Article, Cardiology

2,3-Butanedione monoxime facilitates successful resuscitation in a dose-dependent fashion in a pig model of cardiac arrest

a b s t r a c t

Purpose: Ischemic contracture compromises the hemodynamic effectiveness of cardiopulmonary resuscitation (CPR) and resuscitability from cardiac arrest. In a pig model of cardiac arrest, 2,3-butanedione monoxime (BDM) attenuated ischemic contracture. We investigated the effects of different doses of BDM to determine whether increasing the dose of BDM could improve the hemodynamic effectiveness of CPR further, thus ultimately improving resuscitability.

Methods: After 16 minutes of untreated ventricular fibrillation and 8 minutes of basic life support, 36 pigs were divided randomly into 3 groups that received 50 mg/kg (low-dose group) of BDM, 100 mg/kg (high-dose group) of BDM, or an equivalent volume of saline (control group) during advanced cardiovascular life support.

Results: During advanced cardiovascular life support, the control group showed an increase in left ventricular (LV) wall thickness and a decrease in LV chamber area. In contrast, the BDM-treated groups showed a decrease in the LV wall thick- ness and an increase in the LV chamber area in a dose-dependent fashion. Mixed-model analyses of the LV wall thickness and LV chamber area revealed significant group effects and group-time interactions. Central venous oxygen saturation at 3 minutes after the drug administration was 21.6% (18.4-31.9), 39.2% (28.8-53.7), and 54.0% (47.5-69.4) in the control, low- dose, and high-dose groups, respectively (P b .001). Sustained restoration of spontaneous circulation was attained in 7 (58.3%), 10 (83.3%), and 12 animals (100%) in the control, low-dose, and high-dose groups, respectively (P = .046).

Conclusion: 2,3-Butanedione monoxime administered during CPR attenuated ischemic contracture and improved the resuscitability in a dose-dependent fashion.

(C) 2016

Introduction

Ischemic contracture, which refers to a progressive left ventricular (LV) wall thickening following ischemia, occurs frequently in prolonged cardiac arrest [1,2]. In a study of 59 out-of-hospital cardiac arrest (OHCA) patients who underwent open-chest cardiopulmonary resuscitation (CPR), firm myocardium indicating contracted heart muscle was present in 36 patients (61%) immediately after thoracotomy [2]. Ischemic contracture results in progressive decreases in LV chamber volume and corresponding decreases in stroke volume during CPR, which, in turn, compromise resuscitability following arrest [1-3].

Previous studies in isolated hearts suggested that reperfusion with 2,3- butanedione monoxime (BDM) reduced ischemic contracture and improved myocardial functional recovery after ischemia [4,5]. We previously compared

? Funding sources/disclosures: This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2015R1D1A1A09057248). The funder had no role in the study design, data collection, analysis, decision to publish, or preparation of the manuscript.

* Corresponding author. Tel.: +82 62 220 6809; fax: +82 62 228 7417.

E-mail address: [email protected] (K.W. Jeung).

the effects of 50 mg/kg of BDM with placebo in a pig model of OHCA [6]. In this study, BDM administered during CPR attenuated ischemic contracture significantly, but the improvement of resuscitability by BDM over placebo did not reach statistical significance. Several studies suggest that BDM inhibits myocardial ischemic contracture in a dose-dependent manner [7,8]. Thus, it can be hypothesized that increasing the dose of BDM can improve the hemo- dynamic effectiveness of CPR further, thus ultimately improving resuscitability. However, to our knowledge, the dose-response effects of BDM have not been studied in an in vivo cardiac arrest model.

In this study, we investigated the effects of different doses of BDM in a pig model of OHCA to determine whether a higher dose of BDM could fur- ther attenuate ischemic contracture during CPR and improve resuscitability and short-term survival.

Methods

Thirty-six domestic pigs weighing 21.6 kg (19.7-22.9) were used. The Animal Care and Use Committee of Chonnam National University approved the protocol of this study (CNU IACUC-H-2015-19).

http://dx.doi.org/10.1016/j.ajem.2016.03.025

0735-6757/(C) 2016

Animal care and experiments were conducted according to the author’s Institutional Animal Care and Use Committee guidelines.

