1. Field of the Invention
The present invention generally relates to implantable medical devices, such as cardiac pacemakers and implantable cardioverter/defibrillators, and in particular to a method, a medical device, a computer program product and a computer readable medium for controlling a stimulation therapy using detected heart sounds.
2. Description of the Prior Art
Auscultation is an important diagnostic method for obtaining information of the heart sounds, which is well established as diagnostic information of the cardiac function. The sounds are often described as S1-S4. During the working cycle of the heart mechanical vibrations are produced in the heart muscle and the major blood vessels. Acceleration and retardation of tissue cause the vibrations when kinetic energy is transformed to sound energy, e.g. at valve closing. Vibrations can also arise from turbulent blood flow, e.g. at stenosis and regurgitation. These vibrations may be listened to using a stethoscope or registered electronically using phonocardiography, i.e. graphical registration of the heart sounds by means of a heart microphone placed on the skin of the patient's thorax. Auscultation using a stethoscope is, to a large extent, built on practical experience and long practice since the technique is based on the doctor's interpretations of the hearing impressions of heart sounds. When applying phonocardiography, as mentioned above, a heart microphone is placed on the skin of the patient's thorax. In other words, it may be cumbersome and time-consuming to obtain knowledge of the heart sounds and the mechanical energy during the heart cycle using these manual or partly manual methods and, in addition, the obtained knowledge of the heart sounds may be inexact due to the fact that the knowledge is, at least to some extent, subjective.
The first tone S1 coincides with closure of the Mitral and Tricuspid valves at the beginning of systole. Under certain circumstances, the first tone S1 can be split into two components. An abnormally loud S1 may be found in conditions associated with increased cardiac output (e.g. fever, exercise, hyperthyroidism, and anemia), tachycardia and left ventricular hyperthrophy. A loud S1 is also characteristically heard with mitral stenosis and when the P-R interval of the EKG is short. An abnormally soft S1 may be heard with mitral regurgitation, heart failure and first degree A-V block (prolonged P-R interval). A broad or split S1 is frequently heard along the left lower sternal border. It is a rather normal finding, but a prominent widely split S1 may be associated with right bundle branch block (RBBB). Beat-to-beat variation in the loudness of S1 may occur in atrial fibrillation and third degree A-V block.
The second heart sound S2 coincides with closure of the aortic and pulmonary valves at the end of systole. S2 is normally split into two components (aortic and pulmonary valves at the end of systole) during inspiration. Splitting of S2 in expiration is abnormal. An abnormally loud S2 is commonly associated with systemic and pulmonary hypertension. A soft S2 may be heard in the later stages of aortic or pulmonary stenosis. Reversed S2 splitting (S2 split in expiration—single sound in inspiration) may be heard in some cases of aortic stenosis but is also common in left bundle branch block (LBBB). Wide (persistent) S2 splitting (S2 split during both inspiration and expiration) is associated with right bundle branch block, pulmonary stenosis, pulmonary hypertension, or atrial septal defect.
The third heart sound S3 coincides with rapid ventricular filling in early diastole. The third heart sound S3 may be found normally in children and adolescents. It is considered abnormal over the age of 40 and is associated with conditions in which the ventricular contractile function is depressed (e.g. CHF and cardiomyopathy). It also occurs in those conditions associated with volume overloading and dilation of the ventricles during diastole (e.g. mitral/tricuspid regurgitation or ventricular septal defect). S3 may be heard in the absence of heart disease in conditions associated with increased cardiac output (e.g. fever, anemia, and hyperthyroidism).
The fourth heart sound S4 coincides with atrial contraction in late diastole. S4 is associated with conditions where the ventricles have lost their compliance and have become “stiff”. S4 may be heard during acute myocardial infarction. It is commonly heard in conditions associated with hyperthrophy of the ventricles (e.g. systemic or pulmonary hypertension, aortic or pulmonary stenosis, and some cases of cardiomyopathy). The fourth heart sound S4 may also be heard in patients suffering from CHF.
Thus, the systolic and diastolic heart functions are reflected in the heart sound. The power of energy value of the heart sounds and their relation may carry information of the workload and status of the heart. For example, as discussed above, patients with a wide QRS complex due to e.g. right bundle branch block (RBBB) or A-V block are associated with a widened or split first heart sound S1. Furthermore, changes of the energy of e.g. S1 over time can be a useful tool, for example, in diagnosis of different conditions. A high variability of the energy parameters during otherwise constant conditions indicates, for example, that filling is altering due to e.g. arrhythmia or conduction disorder. As can be understood from the above-mentioned, knowledge of the heart sounds and the corresponding mechanical energy can be used for diagnosis/monitoring and controlling pacing therapy of patients. In addition, this knowledge can also be used to optimize a stimulation therapy and to verify that the stimulation output evokes a desired response in a selected region of the heart. In patients suffering from heart failure, such as Congestive Heart Failure (CHF), the knowledge of the heart sounds and the corresponding energy parameters can be used to estimate the severity of the condition and/or to optimize the AV delay. Consequently, it would be beneficial if signals related to the heart sounds and the corresponding energy could be collected and used for controlling/optimizing pacing therapy in an automated manner.
The known technique presents a number of automated systems for controlling/optimizing stimulation therapy. For example, medical devices and methods in such devices for optimizing AV delay are known. In U.S. Pat. No. 5,700,283, a method and apparatus for pacing patients with severe congestive heart failure is shown. The heart sounds are sensed and used to derive a mechanical AV delay of the patient's heart. The pacemaker applied AV delay is adjusted until the measured AV falls in a predetermined range of between 180 ms to 250 ms. This solution may require extensive signal processing in order to derive the mechanical AV delay of the heart from the sensed heart sounds. In U.S. Pat. No. 6,044,298, a method and apparatus for optimizing a pacing mode of a cardiac pacemaker for patients having CHF are shown. The Total Acoustic Noise (TAN) is measured and an optimum pacing mode is determined by detecting the particular mode associated with the minimum TAN, wherein the integrated value of the acoustic signal during a single heart beat cycle (from P-wave to P-wave or from R-wave to R-wave) is referred to as the total acoustic noise (TAN). However, the total acoustic noise is blunt tool in the optimization of the AV interval due to the fact that it comprises noise from of a large number of sources including, for example, the first heart sound to the fourth heart sound (S1-S4). In addition, other factors such as activity level of the patient, position of the patient, etc. also affect and/or contribute to the noise. This may, for example, introduce errors since it may be difficult to identify which noise source that contributes to the total noise signal and to what extent different sources contribute to it, which, in turn, may entail unreliable optimization results. Hence, the optimization may be based, at least in part, on erroneous information. In WO 01/56651 a system and method for adjusting AV delay by monitoring heart sounds S1 and S2 is disclosed. The presence or absence of one or more of the heart sounds S1 and S2 in the sensor signal indicates whether a stimulation pulse evokes the desired response in the patient's heart. This solution is thus directed to monitoring whether a desired response was obtained where the presence of a predetermined heart sound indicates capture rather than optimizing the AV delay.
Thus, there is a need of a method and a medical device that are capable of automatically controlling the stimulation therapy, and in particular the AV or PV delay, and that provide reliable optimization results using detected heart sounds.