Metabolic determinants of defibrillation. Role of adenosine

Endogenously released adenosine causes pulmonary vasodilation during the acute phase of pulmonary embolization in dogs

Background

Endogenous adenosine levels increase under stress in various organs. Exogenously administered adenosine is a well-known pulmonary vasodilator. However, the physiology and therapeutic potential of endogenous adenosine during alteration in pulmonary hemodynamics such as pulmonary embolism is not elucidated. We hypothesized that the adenosine level increases following an acute elevation of pulmonary resistance, resulting in pulmonary vasodilation.

Methods

We induced acute pulmonary embolization by injecting plastic beads in anesthetized dogs. Plasma adenosine levels, defined as the product of plasma adenosine concentration and simultaneous cardiac output, were assessed from blood samples from the superior vena cava, main pulmonary artery (MPA), and ascending aorta 1 and 10 min following injection. Hemodynamics were assessed with (n = 3) and without (n = 8) administration of the adenosine receptor blocker, 8-(p-sulfophenyl)theophylline (8SPT).

Results

Mean pulmonary arterial pressure (PAP) increased from 11 ± 1 mmHg, peaking at 28 ± 4 mmHg at 52 ± 13 s after injection. During this period, total pulmonary resistance (TPR) elevated from 11 ± 1 to 33 ± 6 Wood unit. Plasma adenosine levels increased in the MPA from 14.5 ± 2 to 38.8 ± 7 nmol/min 1 min after injection. TPR showed greater elevation under 8SPT treatment, to 96 ± 12 Wood unit at PAP peak.

Conclusions

Endogenously released adenosine after acute pulmonary embolization is one of the initial pulmonary vasodilators. The immediate surge in plasma adenosine levels in the MPA could lead to a hypothesis that adenosine is released by the right heart in response to pressure overload.

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Adenosine levels increased after experimental acute pulmonary embolization.

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Plasma adenosine levels immediately rose in the main pulmonary artery.

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Adenosine is one of the initial pulmonary vasodilators after embolization.

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Released adenosine could originate from the right heart following pressure overload.

Electrical cardioversion

External electrical cardioversion was first performed in the 1950s. Urgent or elective cardioversions have specific advantages, such as termination of atrial and ventricular tachycardia and recovery of sinus rhythm. Electrical cardioversion is life-saving when applied in urgent circumstances. The succcess rate is increased by accurate tachycardia diagnosis, careful patient selection, adequate electrode (paddles) application, determination of the optimal energy and anesthesia levels, prevention of embolic events and arrythmia recurrence and airway conservation while minimizing possible complications. Potential complications include ventricular fibrillation due to general anesthesia or lack of synchronization between the direct current (DC) shock and the QRS complex, thromboembolus due to insufficient anticoagulant therapy, non-sustained VT, atrial arrhythmia, heart block, bradycardia, transient left bundle branch block, myocardial necrosis, myocardial dysfunction, transient hypotension, pulmonary edema and skin burn. Electrical cardioversion performed in patients with a pacemaker or an incompatible cardioverter defibrillator may lead to dysfunction, namely acute or chronic changes in the pacing or sensitivity threshold. Although this procedure appears fairly simple, serious consequences might occur if inappropriately perfformed.

The effect of adenosine A1 receptor antagonism on return of spontaneous circulation and short-term survival in prolonged ventricular fibrillation

BACKGROUND

Endogenous adenosine (ADO) is cardioprotective during ischemia and its myocardial concentration increases during untreated ventricular fibrillation (VF). We have previously shown that ADO A1 receptor (ADOA1R) antagonism hastens the time-dependent decay in VF waveform morphology during the circulatory phase of cardiac arrest.

OBJECTIVE

To determine the effect of ADOA1R antagonism on ROSC and short-term survival in prolonged VF.