Animal preparation

After premedication (ketamine, 20 mg/kg; xylazine, 2.2 mg/kg), an- esthesia was induced with 50%:50% N2O/O2 and 2%-5% sevoflurane via a mask. After tracheal intubation, pigs were ventilated at a tidal volume of 15 mL/kg. Anesthesia was continued with 70%:30% N2O/O2 and 0.5%-2% sevoflurane titrated to prevent signs of pain. Ventilatory rates were ad- justed to achieve normocapnia. A double-lumen catheter was advanced from the right femoral artery to the thoracic aorta to monitor aortic pressure and sample blood. The right external jugular vein was cannu- lated with an 8F introducer sheath to monitor right atrial (RA) pressure, to obtain a blood sample, and to insert a right ventricle (RV) pacing catheter. For echocardiographic measurements, a transesophageal echocardiography probe (UST-5293-5; Hitachi Aloka Medical Ltd, Tokyo, Japan) was inserted precordially into the mediastinum, as de- scribed previously [6]. We used this method to obtain an adequate LV long-axis view during CPR without interrupting chest compression. In our previous experience, in which the transesophageal echocardiogra- phy probe was advanced into the esophagus, chest compression fre- quently precluded adequate imaging of the LV. However, with this method, no interruption in chest compression or ventilation was re- quired to visualize the LV during CPR. With regard to the doses of BDM, the dose used in the previous study (50 mg/kg) was chosen as the low dose [6] and 100 mg/kg, 2 times higher than the low dose, was chosen as the high dose. Immediately before inducing ventricular fibrillation (VF), an investigator opened a sealed envelope that assigned animals to 1 of 3 groups (control group, low-dose group, or high-dose group) and prepared either a saline placebo or BDM solution (Samchun Pure Chemical Co Ltd, Pyeongtaek, Korea; 25-g/L concentration solution for low-dose group, 50-g/L concentration solution for high-dose group) in equal volume (2 mL/kg). All other investigators involved in this study remained blinded to treatment allocation until analysis.

Experimental protocol

After baseline measurements, VF was induced by applying an electrical current (60 Hz, 30 mA alternating current) via the RV pacing catheter (Fig. 1). After 16 minutes of untreated VF, basic life support (BLS) using cycles of 30 chest compressions followed by 2 ventilations with ambient air was started. Closed-chest compressions were adminis- tered by 2 investigators (SSC and SML) blinded to the randomization in all animals at a rate of 100/min and a compression depth of 25% of the anterior-posterior diameter of the chest wall. After 8 minutes of BLS, Advanced cardiovascular life support was initiated according to

current Resuscitation guidelines [9]. Positive-pressure ventilations with High-flow oxygen (10 L/min) were provided at a rate of 8/min. At the commencement of ACLS, all animals received 0.5 U/kg of vasopressin intravenously. At 2 minutes after the start of ACLS, either 50 mg/kg (low-dose group) or 100 mg/kg (high-dose group) of the BDM or an equivalent volume of 0.9% saline solution was administered into the RA. After 4 minutes of ACLS, 0.02 mg/kg of epinephrine was administered every 3 minutes if required. During ACLS, defibrillation was attempted using a single biphasic 150-J Electric shock at 2-minute intervals. Sustained restoration of spontaneous circulation (ROSC) was de- fined as an unassisted pulsatile rhythm with a systolic aortic pressure greater than 60 mm Hg maintained for at least 10 minutes [10]. If ROSC was not achieved within 12 minutes of ACLS, resuscitation efforts were discontinued.

Animals that achieved ROSC received mechanical ventilation with 100% O2 at prearrest settings and underwent a 4-hour period of inten- sive care. Five minutes after ROSC, oxygen concentration was reduced to 40%, and the ventilatory rate and/or tidal volumes were adjusted to achieve normocapnia. Recurrent cardiac arrest was treated with stan- dard CPR according to current resuscitation guidelines [9]. Mean arterial pressure was maintained at N 65 mm Hg with norepinephrine infusion. Throughout the intensive care period, titrated doses of sevoflurane were administered to maintain adequate anesthesia. At the end of the 4-hour period, animals were euthanized by infusing potassium chloride.

