A review of pacemakers that physiologically increase rate: the DDD and rate-responsive pacemakers.

Abstract There is little question that the rate responsive pacemaker is beneficial and affords better exercise responsiveness than the VVI by increasing cardiac output during exercise. Each method for controlling rate promises certain advantages, and experiences other compromises in practical implementation. The development of better sensor technology is necessary before some potentially sound methods of control can be investigated practically. Some methods will likely be indicated for certain patient groups, while other methods will better benefit other populations. A single rate responsive method that is superior to all others has not yet surfaced. Rate responsive pacemakers that sense parameters other than atrial rate are indicated in several cases, including sick sinus syndrome, atrial fibrillation, and atrial flutter which cause bradycardia. In fact, most patients with A-V block who would ordinarily receive a VVI pacemaker may benefit from a rate responsive pacemaker. Those patients with sick sinus syndrome and intact A-V conduction who are paced with atrial (AAI) or dual chamber (DDD) will receive the most obvious benefit by the addition of rate responsiveness to atrial pacing. VVI-paced patients in this population will also benefit, although they will lack the A-V synchronous contribution. A-V synchrony plays a less important role as pacing rate increases; the contribution of increased rate to cardiac output is more significant than synchrony at higher rates. 150–152 A-V synchrony may be more significant in compromised hearts, where large increases in rate are contraindicated. Patients with arrhythmias such as atrial fibrillation and flutter-precipitating bradycardia will benefit from rate responsiveness, because atrial rate is unstable and therefore unreliable for atrial-triggered pacing. These arrhythmias may accompany complete heart block due to disease, valve replacement or His-bundle ablation. Rate responsiveness not based on atrial rate is indicated to avoid the inappropriate ventricular rates that result from high atrial rates. For complete heart-block patients with normal atrial conduction and no atrial arrhythmias, the indicated pacing modes include VVI, rate responsive (VVI), and atrial-triggered pacing (such as DDD). Patients who are asymptomatic with VVI pacing may derive additional exercise tolerance and sense of well-being with a rate responsive pacemaker. The DDD appears to be indicated for patients who remain symptomatic at rest with VVI-mode pacing. However, for those not prepared for dual chamber implants, the rate responsive VVI pacemaker represent a second alternative. For patients who are in the lower functional classes based on the New York Heart Association scale and who can sustain an active fitness program, the rate responsive VVI provides higher ventricular rates without the threat of pacemaker-mediated tachycardia experienced with the DDD. For patients in the higher functional classes where exercise is less tolerated, the need is for efficient production of cardiac output during daily activities. The rate responsive VVI may allow lower basal rates increasing cardiac perfusion by lengthening diastole. During activity pacing rate can be increased moderately to provide a sense of well-being. The rate responsive techniques are at various stages of development. Obviously, the atrial-triggered (DDD) is the most developed. The practical use of pH awaits the development of a reliable sensor. Early clinical trials were abandoned because of the difficulty. The magnitude of the minute variations in pH constrains the sensor to having high stability. "pH" responds quickly and potentially has high specificity. QT-controlled pacemakers, which use the ventricular electrode for sensing, have been implanted in over 2,000 patients in Europe. Adequate T wave sensing and QT interval stability are necessary. QT interval responds quickly and has medium specificity. Pacemakers controlled by respiratory rate, using electrodes to sense transthoracic impedance, have been implanted in over 1,000 patients in Europe. The additional subcutaneous implantation of the remote impedance electrode is necessary. Respiratory rate responds quickly and has good specificity. Physical activity, sensed as acceleration within the pulse generator case, is being clinically tested in Europe and the United States. Implantation is similar to the standard VVI. Activity has fast response and medium to low specificity. Temperature-controlled pacemakers, using a thermistor in the pacing lead to sense blood temperature, have been implanted in Europe. Temperature has fast response to most forms of exercise and medium to others. The specificity is good. The stroke volume pacemaker awaits the development of a sensor. Right ventricular impedance proposed as the sensing means is technically obtainable, but requires more research to verify the technique's stability with catheter movement and right-to-left association. The response based on stroke volume should be fast; the specificity depends upon the sensor. Oxygen saturation, measured in venous blood, lacks a proven long-term sensor. Current efforts are focused toward optical and solid state sensors. An optical sensor unaffected by fibrin encapsulation is necessary. Long-term stability of solid state sensors has not been demonstrated. BP, or some function of pressure measured within the cardiovascular system, may prove to be an indicator for rate control. Sensor technology and physiologic understanding are still progressing. Other quantitites, such as circulating levels of catecholamines, are being considered. It is evident that the availability of a sensor is a key limitation to several techniques. More mature sensors will allow more optimal detection of physiologic needs. Also evident is the need for rate responsive pacemakers that complement the DDD to provide adequate treatment and sense of well-being to all pacing patients. These and future concepts for controlling pacemaker rate according to the body's needs provide the basis for tomorrow's pacemakers.