METHODS

Thirty-six swine were assigned by block randomization to one of three groups: a group that received only vehicle (CONTROL), an ADOA1R antagonist pretreatment group (PRE), and a group that was given ADOA1R antagonist during resuscitation (DURING). The animals were instrumented under anesthesia, and ADOA1R antagonist or vehicle, per group assignment, was infused 5 minutes prior to VF induction. At minute 8 of untreated VF, chest compression with ventilation was initiated and a standard drug cocktail, with ADOA1R antagonist or vehicle, was given. The first rescue shock (150 J biphasic) was delivered after 11 minutes of VF. Proportions with 95% confidence intervals (CIs) were calculated for the two outcome measures.

RESULTS

The baseline characteristics and chemistry values for the three groups were mathematically the same. The DURING group had a greater proportion of female animals (seven of 12) in comparison with the CONTROL group (two of 12) (p=0.03). ADOA1R antagonism hastened the decay of VF as previously demonstrated, but the rate of ROSC was the same for all groups: CONTROL=seven of 12, PRE=six of 12, and DURING=seven of 12. There were also no differences in short-term survival: CONTROL=four of 12, PRE=five of 12, and DURING=seven of 12.

CONCLUSIONS

In this study, ADOA1R antagonism had no effect on outcome whether given before induction of VF or upon resuscitation after 8 minutes of untreated VF. The role of endogenous ADO in prolonged VF remains unclear.

Adenosine A1 receptor antagonism hastens the decay in ventricular fibrillation waveform morphology during porcine cardiac arrest

BACKGROUND

Endogenous adenosine (ADO) is known to be cardioprotective during acute myocardial ischemia. Coronary sinus ADO concentration has recently been shown to increase nearly 13-fold over baseline levels after 5 min of untreated ventricular fibrillation (VF). The role of ADO in VF has never been previously examined. The objective of this study was to determine the effect of ADO receptor antagonism, as measured by the scaling exponent (ScE), on the degeneration of VF over time during the circulatory phase of cardiac arrest.

METHODS AND RESULTS

A well-established swine model of prolonged VF arrest was used for this experiment. Eighteen domestic mixed-breed swine were assigned by block randomization to receive either DTI-0017 (5mg/kg), a potent ADO A(1) receptor antagonist or placebo in a double-blind fashion. The animals were instrumented under general anesthesia and acclimatized. The assigned solution was infused over 5 min. One minute after the infusion was completed, VF was induced with a 3s, 60 Hz, 100 mA transthoracic shock and left untreated. Lead II ECG was monitored continuously and recorded at 1000 samples/s. It was determined a priori that evaluation of the plots would be limited to a previously observed plateau phase historically occurring between 5 and 8 min corresponding to the circulatory phase of cardiac arrest. The scaling exponent values over this period were calculated for each of the 18 recordings using custom MATLAB routines. Using the Wald statistic to produce the Chi square distributions the null hypothesis, that there was no difference between the two groups, was tested. The Wald statistic calculation based on eight epochs from 300 to 475 s in placebo and DTI groups was significant to reject the null hypothesis of no difference in the groupxtime interaction at the 0.015 level (Chi square distribution for Wald=17.49, d.f.=7).

CONCLUSIONS

In this swine model, adenosine A(1) receptor antagonism accelerated the natural decay in the ECG VF waveform during the circulatory phase of cardiac arrest. Our findings would suggest that endogenous adenosine has cardioprotective effects during sudden cardiac arrest by slowing the time-dependent degeneration of VF.

Effect of transthoracic shocks on left ventricular function

Although defibrillating shocks are thought to depress ventricular function transiently, the independent effects of high strength shocks (without the metabolic sequelae of pre-shock fibrillation) have not been assessed systematically in humans. Therefore, we delivered three consecutive synchronized monophasic transthoracic shocks (200, 200 and 360 J) at 60s intervals during sinus rhythm and evaluated the effect on left ventricular chamber size and function as determined by transesophageal echocardiography in 11 patients (mean age 67+/-8 years, 9M/2F) with depressed left ventricular function (left ventricular ejection fraction: 14-37%). The shocks did not alter hemodynamics consistently. On average, the shocks did not alter stroke volume, cardiac output, left ventricular ejection fraction or regional wall thickening (all p>0.05 versus baseline). This effect was highly variable and 36% of patients experienced a >25% reduction in cardiac output by the final shock. There was a tendency for regional wall thickening to worsen in the best baseline sextant with an offsetting significant increase in thickening in the worst baseline sextant (p=0.05). Thus, repetitive defibrillation strength transthoracic shocks do not impair left ventricular function consistently in patients with cardiomyopathy. However, the effect is widely variable and potentially important depression of left ventricular function does occur in some patients.

