Reflectocardiography (RCG): A leadless, optical reflection based facile measurement of human cardiac rhythm

Presently, maximum clinical fatalities are caused by cardiac diseases.¹ Many researchers worldwide have been working toward the alleviation and early detection of cardiac malfunctioning.² However, when the heart starts malfunctioning, it becomes extremely important to detect the cardiac rhythms to monitor its activity for an individual’s well-being. Conventionally, the cardiac activity is monitored by electrocardiography (ECG), a process which requires electrical leads to be pasted to certain regions of the subject’s body.³ The involvement of leads increases the complexity of measuring the cardiac activity owing to multiple sources of error (in the case of improper electrical connections to and from the leads).⁴ To avoid such complications arising due to poor electrical connections in the ECG, the field was developed further to use sound waves to monitor the cardiac activities and this phenomenon is termed echocardiography or phonocardiography (PCG).⁵ Other advanced methods have been developed and incorporated into wearable devices for the ease in monitoring of cardiac activities of the test subject without affecting the lifestyle of the subject.⁶ These methods include the measurements of the dissolved oxygen concentration in the blood (SpO₂) measured by pulse oximetry⁷ and measurement of the pulse rate of the heartbeat by monitoring the reflections of IR light from the arteries/veins by monitoring the volume of blood flowing through them; this technique is termed photoplethysmography (PPG).⁸ However, with the advancements in technological fields, researchers have been able to monitor the mechanical vibrations on the surface of our body caused by the heartbeats. Techniques such as gyrocardiography (which is done by placing a gyroscope, a MEMS device, on the chest wall),^9,10 ballistocardiography (measurement of recoil forces of the body in reaction to cardiac ejection of blood into the vasculature),^11,12 seismocardiography (measurement of local chest vibrations due to heartbeat),^13,14 and forcecardiography¹⁵ fall under the purview of mechanical detection of cardiac activities. All the four listed techniques that utilize mechanical detection strategies employ sophisticated MEMS devices, such as accelerometers and gyroscopes. Monitoring cardiac activities and their performance would be severely hampered if there is a slight misalignment in these MEMS devices. The proposed invention overcomes this limitation of lack of functionality by having an optical readout mechanism that would use a light source, an optical detector, and a reflecting surface. It is a facile way to detect the cardiac activities by using a simple optical setup, thus bypassing the need of sophisticated MEMS devices.

Seven male human subjects of east-central Indian ethnicity (age group 20–55 years) were laid supine with the reflecting surface kept on the epigastric region of the abdominopelvic cavity. A black reflecting surface was used to detect the cardiac rhythm, as shown in Fig. 1. The seismic thumps on the body surface were detected by means of the deflection of the image of an object, and the observations were recorded using an open-source screen recorder, OBS Studio¹⁶ at 30 fps.

The employed light source can be a LED bulb or any sharply pointed object whose reflection can be tracked using a simple video recording device, such as a laptop/mobile camera, which acts as a photodetector in this work. The measurements were done in two conditions, with and without clothes on the abdominopelvic regions of the test subject. Video recordings were done using an open-source software program, OBS Studio at 30 fps. Each frame was extracted using another open-source software program, FFmpeg.¹⁷ The movement of the object in each frame was traced using an image processing software program, ImageJ,¹⁸ and the respective vertical coordinates were plotted against the passing time, thus generating a cardiograph, which can be further utilized for analysis of cardiac rhythms to ascertain the well-being of an individual.

The employed methodology and the reported protocol in this work (which is leadless and non-invasive and requires simple putting of the reflecting surface on the abdominopelvic cavity) are to measure the seismic vibrations experienced on the body surface due to the cardiac activities (therefore, can be considered as a variation of seismocardiography)¹⁹ and thus adhere to the guidelines as approved by the ethical committee of Northern Jutland (N-20120069). The measurements were recorded after obtaining informed consent of the seven test subjects. However, only Subject 01 consented for shirtless recording.

Summarily, the reported reflectocardiography (RCG) protocol can be conducted by any person able to handle a simple recording device (such as a mobile phone camera) in any setting. The specification table, mentioning the requirements to perform RCG measurements, is given in Table I.

This work discusses a unique strategy to monitor the cardiac activities. The RCG protocol reported in this paper has an advantage over the prior discussed mechanical vibration monitoring techniques due to the use of an optical setup, thus bypassing the need to design and fabricate complicated MEMS devices using costly setups. The optical setup demonstrated in the present invention includes a fixed object whose reflection is continuously monitored using a camera, and the mechanical vibrations on the body surface are detected by the movement of the reflecting surface via the said camera. Due to the said movements of the reflecting surface, the image of the fixed object would also move, which would give us a video-graphic trend of the heartbeats of the subject. The movement of the reflection (image) of the fixed object is further analyzed by an image processing software program to get the image coordinates. These coordinates are then plotted against time to get a dynamic response of the cardiac activity for Subject 01, showing trends analogous to the ECG (as shown in Figs. 2 and 3).

However, the baseline correction was done with the help of Origin 8.5^®, a graph plotting and data analysis software program, by going for the baseline correction mode after clicking the relevant tabs as per the following steps: Analysis → Peaks and Baseline → Peak Analyzer → Open Dialogue → Recalculate (Auto) → Goal (Subtract Baseline) → Next → Baseline Mode (Asymmetric Least Squares Smoothing) → Next → Asymmetric Factor (0.001), Threshold (0.05), Smoothing Factor (4), Number of Iterations (10) → Next → Subtract → Finish. After the baseline correction was done using the aforementioned steps, the modified RCG patterns obtained are shown in Fig. 3.

