Implantable cardiac stimulation devices are available for treating cardiac arrhythmias by delivering cardiac stimulation pulses for pacing, cardioverting or defibrillating the heart. Such a device, commonly known as an implantable cardioverter defibrillator or “ICD”, senses a patient's heart rhythm and classifies the rhythm according to an arrhythmia detection scheme in order to detect episodes of tachycardia or fibrillation. Arrhythmias detected may include ventricular tachycardia (VT), fast ventricular tachycardia (FVT), ventricular fibrillation (VF), atrial tachycardia (AT) and atrial fibrillation (AT) in addition to bradycardia.
Upon detecting an arrhythmia, the ICD delivers an appropriate therapy. Cardiac pacing is delivered in response to the absence of sensed intrinsic depolarizations, referred to as P-waves in the atrium and R-waves in the ventricle. In response to tachycardia detection, a number of tiered therapies may be delivered beginning with anti-tachycardia pacing therapies and escalating to more aggressive shock therapies until the tachycardia is terminated. Termination of a tachycardia is commonly referred to as “cardioversion.” Ventricular fibrillation (VF) is a serious life-threatening condition and is normally treated by immediately delivering high-energy shock therapy. Termination of VF is normally referred to as “defibrillation.” With regard to atrial arrhythmias, atrial tachycardia or atrial flutter can be treated with anti-tachycardia pacing therapies, pulse bursts, or a cardioversion shock, and atrial fibrillation is typically treated with pulse bursts or a defibrillation shock.
Reliable ICD performance depends on accurate detection of arrhythmias such that an appropriate therapy may be selected and promptly delivered. Undetected malignant arrhythmias can be fatal, and undetected non-malignant arrhythmias may leave the patient in a hemodynamically compromised state. Inappropriately delivered therapies due to false arrhythmia detections can induce arrhythmias in some patients. It is desirable, therefore, to avoid delivering a therapy due to inappropriate arrhythmia detection. For example, it is undesirable to deliver cardioversion therapy during sinus tachycardia, which is a normal heart rate increase in response to exercise. Furthermore, a cardioversion or defibrillation shock is generally painful to the patient and depletes the battery charge. Therefore, accurate prompt detection of cardiac arrhythmias is critical in the selecting and delivering appropriate arrhythmia therapies.
The most common approach to detecting arrhythmias in implantable automatic cardioverters and defibrillators is based on monitoring sensed event intervals determined from cardiac electrogram (EGM) signals. Monitoring of sensed intervals generally involves identifying the event intervals and event rates as they occur and applying a preset group of criteria, which must be met in order to detect a particular arrhythmia. Criteria for identifying various arrhythmias may all be monitored simultaneously. An arrhythmia detection and classification system generally disclosed in U.S. Pat. No. 5,342,402, issued to Olson et al., incorporated herein by reference in its entirety, uses criteria for sensed events, event intervals, and event rates and is employed in the Medtronic Model 7219 devices. An arrhythmia detection and classification system that employs a prioritized set of inter-related rules for arrhythmia detection is generally disclosed in U.S. Pat. No. 5,545,186, issued to Olson et al., also incorporated herein by reference in its entirety.
The majority of clinical experience in detecting cardiac arrhythmias with regard to implantable automatic cardioverting and defibrillating devices is based on bipolar sensing in the area of the right ventricular apex. New cardiac stimulation therapies and applications, such as cardiac resynchronization therapy, however, may require particular lead locations to achieve targeted stimulation at specific locations. These requirements may counter the optimal location of electrodes for reliable cardiac arrhythmia detection. As the variety of implantable cardiac stimulation devices increases, e.g., devices capable of sensing and stimulating in the left side of the heart with the use of a lead deployed through the coronary sinus or leadless devices implanted in the vicinity of the heart such as in a subaxillary location, the type and reliability of EGM signals available for detecting cardiac arrhythmias may change. Thus, the quality of the EGM signals available for arrhythmia detection may suffer.
