Surgery – Diagnostic testing – Respiratory
Reexamination Certificate
2000-05-05
2003-07-08
Nasser, Robert L. (Department: 3736)
Surgery
Diagnostic testing
Respiratory
C600S529000, C600S508000, C607S017000
Reexamination Certificate
active
06589188
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
This invention relates generally to implantable monitoring devices and implantable cardiac therapy devices, and more particularly to a method for monitoring the respiratory patterns of a chronic heart failure patient to track changes in disease state.
II. Description of the Related Art
Congestive heart failure (CHF) is a debilitating, end-stage disease in which abnormal function of the heart leads to inadequate blood flow to fulfill the needs of the body's tissues. 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 may become leaky, allowing regurgitation or backflow of blood. The impairment of arterial circulation deprives vital organs of oxygen and nutrients. Fatigue, weakness, and inability to carry out daily tasks may result.
Not all CHF patients suffer debilitating symptoms immediately. Some may live actively for years. Yet, with few exceptions, the disease is relentlessly progressive.
As CHF 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 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.
CHF has been classified by the New York Heart Association (NYHA). Their classification of CHF corresponds to four stages of progressively worsening symptoms and exercise capacity from Class I to Class IV. Class I corresponds to no limitation wherein ordinary physical activity does not cause undue fatigue, shortness of breath, or palpitation. Class II corresponds to slight limitation of physical activity wherein such patients are comfortable at rest, but where ordinary physical activity results in fatigue, shortness of breath, palpitations, or angina. Class III corresponds to a marked limitation of physical activity wherein, although patients are comfortable at rest, less than ordinary activity will lead to symptoms. Lastly, Class IV corresponds to inability to carry on any physical activity without discomfort, wherein symptoms of CHF are present even at rest and where with any physical activity, increased discomfort is experienced.
Current standard treatment for heart failure is typically centered around medical treatment using ACE inhibitors, diuretics, and digitalis. It has also been demonstrated that aerobic exercise may improve exercise tolerance, improve quality of life, and decrease symptoms. Only an option in 1 out of 200 cases, heart transplantation is also available. Other cardiac surgery is also indicated for only a small percentage of patients with particular etiologies. Although advances in pharmacological therapy have significantly improved the survival rate and quality of life of patients, patients in NYHA Classes III or IV, who are still refractory to drug therapy, have a poor prognosis and limited exercise tolerance. Cardiac pacing has been proposed as a new primary treatment for patients with drug-refractory CHF.
Heart failure patients require close medical management to reduce morbidity and mortality. Because the disease status evolves with time, frequent physician follow-up examinations are often necessary. At follow-up, the physician may make adjustments to the drug regimen in order to optimize therapy. This conventional approach of periodic follow-up may be less satisfactory for heart failure, in which acute, life-threatening exacerbations can develop between physician follow-up examinations. It is well known among clinicians that if a developing exacerbation is recognized early, it can be more easily and inexpensively terminated, typically with a modest increase in oral diuretic. However, if it develops beyond the initial phase, an acute heart failure exacerbation becomes difficult to control and terminate. Hospitalization in an intensive care unit is often required. It is during an acute exacerbation of heart failure that many patients succumb to the disease. Early identification may also allow for pacing therapy from an implanted pulse generator.
In patients with CHF, alterations of breathing patterns such as Cheyne-Stokes respiration or periodic breathing have been identified during both day- and night-time. Periodic breathing is defined as a waxing and waning of tidal volume without periodic phases of apnea, whereas Cheyne-Stokes respiration is defined as a type of periodic breathing that includes periods of apnea between the phases of hyperventilation (hyperpnea) and hypoventilation (hypopnea.) These alterations of breathing are associated with marked oscillations of arterial oxygen saturation and heart rate.
Detection of periodicity and the underlying frequency content of a signal such as arterial oxygen saturation, breath to breath interval, tidal volume amplitude or heart rate (R to R intervals) can be accomplished via any of several widely used techniques of power spectral analysis. These techniques include, but are not limited to, periodogram estimation, the maximum entropy (all poles) method and the Blackman-Tukey algorithm.
Conventional cardiac monitors, such as defibrillators, pacemakers, Holter monitors, and cardiac event records, are tailored for the diagnosis and/or therapy of abnormalities of the cardiac electrical system. In contrast, heart failure is a disease of the cardiac mechanical system: it is primarily a failure of the myocardium to meet the mechanical pumping demands required of it. In monitoring the status of a heart failure patient, measuring the mechanical hemodynamic variables is clearly desirable. Examples of mechanical hemodynamic variables include atrial, ventricular, and arterial pressures, and cardiac output (volume of blood pumped into the aorta per unit time). However, because of the complex feedback network that monitors and controls cardiac performance, measuring variables that do not directly reflect the mechanical performance of the heart is also useful.
Some limitations of monitoring systems have been addressed by the development of an implantable system that monitors hemodynamic status (Medtronic Chronicle, Medtronic, Inc., Minneapolis, Minn.). While this system potentially avoids the need for active patient participation in disease state monitoring, it relies on an intravascular pressure transducer placed in the right ventricle of the heart. Examples of other hemodynamic monitoring systems include U.S. Pat. No. 5,454,838 in which Vallana et al. teach placement of a sensor on the myocardial wall using an intravascular approach. In U.S. Pat. No. 5,496,351, Plicchi et al. propose placing a sensor within the myocardial wall. Mortazavi in U.S. Pat. No. 5,040,538 and Cohen et al. in U.S. Pat. No. 4,815,469 describe placement of an optical sensor within the right ventricle.
An Implantable Ambulatory Electrocardiogram Monitor is described by Nappholz et al. in U.S. Pat. No. 5,113,869, incorporated herein by reference. This device is designed for chronic extravascular implantation. In contrast to cardiac recorders, it performs analysis on the electrocardiogram signal in order to predict imminent cardiac arrhythmias and to detect cardiac ischemia. Like the cardiac recorders, it is capable of storing raw ECG data for later review by a physician. This feature, along with the record of arrhythmic events it detected, allows the physician to tailor pharmacologic therapy. In addition, Nappholz et al. mention the use of transthoracic impedance for minute ventilation, ultrasound transducers for arterial pressure, or other sensors to allow discrimina
Fishler Matthew G.
Street Anne M.
Mitchell Steven M.
Nasser Robert L.
Pacesetter Inc.
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