Effectiveness and Safety of Internal Rectilinear Biphasic Versus Monophasic Defibrillation in Patients Undergoing Cardiac Surgery Michael Winterhalter, MD,* Till Piepenbrock, MD,* Rainer G. Leyh, PhD,† Clemens Gras, MD,* Janusz Zuk, MD,* Jörn Heine, PhD,* Christian Hagl, MD,† Niels Rahe-Meyer, MD,* Hartmut Hecker, Prof,‡ and Siegfried Piepenbrock, MD* 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 en-
ergy 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 ⴞ 0.6 v 1.9ⴞ 1.2; p ⴝ 0.013), maximal energy per patient (7.9 ⴞ 2.5 v 11.6 ⴞ 7.32; p ⴝ 0.006), and cumulative energy (10.1 ⴞ6.1 v 21.3 ⴞ 24.1; p ⴝ 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. © 2005 Elsevier Inc. All rights reserved.
I
trast, asymmetric biphasic waveforms increased dysfunction threshold 14% ⫾ 3% (p ⬍ 0.005) compared with monophasic control waveforms. Because long-duration, low-tilt, biphasic waveforms improve excitation threshold for refractory cells, they should improve defibrillation threshold. Asymmetric waveforms have the additional advantage of improving the safety factor by reducing postshock dysfunction.8 The purpose of this study was to compare a biphasic rectilinear waveform with a monophasic internal shock waveform for termination of VF in patients undergoing cardiac surgery. In a prospective randomized study the authors compared various waveforms for termination of VF and determined the influence of various waveforms on serum markers of myocardial damage.
N GENERAL, monophasic waveform shocks are used for external and internal defibrillation of ventricular fibrillation (VF) in the clinical setting.1-3 Although defibrillation is a lifesaving procedure, delivery of multiple high-energy shocks may be associated with myocardial damage and subsequent hemodynamic impairment.4,5 There is some evidence from the literature that a biphasic pulse form may reduce myocardial damage4,5 and furthermore may be more efficient to terminate VF. However, these data arose from studies in which different biphasic waveforms were used during external defibrillation. Concerning the effects of biphasic internal shocks in patients undergoing cardiac surgery,6,7 only limited data from 1 clinical study and 1 animal study exist.6,7 Both studies used biphasic waveforms with damped sinus waves for termination of VF, and myocardial damage due to internal defibrillation was not investigated. The delivered biphasic waveforms from various defibrillators differ in duration and structure of the first phase of the shock. All devices reverse polarity and deliver a truncated exponential waveform as the second phase of the shock. The devices also differ in the manner in which they respond to patient-specific impedance. The biphasic rectilinear waveform device changes the defibrillator’s internal impedance to compensate for differences in transthoracic impedance, enabling delivery of a constant current and fixed-duration shock. In contrast, the biphasic truncated exponential waveform device prolongs the duration of the shock to compensate for increased transthoracic impedance.8 Jones et al8 performed strength-duration studies to determine the effect pulse duration had on defibrillation-induced cardiac dysfunction. They found that for all waveforms dysfunction threshold decreased with waveform duration. For all durations dysfunction threshold was similar for symmetric monophasic and biphasic waveforms, with the same total duration. In con-
KEY WORDS: biphasic defibrillation, cardiac surgery, ventricular fibrillation
MATERIALS AND METHODS After approval by the local ethical committee, 134 patients 18 years of age or older who were scheduled to undergo elective cardiac surgery were enrolled in this prospective randomized study. Written informed consent was obtained from all patients. Those with a history of ventricular arrhythmias or with an implantable cardioverter-defibrillator, ventricular aneurysms, unstable angina, or preoperative elevated cardiac enzyme levels were excluded from the study. Of the total group, VF developed in 64 patients after reinstitution of coronary artery blood flow. According to the randomization list, the patients were assigned to the monophasic group (group A; n ⫽ 32) or the biphasic internal
