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174 AGE RELATED CHANGES IN CHEST GEOMETRY DURING CONVENTIONAL, CARDIOPUIMONARY RESUSCITATION IN PIGLETS

Conventional cardiopulmonary resuscitation (CON-CPR) generates little cardiac output in adult animals, but changes of chest geometry during CPR have not been studied. In infants and children, chest distortion occurs during CPR. We studied changes in chest geometry during CON-CPR in infant, 1-month, and 3-month old piglets. 17 piglets (7 infant, 6 one month, 4 three month old) were anesthetized and catheters were placed in the thoracic aorta, left ventricle, and right atrium. Anteroposterior (AP) and lateral (IAT) chest diameters were recorded, and ventricular fibrillation was induced. CPR was performed with a Michigan Instruments thumper, with 60% duty cycle, 1:5 compression to ventilation ratio, low airway pressure (20–30 mmHg), at rates of 100 (infants) or 60 (others) per minute. Compression force was varied to increment thumper piston displacement by 0.5 cm steps, and measurements recorded. In all ages, the AP/IAT ratio was greater than unity. 3-month piglets had less compliant chests, and despite thumper forces in excess of 700 newtons (Nt), only 20% total chest displacement could be generated. This was insufficient to produce adequate vascular pressures, and thoracic ejection fraction was negligible. Permanent chest deformation occurred in infants and 1-month old animals, which permitted adequate ejection from the thoracic cavity to generate high vascular pressure. The crossection of the chest is an ellipse, and no ejection occurs until the AP/IAT ratio is less than 1. In 3-month animals, this threshold was barely reached, and negligible vascular pressures were generated. In infants, the AP/IAT ratio is less than 1, and no threshold would be expected. This may explain why CON-CPR is successful in clinical experience and suggests that CON-CPR may be the appropriate mode of CPR in infants.