Heart failure is a debilitating disease in which abnormal function of the heart leads to inadequate blood flow to fulfill the needs of the tissues and organs of the body. Typically, the heart loses propulsive power because the cardiac muscle loses capacity to stretch and contract. Often, the ventricles do not adequately fill with blood between heartbeats and the valves regulating blood flow become leaky, allowing regurgitation or back-flow of blood. The impairment of arterial circulation deprives vital organs of oxygen and nutrients. Fatigue, weakness and the inability to carry out daily tasks may result. Not all heart failure patients suffer debilitating symptoms immediately. Some may live actively for years. Yet, with few exceptions, the disease is relentlessly progressive. As heart failure progresses, it tends to become increasingly difficult to manage. Even the compensatory responses it triggers in the body may themselves eventually complicate the clinical prognosis. For example, when the heart attempts to compensate for reduced cardiac output, it adds cardiac muscle causing the ventricles to grow in volume in an attempt to pump more blood with each heartbeat, i.e. to increase the stroke volume. This places a still higher demand on the heart's oxygen supply. If the oxygen supply falls short of the growing demand, as it often does, further injury to the heart may result, typically in the form of myocardial ischemia or myocardial infarction. The additional muscle mass may also stiffen the heart walls to hamper rather than assist in providing cardiac output. A particularly severe form of heart failure is congestive heart failure (CHF) wherein the weak pumping of the heart leads to build-up of fluids in the lungs and other organs and tissues.
In view of the potential severity of heart failure, it is highly desirable to detect its onset within a patient and to track its progression so that appropriate therapy can be provided. Many patients suffering heart failure already have pacemakers or ICDs implanted therein or are candidates for such devices. Accordingly, it is desirable to provide such devices with the capability to automatically detect and track heart failure. Physiological parameters that can be used to aid in the detection and tracking of heart failure include stroke volume and related cardiac function parameters. Cardiac function is a measure of the overall effectiveness of the cardiac system of a patient and is typically represented in terms of, one or more of, stoke volume, cardiac output, end-diastolic volume, end-systolic volume, ejection fraction or cardiac output index. Stroke volume is the amount of blood ejected from the left ventricle during systole. Cardiac output is the volume of blood pumped by the left ventricle per minute (or stroke volume times the heart rate). End-diastolic volume (EDV) is the volume of blood in the chamber at the end of the diastolic phase, when the chamber is at its fullest. End-systolic volume (ESV) is the volume of blood in the chamber at the end of the systolic phase, when the chamber contains the least volume. Ejection fraction (EF) is percentage of the EDV ejected by the ventricle per beat. Cardiac index is the volume of blood ejected per minute normalized to the body surface area of the patient. Other factors representative of cardiac function include the contractility of the left ventricle or the maximum rate of change of pressure with time (i.e. max dP/dt).
One promising technique for estimating at least some of these physiological parameters is to exploit intra-cardiac impedance detected using leads of the implantable device. Publications by Stahl et al. (Stahl et al., “Assessing Acute Ventricular Volume Changes by Intracardiac Impedance in a Chronic Heart Failure Animal Model”, PACE Vol. 32, 1395-1401, November 2009 and Stahl et al., “Intracardiac Impedance Monitors Hemodynamic Deterioration in a Chronic Heart Failure Pig Model”, Journal of Cardiovascular Electrophysiology, Volume 18, Issue 9, pages 985-990, September 2007) and Bocchiardo et al., “Intracardiac impedance monitors stroke volume in resynchronization therapy patients”, Europace (2010) [doi: 10.1093/europace/euq045] reported that intra-cardiac impedance correlates well with stroke volume (r=0.88, 0.82) or EDP (r=0.82 or 0.81) in animals and in patients. However, the impedance range differed widely among individuals. Modeling by Lippert et al., “Intracardiac Impedance as a Method for Ventricular Volume Monitoring—Investigation by a Finite-Element Model and Clinical Data, 2010 J. Phys.: Conf. Ser. 224 012095, showed the range of intra-cardiac impedance can also be very sensitive to left ventricular (LV) lead positions. This would make the estimation of “absolute values” of stroke volume or cardiac output difficult, i.e., values scaled to the proper units. Yet, many clinicians prefer stroke volume or cardiac output since those parameters provide a direct clinical measure that can be helpful in diagnosing conditions and guiding treatment.
Accordingly, it would be highly desirable to provide improved techniques for estimating stroke volume or cardiac output from impedance signals detected within a patient for informing the clinician, detecting and tracking heart failure or for other purposes such as automatically optimizing pacing delays. It is to these ends that various aspects of the invention are directed.