From the Departments of *Anesthesiology, †Thoracic and Cardiovascular Surgery, and ‡Biometry, Hannover Medical School, Hannover, Germany. Address reprint requests to Michael Winterhalter, MD, Hannover Medical School, Department of Anesthesiology, Carl-Neuberg Str 1, 30625 Hannover, Germany. E-mail:
[email protected] © 2005 Elsevier Inc. All rights reserved. 1053-0770/05/1906-0008$30.00/0 doi:10.1053/j.jvca.2005.06.002
Journal of Cardiothoracic and Vascular Anesthesia, Vol 19, No 6 (December), 2005: pp 739-745
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Table 1. Patient Profile Variables
Age, y (mean ⫾ SD) Male, n (%) Body surface area (m2) Ejection fraction (%) Hypertrophic cardiomyopathy, n (%) Dilated cardiomyopathy, n (%) Valvular heart disease, n (%) Ischemic heart disease, n (%) MI, n (%) Arterial hypertension,* n (%) Atrial fibrillation, n (%) Medications -Blockers, n (%) Antiarrhythmic agent n (%) ACE inhibitor, n (%) Diuretic, n (%) Digoxin, n (%) Nitrate, n (%)
Monophasic Group (n ⫽ 32)
Biphasic Group (n ⫽ 32)
64.03 ⫾ 10.55 26 (81) 1.98 ⫾ 0.2 52.2 ⫾ 18.4 8 (25) 4 (12.5) 16 (50) 27 (84.4) 10 27 (84.4) 4 (12.5)
59.66 ⫾ 11.65 25 (78) 1.98 ⫾ 0.19 59.8 ⫾ 13.9 12 (37.5) 4 (12.5) 23 (71.9) 24 (75) 8 24 (75) 1 (3.1)
24 (75) 0 (0) 23 (71.8) 14 (43.7) 2 (6.2) 15 (46.8)
12 (37.5) 0 (0) 15 (46.8) 8 (25) 2 (6.2) 8 (25)
p (2 test)
0.12 ⬎0.2 ⬎0.2 0.067 ⬎0.2 ⬎0.2 0.073 ⬎0.2 ⬎0.2 ⬎0.2 ⬎0.2† 0.003 p ⬎ 0.2 0.042 0.114 ⬎0.2 0.068
Abbreviations: MI, history of previous myocardial infarction; *Systolic arterial blood pressure ⬎160 mmHg. †Fisher exact test.
waveform (group B; n ⫽ 32) group. Patients’ demographic data are shown in Table 1. Routine monitoring included a 5-lead electrocardiogram (ECG), central venous pressure and radial artery catheters, pulse oximetry, capnography, and blood and urinary bladder temperatures. Patients included in the study were treated by the same group of anesthesiologists and surgeons, all using the same anesthetic and surgical techniques, respectively. Anesthesia was induced with etomidate (0.3 mg/ kg) and fentanyl (0.8 g/kg). In addition, 0.1 mg/kg of pancuronium was used for neuromuscular blockade. A balanced anesthetic was maintained with intermittent administration of fentanyl and continuous administration of sevoflurane (0.5%-2%) before and after cardiopulmonary bypass (CPB). During CPB, propofol (3-5 mg/kg/h) was continuously administered. The CPB technique was the same in all patients: a single aortic and either right atrial or bicaval cannulation,
Stöckert TM roller pumps (Stöckert Instruments, Munich, Germany) and membrane oxygenators (Hilite 7000, Medos; Stolberg, Germany) primed with 1000 mL of Ringer’s lactate solution, 500 mL of glucose 5%, and 40 mL of sodium bicarbonate 8.4% were used. Nonpulsatile pump flow was maintained at 2.4 L/min/m2, with a perfusion pressure of 60 to 80 mmHg. Cardiopulmonary bypass took place at mild hypothermia with a core temperature of 32°C. Myocardial protection was achieved by using intermittent antegrade cold crystalloid or blood cardioplegia every 20 minutes. Initial anticoagulation was accomplished with 400 IU/kg of heparin and was supplemented as needed to maintain an activated coagulation time greater than 480 seconds. Residual heparin was antagonized with protamine sulfate immediately after CPB. Surgical procedures and intraoperative data are shown in Tables 2 and 3. Arterial blood samples were drawn and total creatine phosphokinase
Table 2. Patient Intraoperative Characteristics Variables
CABG, n (%) Valve operation, n (%) CABG & valve operation, n (%) Other procedure, n (%) CPB time (min) Aortic cross-clamp time (min) Operation time (min) Cardioplegic solution, n (%) St Thomas 2 Other Type of intraoperative VF Fine, n (%) Coarse, n (%) Time of VF to defibrillation (min)
Monophasic Group (n ⫽ 32)
Biphasic Group (n ⫽ 32)
p (2 test or unpaired t test)
19 (59) 5 (15.6) 3 (9.4) 5 (15.6) 106.2 ⫾ 48.7 55.2 ⫾ 28.3 230.7 ⫾ 51.8
18 (56) 6 (18.7) 4 (12.5) 4 (12.5) 100.7 ⫾ 38.1 61.0 ⫾ 31.0 212.6 ⫾ 55.9
⬎0.2 ⬎0.2 ⬎0.2 ⬎0.2 ⬎0.2 ⬎0.2 ⬎0.2 ⬎0.2
28 (87.5) 4 (12.5)
27 (84.4) 5 (15.6)
20 (62.5) 12 (37.5) 4.2 ⫾ 2.8
20 (62.5) 12 (37.5) 4.9 ⫾ 2.9
⬎0.2
Data shown as mean ⫾ SD unless otherwise indicated. Abbreviations: CABG, coronary artery bypass grafting; CPB, cardiopulmonary bypass; VF, ventricular fibrillation.
