Biphasic shocks compared with monophasic damped sine wave shocks for direct ventricular defibrillation during open heart surgery

BACKGROUND: Biphasic waveform shocks are more effective than monophasic shocks for transchest ventricular defibrillation, atrial cardioversion, and defibrillation with implantable defibrillators but have not been studied for open chest, intraoperative defibrillation. This prospective, blinded, randomized clinical study compares biphasic and monophasic shock effectiveness and establishes intraoperative energy dose-response curves. METHODS: Patients undergoing cardiothoracic surgery with bypass cardioplegia were randomly assigned to the monophasic or biphasic shock group. Ventricular fibrillation occurring after aortic clamp removal was treated with escalating energies of 2, 5, 7, 10, and 20 J until defibrillation occurred. If ventricular fibrillation persisted, a 20-J crossover shock of the other waveform was used. RESULTS: Cumulative defibrillation success at 5 J, the primary end point of the study, was higher in the biphasic group than in the monophasic group (25 of 50 vs. 9 of 41 defibrillated; P = 0.011). In addition, the biphasic group required lower threshold energy (6.8 vs. 11.0 J; P = 0.003), less cumulative energy (12.6 vs. 23.4 J; P = 0.002), and fewer shocks (2.5 vs. 3.5; P = 0.002). Crossover-shock effectiveness did not differ between groups. Dose-response curves show biphasic shocks to have higher cumulative success rates at all energies tested. CONCLUSIONS: Biphasic shocks are substantially more effective than monophasic shocks for direct defibrillation. The dose-response curve guides selection of first-shock energy for traditional step-up protocols. Starting at 5 J optimizes for lowest threshold and cumulative energy, whereas 10 or 20 J optimizes for more rapid defibrillation and fewer shocks.     

Effectiveness and safety of internal rectilinear biphasic versus monophasic defibrillation in patients undergoing cardiac surgery

BACKGROUND: Recently it has been shown that biphasic external shocks are more effective in the treatment of ventricular fibrillation (VF) compared with monophasic external shocks in terms of number of defibrillation attempts and maximal energy used for termination of VF. Biphasic defibrillators apply different biphasic impulse forms, depending on technology. To the authors' knowledge, there are no existing data concerning the effects of rectilinear biphasic internal shocks in patients undergoing cardiac surgery. The purpose of this study was to compare monophasic with rectilinear biphasic internal shock waveforms for termination of VF in patients undergoing cardiac surgery. METHODS: One hundred thirty-four patients scheduled for elective cardiac surgery were prospectively randomized either to monophasic (group A) or biphasic (group B) internal defibrillation. Defibrillation was started with 7 J and increased stepwise to 30 J in each group until successful termination of VF after aortic declamping. The number of defibrillations, as well as the cumulative and maximal energy for termination of VF, were determined. Preoperatively, intraoperatively, and postoperatively troponin T, total creatine phosphokinase (CPK), and CPK- MB isoenzymes were measured. RESULTS: In 64 patients (47%) VF occurred. The groups consisted of 32 patients each. The number of defibrillations (1.3 +/- 0.6 v 1.9+/- 1.2; p = 0.013), maximal energy per patient (7.9 +/- 2.5 v 11.6 +/- 7.32; p = 0.006), and cumulative energy (10.1 +/-6.1 v 21.3 +/- 24.1; p = 0.016) for successful termination of VF were significantly reduced in group B. Troponin T, CPK, and CPK-MB did not differ between groups. CONCLUSIONS: Results of this study indicate that rectilinear biphasic internal defibrillation is more effective in the treatment of VF during cardiac surgery than is monophasic defibrillation. However, no significant difference in myocardial damage could be detected between groups.     

Wirksamkeit und Sicherheit neuer Impulskurvenformen bei transthorakaler Defibrillation

