2.4. Manual Annotations of ICG, SCG, and PCG

Cardiac timings are traditionally defined relative to the onset of ECG QRS complex, which is considered as the starting point of cardiac contraction. However, for some subjects the start of QRS (Q-wave) was not easy to be detected. In these cases, we started with the neighboring R-wave and used the valley immediately located before the Q-wave.

Estimation of cardiac intervals from the ICG signal required annotation of the characteristic points of B and X, which are assumed to coincide with the opening and closing of the aortic valve. In this study, ICG B was annotated as the local minimum on the notch to the left of point C (Figure 1) and the X point was annotated as the time instant where the lowest ICG value occurred after point C (Carvalho et al., 2011). PEP_icg was measured as the interval from ECG Q to ICG B and TST_icg was obtained as the interval from ECG Q to ICG X. LVET was calculated as the timing interval between B and X points.

On PCG signals, the S1 and S2 sounds were annotated by the expert; EMD_pcg and TST_pcg were estimated as the interval from the ECG Q to the beginning of S1 and S2, respectively. The onset of the S1 sound was defined as the onset of the first peak after the ECG Q which had a height that exceeds the max amplitude of the preceding diastolic period. The onset of the S2 sound was defined as the onset of the first sharp negative wave in the S2 sound.

It was proposed that the SCG fiducial points of MC, MO, AC, and AO would coincide with mitral valve and aortic valve closing and opening, respectively. For the annotation of these points the traditional nomenclature proposed by Crow et al was considered (Crow et al., 1994). On this basis, PEP_scg was obtained from ECG Q to the SCG AO point and TST_scg was measured from ECG Q to the SCG AC point (Figure 1). EMD_scg and Q-MO_scg were measured as the intervals from ECG Q to the SCG MC and MO points, respectively. LVET_scg was measured as the time interval between SCG AO and SCG AC.