Biosensor applications to antitachycardia devices

Patient monitoring across the spectrum of heart failure disease management 10 years after the CHAMPION trial

Despite significant advances in drug‐based and device‐based therapies, heart failure remains a major and growing public health problem associated with substantial disability, frequent hospitalizations, and high economic costs. Keeping patients well and out of the hospital has become a major focus of heart failure disease management. Achieving and maintaining such stability in heart failure patients requires a holistic approach, which includes at least the management of the underlying heart disease, the management of comorbidities and the social and psychological aspects of the disease, and the management of haemodynamic/fluid status. In this regard, accurate assessment of elevated ventricular filling pressures or volume overload, that is, haemodynamic or pulmonary congestion, respectively, before the onset of worsening heart failure symptoms represents an important management strategy. Unfortunately, conventional methods for assessing congestion, such as physical examination and monitoring of symptoms and daily weights, are insensitive markers of worsening heart failure. Assessment tools that directly measure congestion, accurately and in absolute terms, provide more actionable information that enables the application of treatment algorithms designed to restore patient stability, in a variety of clinical settings. Two such assessment tools, implantable haemodynamic monitors and remote dielectric sensing (ReDS), meet the prerequisites for useful heart failure management tools, by providing accurate, absolute, and actionable measures of congestion, to guide patient management. This review focuses on the use of such technologies, across the spectrum of heart failure treatment settings. Clinical data are presented that support the broad use of pulmonary artery pressure‐guided and/or ReDS‐guided heart failure management in heart failure patients with reduced and preserved left ventricular ejection fraction.

Identification of hemodynamically unstable arrhythmias using subcutaneous photoplethysmography

INTRODUCTION

Determination of hemodynamic status is central to arrhythmia management in the inpatient setting. In contrast, therapy decisions in implantable cardioverter defibrillators (ICDs) are based exclusively on the arrhythmia's electrical signature. Hemodynamic sensing in ICDs would allow tailoring of therapy according to perfusion status. Subcutaneous photoplethysmography (PPG) is an attractive technology for this application because it responds to changes in arterial pressure and can be readily incorporated into the housing of implanted devices. This study evaluated the accuracy of PPG in identifying hemodynamically unstable simulated arrhythmias in an animal model.

METHODS AND RESULTS

Rapid atrial and ventricular pacing was used to simulate arrhythmias in an acute preparation of 7 healthy dogs. Aortic pressure and subcutaneous PPG were simultaneously recorded. Simulated arrhythmias were defined as hemodynamically unstable if aortic pressure decreased by >or=15 mmHg, marginally unstable if pressure decreased by 5-15 mmHg, and hemodynamically stable if pressure either increased or decreased by no more than 5 mmHg. An average of 56 arrhythmias were simulated in each animal. Changes in pressure and PPG output were highly correlated, with correlation coefficient of 0.7-0.9. Subcutaneous PPG identified hemodynamically unstable episodes with a sensitivity of 100% for 6 subjects and 80% for 1 subject. Specificity was more than 90% for 6 subjects and was 50% for 1 subject.

CONCLUSIONS

Subcutaneous PPG detects hemodynamically unstable simulated arrhythmias in an acute canine preparation. If successfully validated in humans, this technology may allow ICD therapy to be specifically tailored according to the hemodynamic status of the arrhythmia.

Evaluation of antiarrhythmic drug efficacy in patients with an ICD: unlimited potential or replete with complexity and problems?

There are a number of novel ways in which implantable cardioverter defibrillator (ICD) endpoints can be used in clinical trials to evaluate antiarrhythmic drugs. The advances in ICD technology (storage, retrieval, and accurate interpretation of ICD electrograms) expand the potential to include the use of an ICD endpoint as a clinical surrogate for sudden death. The ICD also provides the necessary safety net to test new drugs. The frequent need for antiarrhythmic drugs in patients already fitted with an ICD (e.g., for atrial fibrillation) necessitates knowledge of the drugs' effect on defibrillator threshold. There are interpretative problems and challenges associated with all types of ICD trials. A particular difficult issue is the degree to which the results of data on antiarrhythmic drug efficacy and safety acquired in the context of an ICD endpoint trial might be extrapolated to patient populations in which the device is not used. These and other challenging issues are discussed, with the goal of enhancing the design and interpretation of clinical trials featuring ICD endpoints.