Limitations of EGM sensing are well known in the art. Noise in the form of electromagnetic interference, skeletal muscle depolarizations, far-field signals, or polarization artifact following a stimulation pulse can interfere with accurate sensing of intrinsic electrical activity. Oversensing of cardiac activity or noise can result in false detections of cardiac events. Undersensing of cardiac activity can result in missed detections of cardiac events. In either situation, cardiac stimulation therapies may be inappropriately withheld or delivered.
Mechanical sensing of cardiac activity has been proposed for use in cardiac stimulation therapy applications such as optimizing timing intervals during cardiac pacing or monitoring hemodynamic performance. Detection of peak endocardial wall motion in the apex of the right ventricle for optimizing A–V intervals has been validated clinically. A system and method for using cardiac wall motion sensor signals to provide hemodynamically optimal values for heart rate and AV interval are generally disclosed in U.S. Pat. No. 5,549,650 issued to Bornzin, et al. A cardiac stimulating system designed to automatically optimize both the pacing mode and one or more pacing cycle parameters in a way that results in optimization of a cardiac performance parameter, including for example heart accelerations, is generally disclosed in U.S. Pat. No. 5,540,727, issued to Tockman, et al.
An accelerometer-based activity sensor used to provide a signal that corresponds to the acceleration due to the heartbeat of a patient is generally disclosed in U.S. Pat. No. 5,991,661 issued to Park, et al. When the patient is determined to be at rest, the acceleration signal is used to determine parameters indicative of the contractility of the heart and the displacement of the heart during a heartbeat.
Implantable sensors for monitoring heart wall motion have been described or implemented for use in relation to the right ventricle. A sensor implanted in the heart mass for monitoring heart function by monitoring the momentum or velocity of the heart mass is generally disclosed in U.S. Pat. No. 5,454,838 issued to Vallana et al. A catheter for insertion into the ventricle for monitoring cardiac contractility having an acceleration transducer at or proximate the catheter tip is generally disclosed in U.S. Pat. No. 6,077,236 issued to Cunningham. Implantable leads incorporating accelerometer-based cardiac wall motion sensors are generally disclosed in U.S. Pat. No. 5,628,777 issued to Moberg, et al. A device for sensing natural heart acceleration is generally disclosed in U.S. Pat. No. 5,693,075, issued to Plicchi, et al. A system for myocardial tensiometery including a tensiometric element disposed at a location subject to bending due to cardiac contractions is generally disclosed in U.S. Pat. No. 5,261,418 issued to Ferek-Petric et al. All of the above-cited patents are hereby incorporated herein by reference in their entirety.
Thus the use of cardiac wall motion sensors in evaluating cardiac hemodynamic performance is known. The use of the signal from a cardiac wall motion sensor as a primary indicator of potentially malignant cardiac arrhythmias is proposed in the above-cited '361 patent to Moberg. The cardiac wall motion sensor signal may be used with conventional R-wave detection circuitry that relies on an IEGM for measuring cardiac activity. An implantable cardiac stimulating device which uses cardiac displacement signals to detect and discriminate arrhythmias is generally disclosed in U.S. Pat. No. 5,480,412 issued to Mouchawar et al., hereby incorporated herein by reference in its entirety. Cardiac wall acceleration signals provided by a cardiac wall motion sensor are integrated over time to derive cardiac velocity signals, which are further integrated over time to derive cardiac displacement signals.
A need remains, however, for an implantable medical device that is capable of detecting cardiac arrhythmias using mechanical cardiac activity information to augment electrical sensing of cardiac activity and that allows classification of detected arrhythmias for monitoring or therapy selection purposes. An implantable system and algorithm employing both mechanical and electrical cardiac activity information can be used to overcome limitations described that are encountered when relying solely on electrical activity sensing, particularly in newer systems that do not include traditional right ventricular apical EGM sensing.
Furthermore, an implantable medical device capable of evaluating mechanical event signals that allows prompt detection of the transition from hemodynamically stable to hemodynamically unstable arrhythmias is also needed. As indicated above, commercial implementations of lead-based accelerometers have been used in relation to the right ventricle. However, left ventricular wall motion is a more direct correlate to cardiac output than right ventricular wall motion. Therefore, monitoring left ventricular wall motion is expected to be more sensitive in discriminating hemodynamically stable and unstable rhythms.