The air gap sign.

An elderly female aspirated a donut and subsequently had loss of consciousness and cardiopulmonary arrest. Family members performed chest compressions until emergency medical services arrived and resuscitated the patient with an adequate heart rate and blood pressure. On hospital admission, the patient was nonresponsive on a mechanical ventilator. Cardiac auscultation revealed distant heart tones, but no rub or mediastinal crunch was heard. The admission chest x-ray revealed a normal cardiac silhouette, but the presence of a pneumopericardium (Fig. 1). Two-dimensional echocardiography was best imaged from the subcostal window. Loss of ultrasound signal was