Arrhythmia recognition strategies and hardware decisions for the implantable cardioverter-defibrillator--a review

The avoidance of inappropriate shocks from the implantable cardioverter-defibrillator (ICD), together with its need to apply antitachycardia pacing to either atria or ventricles, demands considerable sophistication in the design of algorithms to interpret electrical or other cardiac signals in real-time. Methods based on rate and using single short-gap bipolar leads lack discrimination. Right ventricular electrogram morphology algorithms offer improvement but no universal algorithm exists; however, for any given patient an optimum algorithm of this type might be found. One improvement would be to provide atrial information in addition, by employing more than one electrode or a long-gap single bipolar lead. Alternatively, transducer signals could be included, once their efficacy and reliability have been improved. A different approach would be to use the much more sophisticated algorithms at present being tried with surface electrocardiograms. Integrated Circuit technology is reaching the point where this could be done but the requirement for exceptionally high reliability means that special system structures, such as a Memory Intensive Computer Architecture, may be required. When decisions on these approaches are to be made, it must also be remembered that ICDs will soon be implanted and programmed as a routine rather than a highly specialized procedure.

Electrophysiologic basis for T wave alternans as an index of vulnerability to ventricular fibrillation

Substantial evidence indicates that T wave alternans is an intrinsic property of ischemic myocardium. The electrophysiologic basis appears to be spatial and temporal heterogeneity of repolarization resulting from changes in action potential morphology rather than in activation sequence. Ischemia-induced changes in postrepolarization refractoriness and depressed electrical restitution of action potential duration have also been implicated. The main underlying ionic basis for T wave alternans during coronary occlusion appears to be derangements in intracellular cycling of calcium. Accumulation of potassium in the extracellular space adjoining ischemic cells and disruption in electrogenic sodium-calcium exchange may also be involved. In humans, T wave alternans has been observed in Prinzmetal's and classical angina, angioplasty, and bypass graft occlusion. Under these conditions associated with acute myocardial ischemia, alternans is restricted to the ischemic zone, and alternation in action potential morphology is an underlying factor. Recently, repolarization alternans has been shown to be a statistically significant predictor of the results of electrophysiologic testing and arrhythmia-free survival in individuals with and without organic heart disease. Collectively, these observations provide a rationale for quantitation of T wave alternans magnitude for assessment of vulnerability to life-threatening ventricular arrhythmias both in response to and independent of the effects of myocardial ischemia.

The range of sensors and algorithms used in rate adaptive cardiac pacing

Implantable sensors play an important role in physiological cardiac pacing. Sensors can be classified according to the technical methods in which sensing is achieved: the sensing of the evoked ventricular response, intrathoracic impedance and body acceleration forces, and the incorporation of special sensors on pacing electrodes. These sensors differ in their relative merits in terms of speed, proportionality, sensitivity, and specificity of rate response. The efficacy of a sensor can be significantly modified by the algorithm used in relating sensor signal to a pacing rate change. The currently available types of sensors and algorithms are summarized and compared in this review article. The relative merits of these sensors and algorithms form the basis for designing a multisensor pacing system.

Implantable cardioverter defibrillators: current status and future directions

Sudden cardiac death remains the most common mode of mortality in the United States, accounting for up to 450,000 deaths per year. Survivors of cardiac arrest and patients who have recurrent ventricular tachycardia have a high mortality rate with or without antiarrhythmic therapy. The implantable cardioverter defibrillator (ICD) was introduced in 1980 by Mirowski as a potential treatment for these patients. There are presently over 24,000 implants worldwide and the device has proved to be an effective means of preventing sudden death. The components of an ICD include a generator, defibrillation patches or leads, and pacing/sensing leads. The devices can be implanted with acceptable mortality and morbidity either by median sternotomy, left anterior thoracotomy, subxiphoid, or left subcostal approaches. The long-term results have been excellent with an actuarial incidence of sudden cardiac death of 3% at 5 years. Improvements in battery and capacitor technology, lead design, and tachycardia recognition, combined with the addition of hemodynamic sensors and a better understanding of the science of defibrillation, should lead to further improvements over the next several years in the ICD.