Automated external defibrillators: technical considerations and clinical promise.

Early defibrillation is the most important determinant of survival for victims of cardiac arrest due to ventricular fibrillation. The automated external defibrillator (AED) was developed as the result of the American Heart Association's Public Access Defibrillation initiative. The goal of this initiative is to place AEDs in strategic locations so that laypersons with minimal training could promptly defibrillate victims of cardiac arrest. Because of changes in design and the use of alternative waveforms for defibrillation, the modern AED is compact and portable, simple to use, and highly efficacious; in addition, it requires little maintenance. Automated external defibrillators have been used successfully by traditional and nontraditional responders as well as laypersons. In special environments, such as casinos and commercial aircraft, AEDs have performed particularly well. State and federal legislation has eased concerns about AED use by extending legal protection to AED users under Good Samaritan laws. Since the experience continues to be positive, AEDs are being used in increasingly diverse community locations, and public awareness is growing. The American Heart Association's initiative is progressing rapidly.

Effect of carvedilol on survival and hemodynamics in patients with atrial fibrillation and left ventricular dysfunction: retrospective analysis of the US Carvedilol Heart Failure Trials Program.

BACKGROUND: Atrial fibrillation (AF) is present in a significant number of patients with congestive heart failure (CHF) caused by left ventricular dysfunction and is associated with significant morbidity and increased mortality rates. Thus it is necessary to establish therapy to improve the outcome in this high-risk population. METHODS: We conducted a retrospective analysis of data from the US Carvedilol Heart Failure Trials Program and identified patients with AF at the time of enrollment. In these trials, 1094 patients with at least 3 months of heart failure symptoms and an ejection fraction < or = 0.35 were randomly assigned to receive carvedilol or placebo in a double-blind, stratified program according to performance on an exercise test. RESULTS: One hundred thirty-six patients with concomitant AF and CHF were identified during the screening visit (84 assigned to carvedilol and 52 to placebo). Therapy with carvedilol resulted in a significant improvement in left ventricular ejection fraction (from 23% to 33% with carvedilol and from 24% to 27% with placebo, P =.001). The physician global assessment improved in a greater number of patients treated with carvedilol than in those treated with placebo (71% vs 48%, P =.025). A trend toward a reduction in the combined end point of death or CHF hospitalization was also observed (19% in patients treated with placebo and 7% in patients on carvedilol; relative risk, 0.35; 95% confidence interval, 0.12, 1.02; P =.055). CONCLUSIONS: In patients with AF complicating CHF, carvedilol significantly improves left ventricular ejection fraction and physician global assessment and probably reduces the combined end point of CHF hospitalizations or death.

The hot can: ICD failure presenting as severe shoulder pain.

We describe a patient whose presentation of ICD generator failure was excruciating shoulder pain lasting 3 hours. This painful episode prompted evaluation and subsequent generator replacement. Destructive analysis of the explanted device revealed a short circuit of three battery filter capacitors, which resulted in the battery "dumping" its full energy very rapidly, since the impedance across the battery terminal was < 1 ohm. The duration of the painful episode was equal to the estimated ICD battery life under this low impedance condition.

Effect of P-wave timing during supraventricular tachycardia on the hemodynamic and sympathetic neural response.

BACKGROUND: Previous studies have shown the importance of the timing of atrial and ventricular systole on the hemodynamic response during supraventricular tachycardia (SVT). However, the reflex changes in autonomic tone during SVT remain poorly understood. METHODS AND RESULTS: Eleven patients with permanent dual-chamber pacemakers were enrolled in the study. Arterial blood pressure (BP), central venous pressure (CVP), and peripheral muscle sympathetic nerve activity (SNA) were recorded during DDD pacing at a rate of 175 bpm (cycle length 343 ms) with an atrioventricular (AV) interval of 30, 200 and 110 ms, simulating tachycardia with near-simultaneous atrial and ventricular systole, short-RP tachycardia (RPPR). Each pacing run was performed for 3 minutes separated by a 5-minute recovery period. All patients demonstrated an abrupt fall in BP, an increase in CVP, and an increase in SNA regardless of the AV interval. The decreases in SBP, DBP, and MAP and the increase in CVP were significantly less during long-RP tachycardia (AV interval 110 ms) than during the other 2 pacing modes (P:<0.05), and the increase in SNA in 7 of the 11 patients was significantly greater during closely coupled atrial and ventricular systole than during long-RP tachycardia (P:<0.05). CONCLUSIONS: These data suggest that the superior maintenance of hemodynamic stability during long-RP tachycardia is accompanied by reduced sympathoexcitation, which is primarily mediated by the arterial baroreceptors, with a modest cardiopulmonary vasodepressor effect.

