Pacemaker Can Prevent Defibrillator Discharge

-(--Deanne's defibrillator geared up to kick in on Sept. 8 and Oct 13, 2002. Both times, the paced heart was able to re-establish control over itself.)

May 21, 2003 — Delivery of a burst of high-speed pacing for fast ventricular tachycardia (VT) can cut the need for shock by 70% in patients with implantable cardioverter defibrillators (ICDs), according to late-breaking results of a study presented on the final day of the 24^th Annual Scientific Sessions of the North American Society of Pacing and Electrophysiology in Washington, DC.^[1]

Lead investigator Mark S. Wathen, MD, Vanderbilt University (Nashville, TN), presented preliminary data from the Pacing Reduces Shocks for Fast Ventricular Tachycardia II (PainFREE Rx II) trial that compared antitachycardia pacing (ATP) with shock for fast VT in ICD patients. PainFREE Rx II was a prospective, single-blinded trial that randomized 637 ICD patients to either ATP (n = 315) or shock therapy (n = 322) to examine episode duration between both arms (primary endpoint).

Distinction between fast VT, slow VT, and VF; definition of required therapies for ATP and shock arms

Fast VT was defined as the zone between 240 and 320 msec (188-250 beats/min); faster rates were defined as ventricular fibrillation (VF). Slow VT consisted of a QRS duration of 320 msec down to a programmable feature that required >/= 40 msec differentiation between fast and slow VT. "Importantly, 18 out of 24 intervals were necessary for detection of the fast VT and VF zones," Dr. Wathen said.

In the ATP arm, the pacing algorithm for fast VT included an initial burst of ATP that consisted of a single sequence of 8 pulses at 88% of the VT cycle length. If this therapy failed to convert the patient, a second therapy, a shock equal to the defibrillation threshold (DFT) plus 10 joules followed by a shock of maximum ICD output, was delivered. In the shock arm, shock therapy for fast VT was first a shock equal to DFT plus 10 joules, with subsequent therapies at maximum output.

Both groups were well-balanced; indication for ICD was primary prevention in 43% to 44% of patients

Baseline clinical characteristics were well balanced between the 2 groups with respect to age (average, 67 years), ejection fraction (average, 32%), sex (male, 77-80%), coronary artery disease (CAD) (85%), and history of syncope (35%). Both groups received similar pharmacologic therapies (beta-blockers, 57-60%; angiotension-converting enzyme [ACE] inhibitors, 50%; class I antiarrhythmic drugs, 1-2%; and class III antiarrhythmic drugs, 17-22% [majority received amiodarone]). Dr. Wathen also reported that the indication for ICD implantation was primary prevention in 43-44% of patients, and approximately 75% of patients had dual-chambered ICDs.

In the trial, a total of 1278 episodes in 25% of study patients (n=159) were identified as VT or VF; 719 (56%) of these episodes were slow VT; VF was noted in 133 (10%) episodes; and fast VT was found in 426 (33%) of ventricular episodes.

No significant difference was noted in the number of fast VT events, fast VT episodes, or duration of episode between the 2 groups; use of ATP resulted in 70% relative reduction in shock

There was no statistically significant difference between the percentage (and number) of patients who had fast VT events in either the ATP or shock arms (15% [n = 46] vs 16% [n = 50], respectively). In addition, after accounting for 2 patients in the ATP arm who together had a total 131 episodes of fast VT, there were also similar numbers of episodes of fast VT in the ATP and shock arms (151 vs 144, respectively). Researchers noted that there was no significant difference in the episode duration between the shock and ATP arms (10 sec vs 9.7 sec, respectively). See table below for additional trial results.

