Optical Vibrocardiography: A Novel Tool for the Optical Monitoring of Cardiac Activity
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- Morbiducci, U., Scalise, L., De Melis, M. et al. Ann Biomed Eng (2007) 35: 45. doi:10.1007/s10439-006-9202-9
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We present an optical non-contact method for heart beat monitoring, based on the measurement of chest wall movements induced by the pumping action of the heart, which is eligible as a surrogate of electrocardiogram (ECG) in assessing both cardiac rate and heart rate variability (HRV). The method is based on the optical recording of the movements of the chest wall by means of laser Doppler interferometry.
To this aim, the ECG signal and the velocity of vibration of the chest wall, named optical vibrocardiography (VCG), were simultaneously recorded on 10 subjects. The time series built from the sequences of consecutive R waves (on ECG) and vibrocardiographic (VV) intervals were compared in terms of heart rate (HR). To evaluate the ability of VCG signals as quantitative marker of the autonomic activity, HRV descriptors were also calculated on both ECG and VCG time series. HR and HRV indices obtained from the proposed method agreed with the rate derived from ECG recordings (mean percent difference <3.1%). Our comparison concludes that optical VCG provides a reliable assessment of HR and HRV analysis, with no statistical differences in term of gender are present. Optical VCG appears promising as non-contact method to monitor the cardiac activity under specific conditions, e.g., in magnetic resonance environment, or to reduce exposure risks to workers subjected to hazardous conditions. The technique may be used also to monitor subjects, e.g., severely burned, for which contact with the skin needs to be minimized.
KeywordsCardiac displacementHeart rate variabilityLaser Doppler vibrometryElectrocardiography
A great interest has grown during the years on displacement cardiography as enabling methodology for the examination of the cardiovascular dynamics, with several methods proposed for the assessment of cardiac rate. In the past, techniques such as balistocardiography28 and kinetocardiography38 have been proposed, but they have not been used widely. In the 1990s, the seismocardiography has been presented as a novel non-invasive technique (although it is a contact procedure) for recording and analyzing cardiac vibratory activity, 32,33 eligible to be partially or fully alternative to the “gold standard” technique (ECG) for HR assessment. Recently, Augousti and colleagues4 developed a new fiber optic plethysmographic method for monitoring cardiac activity based on the measurement of changes in the torso shape induced by the pumping action of the heart, demonstrating the interest in the development of methods alternative to ECG.
In this optics, wide potentiality could be offered by laser-based techniques that, making use of specific optical sensors, allow non-contact measurements of vibrations to be performed. In the last years, Laser Doppler Vibrometry39 (LDV) has been applied in biomedical areas,6,20 due to its advantageous metrological characteristics. In particular, the high accuracy (about 1% of reading), high resolution (displacement resolution about 8 nm) and the non-contact nature of this technique make these instruments suitable for diagnostic purposes and in vivo tests.20, 30,34
In virtue of the above mentioned facilities, LDV could be an interesting alternative investigational methodology to be tested in the study of cardiac function, the monitoring of heart rate being a typical task in clinical environment for a great variety of patient’s condition. This could be done measuring the movements transmitted to the chest wall by the compression waves generated by the beating heart during its pumping function.
ECG is the most widely used modern method for the monitoring of the cardiac activity (it can be considered the gold standard technique). Careful interpretation of the ECG can reveal not simply the cardiac rate, but subtle and reliable information concerning the condition and functioning of the subject’s heart, by means of HRV analysis. In fact, HRV represents a quantitative marker of autonomic activity, and a powerful tool in the recognition of the relationship between the autonomic nervous system and cardiovascular mortality.2,3,26,36
The monitoring of the ECG requires contact between skin and electrodes, and sensors and cables to be connected. The possibility to obtain information on HR and HRV without contact with the patient represents a future tool in many fields, reaching the goal of a risk reduction: for example preventing secondary exposure of medical personnel to toxic materials under biochemical hazard conditions,23,24 or avoiding potential hazard during magnetic resonance (MR) imaging examinations of monitored patients.14, 15,18. In general, the task of the monitoring of the cardiac activity could not be accomplished when patient’s condition determine impairment of their health if contact with skin or any other surface has to be avoided (e.g. severely burnt subjects31).
