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We appreciate the comments from Dr. Wallis.[1] The space constraints of the case report[2] did not permit a complete description of the radiologic techniques that created such an unusual chest radiograph. The techniques involved certainly may influence the appearance of both normal and aberrant structures on a common radiograph, as may the use of analog or digital technology. For analog portable chest radiographs, the typical radiographic technique involves an approximate, 40-in focal film distance, 75-85 kilovoltage peak (kV[p]), and a typical 1 mA-second exposure adjusted to patient body habitus. For portable digital chest radiographs, as in this case, the exposure is typically made on a standard 14 x 17 inch computed radiography cassette at 75 kV(p) and variable milliampere-second based on body habitus. Digital processing is then performed in a laser scanning reader and sent to a picture archival and communication system for interpretation.
The difficulty in the reported case is reconciling the appearance of a two-headed pulmonary artery catheter (ie. "Swan") in a critically ill patient with supraventricular tachyarrhythmias against the fleeting exposure time used in obtaining a portable chest radiograph. Conventional wisdom suggests that a short radiographic exposure duration would preclude the appearance of dual images, yet the diverging ends of the catheter are clearly apparent. As Dr. Wallis states, the remainder of the film is without motion artifact and appropriately exposed, thus removing those possibilities to account for the two-headed swan. There was no evidence of a mechanical failure of the radiology equipment, leaving our explanation as the most plausible reason.
Although we cannot be completely certain that the two-headed swan in this report resulted from catheter motion, we believe this to be the case "beyond reasonable doubt." Furthermore, the appearance of this radiographic abnormality was used to illustrate the frequency and variety of problems encountered with pulmonary artery catheter use and the potential methods involved in their investigation.