Dr. Wathen reported that not all patients with fast VT episodes received shock therapy in the shock arm; only 67% of episodes were shocked, and 30% of fast VT episodes spontaneously terminated after detection. In comparison, only 20% of patients in the ATP arm received shock therapy. "A single empiric ATP attempt terminated fast VT in 77% of episodes," Dr. Wathen told meeting attendees. This resulted in a 70% relative reduction in shock in the ATP group, compared with the shock arm. Dr. Wathen noted that on a per patient basis for ATP efficacy for fast VT, 80% of patients in the trial benefited from ATP therapy in the fast VT zone. In addition, there was almost no risk of acceleration of VT rate between the 2 groups; only 2 episodes (1.4%) of acceleration after a shock, and 3 episodes (1.8%) by ATP. Finally, the use of ATP did not increase negative outcomes -- for example, ATP was not associated with an increase in death, episode duration, acceleration, or syncopal episodes (Table).

Table. PainFREE Rx II: 12-Month Clinical Outcomes

Outcome	ATP	Shock
Fast VT events, % (n)	15 (46)	16 (50)
Fast VT episodes (#)	282*	144
Episode duration (sec)	9.7	10
Fast VT outcome (% shocks)	20	67
Acceleration, % (n)	1.8 (3)	1.4 (2)
Total mortality, % (n)	10 (33)	7 (24)
Sudden cardiac death, % (n)	1.6 (2)	1.6 (2)
Syncope, (%) (n)	1.4 (2)	0.7 (1)

*2 patients had 131 episodes in the ATP arm; they were the only patients to have more than 20 episodes of fast VT. Remaining 44 patients had a total of 151 episodes.

ATP = antitachycardia pacing ; VT = ventricular tachycardia

Dr. Wathen told attendees that when examining the factors that could predict who would and who would not respond to ATP, left ventricular ejection fraction was found to predict success, whereas anterior myocardial infarction might predict ATP failure. "Importantly, the presence of a myocardial infarction in a patient did not predict ATP success. Another way to state this is that non-CAD patients seemed to have benefit from ATP similar to CAD in this trial, and nonsustained VT was not predicted," Dr. Wathen said.

Most fast rhythms can be pace terminated; shocks are last resort

"This study provides data that should lead to a major paradigm shift," Dr. Wathen said in a press release issued by Medtronic, Inc. (Minneapolis, Minnesota) "What we've shown is that most fast rhythms can be pace terminated and that shocks are a last resort, although ICD patients need the protection of both therapies."

The PainFREE Rx II study results have major implications for quality of life in patients with ICDs. In an interview with Medscape CRM, panel moderator D. George Wyse, MD, University of Calgary (Calgary, Alberta, Canada) said, that reducing the need for shocks for many VT episodes could have a significant impact on the quality of life for patients, particularly since shocks can be quite painful and traumatic. "Some patients even ask to have their devices removed," he said.

"Originally, the fear was that antitachycardia pacing wouldn't work or [that it] would even trigger a faster VT," Dr. Wyse said. "This study shows that by delivering pacing faster than the heart is beating for a period of about 6 seconds, the ICD gets control of the heart and VT can be terminated about 77% of the time." He added that although this antitachycardia pacing approach has been utilized before, this is the first randomized controlled trial to compare ATP with shocks.

"Most ICDs on the market, with the exception of the low-cost one just approved (Cardiac Airbag; BIOTRONIK Inc. [Portland, Oregon]), which is shock only, have the capability to be programmed for pacing," Dr. Wyse pointed out.

Reference

1. Pacing Reduces Shocks for Fast Ventricular Tachycardia II (PainFREE Rx II). Core sessions 31: late-breaking clinical trials II. North American Society of Pacing and Electrophysiology 24th Annual Scientific Sessions; May 17, 2003; Washington, DC.

By Martha Kerr
Reviewer: Albert A. Del Negro, MD

---isn't it wonderful that there are such dedicated individuals gaining so much ground in the field of medicine?

--implantable defibrillators are containdicated in people with certain heart and vascular diseases; and not commonly given to anyone without a history of heart electrical episodes.