In all these situations a method alternative to the typical continuous electrocardiographic recording is desirable to monitor the cardiac rate. In this optics, non-contact devices that could allow from a distance (tens of meters) measurement in general population look to be very useful.
To this aim, we remind that during ventricular contraction, the heart undergoes changes in volume as well as variations in position. The resulting combination of motions is transmitted to the surface of the skin and could be picked up by a laser displacement sensor pointing at a point on the thorax, near the heart.
In this study we investigate the potential of a LDV based method to perform both HR and HRV analysis as the ECG, from measurements of the motion of the external surface of human body (chest wall). In contrast to ECG, the proposed approach can be carried out without electrodes and with the sole laser equipment.
Analysis of HRV was performed considering the time intervals between consecutive fiducial points on ECG and VCG synchronously recorded traces, according to the methods recommended by the task force.37 Time and frequency domain measures of HRV were calculated on both ECG and VCG derived time series, and their equivalence was evaluated.
The movements of the chest wall were measured using a single-point laser Doppler vibrometric system. The LDV technique may accurately measure point-by-point surface velocities using interferometric techniques.39
Experimental Set up
The LDV system performs measurements of velocity, i.e. the ones carried out in the present investigation, with a resolution up to 0.5 μm/s. The laser head was placed at about 1.5 m from the subject chest wall. In order to simplify the measurement procedure and in order to optimize the quality of the signal (increase of the S/N ratio of the vibratory signal) a small (about 2 mm2, weight <1 g) adhesive retro-reflective tape was placed on the chest wall.
Laser power is less than 1 mW, so that no special safety measures are required, but nevertheless also with such low power levels working distances of some tens of meters are possible. On each individual investigated (as explained in the following) ECG and VCG traces were simultaneously recorded. The ECG (obtained from the II-lead output) is connected as in the classical configuration for the recording of the three fundamental leads. An analog-to-digital 12-bit acquisition board with anti-aliasing filters, together with a custom-made software program developed in a LabVIEW® environment (National Instruments, USA), have been used to store the signals. The analog inputs were sampled at 1 kHz. A PC (Pentium IV) was used both for setting of the A/D acquisition board and for providing the storage and processing of the experimental data.
From the recordings on individuals undergoing measurements, we investigated the physiologic relationship of the ECG to the vibratory signal VCG in terms of heart rate variations. To do this, we applied methods proposed by the guidelines on international standards of heart rate variability.17
The measurement method is based on the assumption that the peak of the vibratory signal, which measures chest wall motion, takes place in consequence of the cardiac muscle contraction triggered by the electrical signal measurable by the use of the electrocardiograph. ECG and VCG recordings were filtered with an eighth-order Butterworth low pass filter with a cut-off frequency of 100 Hz. We used heartbeat fiducial timing point provided by ECG recordings, i.e., the time of occurrence of the major local extremum of a QRS-complex (the time of the R-wave maximum). As fiducial point in the VCG signal, we selected the first local maximum value (labeled V peak) in the vibratory trace that follows the R-wave maximum in the ECG trace (the rationale is that VCG signal is responsive to changes in myocardial contraction induced by electrical activation). The results of automatic labeling of R and V fiducial points were reviewed and manually edited for error correction.27
From synchronous recordings, ECG derived RR, and VCG derived VV, interval time series were generated. Before this, ECG signals were filtered with two median filters to remove the baseline wander: in particular, each ECG signal was firstly processed with a median filter of 200-ms width to remove QRS complexes and P-waves, the resulting signal was then processed with a median filter of 600 ms width to remove T-waves. The signal outcoming the second filter operation contained the baseline of the ECG signal, which was then subtracted from the original signal to produce the baseline corrected ECG signal.9
On the calculated RR and VV time series HRV descriptors were calculated, for linear and non-linear quantitative description, both in the time and the frequency domain.
