The Digital Ballistocardiograph

Geoffrey Houlton

Cardiovascular (heart) disease is the cause of more deaths in the western world than all other forms of disease. In the coming years this problem is predicted to escalate as the majority of the ‘baby boomer’ generation will be well into retirement. As a result, technologies that improve the assessment of or aid in the detection of heart disease will become increasingly important in patient monitoring and overall care. Heart Force Medical Inc. (HFM) has applied recent advances in hardware and software technologies, specifically tri-axial accelerometers, to capture non-invasively the low-frequency vibrations created by cardiac contractions. HFM is focused on re-establishing the medical science of ballistocardiography (BCG) using accelerometer technology found in products such as the Nintendo Wii and the Apple iPhone. BCG will assist physicians in their assessment of a patient’s cardiac performance, particularly related to timing events in the cardiac cycle. HFM’s product is a non-invasive medical device that assesses cardiac function by sensing forces generated by heart muscle contractions from a single place on the chest. The instrument uses a proprietary finely tuned sensor placed on the patient’s lower sternum to capture the digital ballistocardiograph (dBG™). The dBG is then transmitted via Bluetooth™ to a computer for analysis.

HFM’s technology presents a significant opportunity for physicians to improve the assessment of patients with suspected heart disease. Currently, devices such as electrocardiogram (ECG) monitors and echocardiograms are used in the identification and assessment of heart disease. The ECG, a relatively quick test, is limited to an electrical assessment of the heart and does not provide any information about the force of contraction. An echocardiogram provides an image of sections of the heart but requires an ultrasound technician, and is expensive and time-consuming. By contrast, the dBG assessment is simple and straightforward: analysis of the waveforms offers timing of cardiac events, force of contraction, and electrical activity and is completed in five minutes. Ultimately, HFM believes it could be used in any doctor’s office as part of a routine cardiac assessment. A pre-commercial dBG device has been tested on a volunteer population. Its capabilities have been demonstrated favorably in comparison with the echocardiogram. HFM has solved many complex issues related to hardware design, software development, signal capture, and analysis of the complex cardiac waveforms. HFM’s innovative technologies have resulted in numerous patent filings with the US Patent and Trademark Office (USPTO) as well as European and Asian filings through the Patent Cooperation Treaty (PCT) process. These patent applications provide considerable barriers to entry from potential competitors.

HFM estimates the dBG will have a significant impact not only on the assessment of cardiac diseases but also on healthcare costs. This impact will be related to patient triage, and potentially, as it is a tool for patient assessments, benefits will also be seen when the results are compared with more expensive diagnostic methods.

History of the Digital Ballistocardiograph

The science of BCG was conceived over a century ago and was studied extensively until the 1960s. Originally, ballistocardiograph devices recorded movement caused by the percussive effects of the heart while the patient lay supine on a bed. The bed contained either an apparatus that would allow for the measurement of the motion generated by the heart’s percussive forces or a facilitating apparatus that was attached across the shin area of the legs.

HFM derived considerable encouragement and guidance in the development of the dBG from reviewing the classic BCG publications from the cardiology literature of the 1950s and 1960s. These references include:

essays defining the physiological significance of BCG;¹
interpretation and standardization of nomenclature of the BCG waveforms;² and
an understanding of the physics involved in the process.³

Enthusiasm for the development of a clinically applicable dBG came from papers such as Isaac Starr’s study of the effects of nitroglycerine on BCG in patients with coronary artery disease.⁴ Later publications describing attempts to develop improved ballistocardiographic recording devices indicated a broad interest in the cardiology community.⁵ Extensive work was completed between 1930 and 1960 by Isaac Starr using ballistocardiograph data to classify BCG waveforms as normal, slightly abnormal, markedly abnormal, and extremely abnormal.⁶ Further research was undertaken to categorize BCG waveforms with patients who had known clinical conditions such as hypertension, angina pectoris, congestive failure, and rheumatic heart disease.⁷ Eventually, the science was accepted by the American Medical Association (AMA). However, the ballistocardiograph device was large and cumbersome, an example is shown in Figure 1. BCG was ultimately abandoned in the 1970s as new and emerging technologies including ultrasound and angiography eclipsed BCG technology. HFM is applying modern accelerometer-based technology to revitalize the science of BCG.

Where the Digital Ballistocardiograph Fits in Cardiology

The dBG represents a significant improvement compared with the ballistocardiograph technology that was offered to cardiologists in the 1950s and again in the 1990s. It will be used as part of the physician’s overall assessment of patients, either in a busy office practice or in the hospital. It provides information immediately to the physician during the patient’s visit. Physicians already have a range of relatively simple instruments to assess patients; the dBG complements these and accurately provides timing of the events of the cardiac cycle without having to refer the patient for an echocardiogram. The information is immediately available and the patient is freed from having to return for another visit to see the physician. The dBG offers physicians a simple and quick means of assessing patients safely and non-invasively at every assessment. HFM believes that repeat assessments will reveal the relative changes in the heart’s performance. As a result the dBG should eventually be used to assess the capability and the relative efficiency of the heart to pump blood at every patient assessment.

