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1
Department of Radiology, Winthrop University Hospital, 259 First St., Mineola,
NY 11501.
2
Brooke Army Medical Center, 3851 Roger Brooke Dr., Fort Sam Houston, San
Antonio, TX 78234.
Received June 14, 2001;
accepted after revision August 2, 2001.
The opinions and assertions contained herein are the private views of the
authors and are not to be construed as official or as representing the views
of the Department of the Army.
Abstract
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MATERIALS AND METHODS. CT scans of the abdomen of 500 inpatients imaged for a wide variety of nontraumatic indications were retrospectively reviewed for a flat cava sign. Two radiologists measured the maximal anteroposterior and transverse diameters of the inferior vena cava at four predetermined levels. The medical records of the subset of patients with a flat cava sign—defined as a maximal transverse-to-anteroposterior ratio of 3:1 or greater at one or more of the four levels—were reviewed for evidence of hypovolemia or hypotension.
RESULTS. Seventy patients (14%; 48 women, 22 men) had a flat inferior vena cava present on at least one of the four levels. Of these 70 patients, 21 had definite and three had possible clinical evidence of hypotension or hypovolemia. A flat cava sign isolated to only one level was seen in 22 of the 70 patients, most commonly at the level just below the renal veins, and only four of these 22 patients had evidence of hypotension or hypovolemia.
CONCLUSION. Of the 500 inpatients, 14% had a flat cava sign on at least one of the four levels examined on abdominal CT scans. The majority of these patients with a flat cava sign did not have hypotension or evidence of hypovolemia, but a minority (30%) did.
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We anecdotally noted the presence of the flat cava sign in occasional patients without trauma who underwent CT of the abdomen, especially elderly women, and wondered if the sign had any relationship to low blood pressure or hypovolemia or if it was purely a variation of normal. To our knowledge, the flat cava sign has not been systematically studied on abdominal CT scans in nontrauma patients. Therefore, we conducted a retrospective study to evaluate the significance of the flat cava sign on abdominal CT scans in patients without trauma.
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After review of the 500 CT cases, the medical records of those patients with a flattened inferior vena cava on at least one of the four levels were reviewed in detail. The medical records were evaluated for evidence of hypotension or hypovolemia within a 12-hr time window spanning 6 hr before and 6 hr after CT, but particular attention was paid to the medical record relating to the hour around the exact time of the CT. Specific criteria used for determining that hypotension was present included a systolic blood pressure less than 100 mm Hg, especially if associated with a heart rate of greater than 140 beats per minute. Data on vital signs during this 12-hr window were available for all patients. Specific criteria used for determining that hypovolemia was present included evidence of fluid depletion at physical examination, such as dry mucous membranes and poor skin tone, and an analysis of fluid inputs and outputs including requirements for fluids and blood. The medical records were also carefully searched for improvement in blood pressure and volume status after treatment.
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The three patients with equivocal findings of hypotension or hypovolemia on chart review had histories as follows: the first, a 78-year-old woman with sepsis and a normal blood pressure had tachycardia, which might have been purely a result of sepsis, although hypovolemia could not be excluded. The second, a 77-year-old woman, had diarrhea and anemia and was normotensive (blood pressure 133/70 mm Hg at the time of CT). However, she was receiving IV fluids, and her inputs were significantly ahead of her outputs for a 48-hr period. The third patient, a 90-year-old woman with anasarca, anemia, and equivocal findings for urosepsis, initially had a blood pressure of 148/50 mm Hg at the time of CT, but this decreased to 108/40 mm Hg within 6 hr after CT.
The total number of males in this study was 235, and the total number of females was 265. The age range of the 500 patients was 2-98 years. Of the 21 children 18 years or younger in the study (4% of the total), none had a flattened inferior vena cava at any level. Of the 70 patients with a flat cava sign on CT scans, there were 48 women and 22 men; this difference was found to be statistically significant (p < 0.01, Fischer's exact test, two-tailed). There were seven men and 14 women composing the group of 21 patients with definite hypotension or hypovolemia around the time of CT, and 15 men and 31 women composing the group of 46 patients who did not have hypotension or hypovolemia. The mean age of the 21 patients with hypotension or hypovolemia was 65 years, and the mean age of the 46 patients without hypotension or hypovolemia was 69 years. The mean age of all 70 patients with a flattened inferior vena cava on at least one level at CT was 68 years, and the mean age of the 430 patients without a flattened inferior vena cava at any level was 57 years; this age difference of 11 years was found to be statistically significant (p < 0.0001, Wilcoxon's rank sum test).
