Heart Block Disease

Heart block is when electrical signals from the atrium do not reach the ventricle.
The sinus node is the heart's natural pacemaker. A heartbeat begins when the sinus node starts electrical signals in the upper right chamber of the heart (right atrium). Electrical signals from the sinus node travel through a group of cells called the atrioventricular node (AV node) before moving to the ventricle.

Heartblock is when the AV node does not let all electrical signals reach the ventricles. This causes a slow or unsteady heartbeat and does not allow enough blood to circulate through the body. Heart block can cause an abnormally low heart rate (bradycardia) with symptoms such as dizziness, extreme fatigue, shortness of breath, or fainting spells. A pacemaker compensates for the impaired AV node by restoring the heart rate to a level that meets the demands of everyday living.

Single Chamber Pacemakers - pacemaker types

A single-chamber pacemaker uses one lead, placed either in the right atrium or the right ventricle of your heart. (Most single-chamber pacemakers have a lead attached to the right ventricle.)

When Attached in the Atrium- Leads attached in the right atrium are designed to correct abnormalities in the heart's sinoatrial (SA) node (a condition known as "sick sinus syndrome" ). This kind of atrial pacing corrects the slow or irregular heart rates that are common with this condition. A pacemaker lead attached in the atrium will ensure that your heart's atrium will contract at an exact millisecond so blood will be pumped into the ventricle.

When Attached in the Ventricle - Leads attached in the right ventricle correct a slow or unsteady heartbeat that results from a condition called "heart block," in which the heart's necessary electrical currents are blocked from reaching the atrioventricular (AV) node. A pacemaker lead attached in the ventricle will ensure that your heart's ventricles contract rhythmically and fully, to increase the amount of blood that your heart can pump into your lungs and body.

Can Patients with Implantable Cardioverter-Defibrillators Use Cellular Phones?

Although several studies have examined the safety of cellular phone use in patients with pacemakers, the impact of digital cellular telephones on implantable cardioverter-defibrillators (ICDs) has been less studied. Electromagnetic fields may interfere with an ICD, causing inappropriate sensing or temporary suspension of arrhythmia detection. This investigation evaluated the effect of digital cellular telephone use on ICD function in patients and in vitro.

A group of 41 patients with ICDs (manufactured by Medtronic, Inc.) performed a series of tests using a digital cellular telephone (AT&T model 6650 with NADC/TDMA-50 technology). Patients were connected to an electrocardiogram monitor and continuously observed while they received and transmitted calls at both the right and left ears; moving the entire route of the lead from electrode in the heart to its connector end; and on top of the implantation site (to simulate carrying the telephone in the upper shirt pocket or at the belt). The effect of increasing the electromagnetic field intensity was analyzed in vitro by suspending an ICD in a saline bath and observing the field required for the ICD to sense electromagnetic interference.

For the tests conducted on patients, no ventricular tachycardia or fibrillation was activated by the electromagnetic interference in any patient. There was no evidence of pacing inhibition or changes to the programmed pacing intervals during any test. The in vitro tests indicated no interaction between the digital phone and ICD function during normal operation. However, the static magnetic field generated by the cellular phone placed over the ICD at a distance of 0.5 cm or less can activate the internal reed switch, which would result in temporary suspension of ventricular tachycardia and fibrillation detection.

The investigators concluded that the TDMA-50 cellular telephones did not interfere with the types of ICDs examined in this study. However, based on the results of the in vitro tests, they recommended the following:

1. maintaining a minimal separation of 15 cm (6 inches) between a 0.6W cellular phone and antenna to the implanted device, and 30 cm (12 inches) for telephones with 3W or more of power;
2. holding the telephone on the ear opposite the side of the implanted device;
3. not holding the telephone near the chest while dialing or conversing, nor carrying the telephone in a breast pocket or on a belt within 15 cm of the implanted device (whether or not it is in use); and
4. storing the telephone in a location opposite the side of the implanted device.

Digital Cellular Telephone Interaction With Implantable Cardioverter-Defibrillators
Authors: Fetter, JG, et al
Source: Journal of the American College of Cardiology

Future expectations in pacing systems

Something currently being done in a clinical trial is placement of a transvenous lead into a coronary sinus branch vein in order to pace the left ventricle. "A completely transvenous lead system of course simplifies the procedure."

She also noted that current studies – such as MADIT II (Multicenter Automatic Defibrillator Implantation Trial II) and SCD HFT (Sudden Cardiac Death in Heart Failure Trial) – are examining whether CHF patients should prophylactically receive defibrillators. "Because these patients remain at risk for sudden cardiac death [SCD], this therapy is being looked at in CHF patients who have not even had sustained ventricular tachycardia [VT] or ventricular fibrillation [VF].

"Consequently, another development I foresee is a combined biventricular pacemaker and defibrillator. This makes sense in theory because many heart failure patients who have survived VT or VF receive defibrillator therapy, and eventually they succumb to pump failure. It’s a complex issue, because sudden deaths often occur in the setting of worsening heart failure. Implanting today’s defibrillators can reduce SCD mortality, but it can’t lower the mortality risk from progressive CHF. A defibrillator that offers biventricular pacing, however, might potentially treat both problems by reducing SCD and stabilizing pump function. To test this hypothesis, the VENTAK¨ CHF study is now under way."

Because of the sheer number of patients with a history of CHF and arrhythmias, Dr. Saxon said she believes this area could develop its own medical specialty. "This is a burgeoning area of interest now. And with the expanding knowledge base we expect in the next several years, I wouldn’t be surprised if a new cardiac specialty was eventually established with its own academic track."