Measurements

Aortic pressure, RA pressure, and electrocardiogram were moni- tored (CS/3 CCM; Datex-Ohmeda, Helsinki, Finland) and transferred to a personal computer (S/5 Collect software; Datex-Ohmeda). coronary perfusion pressure was calculated by subtracting RA end- diastolic pressure from simultaneous aortic end-diastolic pressure. arterial blood gases (RapidLab865; Bayer Health Care, Fernwald, Germany) and lactate (Unicel DXC 800; Beckman Coulter, Fullerton, CA) were measured at prearrest baseline, 5 minutes, and 4 hours after ROSC. At 5 minutes after the start of ACLS, a blood sample was obtained from the introducer sheath inserted into the RA to assess central venous oxygen saturation (ScvO2). Troponin-I (Dimension RXL Max; Siemens Healthcare Diagnostics, Deerfield, IL) was measured at prearrest base- line and at 4 hours after ROSC. Echocardiograms were obtained by a re- searcher blinded to the treatment allocation at the prearrest baseline (5 minutes before the induction of VF), during untreated VF (1, 8, and 16 minutes after the initiation of VF) and CPR (every 2 minutes during BLS and every minute during ACLS), and at 30 minutes and 4 hours after ROSC. An experienced, blinded observer analyzed the echocardio- graphic images. LV chamber area and LV wall thickness during CPR were measured using a technically satisfactory LV long-axis view of the frame

Fig. 1. Experimental timeline. At 2 minutes after the start of advanced cardiac life support (26 minutes after the VF induction), either 50 mg/kg (low-dose group) or 100 mg/kg (high-dose group) of the BDM or an equivalent volume of 0.9% saline solution was administered into the right atrium. The lightning marks indicate the onset of a 10-second pause in chest compres- sions for Rhythm analysis and a 150-J shock if indicated.

Table 1

Baseline characteristics

Control group (n = 12)

Low-dose group (n = 12)

High-dose group (n = 12)

P

Weight (kg)

22.7 (19.6-24.0)

20.5 (19.0-23.0)

21.1 (20.2-22.3)

.439

Aortic systolic pressure (mm Hg)

118.0 +- 11.6

118.3 +- 10.1

114.8 +- 7.8

.648

Aortic diastolic pressure (mm Hg)

76.4 +- 12.3

77.7 +- 8.9

73.3 +- 7.9

.548

Mean aortic pressure (mm Hg)

93.2 +- 12.3

94.7 +- 9.2

90.3 +- 8.3

.556

RA systolic pressure (mm Hg)

9.6 +- 1.9

9.8 +- 1.9

8.9 +- 2.1

.554

RA diastolic pressure (mm Hg)

4.0 (3.0-5.0)

5.0 (4.0-5.8)

5.0 (4.0-6.0)

.327

Mean RA pressure (mm Hg)

6.5 (6.0-8.0)

7.0 (6.0-9.0)

7.0 (6.3-8.5)

.849

Heart rate (/min)

99 +- 16

96 +- 9

98 +- 11

.874

pH

7.500 +- 0.077

7.497 +- 0.042

7.496 +- 0.050

.986

PaCO2 (mm Hg)

37.6 (33.3-40.4)

36.2 (35.7-38.4)

37.2 (35.9-39.1)

.873

PaO2 (mm Hg)

146.1 +- 19.9

138.8 +- 29.1

149.7 +- 33.7

.632

Base excess (mmol/L)

5.8 +- 4.4

5.2 +- 3.2

5.4 +- 3.4

.923

HCO3 (mmol/L)

28.9 +- 3.7

28.4 +- 2.9

28.8 +- 3.1

.929

SaO2 (%)

99.7 (98.6-100.0)

99.5 (98.5-99.8)

99.8 (98.0-100.0)

.736

Troponin (ng/mL)

0.4 (0.1-0.6)

0.2 (0.1-0.5)

0.2 (0.1-0.3)

.672

Lactate (mmol/L)

1.5 (1.2-2.0)

1.3 (0.9-1.8)

1.6 (1.0-2.5)

.554

LVEF (%)

44.8 +- 3.0

43.1 +- 2.1

44.8 +- 4.4

.376

Data are presented as mean +- SD or medians with IQRs. Abbreviation: LVEF, left ventricular ejection fraction.