The agreement between ECG and VCG methods measures was evaluated from two aspects: (i) agreement when these techniques are used for cardiac rate monitoring, assessing intra-individual variations on RR and VV time series; (ii) agreement when on the RR and VV time series HRV indices are calculated. To examine the former agreement, 10 beats-to-10 beats VCG rate (VV) and heart rate (RR) for 5 min segments were compared within each subject, and the agreement between corresponding values were evaluated with the Bland and Altman test5: for the cardiac rate measured with the two methods, the difference against their mean value is plotted. This approach allows to evaluate if two methods for clinical measurement are interchangeable.
Statistical analysis was performed by means of nonparametric test due to the limited number of individuals, which prevented analysis of normality: differences between ECG and VCG derived indices (for each HRV index, values extracted by ECG or VCG from all individuals represent an independent sample containing mutually independent observations, i.e., the individuals themselves) was checked by using the Kruskal–Wallis one-way analysis of variance nonparametric test. Significant level was set at p < 0.05.
Nonlinear Dynamics Analysis
It is known that the normal heart rate is not regular, but varies from beat to beat in an irregular manner.10 Among many tools for the studies of nonlinear dynamics of heart rate, the Poincaré plot of interval time series X deserves special attention: it is a technique portraying the nature of interval fluctuations in a time series, consisting in a scatter-graph built up plotting samples Xi+1 vs. Xi. Poincaré plot analysis is a powerful tool in the investigation of HRV,13 with the shape of the plot categorizable into functional classes that indicate the degree of the heart failure in a subject.19,41 In normal subjects RR intervals Poincaré plot typically appears as an elongated cloud of points oriented along the line-of-identity. For short time recordings, the dispersion of points perpendicular to the line-of-identity reflects the fast beat-to-beat variability in the data.40
Figure 5 also shows the most significant differences in VCG traces due to gender: females exhibit H peak values always higher than the peak labeled V, the opposite being in males recordings.
Noticeable, great similarity in the morphology of the recorded VCG signals can be observed with another method for displacement cardiography, i.e., the contact procedure named seismocardiography.33
Results of the Bland–Altman test in terms of bias and of the Pearson’s product moment correlation, R, which measures the strength of linear relationship between the two sets of data.
0.0118 ± 0.118
0.003 ± 0.040
0.001 ± 0.087
0.010 ± 0.053
−0.005 ± 0.084
Mean valuea (M)
−0.033 ± 0.162
0.000 ± 0.044
−0.000 ± 0.046
0.001 ± 0.041
0.041 ± 0.149
Mean valuea (F)
Mean values of RR and VV time series, for all monitored subjects.
RR mean (ms)
VV Mean (ms)
MEAN % difference
SDNN RR (ms)
SDNN VV (ms)
SDNN % difference
RMSSD % difference
CV % difference
Mean valuea (M)
Mean valuea (F)
Spectral indices of HRV calculated on RR and VV time series. The table summarizes the values of Total spectral power, Shannon channel entropy E, and LF/HF spectral powers ratio calculated on all the monitored subjects, For each quantity, the mean value is presented, together with percent differences . (F = female; M = male)
LF/HF % difference
Total spectral power (SP) RR
Total spectral power (SP) VV
Total spectral power (SP) % difference
Spectral entropy (E) RR
Spectral entropy (E) VV
Spectral entropy (E) % difference
Mean valuea (M)
Mean valuea (F)
Fast variability index SD1 calculated on Poincaré plots from ECG and VCG derived time series, for all the monitored subjects (labelled from 1 to 5 for males, and 6 to 10 for females).
RR SD1 (ms)
VV SD1 (ms)
SD1 % difference
Mean valuea (M)
Mean valuea (F)
No statistical difference was found between ECG and VCG derived indices, over the 10 subjects investigated: MEAN, SDNN, RMSSD, SD1 (p < 0.88), CV (p < 0.82), LF/HF (p < 0.94), SP (p < 0.76), E (p < 0.94).
Difference in the results due to gender was also checked: no statistical significant difference was found between the mean values of the percent differences found for HRV descriptors relative to males and females (p < 0.172).
The herein shown results clearly demonstrate the relationship between the selected events, i.e. the relationship between the R peak on the ECG, and the V peak on the vibratory signal from the thorax. The calculation of HRV descriptors (both linear and non-linear, in the frequency and in the time domain) on the time series from synchronous VCG and ECG recordings confirm their equivalence: VCG may take the place of electrocardiographic HRV analysis without information loss.