Technology

Ballistocardiogram

The dBG uses modern digital technology to capture, process, analyze, and render a recording of the motion of the patient’s lower sternum resulting from the movement of the heart during each cardiac cycle (heart beat). This movement is sensed by a three-axis accelerometer aligned to the three principal anatomical axes and is processed digitally and displayed with the acceleration amplitudes as the vertical axis and time as the horizontal axis. Measurement of acceleration is expressed in milli-gravity (mG) units and time is recorded in milliseconds (ms). A single ECG is sensed, recorded, and displayed synchronously with the dBG. All data are displayed on a computer screen and proprietary software provides tools for annotation and analysis of the waveforms.

The Device

The dBG consists of three main components: the sensor, the digitizing transceiver unit, and the proprietary software application used for device control and data analysis. The sensor captures both the seismic vibrations generated by the heart motion and a recording of the heart’s electrical activity. The vibrations, or forces, are detected using high-sensitivity, calibrated, tri-axial accelerometer-based technology.

The heart’s electrical impulse information is gathered using a rhythm strip configuration, similar to a single-lead ECG (lead 1). The forces are captured in three waveforms, one for each axis. Each waveform represents the accelerations experienced by the sensor from forces created during the cardiac cycle. These three waveforms combined are the essence of the dBG. The dBG is a representation of the forces of the heart, measured as milli-gravity over time in milliseconds. The transceiver unit is responsible for digitizing the captured data and transmitting the data via a secure Bluetooth connection reliably to the application software hosted on a PC (see Figure 2). The controlling software application has all the features and functions required to communicate with and control the device. Once communication between the device transceiver unit and the PC is established, the software can be used to stream continuous data from the device, record the data streams, and manually analyze recorded data.

Capture

Data capture with the dBG is simple and straightforward: the subject lies supine and the sensor is attached to the test patient’s sternum (see Figure 3), with ECG electrodes providing the required electrical contact as well as anchoring the sensor. When the sensor is in place, the software application establishes communication with the dBG device. Once communication is established, data can be streamed or recorded. A rendering of the ECG (upper) and dBG (lower) waveforms is presented in Figure 4. The dBG comprises the accelerations experienced by the sensor in three axes as the heart generates forces during each cardiac cycle. Data capture is simple and quick; typical data capture duration is between 10 and 30 seconds.

Analysis

The software application provides annotation tools that allow for identification of each event in the cardiac cycle (see Figure 5). The dBG waveform can be analyzed to show the timing of the events in the cardiac cycle, including:

aortic valve open (AVO);
aortic valve closed (AVC);
mitral valve open (MVO);
mitral valve closed (MVC);
early diastole;
late diastole;
P1/2T point (J); and
atrial contraction (AC).

These points, measured in each cardiac cycle, can then be used to compute:

rapid ejection period (REP);
isovolumic relaxation time (IVRT);
isovolumic contraction time (IVCT); and
pressure 1/2 time (P1/2T).

The marks and information are set and derived for every beat in the recorded cycle, allowing for complete analysis on a beat-to-beat basis. Investigations are continuing to provide additional data points.

What Distinguishes the Digital Ballistocardiograph from Other Cardiac Analysis Technology?

The dBG is an immediate, sensitive, and specific test of the contractility of the heart. It is a recording of the seismic forces generated by a series of continuous heartbeats over a period of time. Each individual heartbeat can be identified and analyzed independently or as part of the complete sequence of cardiac cycles captured during the assessment. Other cardiac assessment devices, such as echocardiograms and magnetic resonance imaging (MRI) are expensive, the assessments are lengthy (generally 30 minutes or longer), and they require specialized technicians. The dBG is portable and easy to use, and requires limited training to operate in a busy clinical setting.

Study Data

The following studies were conducted to show the precision and capacity of dBG to measure the timing events in the cardiac cycle. The first study compared the durations of the timing events in the cardiac cycle using the dBG with a standard echocardiogram. This included comparison of the measured timing events and duration of the cardiac events using both instruments. The assessments comprised time from the onset of the ‘Q’ wave of the ECG to aortic valve open; aortic valve closed; mitral valve open; mitral valve closed; rapid ejection period; early diastole; and late diastole. Twenty-five subjects (14 male, 11 female; age 20–58 years) with no known cardiovascular disease were tested by both the dBG and an echocardiogram. The results showed limited percentage differences of the means between the echocardiogram and dBG. These ranged from 4.6 to 13.0%, which is considered within the variability of the biological signal and the changes in heart rate (see Table 1).

