Aspiration in chest compression alone without mechanical ventilation in the head down position in dogs

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Background: Previous work by the authors has shown that chest compressions alone without mechanical ventilation during cardiopulmonary resuscitation in the natural supine position was associated with pulmonary aspiration in dogs. The purpose of this investigation was to test the hypothesis that a head down position may prevent aspiration during chest compressions alone and whether oxygenation can be improved by simply insufflation of oral oxygen 10 min after cardiac arrest. Methods: Cardiac arrest was induced in ten mongrel dogs which were anesthetized and paralysed. Eight underwent chest compressions alone in different head down positions using an automatic compressor at 9 kg compression force and 3 cm compression depth. The study was composed of two parts. Part 1 evaluated the effect of insufflation of 10 l/min O 2, into the mouth of the dogs, 10 min after initiation of resuscitation, using chest compressions alone. Part 2 was designed to test our hypothesis that the head down position may protect the lungs from aspiration during chest compression alone. The mouths of the dogs were filled with mixed barium and the dogs underwent serial episodes of chest compressions, for 10 min each, in the 20°head down, 10° head down and the natural supine positions. Chest X-rays with antero-posterior and lateral views were taken to evaluate pulmonary aspiration. Two additional dogs underwent direct chest compression alone in the natural supine position and the time of chest compression was shortened to 5 min. Results: All dogs in the natural position showed evidence of pulmonary aspiration of barium, five or six of the dogs showed tracheal aspiration in the 10° head down position, while no any barium was visualized in the tracheo-broncheal trees of the dogs in the 20° head down position. Supplemental oxygen in the mouth improved the mean Pa O 2 from 67±26 to 160±97 mmHg during chest compressions alone. Conclusion: Chest compression alone without mechanical ventilation in the supine position caused pulmonary aspiration in the unprotected airway in dogs. This complication could be prevented by adopting a 20° head down position. The10° head down position seemed to reduce the severity of the pulmonary aspiration, but not enough to eliminate the danger altogether. Supplemental oxygen in the mouth can improve oxygenation in chest compressions alone.