Animal Preparation
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This study was conducted with the approval of the University of Arizona Institutional Animal Care and Use Committee. Ten domestic piglets (six female, four male), weighing 10.7 ± 1.2 kg, were anesthetized with 5% isoflurane inhalation anesthetic in oxygen administered by face mask. A cuffed endotracheal tube was placed and a surgical plane of anesthesia was maintained with 1% to 2.5% isoflurane in room air. Mechanical ventilation was delivered at a rate of 10-12/min and an initial tidal volume of 15 mL/kg (Narkomed 2A; North American Drager, Draäger Medical, Telford, PA) and subsequently adjusted to maintain end-tidal carbon dioxide at 40 ± 4 mm Hg measured by an infrared capnometer (No. 47210A; Hewlett-Packard, Palo Alto, CA). The positive end-expiratory pressure was set at 5 cm H2O.
Animals were placed in dorsal recumbency on the surgical table. Vascular introducer sheaths (Cordis, Miami, FL) were placed in the external jugular vein, a common carotid artery, and both femoral arteries. Solid-state, micromanometer-tipped catheters (MCP-500; Millar Instruments, Houston, TX) were placed through the external jugular vein sheath into the right atrium and into the descending thoracic aorta through a common carotid artery sheath. Fluid-filled pigtail catheters (5F; Cordis) were advanced through the femoral vascular sheaths into the left ventricle and ascending aorta. Correct placement of all the catheters was verified by fluoroscopy. Adhesive multifunction defibrillation electrode pads (DP2/DP6; Philips Medical Systems, Seattle, WA) were placed in anteroposterior positions. A puck, containing a force transducer and an accelerometer, was adhered to the sternum with an integral adhesive pad (HeartStart4000SP, Version 0.50; Laerdal, Stavanger, Norway). Electrocardiographic leads were attached to three limbs to monitor heart rate and rhythm. A rack (Fig. 1) was placed over the puck to hold it in position and to allow administration of compressions with no residual lean or 1.8 kg (10%) or 3.6 kg (20%) residual lean during the relaxation phase of chest compressions. The lean weight was calculated from pilot studies in similar piglets, which determined that the mean compression force required for maintaining 80-90 mm Hg peak aortic systolic pressure during CPR was 180 newtons. This force corresponds to the force of earth’s gravity on an object with a mass of 18 kg. Therefore, 1.8-kg and 3.6-kg weights on the chest wall were used to represent 10% and 20% residual lean, respectively.
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