The second study tested the reproducibility of the measurements obtained using BCG and the dBG. Seven subjects (four male, three female; age 24–55 years) with no known cardiovascular disease were each tested on three consecutive days. Digital ballistocardiograms were recorded at the same time of the day and under similar experimental conditions. The results indicated that the variables measured with the dBG were reliably measured from day to day. Results are presented in Table 2.

Potential Clinical Applications

The dBG sensor captures the forces generated in each cardiac contraction by a tri-axial accelerometer in three mutually perpendicular planes. Whereas earlier instruments were capable of only a single axis, the dBG waveforms reflect the various events in the cardiac cycle from three anatomical planes. The dBG was initially used in the assessment of the fitness of athletes of various ages in different sports including basketball, Canadian football, baseball, and soccer. Subsequently, tests were performed on volunteers to assess the changes in cardiac timing, events, and performance. The potential of the dBG in clinical cardiological applications became obvious. As the device can provide the assessment of cardiac timing events in less time than the echocardiograph, whenever the timing events are a critical part of a patient’s assessment the dBG could be substituted for the more expensive and time-consuming echocardiogram.

Although the dBG does not provide an image of the heart, it provides equivalent and possibly superior measurements of timing for events in the cardiac cycle. It provides these timings more quickly and requires less technological support and analysis. As such it should be considered as a device to be used for rapid assessment of patients in advance of more time-consuming, technically challenging, and costly procedures.

In the coming months the dBG will be used in a range of clinical and non-clinical situations to assess its true medical capabilities for both clinicians and patients. The development team is concentrating on applications in cardiology to assess cardiac function in a range of clinical situations, including ischemia and heart failure. The timing of cardiac events is well understood and documented in both ischemia and heart failure.

Cardiac monitoring of patients who have been admitted to critical care beds provides an opportunity to assess the changing performance characteristics following the modification of cardiac therapies for these patients. These patients are regularly prescribed therapeutics to control cardiac parameters such as rate and pressure. Physicians need rapid and repeatable assessments of the impact of their prescribed treatments. The dBG could provide a rapid, simple, and objective assessment in this clinical application.

Gradually the device could migrate to the hospital emergency room for the rapid assessment of patients presenting with chest pain with no changes in the ECG (non-ST segment myocardial infarction [NSTEMI]). The diagnosis of true ischemic chest pain can be challenging for physicians and often results in patients waiting in limbo; physicians and patients must wait for biochemical tests that can take several hours to complete a definitive result. The dBG could potentially determine ‘true’ versus ‘false’ ischemia at the bedside simply and quickly, allowing patients to progress with their optimal treatment regimen. Potentially, the dBG could find a place in primary care physician office practices to assist them in the evaluation and triage of patients awaiting referral to a cardiologist.

The following represents a brief overview of the current and planned studies for the dBG in 2010:

Anthracycline-Induced Heart Failure in Oncology Patients;
Stress Testing in Patients with Ischemic Heart Disease;
Assessment of Cardiac Performance in Patients Managed in Critical Care;
Assessment of Chest Pain Presenting to Emergency Room Physician;
Assessment of Heart Failure Indices Before and Following Drug Therapy; and
Assessment of the dBG in Cardiac Resynchronization Therapy.

Potential Applications of Digital Ballistocardiograph Technology

Medical

Cardiology Practice

triage and prioritization of referrals for cardiac investigations;
monitoring response to cardiac interventions;
management of chronic heart failure;
evaluation and monitoring of cardiac valve procedures; and
optimization of cardiac resynchronization therapies.

Emergency Medicine Applications

diagnosis and triage of emergency chest pain patients; and
monitoring of trauma victims, prioritization of care in multiple motor vehicle accident (MVA) admissions.

Intensive Care, Critical Care Unit, and Aesthetic Applications

continuous monitoring of cardiac performance, ischemia, blood volume, overload;
congenital heart disease;
benchmark 3D diagnostic projections; and
monitoring of ventricular performance in chronic patient case load.

Transplantation

identification and quantification of acute and chronic rejection.

Pharmacology and Drug Response Individualization

congestive heart failure (CHF) clinic progress monitoring and drug response evaluation;
monitoring of therapies using cardiotoxic pharmaceutical agents; and
micromanagement of individual drug titrations.

Primary Care Practice

Risk stratification for cardiovascular disease.

Physiology, Kinesiology, Commercial Fitness Organizations, and Sports Medicine

As a tool for investigating cardiac physiology, effects of performance training and development of fitness training programs. Evaluation and assessment of candidates for training programs and fitness evaluation for commercial sports candidates. A quantifier of results of performance enhancement programs for fitness organizations.
As a tool for the management of post-concussion syndromes in athletes and chronic fatigue syndrome in employee groups.

Life and Health Insurance Industry, Human Resource Facilities

Group screening for pre-clinical cardiac disease.
Monitoring of return-to-work post-cardiac-intervention status.
Employee applicant screening.
Risk stratification of life and health insurance policy applicants.