Detection of the Defibrillation Threshold Using the Upper Limit of Vulnerability Following Defibrillator Implantation

OBJECTIVE

This study was designed to test defibrillation threshold (DFT) with the least number of fibrillation inductions using upper limit of vulnerability (ULV) and to describe the most practical set of ICD during DFT following implantation.

BACKGROUND

Although the correlation between ULV and DFT has been well described, there has been no uniform DFT testing protocol taking the advantage of ULV after defibrillator (ICD) implantation.

METHODS

A total of 26 patients undergoing a new ICD implantation had a DFT induced with scanned T wave shock. The hypothesis that ventricular fibrillation (VF) could be defibrillated with 5 J higher than the highest T wave shock needed to induce VF or with 10 J if the T wave shock needed to induce VF was less than 5 J, was tested and 20 patients fulfilled these criteria. The methodology is improved by detecting peak T wave with 12-lead ECG, applying biphasic T wave shock and scanning the T wave shock in a wider window.

RESULTS

Five patients in the first group (n = 15) and one patient in the second group (n = 11) did not fulfill the above hypothesis. The common features of six patients who did not fulfill the hypothesis were that T wave shock needed to induce VF was either under 5 J (5 patients) or high (1 patient).

CONCLUSION

This study revealed the importance of methodology in studies regarding ULV and DFT. Following ICD implantation, we propose the first biphasic T wave detected by 12-lead ECG and rescue shock set at 10 and 15 J, respectively. If any of the scanned T wave (40 ms before and 40 ms after the peak T wave with decrements and increments of 20 ms) shocks could not induce VF, then the T wave and the first rescue shock should be set at 5 and 10 J, respectively. If the induction of VF has been unsuccessful with T wave shock at 5 J, then a safe defibrillation with 10 J should be expected in majority.

Innovative pharmacologic approaches to cardiopulmonary resuscitation

The survival rate of patients undergoing cardiopulmonary resuscitation is 5 to 15%. New cardiopulmonary resuscitation treatment approaches under investigation include the use of vasopressin as a vasopressor, amiodarone for the treatment of ventricular tachyarrhythmias, and adenosine antagonists (i.e., theophylline) for bradyasystolic rhythms. More innovative approaches include the use of thyroid hormone and endothelin.

Update on external cardioversion and defibrillation

External cardioversion is a technique used electively or emergently to terminate arrhythmias such as atrial fibrillation, ventricular tachycardia, and ventricular fibrillation. There have been several advances made to modern defibrillators, including an improvement in the efficacy of the delivered shock. Biphasic shock waveforms have been shown to be superior to monophasic shocks and these are being incorporated into modern units. This paper reviews several reports on biphasic defibrillation. In addition, initiatives to make defibrillators more accessible are also being tested. Although not a technological advance, this initiative may significantly improve the survival of victims of out of hospital cardiac arrests.

External exponential biphasic versus monophasic shock waveform: efficacy in ventricular fibrillation of longer duration