Antiarrhythmic drugs in pregnancy.

During pregnancy a number of rhythm disturbances can occur in both the mother and fetus; these may range from benign ectopy to life-threatening arrhythmias. With a clear understanding of the maternal hemodynamic changes associated with pregnancy, and the appropriate antiarrhythmic therapies available, almost all such cases can be treated successfully. Although no drug is completely safe, most are well tolerated and can be given with relatively low risk. Drug therapy should be avoided during the first trimester of pregnancy if possible and drugs with the longest record of safety should be used as first-line therapy. Conservative therapies should be used when appropriate. Several drug options exist for most maternal and fetal arrhythmias.

Use of automated external defibrillators by a U.S. airline.

BACKGROUND: Passengers who have ventricular fibrillation aboard commercial aircraft rarely survive, owing to the delay in obtaining emergency care and defibrillation. METHODS: In 1997, a major U.S. airline began equipping its aircraft with automated external defibrillators. Flight attendants were trained in the use of the defibrillator and applied the device when passengers had a lack of consciousness, pulse, or respiration. The automated external defibrillator was also used as a monitor for other medical emergencies, generally at the direction of a passenger who was a physician. The electrocardiogram that was obtained during each use of the device was analyzed by two arrhythmia specialists for appropriateness of use. We analyzed data on all 200 instances in which the defibrillators were used between June 1, 1997, and July 15, 1999. RESULTS: Automated external defibrillators were used for 200 patients (191 on the aircraft and 9 in the terminal), including 99 with documented loss of consciousness. Electrocardiographic data were available for 185 patients. The administration of shock was advised in all 14 patients who had electrocardiographically documented ventricular fibrillation, and no shock was advised in the remaining patients (sensitivity and specificity of the defibrillator in identifying ventricular fibrillation, 100 percent). The first shock successfully defibrillated the heart in 13 patients (defibrillation was withheld in 1 case at the family's request). The rate of survival to discharge from the hospital after shock with the automated external defibrillator was 40 percent. A total of 36 patients either died or were resuscitated after cardiac arrest. No complications arose from use of the automated external defibrillator as a monitor in conscious passengers. CONCLUSIONS: The use of the automated external defibrillator aboard commercial aircraft is effective, with an excellent rate of survival to discharge from the hospital after conversion of ventricular fibrillation. There are not likely to be complications when the device is used as a monitor in the absence of ventricular fibrillation.

Biventricular pacing decreases sympathetic activity compared with right ventricular pacing in patients with depressed ejection fraction.

BACKGROUND: Although there have been few studies in which the hemodynamic effects of right ventricular (RV) and left ventricular (LV) pacing were compared with those of biventricular (BV) pacing, the autonomic changes during these different pacing modes remain unknown. We hypothesized that BV pacing results in improved hemodynamics and a decrease in sympathetic nerve activity (SNA) compared with single-site pacing. METHODS AND RESULTS: Thirteen men with a mean ejection fraction of 0.28+/-0.7 were enrolled in the study. Arterial blood pressure (BP), central venous pressure (CVP), and SNA were recorded during 3 minutes of right atrial (RA)-RV, RA-LV, and RA-BV pacing at a rate 10 beats faster than sinus rhythm. BP was greater during LV (151+/-7/85+/-3 mm Hg) and BV (151+/-6/85+/-3 mm Hg) pacing than during RV pacing (146+/-7/82+/-3 mm Hg) (P:<0.05). There were no differences in CVP among all pacing modes (P:=0.27). SNA was significantly less (P:<0.02) during both LV (606+/-35 U) and BV (582+/-41 U) pacing compared with RV pacing (685+/-32 U) (P:<0.02). Although not statistically significant (P:=0. 08 to 0.14), there was a trend for patients with a narrow QRS to have a lower mean BP and higher SNA during LV pacing than during BV pacing (r=0.42 to 0.49). CONCLUSIONS: LV-based pacing results in improved hemodynamics and a decrease in SNA compared with RV pacing in patients with LV dysfunction regardless of the QRS duration.

Initial energy for elective external cardioversion of persistent atrial fibrillation.

