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Ann Thorac Surg 2000;69:1645-1646
© 2000 The Society of Thoracic Surgeons
a J. Gerard Mudd Cardiac Catheterization Laboratory, St. Louis University Health Sciences Center, St. Louis, MO 63110 USA,
e-mail: eisenhmd{at}slu.edu
To the Editor
It was a pleasure to read the article by Dr Peterson and colleagues [1] describing the results of their prospective study that confirms our earlier experience regarding the beneficial impact of graft markers on subsequent angiography [2]. The investigator’s ability to accurately measure and define the elapsed time required to intubate and image each individual vein graft ostium is noteworthy, and confirms subjective observations seen in nearly every catheterization laboratory on a daily basis [3].
The reported 23% reduction in fluoroscopic exposure time for complete graft angiography is not inconsequential. The calculated number of chest x-rays (CXRs) required for an equivalent radiation exposure cited in the text, however, appears to be grossly underestimated. Unlike single CXRs in a radiology suite where the patient stands directly opposed to the film plate some distance from the radiation source (thereby minimizing beam scatter) and the radiology technician stands in an adjacent control booth, the cardiologists stands literally 2 feet from the radiation source as pulses are projected at rates up to 60 pulses per second [4]. Catheterization laboratories and fluoroscopy suites require a mobile X-ray tube commonly located beneath the patient table that projects the beam upwards through the table and through the patient, and is ultimately received by the image intensifier located above the patient. Each surface penetrated will scatter the X-ray beam, thereby increasing the exposure of the cardiologist and technicians who are within a short distance of the beam.
Modern cardiac catheterization laboratories with pulsed and non-pulsed fluoroscopy systems expose the patient to between 2,000 and 5,500 mRad per minute, increasing roughly tenfold with cineangiography [4, 5, 6]. Further, the exposure rates vary with the projection angle used and the dependent oblique "thickness" of the body as opposed to the relatively "thin" antero-posterior projection, with some views requiring up to 8,000–9,000+ mRad per minute [6]. At our institution, an annual safety inspection by an outside and unbiased professional radiation physicist is performed. The Entrance Exposure Rates (EER; the radiation load directly measured at the patient surface using a Lucite phantom) for the most commonly used level of magnification (Magl or 7 inch field), varied from 3,480–5,330 mRad/min for fluoroscopy. The EER for cineangiography at 30 frames per second was measured to be 35,710 mRad per minute.
In comparison, upright postero-anterior (PA) chest x-rays expose the patient to 15–40 mRad per image, with the National Council on Radiation Protection suggesting (and some states requiring) a maximum exposure of 30–35 mRad per image [7].
Using a conservative exposure rate of 30 mRad/PA CXR, the reported 3.3-minute average reduction in fluoroscopy time for angiography of "marked" grafts corresponds to: (3.3 min)(2000 to 5500 mRad/min)(1 PA CXR/30 mRad) = 220 to 605 PA CXR.
If the minimal exposure rate of 15 mRad/PA CXR is used, the corresponding number of equivalent PA CXRs is increased to 440–1,210 per patient, per study. The longest and most complicated "unmarked" case presented may then have required an exposure equivalent of up to 7,333 additional PA CXRs when compared to the longest "marked" case. I would strongly argue that these numbers are not insignificant in terms of additional radiation exposure to the patient, let alone the cardiologist and the catheterization laboratory team who may perform multiple procedures each day.
It seems intuitive that if successful intubation of the graft is faster and easier with graft markers, and requires fewer diagnostic catheter changes [2], the risk of cerebrovascular accident resulting from probing a diseased aorta may be reduced. Similarly, the reported 10.4% reduction in contrast volume required for complete graft angiography also may be significant towards the goal of reducing glomerulopathic effects of contrast media.
Strikingly, when surveyed the most common response by cardiothoracic surgeons to not use graft markers was because there was no perceived benefit to the patient [8]. Further, there is retrospective evidence that circumferential aortocoronary graft markers do not negatively impact graft patency [9, 10].
To quote Dr. Feldman’s editorial in the Journal of Cardiac Surgery: "Consider the reaction of surgeons to an angiographic practice that needlessly added pump time and technical difficulty to surgical procedures. The failure to use graft markers is analogous. The increasing numbers of patients who survive long enough to require repeat angiography highlight the benefits that will accrue if graft markers were used more routinely [11]." The time has come for renewed discussions between cardiac surgeons and their cardiology colleagues regarding routine aortocoronary graft marker insertion at the time of bypass surgery.
Footnotes
The opinions or assertions contained herein reflect the views of the author, and are not to be construed as official, nor as representing the position of the United States Army Medical Department, the Department of Defense, or the United States Federal Government.
References
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