Ventricular fibrillation (VF) duration may be a factor in determining the defibrillation energy for successful defibrillation. Exponential biphasic waveforms have been shown to defibrillate with less energy than do monophasic waveforms when used for external defibrillation. However, it is unknown whether this advantage persists with longer VF duration. We tested the hypothesis that exponential biphasic waveforms have lower defibrillation energy as compared to exponential monophasic waveforms even with longer VF duration up to 1 minute. In a swine model of external defibrillation (n = 12, 35 +/- 6 kg), we determined the stored energy at 50% defibrillation success (E50) after both 10 seconds and 1 minute of VF duration. A single exponential monophasic (M) and two exponential biphasic (B1 and B2) waveforms were tested with the following characteristics: M (60 microF, 70% tilt), B1 (60/60 microF, 70% tilt/3 ms pulse width), and B2 (60/20 microF, 70% tilt/3 ms pulse width) where the ratio of the phase 2 leading edge voltage to that of phase 1 was 0.5 for B1 and 1.0 for B2. E50 was measured by a Bayesian technique with a total often defibrillation shocks in each waveform and VF duration randomly. The E50 (J) for M, B1, and B2 were 131 +/- 41, 57 +/- 18,* and 60 +/- 26* with 10 seconds of VF duration, respectively, and 114 +/- 62, 77 +/- 45,* and 72 +/- 53* with 1 minute of VF duration, respectively (*P < 0.05 vs M). There was no significant difference in the E50 between 10 seconds and 1 minute of VF durations for each waveform. We conclude that (1) the E50 does not significantly increase with lengthening VF durations up to 1 minute regardless of the shock waveform, and (2) external exponential biphasic shocks are more effective than monophasic waveforms even with longer VF durations.

Influence of anaesthesia on defibrillation threshold

Internal cardioverter-defibrillator implantation can be performed under local or general anaesthesia. Whether the technique of general anaesthesia influences the defibrillation threshold remains a matter of debate. We therefore compared, in a prospective, randomised clinical study, the effect of intravenous anaesthesia using propofol with inhalational anaesthesia using isoflurane on the defibrillation threshold in 68 patients scheduled for transvenous single-lead internal cardioverter-defibrillator implantation. Defibrillation threshold was measured at implantation and at device testing 1 week and 1 month after implantation. Patients acted as their own controls. Neither the anaesthetic technique nor the duration of anaesthesia was associated with significant changes in the defibrillation threshold. We conclude that in this group of high-risk patients, both types of anaesthesia are acceptable techniques for internal cardioverter-defibrillator implantation and testing.

Effect of atrial natriuretic peptide on electrical defibrillation efficacy

INTRODUCTION

In vitro studies have suggested that human atrial natriuretic peptide (ANP) modulates the electrophysiologic properties of myocardial cells. This study assessed whether ANP could influence defibrillation efficacy.

METHODS AND RESULTS

In 35 anesthetized dogs, the transcardiac defibrillation threshold (DFT) as well as hemodynamic and electrophysiologic variables were determined before and during treatment with ANP (n = 11), hydralazine (n = 11), or saline (n = 13). ANP (1.5 microg/kg + 0.2 microg/kg per min) increased the plasma concentration of cyclic GMP (a second messenger for ANP) and significantly decreased aortic blood pressure (mean 100+/-11 mmHg to 83+/-15 mmHg). ANP also prolonged ventricular repolarization (effective refractory period 157+/-7 msec to 165+/-11 msec) and markedly reduced DFT (5.4+/-1.2 J to 3.8+/-0.7 J [P < 0.01]) without changing pulmonary artery pressure or sinus cycle length. Neither saline nor hydralazine (1.5 mg/kg) had a significant effect on DFT (saline 4.7+/-2.1 J to 4.6+/-2.4 J; hydralazine 4.3+/-2.0 J to 4.2+/-1.9 J), although hydralazine caused pronounced hypotension (mean aortic pressure 103+/-9 mmHg to 74+/-13 mmHg).

CONCLUSION

These results suggest that ANP increases defibrillation efficacy, and that this effect is not necessarily shared by other vasodilating agents.