The American Heart Association (AHA) has estimated that over 450,000 individuals in the United States experience a sudden cardiac arrest (SCA) each year.^[1]In addition, a recent study showed that during the past decade the proportion of sudden cardiac death to all cardiac death increased by 12.4%.^[2]This may reflect in part the decrease in mortality of acute myocardial infarction (MI) in the modern interventional era, with a resulting increase in the proportion of cardiac deaths due to other causes. It is also known that the majority of SCAs occur outside of a hospital and that a significant number of such events happen at home with witnesses present.^[3]

Based on what is known about the prevalence and location of where most SCA occurs, timely intervention with a home defibrillator could potentially benefit many victims who might otherwise not survive to hospital admission. The concept of early, first-responder cardiac defibrillation has been endorsed by the AHA, The American College of Emergency Physicians, and the International Association of Fire Chiefs.^[4]

Compelling arguments exist in the medical literature for owning a home defibrillator,^[5,6] and encouraging survival data from studies of defibrillator use in public places by lay rescuers^[7-9] support the safety and ease of use of the technology. However, data from large-scale, randomized studies of defibrillator use in the home are not yet available, and probably won't be for at least 1 year.

Until large-scale studies confirm the value of home defibrillator units, physicians will be in a difficult position when asked by a patient or family member to prescribe a home defibrillator. No clinical guidelines for prescribing a home defibrillator currently exist. However, since the known risk factors for SCA parallel those for coronary artery disease (CAD), any discussion about purchase of a home defibrillator must be based on the patient's risk factor profile. Discussion must also be balanced by discussion of risk factor modification. It would be inappropriate to recommend a home defibrillator unit while overlooking the provision of comprehensive, specific risk reduction advice for patients who are sedentary, obese, hypertensive, hypercholesteremic, or who smoke tobacco.

While we await further data, physicians may want to consider cautious support for home defibrillator use for patients with established CAD. The use of home defibrillators as a primary prevention strategy for patients without known heart disease is currently more difficult to unequivocally support.

The following commentaries in the medical literature provide analyses of the pros and cons of both home and public access use of defibrillators:

• Brown J, Kellerman AL. The shocking truth about automated external defibrillators. JAMA. 2000;284:1438-1441. Abstract
• Caffery SL. Feasibility of public access to defibrillation. Curr Opin Crit Care. 2002;8:195-198. Abstract
• Eisenberg MS. Is it time for over-the-counter defibrillators? JAMA. 2000;284:1435-1438. Abstract

Question: What percentage of SCA occurs at home?

Based on information collected in 2000, the AHA stated that 80% of out-of-hospital SCA in the United States occurred at home, and that 60% of these events were witnessed.^[3]Recent data from other countries included findings of 84% of SCA occurring in the home in Ontario, Canada^[9] and 65% in Sweden.^[10]

Information about home defibrillators and training in the use of the device is widely available. The AHA and the American Red Cross provide Web-based information about automated external defibrillators written for consumers. The National Center for Early Defibrillation (a not-for-profit resource center based at the University of Pittsburgh) also provides information related to the need for early defibrillation and the use of home and public-access devices.

The AHA emphasizes early defibrillation as a key part of their educational programs and provides training in the use of a portable defibrillator in conjunction with training in CPR.^[2] During fiscal year 2001-2002, more than 600,000 lay individuals received this combined training.^[3] The American Red Cross also supports and provides training for lay individuals in defibrillator use.

Click here for AHA site

Click here for Red Cross site

Click here for National Center for Early Defibrillation site

Question: What are the risk factors for SCA?

Some risk factors for SCA are known, while others remain unknown and/or not well described. The most frequent causes of lethal cardiac arrhythmias among children and teenagers include coronary anomalies, hypertropic cardiomyopathy, and myocarditis.^[11] Among adults, the majority of SCA and subsequent death are related to CAD.^[12] Most victims of SCA are found to have significant coronary artery stenoses; acute thrombus or plaque disruptions are the specific findings most frequently detected.^[13]

Since it has also been estimated that up to 70% of SCAs are linked to CAD, known risk factors for SCA reflect the most well-defined risks for heart disease ^[14]: age older than 45 years, elevated blood pressure, smoking, elevated total serum and low-density lipoprotein (LDL) cholesterol, diabetes, obesity, family history of coronary heart disease, elevated serum C-reactive protein,^[15] and sedentary lifestyle. However, it is also possible that SCA can present as the first evidence of coronary heart disease. In findings over 4 decades from the Framingham Heart Study, an estimated 63% of women and 50% of men who died suddenly from CAD had no known previous symptoms of CAD.^[3]

