Cardiovascular diseases (CVDs) are a group of heart and blood vessel diseases such as myocardial infraction, commonly known as a heart attack, heart failure, rheumatic heart disease and pulmonary arterial disease1,2. According to World Health Organization (WHO), CVDs are the number one cause of death with estimated 17.9 million deaths around the world1. Heart failure (HF) is a critical CVD with an estimated 64.34 million cases around the world3. HF is a progressive clinical syndrome characterized by a structural abnormality of the heart, in which the heart is unable to pump sufficient blood to meet the body’s requirements. Due to this lack of blood supply, fluid accumulates in the lungs, which impedes oxygenation4,5,6. There are two types of HF: systolic HF with reduced ejection fraction (HFrEF) and diastolic HF with preserved ejection failure (HFpEF). Common causes of HFrEF include cardiomyopathy, heart muscle disease, untreated high blood pressure, faulty heart valves, and coronary artery disease. A common cause of HFpEF is left ventricular hypertrophy (LVH), a condition in which the left ventricle of the heart is thickened and the chamber is unable to sufficiently fill adequate cardiac output7,8,9. According to the Centers for Disease Control and Prevention (CDC), in 2018 there were 379,800 deaths, and 13.4% of total mortality in the US was due to HF2. Moreover, according to the American Heart Association, there are currently 6.2 million adults diagnosed with HF in the US, and this number is estimated to increase to 8 million by 203010. Current HF treatment includes guideline-directed medications and surgically implanted devices which can be very costly. According to the CDC, on average, $30.7 billion were expended on the treatment of HF across the US in 20122. This financial burden is due to the downward trajectory of HF which in later stages leads to repeated hospitalization. Due to this poor diagnosis, 17–45% of deaths occur within one year of initial hospitalization and 45–60% of deaths occur within five years11.
Continuous and real-time monitoring of HF symptoms can alert patients and providers of patient decompensation. Provider can then intervene with medications to avoid patient hospitalization. Fluid accumulation in the lungs is reflected by a decrease in thoracic impedance. Common symptoms of HF are related to fluid overload and include fatigue, weight gain, and feeling short of breath7,8. These symptoms can be monitored for the progression of HF. Currently there are two implantable devices to monitor HF symptoms:, an implantable cardioverter-defibrillator (ICD) and the CardioMEMS™ pulmonary artery monitor8,12,13,14.
An ICD is recommended for patients with HFrEF as they are more likely to have lethal cardiac arrhythmias. An ICD also measures thoracic impedance and can alert providers of the decrease of thoracic impedance, indicating more fluid in the lungs15. It is surgically implanted under the skin, and it detects lethal arrhythmias and restores normal heart rhythm with an electric shock. ICDs have an additional function as a pacemaker, to speed up a heart that is too slow16. An ICD requires an invasive surgical procedure for the initial implantation and whenever battery needs to be replaced, typically within 3–7 years17. There are risks to any surgery and the procedure is also costly. According to the ICD registry, surgical replacement costs approximately $37,00018. Moreover, electromagnetic fields can disrupt the ICD performance, and the risk increases with the increased proximity19,20,21,22,23. It is important to note that ICDs are only recommended for patients with HFrEF; there are no monitoring devices available for 50% of patients with HFpEF24.
CardioMEMS™, is a commercially available diagnostic tool for HF that can alert providers of increased pressure in the lungs. It is a small (15 mm × 3.5 mm × 2 mm) device that is implanted in the pulmonary artery and monitors changes in pulmonary arterial pressure. Increased pulmonary artery pressure is an early indicator of worsening HF 25,26,27. It is costly, approximately $17,75011, and not without risk. CardioMEMS™ was approved by Food and Drug Administration in 2014, for both HFrEF and HFpEF, and in its first three years, 5500 devices were implanted in unique patients. However, CardioMEMS™ failed to predict 22 deaths out of 5500 implants, 4 out of which were due to HF28,29. Moreover, sensor failure occurred in 46 cases, in which 13 required recalibration, 11 patients were hospitalized and 14 sensors were discarded28.
Both currently available HF monitoring systems are not only costly but have significant safety concerns. Moreover, risks of invasive procedures cannot be ignored. Approximately half of patients with HF do not need an ICD and do not qualify for thoracic monitoring it provides. Therefore, there is a critical need for non-invasive solutions for the continuous and real time monitoring of HF progression. Healthcare Wearable Devices (HWDs) can address this need as HWDs are not only cost-effective but are also safe and convenient for the wearer. Moreover, they have been found to be an adequate solution for the continuous and real time monitoring of various biomarkers30,31. In addition to ICD and CardioMEMS, remote dielectric sensing (ReDS) from Sensible Medical, also measures lung fluid content but it is also not portable and cannot be used for point of care at all times32.
Moreover, VitalPatch by VitalConnect is a portable wearable device that can be used to monitor different vital signs related to cardiovascular diseases33. These parameters include heart rate, heart rate variability, respiration rate, body temperature, ECG, posture, and activity fall detection. However, it does not measure thoracic impedance, a significant parameter for the monitoring of HF33.
In this paper, we present an HWD that has the potential to monitor physiological parameters that are important for patients with HF. These parameters include thoracic impedance, electrocardiogram (ECG), heart rate, and motion activity detection.
Thoracic impedance is a critical bio-signal for the monitoring of HF progression, having a magnitude in between 60 and 1000 ohms depending upon the subject under consideration and the number of electrodes used for measuring thoracic impedance34. As discussed, at the onset of HF, fluid starts to accumulate in the thoracic region, this retention of fluid decreases the impedance in this area. Yu et al. in their study of 33 HF patients observed that before the onset of HF, thoracic impedance starts to decrease35. Therefore, this decrease in thoracic impedance is a vital consideration for HF progression36,37,38. Thoracic impedance is evaluated by placing electrodes across the thoracic region, and it measures the resistance to the flow of ions in this area. When the heart is not pumping efficiently, fluid fills the thoracic cavity and facilitates the flow of ions, as fluid is more conductive than air. 35,39,40. The increased flow of charges indicates a decrease in the thoracic impedance. Similarly, with the absence of fluid inside the thoracic region, charges face increased resistance to flow from one electrode to another, which indicates an increase in thoracic impedance41.
Similarly, ECG is a vital bio-signal for the diagnosis and prognosis of cardiovascular diseases. It is a representation of the flow of electrical signals through the heart42. As discussed, one of the symptoms of heart failure is abnormal heart rhythms, known as heart arrhythmias43. Heart arrhythmia is an improper beating of the heart in which the ECG is irregular and differentiable from regular sinus rhythm43. These heart arrhythmias can be identified using ECG. Traditionally, in the outpatient setting, ECG is measured using a Holter monitor which is not suitable for point of care (POC) use. Moreover, cardiomyopathy causes decreased ejection fraction, in which the percentage of blood pumped with each heartbeat decreases44. To compensate for the loss of blood supply, the heart may beat at a higher rate than usual (60–100 beats per minute). This may not be sufficient to provide the cardiac output needed by the body and leads to HF symptoms.
Fatigue is another symptom of HF along with swellings in the legs or edema7,8. O’Donnell et al. conducted a study on 13 HF patients and found that severe HF patients were less able to perform physical activity and hence have low activity45. Moreover, the discomfort due to heart failure affects sleep patterns46,47. These symptoms can be monitored using position sensors which can be used for the better management of heart failure.
This paper will highlight the materials and methods involved in the development of the HWD for the acquisition of the aforementioned parameters along with its preliminary results. Moreover, it will also discuss the challenges and future directions for the use of the discussed HWD for the prediction of HF.