Relationship between the upper limit of vulnerability determined in normal sinus rhythm and the defibrillation threshold in patients with implantable cardioverter defibrillators

The upper limit of vulnerability is the strength above which ventricular fibrillation is no longer inducible with a shock delivered during the vulnerable phase of the cardiac cycle. It has been demonstrated that the upper limit of vulnerability correlates with the defibrillation threshold in a paced rhythm. The purpose of this study is to evaluate the correlation of the upper limit of vulnerability determined in normal sinus rhythm with the defibrillation threshold using a simplified protocol in patients undergoing placement of an ICD. We studied 28 patients who underwent ICD implantation. CPI generators and Endotak leads were used in all patients. Device-based testing was used to determined the defibrillation threshold and the upper limit of vulnerability. The upper limit of vulnerability was tested with three shocks delivered at 0, 20, and 40 ms before the peak of the T wave during normal sinus rhythm. The defibrillation threshold was determined by a simple step up-down protocol. The upper limit of vulnerability (9.0 +/- 4.5 J) did not significantly differ from the defibrillation threshold (9.9 +/- 4.0 J), P = NS. A close correlation was present, correlation coefficient = 0.75, P < 0.0001. The upper limit of vulnerability was within 5 J of the defibrillation threshold in 27 (96%) of the 28 patients. The upper limit of vulnerability underestimated the defibrillation threshold by 10 J in one patient who had a defibrillation threshold of 15 J. The upper limit of vulnerability determined in normal sinus rhythm correlates significantly with the defibrillation threshold in patients undergoing ICD implantation. The protocol is simple and easily implemented clinically.

Adrenergic-dependent Effect of Adenosine-induced Ventricular Fibrillation in the Isolated Rabbit Heart

BACKGROUND: The present study examined the contributory role of endogenous catecholamines in adenosine-induced ventricular fibrillation in isolation rabbit hearts. METHODS AND RESULTS: Cardiac catecholamine depletion was induced in eleven rabbits by the administration of 6-hydroxydopamine (2 x 30 mg/kg, every 12 hours intramuscularly). Hearts were removed 24 hours later, and subjected to 12 minutes of hypoxic perfusion followed by 40 minutes of reoxygenation while heart rate was maintained with atrial pacing. One of six, and one of five hearts from 6-hydroxydopamine treated rabbits developed ventricular fibrillation during hypoxia-reoxygenation when exposed to 3,7-dimethyl-1-propargylzanthine (DMPX) (10 µM) + adenosine (ADO) (1 µM) and DMPX (10 µM) + ADO (10 µM), respectively. In hearts from a control group, not exposed to 6-hydroxydopamine, ventricular fibrillation developed in each of five (100% incidence) hearts when perfused in the presence of DMPX (10 µM) + ADO (10 µM) (P <.05). Nadolol (1 µM), a beta-adrenoceptor DMPX (10 µM) + ADO (10 µM) treated hearts (n = 6, P <.05 vs DMPX + ADO treated hearts). To ensure catecholamine depletion, spontaneously beating isolated hearts from vehicle and 6-hydroxydopamine treated rabbits were perfused under normoxic conditions while exposed to increasing concentrations of tyramine (1, 3, 10 mM) and the change in heart rate was determined. A concentration-related, positive chronotorpic response to tyramine was obtained in hearts from the vehicle treated group that was absent in hearts from 6-hydroxy-dopamine treated rabbits or hearts perfused in the presence of nadolol. CONCLUSIONS: The results demonstrate that inhibition of the cardiac adenosine A(2) receptor, unmasks an adenosine A(1) receptor profibrillatory effect that is dependent upon endogenous cardiac catecholamines and beta-adrenoreceptor activation during myocardial hypoxia-reoxygenation.

Effect of i.v. dipyridamole on cerebral blood flow, blood pressure, plasma adenosine and cAMP levels in rabbits

