first maximum peak of the first derivative
52. ta1/tpp	The ratio of ta1 to the peak-to-peak interval of the PPG waveform
53. tb1/tpp	The ratio of tb1 to the peak-to-peak interval of the PPG waveform
54. tb2/tpp	The ratio of tb2 to the peak-to-peak interval of the PPG waveform
55. ta2/tpp	The ratio of ta2 to the peak-to-peak interval of the PPG waveform
56. (ta1–ta2)/tpp	The ratio of the difference between ta1 and ta2 to the peak-to-peak interval of the PPG waveform
57. (tb1–tb2)/tpp	The ratio of the difference between tb1 and tb2 to the peak-to-peak interval of the PPG waveform