We conducted a prospective randomized study to determine the safety and efficacy rate of 3 commonly used energy levels (100, 200, and 360 J) for elective direct-current cardioversion of persistent atrial fibrillation. When compared with 100 and 200 J, the initial success rate with 360 J was significantly higher (14%, 39%, and 95%, respectively), and patients randomized to 360 J ultimately required less total energy and a lower number of shocks.

Increased sympathetic activity after atrioventricular junction ablation in patients with chronic atrial fibrillation.

OBJECTIVES: The aim of this study was to determine the changes in sympathetic nerve activity (SNA) after atrioventricular junction (AVJ) ablation in patients with chronic atrial fibrillation (AF). BACKGROUND: Polymorphic ventricular tachycardia (PMVT) has been reported after AVJ ablation in patients paced at a rate of < or =70 beats/min. We hypothesized that AVJ ablation results in sympathetic neural changes that favor the occurrence of PMVT and that pacing at 90 beats/min attenuates these changes. METHODS: Sympathetic nerve activity, 90% monophasic cardiac action potential duration (APD90), right ventricular effective refractory period (ERP) and blood pressure measurements were obtained in 10 patients undergoing AVJ ablation. Sympathetic nerve activity was analyzed at baseline and during and after successful AVJ ablation for at least 10 min. Data were also collected after ablation at pacing rates of 60 and 90 beats/min. The APD90 and ERP were measured before and after AV block during pacing at 120 beats/min. RESULTS: Sympathetic nerve activity increased to 134 +/- 16% of the pre-ablation baseline value (p < 0.01) after successful AVJ ablation plus pacing at 60 beats/min and decreased to 74 +/- 8% of baseline (p < 0.05) with subsequent pacing at 90 beats/min. Both APD90 and ERP increased significantly. CONCLUSIONS: 1) Ablation of the AVJ followed by pacing at 60 beats/min is associated with an increase in SNA. 2) Pacing at 90 beats/min decreases SNA to or below the pre-ablation baseline value. 3) Cardiac APD and ERP increase after AVJ ablation. The increase in SNA, along with the prolongation in APD, may play a role in the pathogenesis of ventricular arrhythmias that occur after AVJ ablation.

Selective parasympathetic denervation following posteroseptal ablation for either atrioventricular nodal reentrant tachycardia or accessory pathways.

Baroreflex gain and coronary sinus norepinephrine and epinephrine levels were measured before and immediately after radiofrequency ablation in the posteroseptal region in 9 patients with atrioventricular nodal reentrant tachycardia or posteroseptal accessory pathways. Arterial baroreflex gain was significantly reduced after radiofrequency ablation (p = 0.046), whereas coronary sinus epinephrine and norepinephrine levels did not change significantly compared with preablation levels.

Effect of radiofrequency ablation on atrial mechanical function in patients with atrial flutter.

Atrial stunning, as assessed by left atrial appendage emptying and increased spontaneous echo contrast, is known to occur following direct-current cardioversion of atrial fibrillation (AF) and atrial flutter (AFI). Little is known on atrial mechanical function and the time course of atrial recovery following radiofrequency ablation of AFI. Fourteen patients undergoing radiofrequency ablation of persistent typical counterclockwise AFI were enrolled. Two-dimensional and pulse Doppler transesophageal echocardiography (TEE) were performed before ablation and immediately following restoration of sinus rhythm. Left atrial spontaneous echo contrast grades, left atrial appendage emptying fractions, and peak left atrial appendage emptying velocities were measured. Transthoracic echocardiography (TTE) was performed immediately after ablation, then repeated after 1 day, 1 week, and 6 weeks to measure peak transmitral velocities and percent atrial contribution to ventricular filling. Left atrial appendage emptying velocities decreased significantly following AFI termination (44 +/- 23 cm/s before ablation vs 25 +/- 14 cm/s after ablation, p = 0.01). Left atrial appendage emptying fractions also decreased significantly (0.48 +/- 0.1 preablation vs 0.34 +/- 0.17 postablation, p = 0.02). New spontaneous echo contrast developed in 4 patients (29%) after ablation. Four patients had complete atrial standstill after ablation, and 1 patient developed a new left atrial appendage thrombus. The percent atrial contribution to ventricular filling recovered progressively over 6 weeks with significant improvement in peak transmitral velocities at day 7. Thus, atrial stunning occurs after catheter ablation of AFI and may lead to rapid formation of thrombus in the left atrial appendage. Significant improvement in left atrial function occurs in 7 days.

