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Letters to the Editor |
Division of Thoracic Radiology, Department of Radiology, Massachusetts General Hospital, 55 Fruit Street, Founders Building, Room 202, Boston, MA 02215
Editor:
I read with interest the article by Dr Henk and colleagues (1) in the March 2003 issue of Radiology. The authors discuss a contrast material enhancement pattern seen at computed tomographic (CT) pulmonary angiography. In 45 of 289 patients in the study population, they found that the attenuation values in the main pulmonary artery were less than or equal to those in the aorta, despite the presence of contrast material in the superior vena cava. Their subsequent follow-up study with use of contrast-enhanced echocardiography and a Valsalva maneuver demonstrated intracardiac right-to-left shunts in 39 of the 45 cases with these contrast material dynamics at CT. Their explanation of this attenuation pattern was a shunt between the right and left atria due to a patent foramen ovale or atrial septal defect.
A patent foramen ovale is a persistent opening between the atrial septal primum and secundum at the location of the fossa ovalis. A probe-patent foramen ovale has been shown in 27% of adults in an autopsy series (2). The average diameter of a patent foramen ovale is 3 mm in the 1st decade of life and 6 mm in the 10th decade (2). Contrast-enhanced echocardiography can demonstrate a patent foramen ovale by means of performing and then releasing a prolonged Valsalva maneuver. If a patent foramen ovale is present, contrast-enhanced echocardiography will reveal a right-to-left shunt mediated by a transient reversal of the interatrial pressure gradient (3).
There are two major problems with his article: First, the Valsalva maneuver is an expiratory effort against a closed glottis. Patients are not requested to perform this maneuver during CT pulmonary angiography. Therefore, under normal circumstances, how can blood be shunted from the right to left atria in these patients?
Second, to dismiss poor bolus timing, the authors mention in the discussion section that the 45 patients had streak artifact that was produced by contrast material within the superior vena cava and right atrium. Therefore, the concentration of contrast material within the right atrium would be high, and shunting of a small quantity of well-opacified blood across a patent foramen ovale 3–6 mm in diameter is unlikely to cause the artifact that these authors are describing. It is interesting that within their study, region of interest measurements were performed only within the aorta and main pulmonary arteries. Evaluations of contrast enhancement quality were conducted in the pulmonary artery, right atrium, pulmonary vein, and left atrium; no quality evaluations were conducted in the right or left ventricle. The explanation of this artifact is that a large volume of unopacified blood enters the right atrium from the inferior vena cava; this will be accentuated by inspiration. This will cause poorly opacified blood in the right atrium, right ventricle, and pulmonary arteries. I am interested to know how many of the 45 patients had poor contrast material opacification of the right ventricle. This theory would also explain the intermediate and good contrast enhancement ratings assigned to the pulmonary veins and left atrium, respectively, which were noted by the authors in most of the 45 patients.
I believe that a patent foramen ovale or atrial septal defect is not the cause of the described artifact: It was caused by unopacified blood entering the right atrium from the inferior vena cava and was accentuated by inspiration at the time of or just prior to CT data acquisition.
REFERENCES
Department of Radiology, University of California at San Francisco, 505 Parnassus Avenue, L308, San Francisco, CA 94143. e-mail: christine.henk@radiology.ucsf.edu
The author of the letter has two major problems with our article. The first problem is related to the Valsalva maneuver, the second is in reference to the explanation of the contrast material behavior described (which he refers to as "artifact").
To address the first problem, we have to state that the author is correct in that the patients were not directly asked to perform a Valsalva maneuver. When one watches patients during pulmonary spiral CT angiography, however, the maneuver they perform during the breathing command to maintain breath hold comes very close to a Valsalva maneuver. Besides, mere deep inspiration alone also causes a pressure gradient between the right and left atria (1).
To evaluate the most effective maneuver for detection of a patent foramen ovale at transesophageal echocardiography, Pfleger et al (1) found the mean pressure gradient with the Valsalva maneuver to be only 3 mm Hg higher than that at deep inspiration. Substantial measurable shunt flow occurred in 78% of patients with inspiration, compared with 84% with the Valsalva maneuver. Conclusively, a Valsalva maneuver is not strictly required to provoke contrast material loss into the systemic circulation via a patent foramen ovale, as observed in our study.
For the second problem, a simple physiologic explanation exists, which disproves his hypothesis that mixing of unopacified blood in the inferior vena cava is responsible for loss of contrast enhancement. When one breathes in deeply and holds the breath after inspiration in an enforced manner (as in the Valsava Maneuver), there is an increase in right atrial and central venous pressure, causing a stop or at least a substantial decrease of venous backflow to the heart that occurred during the course of inspiration. This phenomenon is very well known from duplex Doppler sonography of abdominal and lower-extremity veins (2,3), where one can very well appreciate respiratory modulation of flow. During pulmonary spiral CT angiography, in deep inspiration as described, no or very little inflow from the inferior vena cava to the right atrium is present during the vast majority of scan time, rendering the possibility of blood mixing as a cause of poor opacification rather unplausible.
REFERENCES
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