Of the 21 patients with hypotension or hypovolemia, a flattened inferior vena cava was present at two or more of the predetermined levels in 17. There was flattening at two levels in 10 patients, at three levels in five patients, and at four levels in two patients. Of the 46 patients who were not hypotensive or hypovolemic, a flattened inferior vena cava was present at two or more levels in 28. There was flattening at two levels in 25 patients (including three patients with discontinuous flattening at the first and third levels), at three levels in two patients, and at four levels in one patient. Of the three patients with equivocal findings of hypovolemia or hypotension, there was flattening of the inferior vena cava at two levels in two patients and at three levels in the other patient.
A flattened inferior vena cava was seen at only one of the four levels in 22 of the 70 patients. In nine of these 22 patients, the flattened inferior vena cava was isolated to the level just below the renal veins; eight of these nine patients were women. None of these nine individuals were hypotensive or hypovolemic. Of the other 13 patients, there were seven with isolated flattening of the inferior vena cava at the level just below the liver, five with flattening only at the level halfway between the renal veins and the caval bifurcation, and one with flattening only just above the caval bifurcation. Of these 13 patients, only four had evidence of hypotension or hypovolemia.
Mean measurement of the total craniocaudal extent of the flattened cava in the 21 patients with definite hypotension or hypovolemia, estimated to the nearest centimeter, was 7 cm, whereas this mean distance in the 46 patients without hypotension or hypovolemia was 5 cm.
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The flattened inferior vena cava has also been identified on abdominal CT scans in pediatric trauma patients and has been closely associated with hypotension and hypovolemia. Taylor et al. [2] reported this CT finding in three patients less than 2 years old, after severe abdominal trauma. They described a "hypoperfusion complex" in these patients that, in addition to a flattened inferior vena cava, included decreased caliber of the aorta, marked diffuse bowel distention with fluid, moderate to large peritoneal fluid collections, and abnormally intense enhancement of the bowel wall, kidneys, and pancreas. The three children in this report required initial aggressive fluid resuscitation but appeared to be stable enough at the time that the decision was made for an abdominal CT scan to be obtained. However, all three children died of their injuries [2]. Similarly, Sivit et al. [3] reported the hypoperfusion complex in 27 of 1,018 children who underwent contrast-enhanced CT of the abdomen after blunt trauma. Flattening of the inferior vena cava, seen over at least 3 cm in the craniocaudad dimension, was present in all of these 27 children [3]. Most recently, O'Hara and Donnelly [7] described the CT findings of the hypoperfusion complex in six pediatric trauma victims and speculated that inferior vena caval flattening is due to a combination of hypovolemia and vasospasm from increased sympathetic activity related to partially compensated shock.
To our knowledge, the flat cava sign has not been previously studied on abdominal CT scans in patients without blunt abdominal trauma to determine if it has the same significance as it does in trauma patients. On the basis of anecdotal cases observed before this study, we initially believed that a flat cava sign did not correlate with either hypotension or hypovolemia in any patient. Similarly, Hopper [8] anecdotally noted occasional multilevel collapse of the inferior vena cava on abdominal CT scans in nontrauma patients and speculated that this finding was most commonly a normal variant that depended on the degree of respiration, intraabdominal pressure, and hydration status. In our retrospective study of 500 inpatients who did not have blunt abdominal trauma but who did have a wide variety of other emergent and semiurgent indications for abdominal CT, the majority who had a flat cava sign on at least one of the four levels that we examined (46 patients) in fact did not have evidence of hypovolemia or hypotension around the time of CT. However, a minority of patients (21/70 with a flat cava sign) did have a definite correlation with hypotension or hypovolemia, and three had a possible correlation. Therefore, in 24 patients (34%) with a flat cava sign, this finding was significant; it, therefore, cannot be dismissed solely as a normal variant of older nontrauma patients. Although the mean craniocaudal extent of the flattened cava was slightly longer (7 cm) in the patients with hypotension or hypovolemia compared with the patients without either of these findings (5 cm), the presence of this finding at only one of the four levels that were specifically examined in this study was much more likely to be identified in normotensive and normovolemic patients; only four of the 22 patients with a flat cava sign isolated to one level in this study had hypotension or hypovolemia.