After recalling the physiological principles of fibrillation, we develop criteria for optimal biphasic waveforms and discuss existing waveforms today. A new waveform is described: a 5 kHz-chopped biphasic waveform optimally dimensioned in membran potential modeling. In a clinical study based on atrial cardioversions, the new waveform is compared to a monophasic one using two groups containing 14 and 72 patients, respectively. The mean energy for successful defibrillation with the new waveform is 68 J versus 205 J for the monophasic one: ratio 3 (p < 0.001). The ventricular defibrillation threshold derived from this study and from animal studies on ventricular defibrillation is 60 J. Therefore, this new waveform has an Reserve Efficacy (new concept) of a factor 3 when using an applied maximum energy of 180 J, which is a level that is probably not harmful for an ischemic heart. The poor survival rates found in the past for prehospital defibrillation are explaned as follows: the waveforms used required energy levels (200 to 360 J), which were probably often not efficacious, but still harmful for an ischemic heart. On the other hand, the results today in prehospital defibrillation with existing biphasic waveforms do not show any better survival rate that monophasic pulses. This survival rate should be better for the new low-energy waveform with its reserve efficacy, which is entirely below the level of possible harmfulness for an ischemic heart.     

Biphasic waveforms for automatic external defibrillation in human: a review

Ventricular fibrillation is the principal cause of sudden cardiac arrest and the electrical defibrillation is often the only effective therapy. A very interesting question is represented by the electric parameters of defibrillation shock. Today, monophasic waveform is widely used in Europe and in the United States, but, recently, the Food and Drug Administration grants approval for an automatic external defibrillator (AED) producing a biphasic pulse. In this review we discuss about the effectiveness and the safety of biphasic waveform, by examining a series of human studies between 1982 and 1999. We have found that available data are often incomplete, unclear, dishomogeneous and, consequently, difficult to compare. Furthermore, among the authors there is no concordance about the meaning of "safety", "effectiveness", "success", "equivalence" and "superiority" of biphasic versus monophasic shock: however, biphasic shock, that uses a lower energy level, seems to reduce post-defibrillation heart damage. Due to the lack of homogeneous studies it is not possible to state which kind of signal is more reliable, even if some clinical reports and experimental data seem to tribute to the biphasic waveform a better therapeutic effectiveness and safety. By examining the current scientific literature, we conclude that further studies have to be performed to definitively validate the use of biphasic shock.     

Energy dose and other variables possibly affecting ventricular defibrillation during cardiac surgery

Previous studies have suggested that shocks of 5-10 J are required for direct ventricular defibrillation during open heart surgery. However, the efficacy of shocks of less than 5 J, the effects of thermal, biochemical, and temporal factors, and the influence of disease process on defibrillation have not been fully investigated, particularly with modern techniques of myocardial preservation. The purpose of this prospective study in 150 adult cardiac surgical patients was to evaluate the energy, current, and myocardial resistance with low energy DC shocks of 1, 2.5, and 5 J and to relate which biochemical, temporal, thermal, or other factors influence the outcome of a DC shock. Twenty-eight percent of shocks of 1 J and 55% of shocks of 2.5 J produced defibrillation. Above 2.5 J, the success rate reached a plateau at 55%. Other factors associated with the success of DC shocks were high normal serum potassium levels, high PaO2, high ionized calcium levels, and longer reperfusion times at mean arterial and coronary perfusion pressures above 50 mm Hg. Disease process may also play a role because patients with valvular heart disease were more difficult to defibrillate. Heart weight and thickness of ventricular myocardium, measured angiographically, appeared less important in direct defibrillation, except with 1 J shocks when thinner-walled ventricles defibrillated more easily.     

Recent advances in cardiopulmonary resuscitation

Modern cardiopulmonary resuscitation (CPR) has been in existence for just over 40 years. Despite many advances, the overall survival following out-of-hospital cardiac arrest remains dismally poor. However, recent research has uncovered a number of areas of promise that may change this. Until now, all cardiac arrest victims whose initial rhythm is ventricular fibrillation have been managed the same with the emphasis on defibrillation as the initial treatment, irrespective of the time in arrest. Better understanding of the pathophysiology of prolonged arrest and studies in animal and man suggests that we should now question this concept and where there has been a delay, perform chest compressions and ventilation before attempting defibrillation. When defibrillation is required, most devices deliver a monophasic shock, despite the fact that for some time biphasic shocks have been known to be relatively more effective and less injurious. The adoption of biphasic waveforms for external defibrillation is increasing with the added advantage of smaller, lighter and more portable defibrillators. Finally, return of a spontaneous circulation does not always equate to a good outcome and many victims subsequently die from or suffer significant neurological injury. For the first time there now appears to be a method of improving outcome by instituting a period of mild hypothermia in those victims who have a return of their circulation but remain comatose. When put together, these changes offer the greatest chance for many years to improve the outcome of patients who suffer a cardiac arrest outside of the hospital environment.     