This subject showed normal cardiac functions, as the duration between subsequent peaks lie well within the normal range of 0.6–1.2 s.²⁰ Video clips of the recordings of all the seven subjects of the movement of the reflecting surface along with the generated RCG patterns are given as the supplementary material. The measurement principle of the proposed setup works on the identification of seismic thumps on the abdominal cavity by monitoring the movement of the reflection of a stationary object. When the said movement is traced with passing time, a cardiograph is generated. Since the proposed measurement protocol of cardiac rhythm uses the monitoring of the reflection of a stationary object, we term this type of measurement as reflectocardiography (RCG). The vibrational noise that appears in the cardiograph can be reduced/nullified by applying suitable filters during the image processing part of data analysis. The presented data show around 15–25 heartbeats as recorded by the reported measurement protocol. The measurement data (indicated by RR intervals, signifying ventricular depolarization), as recorded by following the reported RCG protocol, are given in Table II.

This method is superior to the already existing methods employing the monitoring of seismic thumps over the body surface due to cardiac activities because it employs a very simple setup comprising only a reflecting surface and a video recording device in lieu of complicated MEMS devices (used in gyrocardiography, ballistocardiography, and seismocardiography). The reflection of any sharply pointed fixed object in the surrounding can be taken as a reference, thus increasing the scope of its application in extremely low-cost settings with a lack of sophisticated instruments. The only measurement parameter to be considered is the monitoring of the reflection of the fixed object by the video recording device. Training to operate such a setup is also not required, as it does not involve any complicated setup and can be operated by any person capable of handling the video recording devices. It can also discern the ventricular depolarization events and their intervals (marked by RR intervals²⁰), which has a direct impact in assessing the well-being of an individual suffering from either of the following ailments: bradycardia, tachycardia, ventricular fibrillation, atrial fibrillation, bigeminy, trigeminy, and premature ventricular contraction. As discussed earlier, the RCG protocol employs a leadless method to detect the cardiac activities with a capability to measure the same while the subject is wearing the clothes. This is a significant advantage over the ECG and can be suitable to be employed in special cases where sticking the electrodes to the skin is not feasible, e.g., burn patients. It is expected to support and, in special cases, even replace the conventional methods (e.g., ECG) to monitor cardiac activities. The baseline of the RCG protocol wandered due to the breathing activities of the test subjects; however, the said breathing activity did not interfere with the recording of seismic thumps over the body surface due to the cardiac activities. This baseline wandering can be corrected by employing suitable algorithms and filters that are regularly being used in biomedical instrumentation. The reported RCG technique has an inherent limitation that pertains to the stiffness of the abdominopelvic cavity. The said stiffness, which could be majorly due to some muscular distress as well as excess body fat, would impede the transmission of seismic thumps due to cardiac activity to the body surface. This would lead to a substantial decrease in the amplitude of the detected thumps. To overcome this limitation, another suitable anatomical location (e.g., jugular vein) can be chosen by the researchers and healthcare professionals to increase the efficacy of the reported RCG protocol. In this paper, we have reported a qualitative study to monitor cardiac activities by means of optical reflection-based detection of seismic thumps experienced over the body cavity and it has inherent advantages over the other reported methods owing to its extremely low-cost and ease of monitoring as highlighted in the specification table (Table I). However, quantitative estimations pertaining to the efficacy of the RCG method over ECG, seismocardiography, gyrocardiography, and ballistocardiography are beyond the scope of the current work and may be taken up by fellow researchers and scientists working in the field of healthcare applications.

In this work, we report a novel way to detect the cardiac rhythms in the human body using a very simple and cost-effective method by placing a reflective surface on the abdominopelvic cavity and track the seismic movements of the reflective surface by monitoring the reflection of an otherwise stationary object. The RCG protocol is significantly improved over the already known methods, such as gyrocardiography, ballistocardiography, and seismocardiography, that employ the monitoring of seismic movements on the body surface from the pumping of blood. Such techniques employ the micrometer-scale mechanical devices, MEMS, that monitor the body movements and transduce the physical movements to a digital readout to record the cardiac activities. Our proposed method does not require any such MEMS devices to monitor the cardiac activity, rather it simply relies on the tracking of the image of a stationary object/light source and does not require any sophisticated handling of the setup and is a big step toward the development of point-of-care cardiac activity monitoring devices. This work would be of immense interest to healthcare professionals, biomedical engineers, and scientists working toward the goal of a sustainable healthcare infrastructure development in low-income and middle-income countries worldwide.

Video recordings and extracted RCG patterns of seven subjects have been incorporated in the associated supplementary material of this manuscript.

A.K. acknowledges Dr. R. R. Bhattacharjee, HoD AINTK, for his valuable input during this work.

The authors have no conflicts of interest to disclose.

The employed methodology and the reported protocol in this work (which is leadless and non-invasive and requires simple placement of the reflecting surface on the abdominopelvic cavity) are to measure the seismic vibrations experienced on the body surface due to the cardiac activities and thus adhere to the guidelines approved by the ethical committee of Northern Jutland (Approval number: N-20120069 reported in Ref. 19). The measurements were recorded after obtaining informed consent from the test subjects.

The data that support the findings of this study are available from the corresponding author upon reasonable request.