Subcutaneous electrocardiogram monitors and their field of view

Abstract

Continuous electrocardiogram (ECG) monitoring of cardiac patients on a long-term, even permanent, basis has become possible. Postsurgical cases, those with significant risk factors, or patients with chronic conditions are candidates for these procedures to assess evolving risk factors and detect life-threatening events. A small sensing device can be implanted subcutaneously to assess the ECG, transmitting status and alerts to local caregivers or a remote monitoring service. We and others have shown that a differential electrode pair with only 2- to 3-cm spacing can produce QRS amplitudes greater than 1 mV, sufficient to accurately identify asystole, tachyarrhythmias, and ST-segment changes. Medtronic's REVEAL and St Jude Medical's CONFIRM are implantable look recorders (ILRs) with a single pair of subcutaneous electrodes mounted on the surface of the case (6 × 2 × 0.7 cm). They store representative rhythm strips when the heart rate exceeds preset limits or when the patient presses a button on the accompanying actuator. These records may be transferred for physician review during a subsequent office visit. Transoma's SLEUTH is a similar ILR, except that one of the electrodes is at the end of a 6-cm lead tunneled under the skin and the wider separation may result in a larger ECG amplitude. Instead of storing the records, SLEUTH transmits them through the skin to a home base unit, which sends them via telephone to a monitoring service. Angel Medical's ALERT system also has a tunneled lead, but one that is introduced pervenously into the right ventricle hoping to detect ST changes in addition to rhythm abnormalities. Advanced multivector ILR devices with integrated event alerting are rapidly approaching commercialization. AJ Medical Devices' CARDIOALARM (4 × 4 × 0.6 cm) has 4 electrodes at the corners of the square package, arranged as 2 orthogonal recording pairs that can produce a robust signal that is relatively immune to signal fluctuations caused by changes in the direction of cardiac activation and patient's body position. This permits accurate identification of dramatic changes in the ECG pattern, such as those occurring in ventricular fibrillation and polymorphic tachycardia. Because of this feature, CARDIOALARM can detect cardiac arrest, and its external receiver can alert bystanders to begin cardiopulmonary resuscitation and can automatically summon Emergency Medical Services. In the future, addition of other sensors, integration of data streams via body surface wireless networks, and real-time interpretive algorithms will allow enhanced monitoring systems to more generally assess evolving risks, the impacts of therapeutic interventions, and patient compliance with rehabilitative programs. When coupled to remote medical monitoring services, these devices have the potential to dramatically impact patient outcomes by lessening the diagnostic dependence on symptom recognition and decreasing event response times. Significant cost savings may also be realized through more efficient use of specialist resources, reduction in the number of office visits to physicians, and long-term improvements in patient health. Structural and behavior barriers to adoption need to be addressed for these methods to reach their full potential, addressing patient privacy concerns, adequate reimbursement, and integration into standard care protocols.