Myocardial protection with insulin cardioplegia: who can really benefit?

AIM: The aim of the study was to evaluate the effects on myocardial protection of insulin-enriched warm blood cardioplegia (IWBC) in coronary artery bypass grafting (CABG) and in subgroups of patients with associated cardiac co-morbidities. METHODS: Between May 2000 and December 2002, 268 consecutive patients underwent CABG with warm blood cardioplegia (group A) or IWBC (10 UI/L) (group B). Hospital outcome, ECG, echocardiography and biochemical markers of ischemia were compared. Differences between subgroups of patients with unstable angina (UA), ventricular hypertrophy (VH) and diabetes were assessed. RESULTS: Hospital mortality, incidence of postoperative myocardial infarction and low output syndrome, IABP requirement, postoperative atrial fibrillation, in-hospital and in-ITU stay, postoperative recovery of left ventricular function and enzyme leakage did not show differences between the 2 groups; inotropic support was lower in IWBC. Moreover, patients with UA and IWBC showed a lower troponin I (TnI) (12 h: 0.82+/-0.57 ng/mL vs 2.56+/-1.18, P < 0.0001; 24 h: 0.71+/-0.64 vs 2.16+/-1.52, P < 0.0001; 48 h: 0.69+/-1.13 vs 1.79+/-1.43, P = 0.001; 72 h: 0.44+/-0.83 vs 1.01+/-1.02, P = 0.001), lower incidence of atrial fibrillation (4.2% versus 60.6%; P < 0.0001) and intraoperative defibrillation (0% versus 27.3%; P = 0.007). Furthermore, patients with VH treated with IWBC showed lower level of TnI (12 h: 0.41+/-0.32 ng/mL vs 2.93+/-0.67, P < 0.0001; 24 h: 0.37+/-0.45 vs 2.40+/-1.28, P < 0.0001; 48 h: 0.22+/-0.18 vs 1.95+/-1.33, P < 0.0001; 72 h: 0.12+/-0.12 vs 1.31+/-1.56, P < 0.0001), lower atrial fibrillation (6.5% vs 48%, P < 0.0001) and ventricular defibrillation (0% vs 20%, P = 0.011). CONCLUSIONS: Insulin addiction to blood cardioplegia does not show any benefit in the global population and in diabetics; nevertheless, better myocardial protection can be demonstrated in patients with unstable angina and left ventricular hypertrophy.     

Prediction of successful defibrillation in human victims of out-of-hospital cardiac arrest: a retrospective electrocardiographic analysis

In the present study we sought to examine the efficacy of an electrocardiographic parameter, 'amplitude spectrum area' (AMSA), to predict the likelihood that any one electrical shock would restore a perfusing rhythm during cardiopulmonary resuscitation in human victims of out-of-hospital cardiac arrest. AMSA analysis is not invalidated by artefacts produced by chest compression and thus it can be performed during CPR, avoiding detrimental interruptions of chest compression and ventilation. We hypothesised that a threshold value of AMSA could be identified as an indicator of successful defibrillation in human victims of cardiac arrest. Analysis was performed on a database of electrocardiographic records, representing lead 2 equivalent recordings from automated external defibrillators including 210 defibrillation attempts from 90 victims of out-of-hospital cardiac arrest. A 4.1 second interval of ventricular fibrillation or ventricular tachycardia, recorded immediately preceding the delivery of the shock, was analysed using the AMSA algorithm. AMSA represents a numerical value based on the sum of the magnitude of the weighted frequency spectrum between two and 48 Hz. AMSA values were significantly greater in successful defibrillation (restoration of a perfusing rhythm), compared to unsuccessful defibrillation (P < 0.0001). An AMSA value of 12 mV-Hz was able to predict the success of each defibrillation attempt with a sensitivity of 0.91 and a specificity of 0.97. In conclusion, AMSA analysis represents a clinically applicable method, which provides a real-time prediction of the success of defibrillation attempts. AMSA may minimise the delivery of futile and detrimental electrical shocks, reducing thereby post-resuscitation myocardial injury.     