⬎0.2
INTERNAL RECTILINEAR BIPHASIC VERSUS MONOPHASIC DEFIBRILLATION
Table 3. Hemodynamic and Metabolic Variables During Cardiopulmonary Bypass
Variables
Monophasic Group (n ⫽ 32)
Biphasic Group (n ⫽ 32)
Esophageal temperature (°C) (t1) 35 ⫾ 1.34 35 ⫾ 1.43 (t2) 34 ⫾ 1.59 34 ⫾ 1.63 Myocardial temperature (°C) (t1) 33 ⫾ 1.2 33 ⫾ 1.31 (t2) 34 ⫾ 1.3 34 ⫾ 1.42 MAP (mmHg) (t1) 59.5 ⫾ 3.5 60.5 ⫾ 3.5 (t2) 67.5 ⫾ 4.0 65.4 ⫾ 3.9 pH (t1) 7.37 ⫾ 0.50 7.37 ⫾ 0.39 (t2) 7.37 ⫾ 0.57 7.36 ⫾ 0.33 PaO2 (mmHg) (t1) 265.1 ⫾ 66.6 272.13 ⫾ 74.19 (t2) 222.9 ⫾ 47.9 238.9 ⫾ 66.6 PaCO2 (mmHg) (t1) 43.3 ⫾ 5.2 41.56 ⫾ 4.23 (t2) 41.02 ⫾ 4.66 41.12 ⫾ 4.55 Base excess (mmol/L) (t1) 0.09 ⫾ 1.94 ⫺1.17 ⫾ 1.8 (t2) ⫺1.9 ⫾ 2.55 ⫺2.6 ⫾ 1.77 Potassium (mEq/L) (t1) 4.72 ⫾ 0.69 5.0 ⫾ 0.61 (t2) 4.9 ⫾ 0.6 5.0 ⫾ 0.6 Sodium (mmol/L) (t1) 135.41 ⫾ 2.07 134.4 ⫾ 2.07 (t2) 135.7 ⫾ 2.29 135.1 ⫾ 2.1 Calcium (mmol/L) (t1) 1.12 ⫾ 0.67 1.14 ⫾ 0.74 (t2) 1.12 ⫾ 0.56 1.15 ⫾ 0.71 Glucose (mmol/L) (t1) 12.38 ⫾ 2.91 12.91 ⫾ 2.66 (t2) 11.95 ⫾ 2.75 11.70 ⫾ 2.83 Hematocrit (%) (t1) 24.84 ⫾ 3.96 25.71 ⫾ 3.31 (t2) 25.56 ⫾ 2.06 26.61 ⫾ 2.95
p (unpaired t test)
⬎0.2 ⬎0.2
⬎0.2 ⬎0.2 ⬎0.2 ⬎0.2 ⬎0.2 0.17 ⬎0.2 ⬎0.2
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received a rectilinear biphasic waveform (Fig 1B) generated by an M series defibrillator (Zoll Medical, Cologne, Germany). The defibrillator has 115 F of capacitance and a resistance of 50 ohms. The surface of the paddles was 36.3 cm2. The shock protocols were identical for the 2 treatment groups. The initial energy used for defibrillation was 7 J in both groups. When VF persisted, higher energy levels for defibrillation were used, beginning with 10 J, followed by 20 J and 30 J (1 shock per energy level) until defibrillation occurred. No antiarrhythmic drugs were given unless VF could not be terminated with 30 J. When VF recurred after successful defibrillation, the initial energy for defibrillation was set at 7 J following the above protocol. The surgeon who delivered the shocks was blinded to the waveform type. All shocks were delivered to the epicardial surface of the heart, with 1 shock paddle placed at the right atrium and the other paddle placed opposite at the lateral aspect of the left ventricle (LV). Complete arterial blood gas analysis was obtained according to standard methods. TnT concentrations were measured using a TnT STAT assay on an Elecsys 2010 system (Roche Diagnostics, Mannheim, Germany). CPK-MB concentrations were determined with the CK-MB assay on the Roche Hitachi 717 system. Creatine phophokinase activity at 25°C was determined with International Federation of Clinical Chemistry standard methods.
