Studies & Publications
Echocardiography is a medical imaging technique which has set the benchmark for measuring cardiac time intervals. In particular, the PEP/LVET ratio extracted from the echocardiogram is of utmost importance in the diagnosis of heart failure (HF) patients . Two ranges of PEP/LVETs can be defined: (1) healthy and (2) HF patients. Ratio assessments near the extremes of the ranges can be used for diagnosis, but further testing is required for values within the overlapping ranges.
Research has shown the relationship between the extracted fiducial points of the seismocardiogram and the echocardiogram. Specific fiducial points of the SCG (AO and AC) occur in the proximity of the echocardiogram markers on the produced Doppler images or the M-mode.
Based on 43 healthy and 134 heart failure patients, where echocardiography was used to find PEP/LVET, recent studies showed:
These studies confirm the potential of STI evaluation to non-invasively and remotely monitor patients over an extended period.
Reproducibility of PEP/LVET
This part of the study was dedicated to identifying the fiducial points of the seismocardiogram signal which correspond with the echocardiogram in Doppler and M-mode. The reproducibility criteria was defined as follows: Seismocardiogram derived by PEP/LVET is reproducible if the fiducial point of the seismocardiogram correspond to the range of aortic valve opening and closing of the echocardiogram. In that case, the calculated PEP/LVET of the echocardiogram would be reproducible by the seismocardiogram.
One Minute Repeatability
Our main objective was to demonstrate that a calculated PEP/LVET ratio derived from a seismocardiogram does not change within a short period. Three consecutive measurements of 10 cardiac cycles, with a one minute interval in between, were used for comparison.
Minimum 24 Hours Repeatability
The objective of this part of the study was to demonstrate that the PEP/LVET of the healthy subjects did not change significantly if the recordings were conducted under the same conditions over a period longer than 24 hours. To accomplish this objective, two 1-minute measurements with at least a 24 hours interval between, were conducted for each subject. The recordings were taken at exactly the same time each day. The subjects were advised not to drink caffeinated beverages, alcohol or smoke cigarettes four hours prior to data recordings.
The recording was conducted on 23 healthy individuals (age: 25±5 years , height: 172±5 cm, weight:70±12 kg, and sex: 15 males and 8 females) . The echocardiography expert used both the Doppler and M-mode to find mitral and aortic valve opening and closing.
For the reproducibility of PEP/LVET, the seismocardiogram and echocardiogram images needed to synchronize. To accomplish this goal, two electrocardiogram signals were used and both recordings were stopped at the same time. The ECG R waves were used to find the interval associated with the last stop point in order to verify the correct synchronization. The echocardiography device captured images in the range of 50 to 100 frames per second, while the seismocardiogram signal was measured with a sampling frequency of 1KHz. As a result, comparison of the fiducial points for these two different technologies in millisecond accuracy was not possible. Moreover, the opening and closing of the valves are associated with an interval and hence associating a simple fiducial point for comparison is not feasible. To overcome these obstacles, the echocardiography expert detected two points: start and end of opening and closing of the valves of multiple synchronized cardiac cycles. The median of the start and end points were calculated to remove the outliers.
For the one minute repeatability study, three recordings with 1-minute wait intervals between them were conducted. For the minimum 24 hours repeatability, two measurements of 1-minute seismocardiogram recordings were conducted with at least 24 hours between them. The echocardiography expert also measured additional parameters while the subjects were in the supine position. These parameters were: ejection fraction, cardiac out and E/A ratio.
Summary of Results
Reproducibility of PEP/LVET
For the PEP measurements, it was shown that the median PEP recorded by considering the interval of ECG Q to the midpoint between SCG isovolumic moment (IM) and aortic valve opening (AO) lay within the interval defined from Q to the median of the start and end of aortic valve opening for all 23 individuals. Furthermore, it was demonstrated that the maximum difference between the median PEP/LVET derived from seismocardiography and echocardiography was 2.3%.
One Minute Repeatability
In three consecutive recordings one minute apart, it was demonstrated that the PEP/LVET was in the healthy range for all 22 individuals. The maximum change of the median of PEP/LVET between the recordings was 3%. In other words, the PEP/LVET parameter extracted from seismocardiogram is reproducible over short periods of time.
Minimum Twenty Four Hours Repeatability
For the 21 individual recordings, it was demonstrated that the PEP/LVET varied by a maximum of 5% for 19 individuals and by less than 10% for the next two candidates. Moreover, the ejection fractions for all 21 individuals were above 55%, indicating a normal range with less than 5-10% change for two days of recordings.
 Farzad Khosrow-khavar, Kouhyar Tavakolian, Carlo Menon, Automatic and Robust Segmentation of Seismocardiogram for Noninvasive Estimation of Cardiac Time Intervals, IEEE Transaction in Biomedical Engineering, Nov 2016.
 Kouhyar Tavakolian, Systolic Time Intervals and New Measurement Methods, submitted to Cardiovascular Engineering and Technology, Springer, Accepted for Publication March 2015.
 Farzad Khosrow-khavar, Kouhyar Tavakolian, Andrew P. Blaber, Reza, Fazel-Rezai, John M Zanetti and Carlo Menon Automatic Annotation of Seismocardiogram with Higher Frequency Precordial Accelerations IEEE Journal of Biomedical and Health Informatics, Vol. 19, No. 4, pp. 1428-1434, July 2015.
 Kouhyar Tavakolian, Geoff Houlton, Guy A. Dumont, Andrew P. Blaber, Precordial vibrations provide noninvasive detection of early-stage hemorrhage Journal of Shock, Lippincott Williams and Wilkins, Vol. 41, No. 2, pp. 91:96, Feb 2014.