Fig. 2. Left ventricular wall thickness (A) and LV chamber area (B) during 16 minutes of untreated VF, simulated BLS, and simulated ACLS. Data are presented as means +- SDs. *P b .05 (control group vs high-dose group), **P b .05 (control group vs low-dose group).

showing maximal dimension of the LV chamber following release of chest compression. The LV chamber area was estimated by electronic integration after manual tracing of endocardial borders. The LV wall thickness was measured in the lateral wall at the midventricular level.

Statistical analysis

Sample size was calculated to detect a difference with respect to end-diastolic volume among the groups based on the data of previous work [6]. We calculated a sample size of 12 animals in each group to reach an ? of .05 and a power of 90%. Normally distributed variables were summarized as mean +- standard deviation (SD) and an indepen- dent t test was performed, whereas nonnormally distributed variables were summarized as medians with interquartile ranges (IQRs) and a Mann-Whitney U test was conducted. Categorical variables were shown as numbers of cases with percentages. Comparisons of categori- cal variables were performed using the ?2 test or Fisher exact test, as indicated. Repeated-measures analysis of variance (RM-ANOVA) was used for comparison of LV wall thickness, LV chamber area, and CPP during the untreated VF and BLS. We could not obtain these variables fully in 2 animals because they achieved ROSC after 4 minutes of ACLS. Mixed-model analysis, which can retain cases with missing data points, was thus used for comparison of these variables during the ACLS. Pairwise comparison with Bonferroni adjustment was performed for post hoc analysis. Data were analyzed by using PASW/SPSS software, version 18 (IBM Inc, Chicago, IL). A P value of b .05 was considered significant.

Results

No differences were observed among the 3 groups at baseline (Table 1). During the untreated VF and BLS, the LV wall thickness pro- gressively increased and the LV chamber area progressively decreased (Fig. 2). The RM-ANOVA of the LV wall thickness and LV chamber area during the untreated VF and BLS revealed no significant group effects or group-time interactions but significant effects for time (P b .001 for both LV wall thickness and LV chamber area). During the ACLS, the LV wall thickness in the control group increased further, from 14.0 mm (11.8-16.3) to 16.0 mm (16.0-18.5), and the LV chamber area in the con- trol group decreased further, from 6.20 cm2 (4.72-8.89) to 5.48 cm2 (3.88-5.85). In contrast, the LV wall thickness of the low-dose group

and the high-dose group decreased from 12.5 mm (10.0-13.3) and

13.5 mm (11.8-14.0) to 10.0 mm (9.0-14.0) and 10.0 mm (8.8-12.3), re-

spectively, and the LV chamber area of the low-dose group and the high-dose group increased from 6.96 cm2 (5.23-8.74) and 7.35 cm2 (6.52-8.63) to 7.67 cm2 (5.03-8.60) and 8.01 cm2 (7.41-13.39), respec-

tively, during the ACLS. Mixed-model analyses of the LV wall thickness and LV chamber area during the ACLS revealed no significant effects of time but significant group effects (P b .001 for both LV wall thickness and LV chamber area) and group-time interactions (P = .002 for LV wall thickness, P = .046 for LV chamber area). In the post hoc analyses, the LV chamber area in the high-dose group increased significantly compared with that in the control group, starting at 28 minutes, but neither the difference between the control and low-dose groups nor that between the low-dose and high-dose groups reached statistical sig- nificance. ScvO2 at 3 minutes after the drug administration was 21.6% (18.4-31.9), 39.2% (28.8-53.7), and 54.0% (47.5-69.4) in the control,

low-dose, and high-dose groups, respectively (P b .001). The ScvO2 in the high-dose BDM group was significantly higher compared with that in the control group (P b .001), whereas neither the difference between control and low-dose groups nor that between the low-dose and high- dose groups reached statistical significance. CPP did not differ across the groups during the BLS but differed significantly during the ACLS (Fig. 3).

Of 36 animals, 33 restored spontaneous heartbeats lasting for more than 1 minute (10 [83.3%] in the control group, 11 [91.7%] in the low- dose group, 12 [100%] in the high-dose group; P = .758). Thereafter, 8 animals (80.0%) in the control group, 3 animals (27.3%) in the low- dose group, and 2 animals (16.7%) in the high-dose group experienced recurrent cardiac arrest (P = .008), which was pulseless electrical activ- ity in 2 animals of the control group and recurrent VF in the remaining 11 animals. They immediately received CPR to which, however, 3 ani- mals in the control group and 1 animal in the low-dose group did not re- spond. As a result, sustained ROSC was attained in 7 animals (58.3%) in the control group, 10 animals (83.3%) in the low-dose group, and 12 an- imals (100%) in the high-dose group (P = .046). Resuscitation variables in the successfully Resuscitated animals are shown in Table 2.