There is a variety of potential explanations for the flattened inferior vena cava that we identified in our normovolemic and normotensive adult patients. These include a normal variation, especially in elderly women; a change in the vessel tone or connective tissues within the caval wall that occurs with aging; a redistribution of blood volume that does not manifest as clinical hypotension or hypovolemia; and variations in caval shape and volume with ventilation, intraabdominal pressure, and position of the patient. To our knowledge, none of these potential explanations have been investigated with CT, but there have been two sonographic studies that addressed some of the potential explanations. Rak et al. [5] prospectively evaluated the infrahepatic inferior vena cava of 26 normal volunteers on sonography and showed significant variation in caval caliber related to ventilation and intraabdominal pressure. Similarly, Grant et al. [6] studied the inferior vena cava in 25 normal volunteers at sonography. During inspiration and Valsalva's maneuvers, the inferior vena cava significantly decreased, whereas it significantly increased during expiration and breath-holding [6].
There are some potential limitations of our study. This study was retrospective, and although we were initially unaware of the clinical history when we performed the image review, we were aware that all patients whose medical records we examined did have a flattened inferior vena cava. We did not examine the medical records of the 430 patients without a flattened inferior vena cava to determine the incidence of hypotension or hypovolemia, but investigating this group was not the purpose of our study. Also, we chose a maximal transverse-to-maximal anteroposterior caval ratio of three or greater as the criteria for a flattened cava; although this may have been somewhat arbitrary and without specific precedent in the relatively limited radiology literature on this topic, we believed that an objective cutoff was necessary, rather than merely declaring that a cava was "flat" or "not flat." There was little guidance in the previous literature, other than a single paper which defined flattening of the inferior vena cava as "9 mm or less (in the anteroposterior diameter), measured at the level of the renal veins" [4]; all other studies simply stated that the inferior vena cava was considered diminished if it appeared "flattened," typically on "multiple" or "at least three contiguous images" [1,2,3, 7]. A ratio may be more reproducible than a single dimensional measurement and may account for differences in body habitus.
It is unclear, however, how many more patients with hypotension or hypovolemia would have been identified if a higher or lower ratio had been used as a threshold; presumably, the specificity of a flattened cava sign for hypotension or hypovolemia would have increased or decreased, respectively, while its sensitivity would have decreased, or increased, respectively.
Additionally, a minority of the patients in this study did not received IV contrast for CT, but the relatively small volume of fluid that was administered during most of the CT examinations probably would not have made a significant difference in caval status. More vigorous fluid resuscitation as opposed to a small volume of fluid would likely be needed to have a significant effect on central venous pressure and volume [9]. Furthermore, some elderly patients may have a relatively low blood pressure as a baseline, so the clinical significance of hypotension is not always clear. Finally, although we attempted to determine the volume status of patients with a flat cava sign through a detailed chart review, a prospective study would have allowed more systematic evaluation of each patient's volume status; ideally such an evaluation would also include central venous pressure monitoring, although this obviously is not possible in most patients.
In summary, of the 500 nontrauma inpatients in our study who underwent abdominal CT, 14% had a flat cava sign. The flattened inferior vena cava was more commonly seen in women and in older patients. When isolated to one of the four levels examined in this study, particularly at the level just below the renal veins, the flat cava sign most often occurred in elderly women who did not have evidence of hypotension or hypovolemia. In contrast to patients with blunt trauma, the presence of a flattened inferior vena cava was more common in patients without hypotension or hypovolemia, but 30% of patients did have definite evidence of hypovolemia or hypotension. The findings of a flat cava sign should, therefore, not be ignored but closely correlated with the clinical findings. Further prospective studies in both trauma and nontrauma patients would help clarify the exact significance of the flat cava sign on abdominal CT scans.
Acknowledgments
We thank the biostatistical department and CT technologists at our
institution for their assistance with this project.
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