0.15 ⬎0.2 0.009 ⬎0.2 0.025 ⬎0.2 0.069 ⬎0.2 ⬎0.2 ⬎0.2 ⬎0.2 ⬎0.2 ⬎0.2 0.13
Data shown as mean ⫾ SD unless otherwise indicated. Abbreviations: MAP, mean arterial pressure; PaO2, partial pressure of arterial oxygen; PaCO2, partial pressure of arterial carbon dioxide.
(CPK), CPK-MB, and troponin T (TnT) levels were determined immediately after administration of cardioplegia (t1), 5 minutes after termination of VF (t2), 8 hours after the operation (t3), and 20 hours after the operation (t4). ␣-Stat management was used for interpretation of the arterial blood gas analysis and treatment of the patients during CPB. Hematocrit was maintained between 20% and 25%. Potassium was administered to maintain a level between 4.5 and 5.5 mEq/L. Ventricular fibrillation was detected on a 5-channel ECG monitor and by direct visual observation of the heart. According to the ECG, VF was categorized as coarse or fine. Successful defibrillation was defined as a shock that terminated VF for at least 60 seconds. Group A patients (n ⫽ 32) received monophasic shocks (Fig 1A) generated by a Cardio Serv defibrillator (Cardio Serv, Hellige Marquette, Freiburg, Germany). The defibrillator has 32 F of capacitance, 26 mH of inductance, and a resistance of 6.5 ohms. The paddles had a circular form with a surface of 36.0 cm2. Group B patients (n ⫽ 32)
Fig 1. Damped sine (A) and rectilinear biphasic (B) waveforms used in this study.
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Fig 2.
Number of shocks per patient.
Patient group characteristics are described as the mean and standard deviation or absolute and relative frequencies. Comparisons between the groups were performed with the unpaired t test for normally distributed data or with the Mann-Whitney U test for other quantitative data, and with the ␥ 2 test or Fisher exact test for qualitative variables. All tests were performed 2-sided, with the significance level ␣ ⫽ 0.05. However, for the primary endpoint, a 1-sided hypothesis was tested according to the study protocol. The trial was designed to detect a difference of 3 J in favor of the bipolar group with a power of 80%. Inasmuch as there was less information on standard deviation, an internal pilot study was planned with blinded evaluation of the common standard deviation. Based on this result the sample size of 32 for each group was calculated. RESULTS
Of 134 patients enrolled, 64 received 1 or more defibrillation shocks (monophasic or biphasic) after aortic declamping; these patients are analyzed in this study. The demographic, preoperative clinical data, and comorbidities between groups showed no significant differences (Table 1). In most patients, coronary artery bypass grafting (CABG) (group A, n ⫽ 19; group, B n ⫽ 18) was performed. The surgical procedures performed and intraoperative data did not differ between groups (Table 2). Potassium levels, perfusion pressure, body temperature, blood temperature, hematocrit, and pH after aortic declamping revealed no statistically significant differences between the study groups (Table 3). The incidence of fine VF was the same, with 62.5% (20 of 32) in each group (Table 2). The quantity of shocks delivered in each group is shown in
Fig 4.