The 2005 European Guidelines for cardiopulmonary resuscitation: major changes and rationale

During the 2005 International Consensus Conference on Cardiopulmonary Resuscitation (CPR) and Emergency Cardiovascular Care Science, a rigorous evidence-based evaluation process was conducted. The consensus reached during that Conference constituted the basis of the current CPR guidelines of the European Resuscitation Council (ERC), published in December 2005. Those guidelines included many important changes, made on the basis of emerging evidence. For example, the compression-ventilation ratio for CPR in non-intubated patients was increased from 15:2 to 30:2 and a strong recommendation to minimize interruptions in chest compression was issued in order to maximise organ perfusion. Energy levels for monophasic defibrillation were increased and specific energy levels for biphasic defibrillation have been recommended, in order to maximise the efficacy of the first shock. New timing of defibrillation shocks is now advised: the three-stacked shock sequence has been replaced by high-energy single shocks followed by two-minute cycles of CPR, in order to reduce CPR interruptions. Timing for administration of drugs has been adapted to the new shock sequence and the advanced life support (ALS) universal algorithm has been modified. Some controversial topics are still a matter of investigation and debate, including the use of therapeutic hypothermia in non-shockable cardiac arrests, the efficacy of a period of CPR before defibrillation in long-lasting cardiac arrests, and the chest-compression-only CPR for first responders of out-of-hospital cardiac arrests.     

The effects of nifedipine on ventricular fibrillation mean frequency in a porcine model of prolonged cardiopulmonary resuscitation

We assessed the effects of a calcium channel blocker versus saline placebo on ventricular fibrillation mean frequency and hemodynamic variables during prolonged cardiopulmonary resuscitation (CPR). Before cardiac arrest, 10 animals were randomly assigned to receive either nifedipine (0.64 mg/kg; n = 5) or saline placebo (n = 5) over 10 min. Immediately after drug administration, ventricular fibrillation was induced. After 4 min of cardiac arrest and 18 min of basic life support CPR, defibrillation was attempted. Ninety seconds after the induction of cardiac arrest, ventricular fibrillation mean frequency was significantly (P < 0.01) increased in nifedipine versus placebo pigs (mean +/- SD: 12.4 +/- 2.1 Hz versus 8 +/- 0.7 Hz). From 2 to 18.5 min after the induction of cardiac arrest, no differences in ventricular fibrillation mean frequency were detected between groups. Before defibrillation, ventricular fibrillation mean frequency was significantly (P < 0.05) increased in nifedipine versus placebo animals (9.7 +/- 1.2 Hz versus 7.1 +/- 1.3 Hz). Coronary perfusion pressure was significantly lower in the nifedipine than in the placebo group from the induction of ventricular fibrillation to 11.5 min of cardiac arrest; no animal had a return of spontaneous circulation after defibrillation. In conclusion, nifedipine, but not saline placebo, prevented a rapid decrease of ventricular fibrillation mean frequency after the induction of cardiac arrest and maintained ventricular fibrillation mean frequency at approximately 10 Hz during prolonged CPR; this was nevertheless associated with no defibrillation success. IMPLICATIONS: This study evaluates the effects of a calcium channel blocker on ventricular fibrillation mean frequency, hemodynamic variables, and resuscitability during prolonged cardiopulmonary resuscitation (CPR) in pigs. Nifedipine, but not saline placebo, prevented a rapid decrease of ventricular fibrillation mean frequency after the induction of cardiac arrest and maintained ventricular fibrillation mean frequency at approximately 10 Hz during prolonged CPR but did not improve resuscitability.     

The effects of repeated doses of vasopressin or epinephrine on ventricular fibrillation in a porcine model of prolonged cardiopulmonary resuscitation

