Heart failure is a debilitating disease in which abnormal function of the heart leads in the direction of 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 eject or 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 muscle causing the ventricles (particularly the left ventricle) to grow in volume in an attempt to pump more blood with each heartbeat. 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. The additional muscle mass may also stiffen the heart walls to hamper rather than assist in providing cardiac output. Progression of HF could lead to congestion wherein the weak pumping of the heart leads to build-up of fluids in the lungs and other organs and tissues.
Pulmonary edema is a swelling and/or fluid accumulation in the lungs often caused by heart failure. Briefly, the poor cardiac function resulting from heart failure can cause blood to back up in the lungs, thereby increasing blood pressure in the lungs, particularly pulmonary venous pressure. The increased pressure pushes fluid—but not blood cells—out of the blood vessels and into lung tissue and air sacs (i.e. the alveoli). This can cause severe respiratory problems and, left untreated, can be fatal. Pulmonary edema can also arise due to other factors besides heart failure, such as infections.
Pulmonary edema due to heart failure may be referred to as cardiogenic pulmonary edema. Significant increase in thoracic fluids is referred to as a “fluid overload.” Fluid overload can exacerbate the symptoms of HF and lead to acutely decompensated heart failure, which is a serious and potentially fatal condition. Accordingly, it is highly desirable to detect fluid overload states so that the patient or caregiver can be warned and appropriate therapies delivered. With a fluid overload state, the patient typically becomes hypervolemic (wherein hypervolemia is broadly defined herein as an abnormal increase or shift in blood or fluid volume.) Hence, it would be desirable to equip implantable devices such as diagnostic sensors, pacemakers and CRTs to detect and respond to hypervolemia as an indicator of fluid overload.
One technique for use by an implantable medical device for detecting a fluid overload uses cardiac pressure measurements such as left atrial pressure (LAP) measurements. A significant increase in LAP is deemed to be indicative of such fluid overload. Other pressure parameters that can be used are left ventricular pressure (LVP) and pulmonary artery pressure (PAP.) In response to detection of fluid overload, diuretics can be administered to the patient to reduce the amount of thoracic fluids. Delivery of too large a dosage of diuretics may cause the patient to instead become hypovolemic (wherein hypovolemia is broadly defined herein as an abnormal decrease in blood or fluid volume.) Hypovolemia may arise for other reasons as well. Hence it would also be desirable to equip implantable medical devices to detect and respond to hypovolemia. Ideally, it would be desirable for the device to reliably distinguish or discriminate among hypervolemia, hypovolemia and the healthy volemic state therebetween. The volemic state between hypervolemia and hypovolemia is generally referred to herein as euvolemia (i.e. a normal or healthy volemic state.) Other terms such as optivolemia (i.e. an optimal volemic state) or normovolemia (i.e. a normal volemic state) may also be used herein.
Accordingly, aspects of the invention are generally directed to providing techniques for detecting and discriminating hypervolemia, hypovolemia and euvolemia. Other aspects are directed to detecting other conditions or states, such as a pressure alternans state indicative of imminent heart failure decompensation.