Fig 2. For successful defibrillation, the quantity of shocks delivered was significantly reduced in patients who received biphasic waveforms (group B; 1.3 ⫾ 0.6 v 1.9 ⫾ 1.2; p ⫽ 0.013). However, recurrent VF after successful defibrillation occurred in 1 patient after monophasic defibrillation (group A) and in 2 patients after biphasic defibrillation (group B) because of manual manipulation of the heart. Biphasic defibrillation was more effective to terminate VF than was monophasic defibrillation. Biphasic defibrillation was successful at the 7-J energy level in 78.1% of patients (25 of 32) versus 50% (16 of 32) in the monophasic group (Fig 3). Thus, the threshold for successful defibrillation was significantly reduced in group B patients (group A, 11.6 ⫾ 7.3; group B, 7.9 ⫾ 2.5; p ⫽ 0.006). Furthermore, the cumulative energy per patient necessary for successful defibrillation was significantly lower in the biphasic defibrillation group (10.1 ⫾ 6.1 v 21.3 ⫾ 24.1; p ⫽ 0.016; Fig 4). The TnT, CPK, and CK-MB levels are shown in Table 4 and Fig 5. One group A patient was excluded from the analysis because of an intraoperative myocardial infarction. The perioperative TnT (p ⬍ 0,001), CPK (p ⬍ 0.001), and CK-MB (p ⬍ 0.001) levels increased significantly over time, with a peak
Table 4. Biochemical Markers Variables
Troponin-T (g/mL) t1 t2 t3 t4 Creatine kinase (U/L) t3 t4 CK-MB (U/L) t4 t3 Fig 3.
Shock effectiveness.
Total cumulative energy per patient.
Monophasic Group (n ⫽ 31)
Biphasic Group (n ⫽ 32)
p (unpaired t test)
0.0325 ⫾ 0.6792 0.0875 ⫾ 0.9371 0.3694 ⫾ 0.32162 0.4754 ⫾ 0.29538
0.0216 ⫾ 0.03777 0.0888 ⫾ 0.9688 0.2859 ⫾ 0.18470 0.3833 ⫾ 0.19895
⬎0.2 ⬎0.2 0.208 0.206
214.6 ⫾ 137.0 339.3 ⫾ 309.1
214.3 ⫾ 145.4 238.7 ⫾ 120.5
⬎0.2 ⬎0.2
20.6 ⫾ 15.3 18.2 ⫾ 8.7
16.9 ⫾ 8.2 18.75 ⫾ 7.86
⬎0.2 ⬎0.2
Data shown as mean ⫾ SD unless otherwise indicated.
INTERNAL RECTILINEAR BIPHASIC VERSUS MONOPHASIC DEFIBRILLATION
Fig 5.
Perioperative troponin-T kinetics.
level 20 hours postoperatively. A tendency toward higher postoperative TnT, CPK, and CK-MB levels was found in group A. However, this was not statistically significant at any time. DISCUSSION
In this prospective randomized study it was shown that internal rectilinear biphasic defibrillation is more effective to terminate intraoperatie VF compared with monophasic defibrillation. However, there was no difference in myocardial damage from defibrillation between groups. For external defibrillation, biphasic defibrillation is superior to monophasic defibrillation in terms of success rate and cumulative energy necessary for successful defibrillation of VF.9-11 Data about the effectiveness of these different defibrillation modes for internal defibrillation used in cardiac surgery are sparse. Schwartz et al7 demonstrated in a prospective randomized clinical study that biphasic internal defibrillation using truncated exponential curves in cardiac surgery is substantially more effective than monophasic shocks, with a cumulative defibrillation success at 5 J in 52.1% versus 22% (p ⫽ 0.01) and a significantly reduced cumulative energy (12.6 J ⫾ 12.3 v 23.4 ⫾ 16 J; p ⫽ 0.002). The results of the present study showed that for successful termination of VF the number of shocks (p ⫽ 0.013) and cumulative energy (p ⫽ 0.013) were significantly reduced in the biphasic group. Although these results are consistent with the recently published study by Schwartz et al,7 it is important to emphasise that different biphasic pulse waveforms were used. In this study biphasic rectilinear pulse waveforms were used, whereas Schwartz and colleagues used biphasic truncated exponential curve pulse waveforms. Because the effect of different biphasic pulse waveforms on the effectiveness to terminate VF has not been evaluated yet, the result of these studies must be interpreted with caution. These 2 biphasic shock waveforms are different in terms of duration and structure of the first phase of the shock and deliver a truncated exponential waveform for the second phase of the
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shock. Furthermore, the devices used in both studies respond differently to patient-specific impedance. The biphasic rectilinear waveform device changes the defibrillator’s internal impedance to compensate for differences in transthoracic impedance, enabling delivery of a constant-current, fixed-duration shock. In contrast, the biphasic truncated exponential waveform device prolongs the duration of the shock to compensate for increased transthoracic impedance.7 It can be speculated that these differences might have some clinical effect. This hypothesis is supported by Jones et al,8 who performed strengthduration studies to determine the effect of pulse duration on defibrillation-induced cardiac dysfunction. They demonstrated that cardiac dysfunction threshold decreased with waveform duration, with no difference between monophasic and biphasic waveforms. However, asymmetric biphasic waveforms increased dysfunction threshold 14% ⫾ 