This study evaluated ventricular fibrillation mean frequency and amplitude to predict defibrillation success in a porcine cardiopulmonary resuscitation (CPR) model using repeated administration of vasopressin or epinephrine. After 4 min of cardiac arrest and 3 min of CPR, 10 pigs were randomly assigned to receive either vasopressin (early vasopressin: 0.4, 0.4, and 0.8 units/kg, respectively, n = 5) or epinephrine (early epinephrine: 45, 45, and 200 microg/kg, respectively, n = 5). Another 11 animals were randomly allocated after 4 min of cardiac arrest and 8 min of CPR to receive every 5 min either vasopressin (late vasopressin: 0.4 and 0. 8 units/kg, respectively, n = 5) or epinephrine (late epinephrine: 45 and 200 microg/kg, n = 6). Ventricular fibrillation mean frequency and amplitude on defibrillation were significantly higher in the vasopressin groups than in the epinephrine groups, respectively. In vasopressin versus epinephrine animals, mean frequency immediately before defibrillation was 9.6 +/- 1.5 Hz vs 7. 0 +/- 0.7 Hz (P < 0.001), mean amplitude was 0.65 +/- 0.26 mV vs 0. 21 +/- 0.14 mV (P < 0.001, and coronary perfusion pressure was 27 +/- 9 mm Hg vs 8 +/- 4 mm Hg (P < 0.00001), respectively. In contrast to no epinephrine animals, all vasopressin animals were successfully defibrillated and survived 1 h (P < 0.05). Mean fibrillation frequency and amplitude predicted successful defibrillation and may serve as noninvasive markers to monitor continuing CPR efforts. Furthermore, vasopressin was superior to epinephrine in maintaining these variables above a threshold necessary for successful defibrillation.     

Lidocaine enhances intraoperative ventricular defibrillation

The efficacy of lidocaine during myocardial reperfusion in coronary artery bypass surgery was evaluated in 20 patients randomly assigned to a control group (n = 10) or to receive lidocaine, 1 mg/kg intravenously 5 min before aortic unclamping and cardiac reperfusion, followed by infusion at 40 micrograms X kg-1 X min-1 (n = 10). We recorded ECG leads II and V5 continuously, and number, energy, and current of direct current (DC) shocks starting at 1 joule. The number of low energy DC shocks to sustained defibrillation (5.5 +/- 2.0 vs 3.5 +/- 2.0, mean +/- SD, P less than 0.05) decreased significantly with lidocaine infusion. The energy (11.0 +/- 6.3 vs 5.6 +/- 3.9 joules, P less than 0.05) and current (12.7 +/- 4.2 vs 8.9 +/- 4.7 amperes, not significant) likewise decreased with lidocaine infusion. Energy and current for the first successful shock, although lower in the lidocaine group, were not statistically significantly lower than in the control group. Initial reperfusion rhythm was not influenced by lidocaine. Plasma electrolyte levels, arterial blood gas tensions, myocardial temperature, and surgical technique--factors known to influence defibrillation--were similar in all patients. Administration of lidocaine during myocardial reperfusion allows defibrillation with fewer DC shocks of lower energy and current.     

Neue Aspekte der elektrischen Defibrillation

Early defibrillation is the standard of care for patients with ventricular fibrillation (VF) and pulseless ventricular tachycardia (VT). Technical developments aim at further miniaturization and simplification of defibrillators as well as adaptation of energy requirements to the patient´s needs. Implantable Cardioverter-Defibrillators (ICD) and automated external defibrillators (AED) are based upon the same technology. Both devices analyze the ECG signal internally, followed by a ”shock” or ”no shock” decision. Use of automated devices is the prerequisite for defibrillation by non-physicians. Chest impedance measurements and use of alternative shock waveforms, such as biphasic, aim at adaptation of energy or current to the patient’s individual needs and avoid application of unnecessarily high amounts of energy to the myocardium. Calculation of median frequency is a non-invasive method for analyzing the heart´s metabolic and electrical state. It helps to determine the optimal moment for defibrillation during cardiopulmonary resuscitation (CPR). Developments concerning the structure of in-hospital emergency systems or pre-hospital emergency medical services (EMS) aim at further reductions in time from collapse of a patient until first defibrillation. Such developments include early defibrillation programs for emergency medical technicians (EMT), nurses, and fire or police department first responders as well as wide distribution of easy-to-operate defibrillators in public areas, as discussed during the American Heart Association’s Public Access Defibrillation conferences. All programs of that kind have to be organized and supervised by a physician who is responsible for training and supervision of the personnel involved.     

The value of predischarge ICD tests in patients with a successful peroperative test

An internal cardioverter defibrillator (ICD) is normally extensively tested during implantation. The necessity of retesting prior to discharge of the patient is a matter of debate. In our material of 30 patients undergoing first-time implantation of a transvenous internal defibrillator system, we retrospectively compare the predischarge defibrillation test with the peroperative test. A successful peroperative defibrillation test with no failed shocks at 10 J below maximal energy level was followed by a successful predischarge test with the same safety margin in 18/19 patients, while one patient required a maximal energy ICD shock for conversion at the predischarge test. We conclude that the predischarge defibrillation test can be omitted if the peroperative test was successful, with no failed shocks at 10 J below maximal energy level and if the shock therapy is set to maximal energy level.     

