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Original Research |
1 Department of Radiology, Hadassah–Hebrew University Medical Center, Ein
Kerem POB 12000, Jerusalem, Israel 91120.
2 Department of Radiology, Beth Israel Deaconess Medical Center, Boston,
MA.
3 Department of Radiology, Tel Aviv Medical Center, Tel Aviv, Israel.
Received October 14, 2007;
accepted after revision May 7, 2008.
Address correspondence to J. Sosna
(jacobs{at}hadassah.org.il).
Abstract
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SUBJECTS AND METHODS. Rectal pressure was measured in the supine and prone positions for CT colonography of 379 patients in two groups. One hundred seventy-seven patients underwent imaging supine and then prone, and 202 patients were prone and then supine. Insufflation and patient pain parameters were based on patient self-report and investigator visualization of observable indicators. Colonic distention, residual feces or fluid, and diverticulosis were evaluated with a semiquantitative scoring system. Fisher's exact, Student's t, and chi-square tests as well as multivariate logistic regression analysis were performed.
RESULTS. Pressure was higher in the prone than in the supine position in both groups (p < 0.001). The measured pressure in the initial position did not differ between groups (p = 0.88). Pressure increased from 38.3 to 40.07 mm Hg in the second position in the supine-first group and decreased from 38.3 to 32.25 mm Hg in the second position in the prone-first group (p < 0.001). The percentage of patients with pain in the second position was 40% in the supine-first group compared with 18% in the prone-first group (p < 0.003). Distention did not differ between the groups. Pain was associated with increased pressure and diverticulosis (p < 0.001) but not with retained feces or fluid.
CONCLUSION. Sustained pressure in the air-insufflated colon was higher in the prone than in the supine position. Imaging in the prone position first results in a significant decrease in pressure in the latter phase and less pain. Pain was associated with pressure and diverticulosis.
Keywords: CT colonography • diverticulosis • pain • position • pressure
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In vivo and cadaveric animal experiments have yielded data regarding intraluminal pressure that can lead to rupture of the colon [7–12]. An adult human cadaveric cecum exposed to less than 40 mm Hg of intraluminal pressure generally does not rupture, but cecum exposed to more than 150 mm Hg of pressure always ruptures [7–12]. A barium enema administered from 3 feet (91 cm) above the examination table causes intraluminal colonic pressure of 56–80 mm Hg and is considered safe [11]. However, a barium enema administered from 6 feet (1.8 m) above the table causes intraluminal colonic pressure of 140–168 mm Hg and can cause perforation [11]. The pressures needed for perforation of the right colon and cecum are lower than those needed for perforation of the sigmoid and descending colon [8, 9]. It is estimated that the upper limit of safe intraluminal human colonic pressure is 80 mm Hg, because perforation can occur at pressures greater than 140 mm Hg [13].
Colonic distention is a prerequisite for diagnostic CTC [14]. Distention can be achieved with gentle insufflation of room air or CO2. There is a delicate balance of proper distention, patient comfort and pain, and safety. A poorly distended colon can obscure polyps and masses and reduce the diagnostic yield of the examination [15, 16]. However, overdistention of the colon can cause pain and colonic perforation. Methods of optimizing colonic distention include CO2 insufflation and dual positioning; both have been found beneficial [15–18]. Use of a spasmolytic agent is controversial [19–21].
When a CO2 pump is used for CTC, the upper limit of colonic pressure is 25 mm Hg, but to our knowledge there is no information about the colonic pressure recorded when air insufflation is used [22, 23]. With air insufflation, 50 puffs of air or sufficient puffs to produce insufflation tailored to the patient's tolerance have been reported to be adequate [24]. Whether insufflation is manual or automatic also may affect colonic pressure and distention.
Knowledge of the physiologic pressure induced by air insufflation and the factors affecting pressure may help to optimize CTC insufflation techniques. Because transmission of abdominal pressure in humans is nearly homogeneous and based on the hydraulic model of abdominal mechanics, measurement of rectal pressure is representative of pressure in the colon and abdominal cavity [13].