An expanded overview of the etiology and risk factors associated with SCA can be found in the Medscape Clinical Update Sudden Cardiac Arrest: Use of Home Defibrillators (http://www.medscape.com/viewarticle/448882). A comprehensive description of the incidence and prevalence of CAD in the United States is provided annually in a publication from the AHA.^[3]

Question: Which patients are the most appropriate potential candidates for a home defibrillator unit?

Individuals who have suffered a MI have a sudden death rate from cardiac disease that is 4-6 times greater than the risk for the general population. Within 6 years after a documented MI, 7% of men and 6% of women will die suddenly.^[3] Patients known to be at the highest risk for SCA include individuals with previous MI with left ventricular dysfunction and a history of sustained or non-sustained ventricular tachycardia, or a history of previous cardiac arrest. A cardiologist should evaluate all such individuals for consideration of treatment with an implantable defibrillator device. It is possible that patients with systolic dysfunction and CAD who are not candidates for an implantable device could benefit from owning a home defibrillator.

Beyond consideration of recognized risk factors that have already been described and known cardiac pathology, it is not possible to specifically identify other individuals at increased risk for SCA at the present time. Although studies focused on SCA risk are beginning to provide more data, results are still considered preliminary.

The value of prescribing a home defibrillator for patients at low to medium risk for CAD is the area that is most controversial. This currently existing "gray area" would particularly include patients with known cardiac risk factors but who are without apparent clinical evidence of CAD. The financial cost of owning a home defibrillator is significant, and it can be argued that a strategy of aggressive risk factor reduction would be a more effective and cost prudent strategy.

Still, as consumer awareness of such devices grows, physicians will be increasingly asked by patients to prescribe home defibrillators even if their individual risk for SCA is moderate to very low. Due to lack of consensus to guide prescribing and lack of evidence-based information about efficacy, it is most reasonable to discuss the possibility of owning a home defibrillator unit with all patients with known CAD who have none of the above high-risk predictors that would warrant an implantable device. The extremely large number of individuals in the United States who are free of known heart disease but who have 1 or more risk factors for CAD makes the strategy of home defibrillator ownership for all such individuals impractical and cost inefficient at this point in time. Future efficacy and cost-effectiveness studies may shed light on the appropriateness of home defibrillator ownership for such patients.

The Advanced Cardiac Life Support Subcommittee and the Emergency Cardiac Care Committee of the AHA has identified "early" defibrillation, in which an electrical shock is delivered within a very few minutes after arrest, as the most important predictor of survival of SCA.^[18] The chance of survival decreases 7% to 10% for every minute that passes without defibrillation when a shockable rhythm is present. When shock is delivered within 3-5 minutes, rates of survival of SCA secondary to ventricular fibrillation range from 48% to 74%.^[3] Correlations between defibrillation delay and survival are further strengthened by findings of initial survival rates of nearly 100% when a shock for ventricular fibrillation was delivered within 1-2 minutes after cardiac arrest in an inpatient setting.^[16,17]

In addition to decreased overall survival, delay in defibrillation also results in increased neurologic impairment in SCA survivors.^[19] The chances of survival (most likely with irreversible cognitive impairment) would have been extremely poor for a patient who did not have appropriate intervention (CPR, including defibrillation) for an extended period of time following arrest. The AHA estimated that the likelihood of surviving SCA due to ventricular fibrillation is only 2% to 5% if defibrillation is provided more than 12 minutes after collapse.^[3]

Gerald W. Smetana, MD, Division of General Medicine and Primary Care, Beth Israel Deaconess Medical Center; Associate Professor of Medicine, Harvard Medical School, Boston, Massachusetts .Dr. Smetana has no significant financial relationships within the last 12 months to disclose. No investigational products or unlabeled uses are discussed in the article.