The Value of Predischarge ICD Tests in Patients with a Successful Peroperative Test

An internal cardioverter defibrillator (ICD) is normally extensively tested during implantation. The necessity of retesting prior to discharge of the patient is a matter of debate. In our material of 30 patients undergoing first-time implantation of a transvenous internal defibrillator system, we retrospectively compare the predischarge defibrillation test with the peroperative test. A successful peroperative defibrillation test with no failed shocks at 10 J below maximal energy level was followed by a successful predischarge test with the same safety margin in 18/19 patients, while one patient required a maximal energy ICD shock for conversion at the predischarge test. We conclude that the predischarge defibrillation test can be omitted if the peroperative test was successful, with no failed shocks at 10 J below maximal energy level and if the shock therapy is set to maximal energy level.     

Shock wave lithotripsy at 60 or 120 shocks per minute: a randomized, double-blind trial

PURPOSE: The rate of shock wave administration is a factor in the per shock efficiency of shock wave lithotripsy (SWL). Experimental evidence suggests that decreasing shock wave frequency from 120 shocks per minute results in improved stone fragmentation. To our knowledge this study is the first to examine the effect of decreased shock wave frequency in patients with renal stones. MATERIALS AND METHODS: Patients with previously untreated radiopaque stones in the renal collecting system were randomized to SWL at 60 or 120 shocks per minute. They were followed at 2 weeks and 3 months. The primary outcome was the success rate, defined as stone-free status or asymptomatic fragments less than 5 mm 3 months after treatment. RESULTS: A total of 220 patients were randomized, including 111 to 60 shocks per minute and 109 to 120 shocks per minute. The 2 groups were comparable in regard to age, sex, body mass index, stent status and initial stone area. The success rate was higher for 60 shocks per minute (75% vs 61%, p = 0.027). Patients with larger stones (stone area 100 mm or greater) experienced a greater benefit with treatment at 60 shocks per minute. The success rate was 71% for 60 shocks per minute vs 32% (p = 0.002) and the stone-free rate was 60% vs 28% (p = 0.015). Repeat SWL was required in 32% of patients treated with 120 shocks per minute vs 18% (p = 0.018). Fewer shocks were required with 60 shocks per minute (2,423 vs 2,906, p <0.001) but treatment time was longer (40.6 vs 24.2 minutes, p <0.001). There was a trend toward fewer complications with 60 shocks per minute (p = 0.079). CONCLUSIONS: SWL treatment at 60 shocks per minute yields better outcomes than at 120 shocks per minute, particularly for stones 100 mm or greater, without any increase in morbidity and with an acceptable increase in treatment time.     

Patient-specific characteristics determine success of surgical atrial fibrillation ablation in patients with persistent atrial fibrillation

Surgical atrial fibrillation ablation (SAFA) has not achieved the efficacy of Cox's original maze procedure, although technical improvements continue to be made. It is possible that biologic factors determine SAFA success. Therefore we examined how patient-specific characteristics affected SAFA success in 353 atrial fibrillation (AF) patients who underwent SAFA at a single institution. Among these, 257 (72.8%) had continuous AF and 96 (27.2%) had intermittent AF. For 297 patients (84.1%) postoperative follow-up was > 3 months. We compared SAFA success in patients whose procedure involved only pulmonary vein isolation with those whose procedure involved extensive lesion sets. Multivariate analysis included AF duration, left atrial size, preoperative atrial flutter, concomitant procedures, lesion sets, and energy source. Early SAFA success was classified as freedom from AF between postoperative months 3 and 6, and intermediate success between postoperative months 6 and 12. Receiver-operating characteristic (ROC) curves and stratum-specific likelihood ratios (SSLR) were generated to compare intermediate failure by left atrial size (LAS) thresholds. SAFA was more successful in the intermittent than the continuous AF group (n = 66, 86% vs n = 165, 71%; P = .014). When pulmonary vein isolation was compared only to more extensive lesion sets, there was no difference in success in the intermittent (34, 91% vs 32, 81%; P = .24) or continuous groups (67, 73% vs. 98, 69%; P = .603). Success for intermittent AF patients was not correlated with variables considered; in continuous AF patients, predictors included presence of concomitant mitral valve repair/replacement (P = .075), decreasing LAS (P = .025) and absence of preoperative atrial flutter (P = .001). In the continuous AF group, ROC curves and corresponding areas under the curve (AUC) were 0.60 (0.50-0.71) for failure at 6 months to 1 year. SSLR analysis generated 2 strata for LAS: < 8 cm with SSLR = 0.87 (0.74-1.0) and < or = 8 cm SSLR = 2.98 (1.07-8.3). In patients with intermittent AF, SAFA achieved acceptable results regardless of tested preoperative and intraoperative variables. In continuous AF, patient-specific characteristics affected success more than intraoperative variables. Failure was more than 3-fold greater in continuous AF patients with an LAS < or = 8 cm. In both patient types, more extensive lesion sets were not shown to improve outcomes. Future improvements in SAFA may depend on pharmacologic and/or surgical substrate modification.     