The effect of biphasic defibrillation on the immediate pacing threshold of a dedicated bipolar, steroid-eluting lead.

It is apparent that pacing threshold increases following an ICD shock, although the degree of change observed is dependent on the method used to assess pacing and the lead design used. We previously demonstrated a rise in postshock pacing threshold using a lead with integrated bipolar pacing in which the distal shocking coil also serves as the pacing anode. In this study, we sought to investigate whether the postshock pacing threshold increased significantly in an endocardial, steroid-eluting lead with dedicated bipolar pacing electrodes. Twenty patients (16 men, 4 women; median age 73, ejection fraction [EF] 0.17-0.58) were studied during pectoral ICD implantation (Medtronic active can model 7221Cx or 7223Cx with model 6932-65 lead). The diastolic pulse width pacing threshold at 1 or 2 V was determined. Pacing rate was set > or = 100/min at twice diastolic threshold output to assess pacing immediately following the first DFT test shock. For subsequent shocks, the output was adjusted to establish postshock thresholds as 1, 2, 3, or 4 times the diastolic threshold. The postshock threshold was defined as the output yielding 100% capture > or = 2.5 seconds following a shock. In 8 of 20 patients (ratio 0.40 +/- 0.11), a rise in the post-shock threshold was shown by failure of consistent capture when pacing at 2 times diastolic threshold > or = 2.5 seconds after a DFT test shock. Two of these patients failed at 3 times threshold, but none failed at 4 x threshold. Five of 12 patients with successful capture of 2 times threshold failed to capture at threshold. The postshock threshold increased by a mean factor of 2.83 +/- 0.83 in the group of patients with a threshold rise. Following ICD shock in an active can, steroid-eluting lead system with dedicated bipolar pacing, the post-shock threshold increases significantly. Our studies suggest a need for postshock pacing to be set at least 4 x threshold regardless of the lead design.

Reflex control of sympathetic activity during simulated ventricular tachycardia in humans.

BACKGROUND: Ventricular tachyarrhythmias present a unique set of stimuli to arterial and cardiopulmonary baroreceptors by increasing cardiac filling pressures and decreasing arterial pressure. The net effect on the control of sympathetic nerve activity (SNA) in humans is unknown. The purpose of this study was to determine the relative roles of cardiopulmonary and arterial baroreceptors in controlling SNA and arterial pressure during ventricular pacing in humans. METHODS AND RESULTS: Two experiments were performed in which SNA and hemodynamic responses to ventricular pacing were compared with nitroprusside infusion (NTP) in 12 patients and studied with and without head-up tilt or phenylephrine to normalize the stimuli to either the arterial or cardiopulmonary baroreceptors in 9 patients. In experiment 1, the slope of the relation between SNA and mean arterial pressure was greater during NTP (-4.7+/-1.4 U/mm Hg) than during ventricular pacing (-3.4+/-1.1 U/mm Hg). Comparison of NTP doses and ventricular pacing rates that produced comparable hypotension showed that SNA increased more during NTP (P=0.03). In experiment 2, normalization of arterial pressure during pacing resulted in SNA decreasing below baseline (P<0.05), whereas normalization of cardiac filling pressure resulted in a greater increase in SNA than pacing alone (212+/-35% versus 189+/-37%, P=0. 04). Conclusions--These data demonstrate that in humans arterial baroreflex control predominates in mediating sympathoexcitation during ventricular tachyarrhythmias and that cardiopulmonary baroreceptors contribute significant inhibitory modulation.

Baroreflex gain predicts blood pressure recovery during simulated ventricular tachycardia in humans.

BACKGROUND: Despite similar degrees of left ventricular dysfunction and similar tachycardia or pacing rate, blood pressure (BP) response and symptoms vary greatly among patients. Sympathetic nerve activity (SNA) increases during sustained ventricular tachycardia (VT), and the magnitude of this sympathoexcitatory response appears to contribute to the net hemodynamic outcome. We hypothesize that the magnitude of sympathoexcitation and thus arterial baroreflex gain is an important determinant of the hemodynamic outcome of VT. METHODS AND RESULTS: We evaluated the relation between arterial baroreflex sympathetic gain and BP recovery during rapid ventricular pacing (VP) in patients referred for electrophysiological study. Efferent postganglionic muscle SNA, BP, and central venous pressure (CVP) were measured in 14 patients during nitroprusside infusion and during VP at 150 (n=12) or 120 (n=2) bpm. Arterial baroreflex gain was defined as the slope of the relationship of change in SNA to change in diastolic BP during nitroprusside infusion. Recovery of mean arterial pressure (MAP) during VP was measured as the increase in MAP from the nadir at the onset of pacing to the steady-state value during sustained VP. Arterial baroreflex gain correlated positively with recovery of MAP (r=0.57, P=0.034). No significant correlation between ejection fraction and baroreflex gain (r=0.48, P=0.08) or BP recovery (r=0.41, P=0.15) was found. When patients were separated into high versus low baroreflex gain, the recovery of MAP during simulated VT was significantly greater in patients with high gain. CONCLUSIONS: These data strongly suggest that arterial baroreflex gain contributes significantly to hemodynamic stability during simulated VT. Knowledge of baroreflex gain in individual patients may help the clinician tailor therapy directed toward sustained VT.