Although measurement of RR intervals of ECG is the standard for HRV analysis, this method has practical limitations.21 Particular emphasis we wish to put in the limitations inherent to the use of standard ECG instrumentation for those cases in which electrodes and cables cannot be easily used. Moreover, the risk of interference with other biomedical instrumentation (such as defibrillators, electrical surgical units, magnetic resonance instrumentation) operating together with ECGs is always possible.
Due to the challenges and limitations in the employment of standard ECG in specific conditions, and being HRV a very powerful tool in the assessment of cardiovascular disease, several functional information could be achieved by non-contact cardiac monitoring, once its capability to investigate the simpato-vagal control from the neural structure of a patient is demonstrated. To overcome, fully or partially, the inherent limitations of electrocardiographic technique, analysis of cardiac rate from pulse wave signal was proposed as a potential surrogate of HRV analysis by ECG,7 in virtue of the wide use of pulse wave equipments both in hospital cares and clinical homecare practices. However, it was assessed that the smoothed morphology of pulse waves precludes accurate measurement of pulse-to-pulse intervals such as those used for measuring RR intervals in ECG recordings. Recently, Matsui and collegues23,24 proposed a non-contact method using a microwave radar to monitor the heart and respiratory rates of a healthy person placed inside an isolator or of experimental animals exposed to toxic materials.
Laser based vibration measurement could be of primary interest in this field. Due to the non-contact nature of the optical probe, laser techniques have a series of undoubted advantages, offering interesting perspectives of progress for vibration measurements in terms of innovative applications all the times a non-contact monitoring is the best or the needed choice.
In this paper, the authors propose a novel optical measurement procedure, based on the remote measurement of chest wall movement, to be used as a valid substitute of standard ECG in the assessment of the cardiac rate and in the analysis of heart rate variability, both in daily life and in “polluted environments”. An advantage of this approach is that it could provide a non-contact method to measure the compression waves that, generated by the heart during its movement excited by electric depolarization waves, are transmitted to the chest wall. The herein presented study moves in the same direction of non-invasive/non-contact methods recently proposed 4,11,23,24 for monitoring the cardiac activity.
Is Optical VCG Suitable for HR and HRV Analysis?
In clinical comparison between ECG and optical VCG techniques for the assessment of the vital sign monitoring (the actual cardiac rate), it is necessary to see whether they agree sufficiently for the new to be equivalent to the old: the results of the test of Bland–Altman put in evidence that significant differences are not present, from a clinical viewpoint.
In this study it was also demonstrated that, at present, optical VCG is suitable for HRV analysis. To do this, time- and frequency-domain measures of HRV were calculated as recommended by the Task Force of the European Society of Cardiology,21 and compared on ECG and optical VCG simultaneous recordings, as by Migliaro et al. 25
It is opinion of the authors that a comparison with basic ECG recording or RR intervals time series only is not sufficient to evaluate the sensitivity of optical VCG to perform HRV, which represents an added value for the proposed technique. In fact, in the analysis of method comparison data, neither the correlation coefficient between RR and VV time series nor techniques such as regression analysis are completely appropriate.5 In the same way, a comparison based only on mean values and standard deviations of the RR and VV time series is not sufficient to test the equivalence of the methods in terms of HRV analysis. Being this true, in general, for ECG based HRV analysis (where SDNN was found to be insensitive to specific physio-pathological conditions, i.e., two subjects with the same SDNN may have different clinic reference frames), we think that the same in deep analysis has to be performed when a new method for HRV has to be tested, even if this implies the calculation of several HRV descriptors, covering the quantification of both linear and non linear content, in the time and in the frequency domain. In fact, it may be possible that RR and VV time series have identical mean, standard deviation and ranges, but different autocorrelation functions and therefore different power spectra: this was clearly showed by Kaplan.16
The HRV measured using VCG agreed, with high coincidence, with the HRV derived from an ECG record. In fact, results showed mean percent differences of VCG derived descriptors, with respect to ECG ones, that do not threshold the 4.80% (3.03% mean value) for the LF/HF index, the 4.00% (1.48% mean value) for the fast variability index, the 4.00% (0.93% mean value) for the spectral entropy, the 2.55% (0.81% mean value) for the SDNN index, the 0.03% (0.01% mean value) for the mean value of the time series, the 2.60% for the CV (0.83% mean value), and the 5.53% for the RMSSD (1.93% mean value). The highest percent difference was found in subject 10, that exhibited a 5.88% difference in the spectral power values, but for this HRV descriptor a very low 1.95% difference mean value was found.