Our hypothesis was that insufflation of the colon is a dynamic process that can be affected by patient position and is related to the patient's perceived degree of pain. Dual positioning for CTC can begin with the patient either supine or prone. Our aim was to prospectively measure rectal pressure and to evaluate pain during air-insufflated CTC performed with both positioning sequences and to analyze factors affecting intraluminal pressure.
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The two groups comprised 379 patients (221 men [58.3%], 158 women [41.7%]; average age, 63.2 years; range, 39–90 years): 177 patients in the supine-first group and 202 in the prone-first group. The study included 251 (66.2%) participants at average risk of colorectal cancer, 64 (16.9%) at high risk because of a family history of colorectal cancer or previous polyp resection, and 64 (16.9%) who had symptoms suggestive of malignancy, such as anemia, positive result of occult blood test, weight loss, and abdominal pain. The age and sex distributions were the same for the two groups.
Bowel Preparation
Before CTC, patients adhered to a 24-hour clear liquid diet. Patients who
did not have chronic renal failure or congestive heart failure took two 5-mL
doses of 0.9 g of disodium hydrogen phosphate and 2.4 g of sodium dihydrogen
phosphate (Soffodex, Dexxon) and two 10-mg bisacodyl tablets the morning and
the evening of the day before the examination. For patients with chronic renal
failure or congestive heart failure, bowel preparation consisted of 4 L of
polyethylene glycol solution (Golytely, Braintree Laboratories).
CTC Insufflation Technique
Room-air insufflation and monitoring were performed by a single registered
physician with 3 years of experience performing and monitoring approximately
3,800 CTC examinations. Rectal examination was not routinely performed. The
patients were placed in the right decubitus position, and room air was
insufflated through a Foley rectal catheter (14- to 20-French) for the first
20 puffs. The rest of the insufflation was performed with the patient in the
prone or supine position after gentle turning to that position. The balloon
was not routinely inflated. If rectal prolapse or air leakage was noticed with
use of the Foley catheter, the catheter was exchanged for a rectal tube
(Miller Air Tip, E-Z-EM). Exchange was necessary in 60 cases (15.8%). A
handheld plastic bulb was used for insufflation.
The number of initial puffs, insufflation time measured in minutes with a stopwatch, and the patient's report of pain were recorded separately for each position. In addition to the patient's report, the investigator's visualization of observable indicators such as facial expression and body and leg movements was used to discern the presence of pain. The physician performing insufflation specifically asked the patients whether they experienced pain before insertion of the rectal catheter. During insufflation in either position, the patients were specifically asked to differentiate inconvenience related to the rectal tube from abdominal pain related to distention. Only pain related to insufflation and distention was recorded. The questions were repeated at completion of insufflation in the second position. At the completion of insufflation in each position, the presence of pain was recorded on a separate form prepared for this purpose.
Rectal pressure was measured with a manometer (Microcuff, Microcuff) attached to the sidehole. Recorded colonic pressure, measured in millimeters of mercury, was defined as the recorded sustained pressure at the end of insufflation. An initial scout image was acquired to confirm adequate distention of the colon and for planning of data acquisition. A second scout image to confirm the adequacy of colonic distention was obtained with the patient in the second position. Additional puffs were added as needed to achieve optimal colonic distention. Sustained final colonic pressure and pain were recorded. The pressure measurement was used only as an adjunct to manual insufflation. Insufflation was terminated only according to conventional measures of patient discomfort and visualization of abdominal distention, not because of pressure measurements. No spasmolytic or IV contrast agent was used.
We performed an in vitro analysis to evaluate the volume of one puff of air from a handheld bulb. A Foley catheter or a rectal tube was inserted into an Erlenmeyer laboratory flask, which was filled with water, immersed in a water tank, and inverted. Each puff of air displaced water from the flask. We calculated the average volume of air in the flask from 10 experiments. Air was insufflated by the same physician who performed insufflation for all of the study patients.