During the 4 hours of intensive care period, 1 pig in the control group died 18 minutes after ROSC because of severe cardiogenic shock refractory to norepinephrine infusion, and 2 pigs in the low-dose group developed VF refractory to CPR and defibrillation at 15 minutes after ROSC. Thus, 6, 8, and 12 animals survived during the intensive

Fig. 3. Coronary perfusion pressure during cardiopulmonary resuscitation. Data are presented as means +- SDs. Repeated-measures ANOVA on CPP during the 8-minute simulated BLS revealed no significant group effect (P = .696) or group-time interaction (P = .758) but significant effect for time (P b .001). Mixed-model analysis on CPP during the simulated ACLS revealed significant group effect (P b .001) and group-time interaction (P = .005). CPP was numerically higher in the low-dose and high-dose groups than in the control group, but neither the difference between the control and low-dose group nor that between the control and high-dose group reached statistical significance in the post hoc analyses.

Table 2

Resuscitation variables in successfully resuscitated animals

Control group (n = 7)

Low-dose group (n = 10)

High-dose group (n = 12)

P

No. of countershocks (n)

4.0 (4.0-5.0)

3.0 (3.0-6.5)

3.0 (3.0-5.8)

.317

No. of epinephrine administrations (n)

1.0 (1.0-3.0)

1.0 (1.0-3.0)

1.0 (1.0-2.0)

.992

Duration of ACLS (min)

6.0 (6.0-12.0)

6.0 (6.0-12.0)

6.0 (6.0-8.0)

.982

Data are presented as medians with IQRs.

care period in the control, low-dose, and high-dose groups, respectively (P = .017). Neither the duration of norepinephrine administration (P =

.709) nor the total dose of norepinephrine administered during the in- tensive care period (P = .717) differed significantly among the 3 groups. The hemodynamic and laboratory variables after ROSC did not differ across the 3 groups (Table 3).

Discussion

In this study, we found that BDM attenuated ischemic contracture and improved the resuscitability in a dose-dependent fashion, resulting in highest resuscitability with a high-dose BDM. These findings confirm our earlier work that BDM can effectively reduce ischemic contracture and can have favorable effects on resuscitability [6].

2,3-Butanedione monoxime is thought to facilitate successful resus- citation primarily by attenuating reductions in LV volume, thus main- taining the hemodynamic effectiveness of CPR. In the present study, LV chamber area was reduced to approximately one-third of the base- line value already at 2 minutes after the start of ACLS. The LV chamber area was preserved in the BDM-treated groups, but in the control group, it was reduced further and was obliterated almost totally at the end of resuscitation attempts. To our knowledge, this is the first study evaluating the dose-response effects of BDM in an in vivo cardiac arrest model. In this study, BDM attenuated ischemic contracture in a dose- dependent fashion, but only high-dose group animals achieved a statistically significant increase in LV chamber area compared with the control group. Consistent with this finding, ScvO2 after the drug admin- istration increased as the dose of BDM increased, resulting in a signifi- cant difference between control and high-dose groups. These findings

Table 3

Hemodynamic and laboratory variables after ROSC

Control group (n = 7)a

Low-dose group (n = 10)b

High-dose group (n = 12)

P

5 min after ROSC

Aortic systolic pressure (mm Hg)

148.0 (95.0-158.0)

134.5 (102.8-162.3)

151.5 (140.8-160.8)

.498

Aortic diastolic pressure (mm Hg)

89.0 (65.0-119.0)

91.0 (67.0-116.5)

106.0 (97.8-110.8)

.788

Mean aortic pressure (mm Hg)

108.0 (78.0-132.0)

105.5 (78.8-134.8)

123.5 (109.5-130.3)

.570

RA systolic pressure (mm Hg)

15.9 +- 4.8

15.1 +- 3.0

13.7 +- 2.9

.388

RA diastolic pressure (mm Hg)