The similarity for the times series from ECG and VCG, is due to the capability of the laser based method to catch features of the cardiac displacement that are directly related to the electric activity recorded by ECG. This is clearly testified by the VCG and ECG synchronous recordings from which the time series VV and RR are extracted. Our analysis reveals small differences causing small variations in the indices calculated. However, these small differences in the indices can be considered to specify the same synoptic picture for each subject investigated: HRV analysis between normal and pathologic subjects shows sharp-cut differences in the quantitative indices.
No significant differences between ECG and VCG analysis were obtained, in terms of gender. Males and females showed similar very low percent differences in the values of HRV descriptors calculated by means of VCG and ECG, even if higher differences were found for females, with respect to males. This is probably due to the specific site on the thorax chosen in the present study: in general different local chest wall movements could be expected for females in regions of the chest walls where anatomic features differences between genders are generally pronounced.
These observations indicate that the VCG provides a reliable assessment of cardiac rate and HRV, suggesting that this methodology is potentially useful for assessing cardiovascular variability and dynamics.
Fields of Application of the Technique
Optical VCG seems to overcome the inherent limitations of previously proposed displacement cardiography methods for the examination of the cardiovascular dynamics, the main limits to the diffusion of such methods being the feasibility of the technique (kinetocardiography, balistocardiography), the low sensitivity of the device and the influence of the artefacts on the acquired signal (apexcardiography), the influence of environmental noise (ballistocardiography, apexcardiography), and the contact nature of the sensing technique (balistocardiography, kinetocardiography and seismocardiography). Therefore, a potential field of application for the proposed technique could be the ones were displacement cardiography methods were applied.
The procedure can easily be operated without applying electrodes, avoiding any contact between sensor and patient, therefore resulting suitable to monitor the cardiac activity of subjects under specific conditions.
For example, VCG could be used to monitor HRV without touching the patient, thus preventing secondary exposure of medical personnel to toxic materials under biochemical hazard conditions (e.g., toxic vapors and infectious bacteria). Alternative to the apparatus recently proposed by Matsui and collegues23,24 (a non-contact method using a microwave radar to monitor the heart and respiratory rates of healthy people), the technique appears promising for future pre-hospital monitoring of nerve gas victims, septic patients or in predicting multiple organ dysfunction syndrome patients, thus reducing the risk of secondary exposure in the case of large-scale disasters.
Moreover, being the optical sensor intrinsically insensitive to electromagnetic interferences, optical VCG could offer the benefit of immunity to this kind of noise, as well as avoiding the generation of spurious artefacts in an MR scanning environment, as suggested by Agousti et al.4
Concerning MRI, in general practice ECG monitoring is considered safe if the setup is MR compatible and the electrodes are fixed in a proper way. However, the literature on ECG monitoring during MRI contains little information other than the guideline to avoid loops in the leads and conductive loops formed by conductors with skin contact.14 Recently, Kugel et al.18 demonstrated that there is a potential hazard during examinations of patients with attached or implanted long conductors: high voltages can be induced in straight conductors without loops as ECG cables by coupling with the electric component of the HF field in the MR bore. Local heating or sparking can cause an open flame at the position of the electrodes, and this danger exists even with ECG equipment that is specifically marked as MR compatible.
Limitations in monitoring ECG in interventional and intraoperative magnetic resonance imaging are also pointed out by Kettenbach et al.,17 which state that is difficult to achieve true lead I, II, and III waveforms owing to the close placement of the electrodes that is necessitated by their use in the magnetic field.15 Furthermore, the magnetohydrodynamic effect of blood flowing through the heart creates an ECG artifact.