According to Dr. Saxon, it will be some time before researchers know whether new pacing modalities, such as biventricular pacing, can affect mortality. "In both the VIGOR CHF and the VENTAK CHF studies, researchers are measuring the effects on only cardiac performance and quality of life. If we find positive results, it would of course be ideal if investigators went on to show prolonged survival, too.

"The use of pacing to treat CHF is a new frontier for us as investigators. There is room for much good work to be done in this area, and we are excited at the prospect of helping these patients whose choices are currently so limited." (guidant.com)

pacing research

Other pacing research : Some researchers are exploring different options for biventricular pacing. For example, one of the inclusion criteria for the VIGOR CHF trial is that patients have BBB. Dr. Saxon mentioned, however, that there is no firm consensus as to whether BBB is a necessary criterion for biventricular pacing. She and her colleagues are conducting other acute studies to determine if patients with no overt BBB can improve with biventricular pacing.

"There are also many questions about how biventricular pacing should be applied," she pointed out. "For instance, no one knows the precise location of the ideal LV pacing site. It could be the LV apex, the far lateral wall or closer to the base of the heart. Researchers are also wondering which is more important in reducing CHF symptoms: finding the best LV pacing site or pacing both ventricles. And other investigators are focusing solely on the timing of ventricular activation – whether it’s better to pace both ventricles simultaneously or to delay pacing one ventricle by several milliseconds."

Dr. Saxon explained that while biventricular pacing is the area of most intense interest right now, researchers are continuing to explore other types of pacing to treat CHF. "Whereas a standard dual-chamber pacemaker paces the right atrium, some experimental systems are using left atrial dual-chamber pacing via the coronary sinus. No data have yet been published on this approach, though." (guidant.com)

biventricular pacing studies

Current biventricular pacing studies : Dr. Saxon pointed out that biventricular pacing was first considered as a potential therapy because CHF patients have marked dyssynchrony in ventricular contraction, due to tissue damage from a previous infarct or simply from muscle deterioration. This damage or scarring promotes delays in left ventricular (LV) activation and reduces contractility. "The theory behind biventricular pacing is to resynchronize ventricular function by activating both ventricles at the same time. This, in turn, might improve cardiac contraction and eliminate patient symptoms."

She added that researchers have evaluated this type of pacing primarily in small groups of end-stage patients who were not transplant candidates. Bakker et al. conducted a study of 12 such patients and reported improvements in both hemodynamics and functional capacity. The investigators found that mean LV ejection fraction (LVEF) increased by 8 ± 2%, mean New York Heart Association Class improved from 4 to 2.5 (p = 0.03) and mean peak VO2 increased by 4.4 ml/min/kg (95% CI: 2.2-6.6).5, 6 Dr. Saxon has also published data on several patients whose cardiac function improved with biventricular pacing.

"Consequently the VIGOR® CHF study was developed, using a pacemaker still under investigation that paces both ventricles simultaneously," she explained. The VIGOR CHF trial is expected to enroll over 100 patients at 25 centers worldwide. All patients must have advanced CHF and, despite maximal medical therapy, must remain symptomatic.

"Patients must also have left or right bundle branch block [BBB], since in concept it seems that patients with this type of conduction system delay might benefit the most from restored synchrony."

Dr. Saxon went on to explain that patients in the VIGOR CHF study receive a dual-chamber pacemaker by standard implant techniques, with a right atrial lead to promote AV synchrony and a ventricular lead implanted in the right ventricle. Patients will be paced in the VDD mode.

"When designing this study, the investigators looked at ways to protect these very sick patients, who are susceptible to adverse effects from invasive studies. We are using co-investigators – an electrophysiologist, a heart failure specialist, an experienced cardiovascular surgeon and an echocardiographer. It takes a lot of effort to coordinate these teams at each center, but this unique approach seemed necessary to help ensure patient safety."

The endpoints of the VIGOR CHF study will assess changes in LVEF, peak exercise ability and quality of life issues, as well as medical cost. "We hope not only to resynchronize ventricular contractions, but also to reduce paradoxical septal wall motion," she said. The study is expected to take approximately three years. (guidant.com)

research with pacing therapy

Prior research with pacing therapy : Around 1990 there were early published reports about the benefits of dual-chamber pacing with a shortened atrioventricular (AV) delay.2-4 "The studies found that CHF patients improved significantly with a 'superphysiologic’ AV delay of about 100 ms," said Dr. Saxon. "This sparked a lot of enthusiasm. However, subsequent studies have not been able to replicate the early results, except in one subgroup – patients who have a marked delay in their AV nodal conduction, and thus have long baseline PR intervals. In these patients, the ventricular contraction is delayed, leading to inadequate filling and mitral valve regurgitation. A pacemaker with a shorter AV delay can help solve this problem. A consensus is now emerging that while reducing the AV delay can help patients in this one small niche group, it’s not a therapy that can be applied broadly. However, these studies were among the first to show that pacing could be used to treat a segment of CHF patients who had no traditional bradycardia indications."

She added that decreasing the AV delay has the potential to help more CHF patients, but the AV delay must be tailored to the patient. "If a physician is willing to spend the time and order sophisticated tests to help pinpoint a patient’s optimal AV delay, there will usually be some benefit – sometimes incremental, sometimes dramatic. But this strategy is certainly not widely applicable to CHF patients." (guidant.com)