5.9 +- 3.1

7.9 +- 1.9

7.1 +- 1.7

.182

Mean RA pressure (mm Hg)

9.0 (9.0-12.0)

11.0 (10-12.0)

10.5 (8.3-11.0)

.322

Heart rate (/min)

178 +- 37

192 +- 34

195 +- 26

.505

pH

7.033 (6.962-7.093)

7.091 (7.019-7.159)

7.064 (6.987-7.146)

.252

PaCO2 (mm Hg)

53.3 +- 6.2

56.2 +- 7.7

52.5 +- 7.6

.531

PaO2 (mm Hg)

183.2 +- 76.1

212.8 +- 100.1

235.6 +- 72.3

.418

Base excess (mmol/L)

-16.2 +- 3.7

-13.2 +- 2.0

-14.5 +- 3.1

.180

HCO3 (mmol/L)

13.4 +- 2.4

15.4 +- 1.7

14.3 +- 2.3

.213

SaO2 (%)

98.7 (95.3-100.0)

99.5 (94.6-100.0)

99.8 (97.1-100.0)

.879

Lactate (mmol/L)

12.2 +- 2.1

11.1 +- 1.9

12.0 +- 1.9, 10

.507

30 min after ROSC

Aortic systolic pressure (mm Hg)

106.8 +- 15.8

117.0 +- 15.4

118.0 +- 21.0

.462

Aortic diastolic pressure (mm Hg)

63.3 +- 12.6

81.6 +- 12.9

78.9 +- 20.8

.127

Mean aortic pressure (mm Hg)

79.3 +- 14.8

94.3 +- 13.3

94.0 +- 20.3

.207

RA systolic pressure (mm Hg)

10.3 +- 3.4

12.3 +- 2.5

12.3 +- 3.3

.416

RA diastolic pressure (mm Hg)

4.5 +- 1.8

5.9 +- 1.6

5.8 +- 2.7

.384

Mean RA pressure (mm Hg)

6.5 +- 2.2

8.8 +- 1.9

8.8 +- 2.6

.140

Heart rate (/min)

152 +- 24

161 +- 25

159 +- 23

.776

LVEF (%)

48.4 +- 11.5

40.8 +- 12.3

42.4 +- 10.0

.430

4 h after ROSC

Aortic systolic pressure (mm Hg)

105.8 +- 6.5

109.5 +- 17.0

110.0 +- 9.3

.768

Aortic diastolic pressure (mm Hg)

72.0 +- 6.4

72.8 +- 13.8

74.8 +- 11.2

.866

Mean aortic pressure (mm Hg)

85.0 +- 5.5

88.5 +- 15.1

89.2 +- 11.7

.776

RA systolic pressure (mm Hg)

13.5 (9.8-16.0)

14.5 (9.3-16.0)

12.0 (11.0-13.8)

.873

RA diastolic pressure (mm Hg)

6.7 +- 2.0

6.5 +- 2.4

7.8 +- 1.9

.360

Mean RA pressure (mm Hg)

9.8 +- 2.1

9.9 +- 2.5

10.1 +- 1.4

.956

Heart rate (/min)

156 +- 38

142 +- 30

138 +- 38

.528

pH

7.390 +- 0.045

7.417 +- 0.039

7.398 +- 0.042

.458

PaCO2 (mm Hg)

38.9 +- 3.5

38.9 +- 2.9

39.4 +- 3.0

.898

PaO2 (mm Hg)

156.0 +- 32.8

153.2 +- 34.7

151.8 +- 31.1

.967

Base excess (mmol/L)

-1.3 +- 2.7

0.4 +- 2.2

1.2 +- 5.5

.524

HCO3 (mmol/L)

23.1 +- 2.3

24.5 +- 2.0

23.9 +- 2.6

.536

SaO2 (%)

99.9 (99.0-100.0)

98.9 (96.8-99.9)

99.6 (98.4-100.0)

.440

Troponin (ng/mL)

148.5 (92.3-183.5)

75.6 (51.6-95.9)

134.0 (38.5-199.0)

.187

Lactate (mmol/L)

2.4 (2.2-5.4)

2.7 (1.9-5.0)

3.5 (2.6-4.7)

.634

LVEF (%)

28.0 (25.2-28.9)

30.2 (19.3-37.8)

28.9 (17.3-31.3)

.719

Data are presented as mean +- SD or medians with IQRs.

a n = 6 at 30 minutes and 4 hours after ROSC.

b n = 8 at 30 minutes and 4 hours after ROSC.

indicate that BDM improved the hemodynamic effectiveness of CPR in a dose-dependent fashion and explain the favorable effects of BDM on resuscitability shown in this study.

