The experiments were performed in an established swine model of cardiac arrest. All animals (n=32) received humane care in compliance with the "Principles of Laboratory Animal Care" formulated by the National Society for Medical Research and the Guide for the Care and Use of Laboratory Animals prepared by the Institute of Laboratory Animal Resources and published by the National Institutes of Health (NIH publication 86-32, revised 1985). This experiment was carried out when I visited Weil Institute of Critical Care Medicine and the Institutional Animal Care and Use Committee of the Weil Institute of Critical Care Medicine approved the protocol. The animal laboratories of the Weil Institute are fully accredited by American Association for Accreditation of Laboratory Animal Care (AAALAC) International.

Thirty-two male Yorkshire-X domestic swine (Sus scrofa) weighing 40±3 kg were used in this study. The animals were fasted overnight except for free access to water. Anesthesia was initiated by intramuscular injection of ketamine (20 mg/kg) and completed by ear vein injection of sodium pentobarbital (30 mg/kg). A cuffed endotracheal tube was advanced into the trachea. The animals were mechanically ventilated with a volume-controlled ventilator (Model MA-1, Puritan-Bennett, Carlsbad, CA, USA) with a tidal volume of 15 mL/kg, and FiO₂ of 0.21. End-tidal CO₂ was monitored with an infrared capnometer (Model 01R-7101A, Nihon Kohden Corp, Tokyo, Japan). Respiratory frequency was adjusted to maintain PetCO₂ between 35 and 40 mmHg. For the measurement of aortic pressure (AOP), a fluid filled catheter was advanced from the right femoral artery into the thoracic aorta. For the measurements of right atrial pressure (RAP) and pulmonary arterial pressure (PAP), a 7-French, thermodilution-tipped catheter was advanced from the right femoral vein and flow-directed into the pulmonary artery. For inducing VF, a 5-French pacing catheter (EP Technologies, Inc., Mountain View, CA, USA) was advanced from the right jugular vein into the right ventricle. The position of catheters was confirmed by characteristic pressure morphology and X-ray. The pacing catheter was removed after onset of VF.

Fifteen minutes before inducing cardiac arrest, baseline measurements were obtained and the animals were then randomized to initial defibrillation or chest compression. VF was induced by 1 mA alternating current delivered to the right ventricular endocardium through a pacing catheter. Mechanical ventilation was discontinued after onset of VF. Prior to starting the resuscitation procedure, the pacing catheter was withdrawn to avoid injury during chest compression.

In the chest compression group (16 animals, delayed defibrillation), fifteen minutes after onset of VF, chest compression was started with a pneumatic piston-driven chest compressor (Thumper®, Model 1000, Michigan Instruments, Grand Rapids, MI, USA). The chest compression was programmed to provide 100 compressions/minute and synchronized to provide a compression/ventilation ratio of 30:2 with equal compression-relaxation intervals, i.e. a 50% duty cycle. The compression depth was adjusted to decrease the anterior-posterior diameter by 25%. Coincident with starting precordial compression, the animals were mechanically ventilated with tidal volume of 15 mL/kg and FiO₂ of 1.0. After 2 minutes of chest compression, one dose of epinephrine (30 µg/kg) was injected into the right atrium. After another 3 minutes of chest compression, one 120 J biphasic electrical defibrillation was delivered between the right infra-clavicular electrode and the apical electrode with a Heartstart® XL defibrillator (Philips Medical Systems, Andover, MA, USA). If an organized cardiac rhythm with mean aortic pressure (MAP) of more than 60 mmHg persisted for an interval of 5 minutes or more, the animal was regarded as successfully resuscitated. If the ROSC was not achieved, CPR was resumed for 1 minute prior to the next defibrillation attempt. This sequence was repeated until the animal was either successfully resuscitated or pronounced dead after a total of 15 minutes of CPR. Other doses of epinephrine were injected separately at the 7th and 12th minute following the initiation of CPR.

In the initial defibrillation group (16 animals, immediate defibrillation), fifteen minutes after onset of VF, immediate defibrillation was delivered. The chest compression was started after defibrillation. The defibrillation was done with biphasic 120 J. CPR was conducted with the same method as the chest compression group (Figure 1).

Echocardiograms were obtained immediately before starting chest compression in both groups. Dimensions of LV were assessed by echocardiographic methods in parasternal short axis view. The degree of LV contracture was calculated by comparison with volume of the LV before and after defibrillation (Figure 2).

Continuous variables were presented as mean±SD. Student's t-test was used to compare continuous variables between two groups. Categorical variables are reported as counts and percentages. Primary and secondary outcomes were binary, and the chi-square test or Fisher's exact test, as appropriate, was used to compare outcomes in chest compression first and initial defibrillation groups. A value of P<0.05 was considered significant. Analyses were carried out using SPSS V.11.0 software (IBM Corp., Chicago, IL, USA).