In response to intravenous administration of dipyridamole, the quantitative and temporal changes in plasma adenosine and cyclic AMP (cAMP) levels in relation to the changes in cerebral blood flow (CBF) and mean arterial blood pressure (MABP) have not been studied. Therefore, we investigated simultaneously the changes in CBF (hydrogen and thermal clearance methods), MABP, plasma adenosine (HPLC) and cAMP (radioimmunoassay) levels for 1 h after intravenous injection of 0.7 and 1.4 mg/kg dipyridamole in rabbits. In separate experiments, only plasma adenosine concentrations were measured to determine how and for how long intravenous administration of 0.7 mg/kg dipyridamole is able to inhibit the removal of plasma adenosine. Dipyridamole decreased MABP, increased plasma adenosine and cAMP levels in a dose-dependent manner. The dose-dependency of increases in CBF could not be demonstrated owing to the marked hypotension. The increase in plasma adenosine concentrations was biphasic. The first peak could be detected at the end of the dipyridamole injection. The second peak occurred 20 min after drug administration, simultaneously with the maximal increases in plasma cAMP level and CBF, whereas the maximal fall in MABP developed earlier. Intravenous administration of 0.7 mg/kg dipyridamole inhibited adenosine uptake only by 25%, which lasted less than 10 min. We concluded that intravenously given dipyridamole is responsible only for the initial short-lasting elevation of plasma adenosine concentration, and is able to induce vasodilation without either dipyridamole itself or adenosine necessarily gaining access to the muscular layer.

Comparative reproducibility of defibrillation threshold and upper limit of vulnerability

The upper limit of vulnerability (ULV) is the strength at or above which VF is not induced when a stimulus is delivered during the vulnerable phase of the cardiac cycle. Previous studies have demonstrated a statistically significant correlation between the ULV and the defibrillation threshold (DFT) in groups of patients. However, the correlation between ULV and DFT may not be close in individual patients. This imperfect correlation may be due to physiological factors or to limitations of the measurement methods. The reproducibility of either DFT or ULV has not been studied critically. The purpose of this study was to compare the reproducibility of clinically applicable methods for determination of DFT and ULV. We prospectively studied 25 patients with a transvenous implantable cardioverter defibrillator (Medtronic 7219D) at postoperative electrophysiological study. DFT was defined as the lowest energy that defibrillated after 10 seconds of VF. The ULV was defined as the lowest energy that did not induce VF with three shocks at 0, 20, and 40 ms before the peak of the T wave in ventricular paced rhythm at a cycle length of 500 ms. Both the DFT and the ULV were determined twice for biphasic pulses using a three-step, midpoint protocol. There was no significant difference between the two determinations of DFT (10.1 +/- 5.9 J vs 10.4 +/- 5.8 J), the two determinations of ULV (13.4 +/- 6.8 J vs 13.8 +/- 6.6) or the DFT-ULV Pearson correlation coefficients for each determination (0.84, P < 0.001 vs 0.75, P < 0.001). To analyze reproducibility, Lin concordance coefficients for second determination versus first determination were constructed for both ULV and DFT. This coefficient is similar to the Pearson correlation coefficient, but measures closeness to the line of identity rather than the line of regression. The Lin concordance coefficient for ULV was higher than that for DFT (0.93, 95% CI 0.85-0.97 vs 0.64, 95% CI 0.33-0.82; P < 0.01). For paired comparison of defibrillation efficacy under different experimental conditions, the sample sizes required to detect differences of 2 J, 3 J, and 4 J (80% power, P < 0.05) were 52, 24, and 15 for DFT versus 15, 8, and 6 for ULV. We conclude that a simple, clinically applicable method for determination of ULV is more reproducible than the single point DFT. Measured correlations between the ULV and single point are limited by the reproducibility of the DFT measurement.

Upper limit of vulnerability is a good estimator of shock strength associated with 90% probability of successful defibrillation in humans with transvenous implantable cardioverter-defibrillators

OBJECTIVES

The goals of this study were to determine the probability of successful defibrillation at the upper limit of vulnerability and to evaluate a minimal safety margin for implantable cardioverter-defibrillator first shocks based solely on the upper limit of vulnerability.

BACKGROUND

The upper limit of vulnerability is the strength at or above which ventricular fibrillation is not induced when a stimulus is delivered during the vulnerable phase of the cardiac cycle. It has been proposed as an estimate of defibrillation efficacy because it correlates with the defibrillation threshold and can be determined with a single episode of fibrillation.