Electrical and mechanical instability persists even after successful resuscitation, and thus, recurrent cardiac arrest commonly occurs early in the postresuscitation period [11,12]. In the present study, 80% of the control group animals that initially restored spontaneous heart- beats experienced recurrent cardiac arrest within 2-3 minutes, and most of the recurrent cardiac arrests (75%) were VF. Meanwhile, recur- rent cardiac arrest occurred only in 27.3% and 16.7% of the animals that initially restored spontaneous heartbeats in the low-dose group and high-dose groups, respectively. This finding suggests that BDM may at- tenuate electrical and mechanical instability occurring early after suc- cessful resuscitation. Consistent with this finding, several studies have reported that BDM has an antiarrhythmic effect, probably associated with BDM-induced inhibition of intracellular calcium overload [4,13-15]. A number of studies have indicated that BDM improves myocardial function after ischemia probably by reducing myocardial injury [8,13,16-18]. In the present study, however, BDM neither reduced the release of troponin I nor improved postresuscitation LV function. We administered BDM only once during CPR as an Intravenous bolus, which may not be enough to achieve beneficial effects on the postischemic myocardial injury. Several studies have suggested that beneficial effects of BDM on postischemic myocardial injury depend on the duration of BDM administration [5,19]. In isolated guinea pig hearts that underwent cardioplegic arrest, initial reperfusion with BDM suppressed creatine kinase and lactate dehydrogenase release markedly, but it was necessary to apply BDM for up to 20 minutes to in- duce lasting suppression of enzyme release [19]. Further study is re- quired to investigate whether BDM infusion after ROSC can reduce myocardial injury and thus improve LV function. Several studies showed significant decreases in lactate level in BDM-treated hearts compared with control hearts [20,21]. However, in our studies [6], BDM administration did not reduce lactate level after ROSC, although we cannot explain the reasons. Further studies are required to deter-

mine the effects of BDM administration on lactate level after ROSC.

The severity of ischemic contracture increases as the duration of un- treated VF increases [1]. We chose a prolonged period of untreated VF to achieve pronounced development of ischemic contracture so as to allow for monitoring of treatment effects. In our previous experiences, control group animals that received standard-dose epinephrine after prolonged untreated VF showed very low rates of ROSC [22,23]. In the present study, we administered vasopressin as a first vasopressor for all animals to evaluate the effects of BDM on post-ROSC variables by enhancing the rate of ROSC. Previous studies on pigs suggest that, compared with epi- nephrine, vasopressin increases vital organ blood flow and the rate of ROSC after prolonged cardiac arrest [24]. We think that intervention re- versing ischemic contracture, such as BDM administration, has its clini- cal value, particularly in prolonged untreated VF, where standard ACLS fails. However, many questions must be answered before BDM can be applied clinically. First, in brief VF, where ischemic contracture does not develop usually, BDM administration might not facilitate successful resuscitation but rather impede ROSC because of its negative inotropic effect. Thus, BDM administration should be tested in a model with a shorter No-flow time. Second, BDM appeared to have a Short duration of action in our previous work [6]. Repeated administration might be required in the refractory case to optimize the chances for ROSC. Thus, further studies are required to determine the optimal dosing strategy.

The present study has several limitations. First, we neither per- formed a histological assessment of myocardial injury nor examined long-term survival. Both assessments require observation of the animals for a prolonged period of time. However, we were unable to keep the animals for more than 4 hours because of our limited resources, including a shortage of personnel. Second, the animals were young, healthy, and free of atherosclerotic disease. Thus, caution is warranted with regard to direct extrapolation to humans.

Conclusions

We showed that BDM administered during CPR reduced ischemic contracture and improved resuscitability and short-term survival in a dose-dependent fashion. In view of these results, we believe that administration of BDM during CPR deserves further evaluation.

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