Ventricular fibrillation is a life-threatening cardiac arrhythmia resulting in the immediate loss of blood pressure. Fibrillation must be treated within minutes of its onset, or the patient may die. The only effective treatment of ventricular fibrillation is the delivery of an adequately strong electric shock to the ventricles of the heart. At present, there exists an identifiable population of patients who survive an episode of ventricular fibrillation, because of prompt therapy. Although these patients may survive their first episode of ventricular fibrillation, due to the efforts of emergency resuscitation teams, their long-term prognosis is very poor. For these patients, who are becoming more identifiable, an alternative treatment is implantation of an automatic defibrillator. A major obstacle to be overcome in the creation of such a device is development of a low power demand device and a reliable detection circuit to identify accurately ventricular fibrillation. It would, therefore, be desirable to provide an automatic implantable defibrillator incorporating a reliable detection circuit that quantitatively preprograms and weighs multiple signals received from the heart before a defibrillatory shock is delivered to the cardiac ventricles.
When functioning normally, the muscle fibers of the heart are stimulated by a wave-like electrical excitation that originates in the sino-atrial node in the right atrium. The excitation proceeds via the atrium of the heart to the ventricles. The wave-like excitation triggers the ventricular muscle fibers by causing a sequential depolarization of adjacent cells, thus creating an efficient contraction and pumping action, i.e., proper mechanical activity. Under certain conditions, e.g., partial deprivation of the oxygen supply to parts of the heart, this organized wave-like pattern is interrupted and fibrillation, a disorganized, random contraction and relaxation of the fibers of the ventricular muscle can result; this is fibrillation. During ventricular fibrillation, the muscle fibers are electrically depolarizing and repolarizing in random fashion, resulting in a chaotic twitching of the ventricular muscle, with the result that no effective pumping of blood is accomplished. By applying a suitable discharge of electrical current to the ventricular muscle fibers, it is possible to depolarize enough of the fibers at once to re-establish synchrony, thus enabling the ventricles to resume the normal rhythmic pumping.
Defibrillation as it is used today in emergency situations generally employs two electrodes placed on the chest of a patient. An electrical current is discharged through the electrodes and defibrillation of the heart is accomplished. It is well known that within minutes of the onset of ventricular fibrillation, irreversible changes start to occur in the brain and other vital organs; hence it is desirable to effect defibrillation as promptly as possible. Therefore, a reliable automatic defibrillator is desirable because of the need to terminate fibrillation promptly. It is not reasonable to expect that a trained hospital attendant will be present to aid a patient experiencing ventricular fibrillation.
Automatic cardiac defibrillator devices, which sense and analyze the electrical activity of the heart, are known in the art such as shown by U.S. Pat. No. 3,857,398. The electrical activity of the heart has typically been detected by a pair of electrodes placed in or around the heart. Such a method enables detection of an electrocardiogram (ECG) showing a record of R and T waveforms or complex indicating stages of electrical depolarization and repolarization of the ventricles of the heart. However, those devices which sense electrical activity alone have not been proven reliable, since many types of arrhythmia, i.e., a rapid beating of the heart, may mimic ventricular fibrillation and may deceive a detector monitoring only the electrical activity of the ventricles and cause it to deliver an unnecessary defibrillatory shock. With the relatively high current levels required to defibrillate the cardiac ventricles, it is imperative that no defibrillatory shock be delivered unless necessary. If the detecting system should make a false positive decision, i.e., diagnose the presence of ventricular fibrillation when it is, in fact, not present, the patient will unnecessarily experience an uncomfortable, perhaps painful and possibly harmful electric shock. If the detector should arrive at a false negative decision, i.e., fail to recognize the presence of ventricular fibrillation, the patient will probably die.
Automatic cardiac defibrillator devices which sense not only the electrical activity but also the mechanical activity of the heart are also well known in the art. Such devices have typically utilized an ECG in conjunction with one of the many methods known in the art for measurement of ventricular mechanical activity. Development of devices for measuring stroke volume are briefly discussed in an article by Geddes et al. titled "Continuous Measurement of Ventricular Stroke Volume by Electrical Impedance," (pages 118-131) appearing in the April-June 1966, Cardiovascular Research Center Bulletin published by Baylor University College of Medicine.
The electrical impedance method of measuring ventricular stroke volume has been recognized for a number of years as an effective method of instantaneously detecting mechanical pumping activity of the heart. It is well known that the resistance of a conductor depends upon the resistivity of its component material, and varies with the length, and inversely with the cross-sectional area. If the length is kept constant and the amount of conducting material between a pair of electrodes is varied, the resistance varies accordingly. Since the apex-base length of the heart remains substantially constant during systole, the resistance measured between a pair of electrodes inserted into the base and apex of a ventricle varies inversely with cross-sectional area. Further, since the conductivity of the conductive material (blood) is more than five times that of cardiac muscle, the majority of the current between the pair of electrodes is confined to the blood within the ventricle. Thus a decrease in diameter during systole decreases the cross-sectional area of the blood between the electrodes and increases the resistance measured between the electrodes inserted into the cavity at the apex and base of the heart. The blood in the ventricle constitutes, therefore, a conductor of irregular and changing shape, establishing a definite relationship between the changes in impedance and in volume during a cardiac cycle. A further understanding of the continuous measurement of ventricular stroke volume by electrical impedance is contained in the above article, which is hereby incorporated herein by reference.
Workers in the field have utilized and combined the teachings described in the article with the circuitry of an ECG for automatically detecting fibrillation. Heilman et al., U.S. Pat. No. 4,030,509, issuing June 21, 1977, also describe the placing of base and apex electrodes around portions of the heart for discharging energy. The use of an electrode covering the apex of the heart in combination with a superior vena cava catheter electrode is also described in the literature. All of the devices for measuring ventricular stroke volume by electrical impedance have required either a thoracotomy or a laparotomy. In either procedure the heart itself must be surgically exposed.
Pressure transducers attached to a catheter introduced into the heart via the superior vena cava have been employed for measuring cardiac mechanical activity, but such transducers have proven susceptible to mechanical failure and to premature disintegration during the high-current levels delivered for defibrillation. It would, therefore, be desirable to employ a single catheter implantable in the right ventricle of a heart by insertion through a superficial vein, or the superior vena cava, in the right atrium thereby decreasing surgical risk and trauma to the patient and having a plurality of electrodes for detecting and measuring ECG signals, for detecting and measuring ventricular stroke volume by electrical impedance, and for discharging a defibrillatory shock to the heart.