Cardiac pacing for rhythm abnormalities began in the 1960s with demand pacing. In these devices a sensed ventricular depolarization (VS) started an escape interval timer. In the absence of another ventricular sense event during the escape interval, the pacer device would time out and deliver a ventricular pacing stimulus (VP) to the right ventricle (RV) of the heart. This modality (VVI) provides a rate floor for the patient. Normal sinus rhythm (NSR) above this rate floor will be sensed and used to reset the escape interval timer to “inhibit” the pacemaker output stimulus.
These devices are successfully used to treat patients with a too slow heart rate (bradycardia). To further improve the cardiac output the atrio-ventricular, or AV sequential pacemaker, was introduced. The AV sequential pacemaker (DVI) will likely stimulate the atrium (AP) prior to stimulating the ventricle (VP). This improves the cardiac output due to increased filling of the ventricles as described by “Starlings Law”. The time interval between the atrial pace (AP) or atrial sense (AS) events and the ventricular stimulus (VP) in this dual chamber modality (DVI) is called the AV delay, and the AV delay is typically a physician programmable parameter, although some devices employ algorithms that perform a form of adaptive AV delay. Further refinement of dual chamber pacing has resulted in devices which sense and pace in both chambers of the right heart. Dual demand (DDD/DDI) pacers can manage heart rhythm over a wide range of conduction disorders.
In the 1980s automatic implantable defibrillators (AID) entered use to deliver defibrillation shock to interrupt bouts of ventricular fibrillation (VF) or ventricular tachycardia (VT) in patients exhibiting sudden death syndrome. These AID devices detected ventricular electrogram (EGM) information from an indwelling lead system and delivered a high voltage current to the heart. Advances in this therapy have included the adoption of transvenous lead systems and the incorporation of tiered therapy.
A tiered ICD device can pace or cardiovert or defibrillate as required. Such devices can deliver stimuli that extend from low energy pacing therapy through cardioversion and antitachycardia therapies such as burst stimulation and ramped stimulation of ventricular chambers. The stimulation train hierarchy attempts to convert an arrhythmia before a need arises to administer defibrillation shocks.
For patients with a fast but regular heart rhythm (ventricular tachycardia, VT), various protocols for administering antitachycardia pacing therapy (ATP) have been proposed and implemented as part of a tiered therapy device.
One strategy widely used calls for fast pacing at a rate near the observed tachycardia rate. The delivery of ventricle burst stimuli will interrupt a reentry circuit or entrain the rhythm. For patients where ventricular tachyarrhythmia accelerates to ventricular fibrillation (VF), the implanted cardiovertor defibrillators must quickly default to a high energy defibrillation regime. The defibrillation shock delivers a relatively large amount of energy to the heart to depolarize critical mass of myocardium at once. With luck a normal sinus rhythm develops after the global refractory period of the tissues. The electrical field distribution is such that nearby skeletal muscles are stimulated as well. This results in very painful therapy that is typically invoked when a high ventricular rate is observed for several seconds. Sometimes the patient experiences an electric pause and needs bradycardia pacing support after the shock.
It has been discovered more recently that patients with heart failure (HF) benefit from cardiac resynchronization therapy (CRT) where two synchronized ventricular stimuli are provided. In such a case that employs multiple leads, multiple output circuits, one for each lead, may be used. One VP stimulus is provided to the right ventricle (RV), and the other stimulus is provided to the left ventricle (LV). It has been also demonstrated by Berry M. van Gelder et. al. in “The Hemodynamic Effect of Intrinsic Conduction During Left Ventricular Pacing as Compared to Biventricular Pacing,” J. Am. Coll. Cardiol. 2005; 46; 2305-2310, that left ventricular pacing is hemodynamically superior to biventricular pacing in CRT when fusion with intrinsic conduction over the right bundle is present. If both chambers are stimulated, the delay between the stimulation of the two chambers VtoV (VV) is selected to mimic the progression of a normal ventricular depolarization wavefront around the heart.