The predictive value of ventricular fibrillation electrocardiogram signal frequency and amplitude variables in patients with out-of-hospital cardiac arrest.

We evaluated ventricular fibrillation frequency and amplitude variables to predict successful countershock, defined as pulse-generating electrical activity. We also elucidated whether bystander cardiopulmonary resuscitation (CPR) influences these electrocardiogram (ECG) variables. In 89 patients with out-of-hospital cardiac arrest, ECG recordings of 594 countershock attempts were collected and analyzed retrospectively. By using fast Fourier transformation analysis of the ventricular fibrillation ECG signal in the frequency range 0.333-15 Hz (median [range]), median frequency, dominant frequency, spectral edge frequency, and amplitude were as follows: 4.4 (2.4-7.5) Hz, 4.0 (0.7-7.0) Hz, 7.7 (3.7-13.7) Hz, and 0.94 (0.24-1.95) mV, respectively, before successful countershock (n = 59). These values were 3.8 (0.8-7.7) Hz (P = 0.0002), 3.0 (0.3-9.7) Hz (P < 0.0001), 7.3 (2.0-14.0) Hz (P < 0.05), and 0.53 (0.03-3.03) mV (P < 0.0001), respectively, before unsuccessful countershock (n = 535). In patients in whom bystander CPR was performed (n = 51), ventricular fibrillation frequency and amplitude before the first defibrillation attempt were higher than in patients without bystander CPR (n = 38) (median frequency, 4.4 [2.4-7.5] vs 3.7 [1.8-5.3] Hz, P < 0.0001; dominant frequency, 3.8 [0.9-7.7] vs 2.6 [0.8-5.9] Hz, P < 0.0001; spectral edge frequency, 8.4 [4.8-12.9] vs 7.2 [3.9-12.1] Hz, P < 0.05; amplitude, 0.79 [0.06-4.72] vs 0.67 [0.16-2.29] mV, P = 0.0647). Receiver operating characteristic curves demonstrate that successful countershocks will be best discriminated from unsuccessful countershocks by ventricular fibrillation amplitude (3000-ms epoch). At 73% sensitivity, a specificity of 67% was obtained with this variable. IMPLICATIONS: In patients with out-of-hospital cardiac arrest, successful countershocks will be best discriminated from unsuccessful countershocks by ventricular fibrillation amplitude (3000-ms epoch). At 73% sensitivity, a specificity of 67% was obtained with this variable.     

Physical principles of defibrillators

Defibrillation is the only effective, and therefore life-saving, treatment for ventricular fibrillation. Defibrillators are devices that store and then discharge pre-programmed quantities of electrical energy through the heart to synchronously depolarize myocytes and allow return of sinus rhythm. Energy is supplied from batteries and is stored in a capacitor before being discharged through leads to electrodes and then across the heart either directly or via the chest. The ratio of charge stored to potential difference is known as capacitance and will determine the energy discharged by the defibrillator. Capacitance is equal to charge/voltage, and energy is equal to charge × voltage. Inductors and other electronic components are used to ensure that the waveform of the discharge is of optimal shape and duration. Most modern defibrillators use a biphasic waveform, which has a positive and a negative component and reduces the energy required for defibrillation. Patient factors, including thoracic and cardiac impedance, will also determine the energy delivered. Defibrillators can be external (transthoracic) or implantable. Owing to the energy used in defibrillators, care must be taken to prevent electrocution of medical staff and burns to the patient.