3% (p ⬍ 0.005) compared with monophasic control waveforms. Aside from the defibrillation mode, the serum potassium level, arterial perfusion pressure, myocardial temperature, body temperature, duration of CPB, use of antiarrhythmic drugs, mean fibrillation frequency, duration of fibrillation, and size of defibrillation paddles are factors that influence the effectiveness to terminate VF.12-15 However, none of these factors differed between the biphasic and monophasic groups. Complete heart block, severe arterial hypotension, and myocardial necrosis have been reported as the result of high-energy transthoracic defibrillation.16,17 Experimental studies have documented that direct application of defibrillator shocks may result in myocardial necrosis and dysfunction of cell respiration and oxidative metabolism.18,19 Data from the literature indicate that the shock waveform used may have an effect on the degree of myocardial injury after direct shock application. Osswald et al5 showed in an experimental animal setting that internal biphasic shocks are less injurious on myocardial oxidative metabolism with accelerated recovery of myocardial function compared with internal monophasic shocks. Boriani et al20 demonstrated that repeated biphasic internal shocks for the treatment of atrial fibrillation is associated with an increase in troponin I levels, suggesting subtle asymptomatic myocardial injury. In the present study, standard markers (TnT, CPK-MB) were used to assess myocardial damage. Previously, TnT has been suggested as being more sensitive and specific to myocardial injury compared with the CPK-MB isoenzyme.21 TnT is 1 of 3 components of troponin, which is a major protein associated with actin filaments and mediates Ca2⫹ regulation of muscle contraction.22 Because most cardiac TnT is myofibril bound, its release in serum is expected to correlate with the extent of myocardial necrosis. TnT release beyond several hours originates from intracellular compartmented myofibril-bound TnT, whereas early release after ischemic events may derive from cytosolic TnT.23 The present study data show that serum concentrations of TnT and CKP-MB increased significantly in the perioperative period; however, there was no statistically significant difference between the 2 groups. Factors influencing perioperative TnT and CPK-MB release are many, including aortic cross-clamp time, CPB time, type of surgery performed, and type of cardioplegia used for myocardial protection.24-27 Inasmuch as these factors showed no difference between
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groups, the authors speculate that the defibrillation mode per se has no effect on the degree of myocardial injury. The authors address several limitations of this study. First, it is controversial whether myocardial wall thickness or myocardial hypertrohy are significant factors influencing the defibrillation threshold. Lucy et al28 showed in an animal study that myocardial hypertrophy may profoundly increase defibrillation energy requirements for successful defibrillation. Their results were supported by Chapman et al,29 who demonstrated in a clinical setting that left ventricular hypertrophy (LVH) and LV weight are significantly correlated with defibrillation threshold. However, others demonstrated in animal and clinical studies30-32 that LVH or LV weight have no significant influence on the defibrillation threshold. In the present study, neither myocardial wall thickness, myocardial weight, nor the size of the heart were determined. Thus, it can be speculated that this could have influenced the results. However, the incidence of preoperative dilated cardiomyopathy (12.5% v 12.5%; p ⬎ 0.2) and systemic arterial hypertension (84.4% v 75%; p ⬎ 0.2) showed no differences between groups. Moreover, the biphasic
group showed a higher incidence of hypertrophic cardiomyopathy (25% v 37.5%), but this did not reach statistical significance (p ⬎ 0.2). Second, although TnT and CPK-MB kinetics and values showed no differences between groups, it cannot be judged whether more sensitive methods (eg, ventricular biopsy to assess intracellular adenosine triphosphate and lactate levels, measurements of lactate levels, and oxygen extraction in the coronary sinus) would have detected additional subtle myocardial injury at the cellular level. The results of this prospective randomized study provide some evidence that internal rectilinear biphasic shock is more effective to terminate VF during cardiac surgery than is internal monophasic defibrillation. However, the study did not detect any differences in the degree of myocardial injury by TnT and CPK-MB levels between internal rectilinear biphasic shock and internal monophasic defibrillation. Multicenter prospective randomized studies comparing different types of biphasic defibrillators (rectilinear, truncated exponential curves) with monophasic defibrillators with larger patient cohorts are needed in the future.
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