Interaction of a commercial heart rate monitor with implanted pacemakers.

Dry-electrode heart rate monitors allow display of heart rate by transmitting a signal to the receiving device, which typically is on the wrist or exercise machine, but due to the potential for electromagnetic interference, their use has been contraindicated in patients with pacemakers. In 12 patients, we found no adverse effect on pacemaker function; in addition, the monitors generally were accurate in measuring heart rate during pacing.

Effects of repeated electrical defibrillations on cardiac troponin I levels.

Multiple endocardial countershocks applied during intraoperative endocardial implantable cardioverter-defibrillator testing for the purpose of defibrillation threshold determination resulted in detectable myocardial injury in 5 of 12 patients, as indicated by elevations in cardiac troponin I levels. This injury was not associated with acute changes on the surface electrocardiogram.

Treatment of cardiac arrhythmias during pregnancy: safety considerations.

Maternal and fetal arrhythmias occurring during pregnancy may jeopardise the life of the mother and the fetus. When arrhythmias are well tolerated and patients are minimally symptomatic, conservative therapy, such as observation and rest or vagal manoeuvres, should be employed. When arrhythmias cause debilitating symptoms or haemodynamic compromise, antiarrhythmic drug therapy is indicated. Although no antiarrhythmic drug is completely safe during pregnancy, most are well tolerated and can be given with relatively low risk. Physiological changes that occur during pregnancy mandate caution when administering antiarrhythmic drugs, with close monitoring of serum concentration and patient response. Drug therapy should be avoided during the first trimester of pregnancy if possible, and drugs with the longest record of safety should be used as first-line therapy. Several therapeutic options exist for most arrhythmias in the mother and fetus. Of the class IA agents, quinidine has the longest record of safety during pregnancy, and is generally well tolerated. Procainamide is also well tolerated, and should be a first line option for acute treatment of undiagnosed wide complex tachycardia. All IA agents should be administered in the hospital under cardiac monitoring due to the potential risk of ventricular arrhythmias (torsade de pointes). The IB agent, lidocaine (lignocaine), has local anaesthetic role but is also generally well tolerated as an antiarrhythmic agents. Phenytoin should be avoided due to the high risk of congenital malformations and limited role as an antiarrhythmic drug. Of the IC agents, flecainide has been shown to be very effective in treating fetal supraventricular tachycardia complicated by hydrops. Beta-Blockers are generally well tolerated and can be used with relative safety in pregnancy, although recent data suggest that they may cause intrauterine growth retardation if they are administered during the first trimester. Amiodarone, a class II agents with characteristics of the other antiarrhythmic drug classes, has been reported to cause congenital abnormalities; it should be avoided during the first trimester and used only to treat life-threatening arrhythmias that fail to respond to other therapies. Adenosine is generally safe to use in pregnancy, and is the drug of choice for acute termination of maternal supraventricular tachycardia. Digoxin has a long track record of treating both maternal and fetal arrhythmias, and is one of the safest antiarrhythmics to use during pregnancy. Direct current cardioversion to terminate maternal arrhythmias is well tolerated and effective, and should not be delayed if indicated. The use of an implantable cardioverter-defibrillator should be considered for women of childbearing potential with life-threatening ventricular arrhythmias.

Intracardiac shunts resulting from transseptal catheterization for ablation of accessory pathways in otherwise normal hearts.

In a series of 14 patients undergoing transseptal catheterization for ablation of left-sided accessory pathways, hydrogen appearance time was used to detect left-to-right shunting after removal of the catheter. Six of the 12 patients who had no evidence of shunt before catheterization had evidence of shunting after the procedure.