The optical VCG technique could be alternative to ECG in MRI practice, thus avoiding both the artefacts and risks mentioned above, that are primarily due to the presence of cables in the MR bore.
The technique may be used also to monitor serious thermal and all electrical burns, because the placement of ECG leads may be difficult when there are extensively burned areas. In fact, as ECG leads do not adhere to burned tissues, they must be placed in non-traditional areas (e.g., anywhere not burned, such as the back or legs), and if there are no unburned areas of the body, ECG leads can be placed in the customary positions and held in place with a wrap of gauze around the chest. Alternatively, in the emergency departments a more invasive technique such as stapling ECG leads in place, may be used, with appropriate systemic analgesics to be administered before this procedure31. Also in this case, the possibility to perform the non-contact monitoring of the cardiac activity could be relevant for patients health.
Limitations of the Study
A limitation to the application of the optical VCG may be caused by the physiological differences between VV and RR interval. Theoretically, the variability of the vibratory rate is the sum of the variability existing in RR interval and in the velocity of propagation of the mechanical pulse through the chest. Although the present study indicates usefulness of the VCG for assessing cardiac rate and good agreement in HRV analysis in short recordings at rest, its usefulness as a surrogate of ECG for assessing autonomic functions and mortality risk need to be examined more in deep.
Notwithstanding the use of adhesive retro-reflecting tape applied on the chest wall, we think that the dimension of the tape we used in our study (3 × 2 mm2, weight <1 g) does not represent a limitation in the application of the method. To confirm this, it was recently demonstrated35 that the effect of the skin surface on the vibratory signal is an amplitude reduction with respect to the optical VCG traces measured from an “optimal” surface, i.e. the retro-reflective tape applied on the skin. Moreover, optical VCG traces measured directly from skin are affected by drop-out, but a strategy for vibrometric signal filtering including the implementation of tracking filters allows drop-out effects avoidance without any loss of information.35
The main limitation to the application of the optical VCG, if performed using commercial laser Doppler vibrometry equipments, lies in the fact that it is an expensive technology. The authors choose to make use of a commercial laser Doppler vibrometer for their investigation, because of its very high sensitivity for vibratory velocity and displacement measurements at long operative distances (some meters). We considered this a conservative choice, the aim of the study being the feasibility of an optical, non-contact, from a distance HRV analysis, which needs signals with SNR the highest, to be performed. However, a posteriori considerations on the quality of the vibratory signal recorded from the chest wall35 allow us to affirm that lower cost optical sensors are available for optical VCG, with costs comparable to those of commercial ECG devices.
Further improvement for the proposed VCG application is in the interpretation of the vibratory signal in terms of cardiac mechanics, in order to correlate each morphologic feature in the VCG to specific events in the cardiac cycle for a wide range of physio-pathological conditions. In future, goals of the authors will be the evaluation of other optical sensors, and the evaluation of the hypothesis that the sole optical VCG signal could furnish the same informative content that is now obtained by three distinct physiological measurements, i.e., breathing, arterial pressure and electrical activity of the heart. In fact it could be thought that several functional information could be achieved from the optical signal we carried out from the vibrometer, and relevant patient’s condition related to the pathology could be obtained by means of biomechanical studies.
We proposed a new technique (VCG) capable to perform non-contact monitoring of the cardiac rate using an optical method. The purpose of this study was to evaluate the feasibility of the technique, and to test its sensitivity for HR monitoring and HVR analysis.
- cardiac rate assessed by VCG agreed with heart rate measured by ECG;
- HRV descriptors calculated on the VV time series, both in the time and frequency domain, show values equivalent to those given by HRV analysis on the RR time series.
VCG is fast and easy to perform and recordings are easily reproducible. VCG holds promise for being a useful and powerful tool for non-contact monitoring of cardiac activity of subjects under specific conditions. Clinical studies are currently in progress to evaluate the usefulness of VCG in cardiac resynchronization therapy. Finally, the authors think that using this novel non-contact application in fields such as MRI, or in “polluted environments”, a risk minimization can be reached both for patients and operators.
The authors wish to thank Professor Enrico Primo Tomasini and Dr. Giorgio Corbucci for the useful comments and the endless support to the research activity.