Removing Need for Leads in Cardiac Monitoring
- Wednesday, 01 May 2013
Electrocardiogram (ECG or EKG) measurements typically involve time-consuming skin preparation, lead application, conductive gels, and even shaving of body hair. More recently, dry contact sensors have come into use in some sports and home health monitoring units, but these frequently experience contact problems, particularly in users with dry skin.
Plessey Semiconductors Ltd., Devon, UK, recently launched a new device that works in a different way, without contactrelated limitations. The imPulse unit is a personal hand-held ECG touch monitor that looks like a game controller, and is capable of accurately measuring lead I ECG (left arm to right arm) just by the user placing his or her thumbs onto the unit’s two sensors, as shown above in Figure 1. The data that is collected can be transmitted in real time via a Bluetooth link to a smartphone.
Custom software installed on the smartphone displays the ECG trace, monitors the heart rate, and graphs the trend of the heart rate. From there, data can be recorded for analysis in “the cloud” against commercially available algorithms or interpreted by a cardiologist remote from the patient.
Unique Sensor Technology
The key components of the imPulse are the company’s award-winning EPIC sensors. EPIC is an acronym for “Electric Potential Integrated Circuit,” but the term has become synonymous with the integrated circuit technology, the sensor itself, and, in a wider context, the physical principles of operation of the device within a system.
EPIC is a noncontact electrometer, meaning that there is no direct DC path from the outside world to the sensor input, a condition that is somewhat analogous to the gate electrode of an MOS transistor. The electrode is protected by a capping layer of dielectric material to ensure that the electrode is isolated from the body being measured. The device is AC coupled with a lower corner frequency (-3dB) of a few tens of MHz and an upper corner frequency above 200 MHz. This response is adjustable and can be tailored to suit a particular application.
The lightweight handheld unit contains two sensors, enabling measurement of a differential signal between contact points either side of the heart, as is required for ECG. The two contact points are, quite simply, the user’s two thumbs that rest comfortably on the sensors located on the top surface of the unit. The sensors themselves utilize capacitive coupling, which is what sets them apart from almost all current sensors used for detecting electrophysiological signals. Most commercially available sensors are galvanic, which means that they rely on a low electrical resistance between the skin and the electrode to obtain a good, usable, signal. Common things such as dry skin or hair increase the resistance; hence the need for gels, shaving, and various skin preparation techniques.
The electrode surface that is touched by the user is actually an insulator, not a conductor, and so any amount of extra resistance is irrelevant. Instead the contact between the user’s thumbs and the sensor electrodes couples the electrical signal from the heart capacitively onto the very high impedance input stage of the sensor. The high input impedance is important as it allows the sensors to respond at the low frequencies required for an accurate ECG. By using two sensors, the electronics in the imPulse are able to reject the “common mode” noise that is always present on the body from surrounding electrical apparatus, mostly from mains electricity, giving a clean and clear ECG signal.
Among the applications that Plessey envisions for its imPulse unit, two stand out. One is as a screening device for use by clinicians. Full 12-lead ECGs are time-consuming, expensive, and are a drain on health service resources. While a single-lead ECG measurement can never provide the full information of a 12-lead ECG, it can most certainly be a key tool in detecting common heart complaints such as arrhythmias, atrial fibrillation (AF), and the effects of myocardial infarction. AF is often an early warning of an impending stroke, which at the very least will result in long recovery periods including hospitalization, and, at worst, long-term severe disability or death. Being able to provide a quick and simple test for these conditions will enable those who urgently need the attentions of cardiologist to be prioritized, saving not only resources, but much suffering and ultimately, lives.
The second application area is as a home health screening device. The situation can range from an individual wanting to measure well-being for fitness reasons, to serious athletes wanting to measure ECG parameters as part of a training program, to regular health monitoring of those with cardiac problems who would otherwise need to be hospitalized. While this product is not intended to replace regular doctor visits, it can supplement the visits with the ability to record a short ECG trace and have it automatically transmitted to a healthcare provider for software-based analysis on a daily basis. This can have a major impact on the ability for the sick and the elderly to preserve their independence, while not being subjected to an increased risk of preventable heartrelated illness.
Although the imPulse is primarily designed for measuring an ECG from the hands, it can also be placed on the user’s chest, and, in the right conditions, can even measure ECG through clothing. The imPulse will soon be qualified and approved for use in the UK and US.
This article was written by Robert Breakspear, Principal Applications Engineer, Plessey Semiconductors Ltd., Devon, UK. For more information, visit http://info.hotims.com/45603-162.