METHODS

We studied 40 patients prospectively at implantation of transvenous cardioverter-defibrillators. Defibrillation threshold was defined as the weakest biphasic shock that defibrillated after 10 s of ventricular fibrillation. The upper limit of vulnerability was defined as the weakest biphasic shock that did not induce ventricular fibrillation when given at 0, 20 and 40 ms before the peak of the T wave in ventricular paced rhythm at cycle length 500 ms. After determination of the upper limit of vulnerability and defibrillation threshold, patients underwent six additional fibrillation-defibrillation episodes. The strength of five of the defibrillation shocks was equal to the upper limit of vulnerability; the strength of one of the six shocks was randomly selected to be equal to the upper limit of vulnerability plus 3 J. The implantable cardioverter-defibrillator was tested at the upper limit of vulnerability plus 3 J in 28 patients.

RESULTS

The defibrillation threshold was 8.8 +/- 5.0 J (mean +/- SD), and upper limit of vulnerability was 11.3 +/- 4.6 J; the defibrillation threshold and upper limit of vulnerability were highly correlated (r = 0.89, p < 0.001). The success rate for the 200 defibrillation shocks with strength equal to the upper limit of vulnerability was 90% (95% confidence intervals based on proportion of successes in 40 patients: 86% to 94%). All five defibrillation test shocks at the upper limit of vulnerability were successful in 24 patients (60%); four of five were successful in 12 patients (30%); and three of five were successful in 4 patients (10%). All 40 test shocks and 28 implantable cardioverter-defibrillator shocks with a strength equal to the upper limit of vulnerability plus 3 J were successful.

CONCLUSIONS

The upper limit of vulnerability is a good estimator of the shock strength associated with 90% probability of successful defibrillation (DFT90). A strength of 3 J above the upper limit of vulnerability is a good estimate of the minimal acute safety margin for implantable cardioverter-defibrillator first shocks.

Myocardial vulnerability to T wave shocks: relation to shock strength, shock coupling interval, and dispersion of ventricular repolarization

INTRODUCTION

Induction of ventricular fibrillation (VF) by T wave shocks is of clinical interest due to the correlation between the upper limit of vulnerability (ULV) and the defibrillation threshold (DFT). However, the ULV has not yet been defined precisely in reference to the entire "area of vulnerability" (AOV), which is defined bifunctionally by both shock strengths and shock coupling intervals, nor has it been related to the dispersion of ventricular repolarization, considered to be an important determinant of vulnerability.

METHODS AND RESULTS

In 11 isolated perfused rabbit hearts immersed in a tissue bath containing a 3-lead ECG recording system and two opposite plate electrodes for field shock administration, 7 monophasic action potentials (MAPs) were recorded simultaneously from different epicardial and endocardial regions of the right and left ventricles. An average of 90 +/- 25 monophasic waveform shocks of varying shock strengths and coupling intervals were delivered to each heart to determine the horizontal and vertical boundaries of the AOV. The AOV approximated a rhomboid with homogenous VF inducibility. The ULV and lower limit of vulnerability (LLV) represented discrete corners of the AOV with significant changes in VF inducibility if either shock coupling intervals or shock strength were changed by only 10 msec or 10 V, respectively (P < 0.001). The ULV occurred at 7 +/- 10 msec shorter coupling intervals than the LLV (P < 0.05), and VF-inducing shock strengths at the left corner of the AOV were 50 +/- 67 V higher as compared to the right corner (P < 0.01). The maximal range of VF-inducing coupling intervals coincided (within < 2 msec) with the dispersion of MAPs at 70% repolarization, and the ULV coupling interval coincided (within < 4 msec) with the longest repolarization at 50%.

CONCLUSIONS

(1) VF vulnerability to monophasic T wave shocks is defined by an AOV that has the shape of a leftward tilted rhomboid. (2) Both the ULV and LLV are sharply defined upper and lower corners of the AOV rhomboid. (3) The width of the AOV corresponds to the dispersion of ventricular repolarization at the 70% level. (4) Considering the dispersion of ventricular repolarization may yield more precise ULV determinations and a better understanding of the correlation between the ULV and DFT.