CTC Technique
Data acquisition from the entire colon was performed in single breath-hold.
Studies were performed on a 16-MDCT scanner (Brilliance 16, Philips
Healthcare) (212 patients) or a 64-MDCT scanner (LightSpeed VCT, GE
Healthcare) (167 patients). Volumetric data were acquired from the full colon
with 1.5- to 2.0-mm slice thickness and a 0.75- to 1.0-mm increment. The
rotation time was 0.5 second. Study parameters were 120 kVp, 100 mAs for the
first position, and 50 mAs for the second acquisition.
CTC Interpretation
Acquired CT data were transferred to a dedicated workstation (V3D,
Viatronix) equipped with navigator software for multiplanar reformations of
the air-distended colon and for an endoluminal perspective through the
distended colonic lumen in antegrade and retrograde directions. Combined 2D
and 3D reading was used with at least one full endoluminal navigation in both
the supine and prone positions and a full 2D reading.
CTC Evaluation
All studies and parameters were interpreted by a single board-certified,
fellowship-trained abdominal radiologist with 6 years of experience
interpreting approximately 5,500 CTC patient studies, who did not perform
insufflation. Except for position order, the reader was blinded to the
technical values recorded at insufflation. The 3D software (V3D, Viatronix)
was used to extract the air-filled colon automatically on the basis of a
predetermined threshold of the density of the air-filled colon and provided
the number of regions into which the entire colon was divided. Overall
distention of the 3D extracted air-filled colon was graded in each position on
a 1–5 scale (Table 1).
The least area of the colon distended that was at least a full colon segment
long was assessed and scored. Shorter areas with poorer distention did not
influence the distention score and were attributed to spasm or peristalsis. As
an adjunct measure for assessing distention, the numbers of colon regions
extracted automatically with the 3D software were recorded and compared for
the groups. Excellent distention resulted in extraction of one segment.
Less-optimal distention resulted in extraction of larger numbers of segments.
The presence of diverticulosis in a segment was scored with a predetermined
system, as was the presence of residual feces or fluid
(Table 1).
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Statistical Analysis
We used Fisher's exact test to examine the association between categoric
variables, Student's t test for independent and matched continuous
variables, and the chi-square test when applicable. We used multivariate
logistic regression analysis to assess the association between the covariates
and pain. All analyses were performed with statistical analysis software (SAS
version 9.1, SAS Institute). A value of p < 0.05 was considered
significant.
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In the prone-first group, the mean number of insufflated puffs before the examination was 46.3 ± 6.63 (range, 21–93), for an approximate volume of 1,759 mL. An average of 1.86 additional puffs (range, 0–12) with an approximate total volume of 71 mL were given between the prone and supine studies. The sum of the average number of initial and second-round puffs was 48.12 (range, 21–105), for an approximate volume of 1,829 mL. The average insufflation time was 3.2 minutes (range, 2–4 minutes).
Overall, initial insufflation volume, added insufflation volume, and insufflation time were significantly lower in the prone-first group than in the supine-first group (p < 0.001). No perforations were recorded.
Colonic Pressure
In the supine-first group, mean rectal pressure in the supine position was
38.3 ± 3.18 mm Hg (range, 25.7–48.5 mm Hg) compared with 40.08
± 3.87 mm Hg (range, 22–45.6 mm Hg) in the prone position. The
difference in values for the two positions was statistically significant
(p < 0.0001). In the prone-first group, mean rectal pressure in
the prone position was 38.2 ± 3.63 mm Hg (range, 28.7–45.6 mm Hg)
compared with 32.25 ± 5.5 mm Hg in the supine position (range,
19.1–44.1 mm Hg). The difference in values between the two positions was
statistically significant (p < 0.0001).
In the comparison of the groups, measured pressure in the first position did not differ significantly between the groups (p = 0.88). In the second position, however, pressure increased in the supine-first group but decreased in the prone-first group, the difference being statistically significant (p < 0.001). There was no difference in distention between the two groups (p = 0.24). The mean overall distention scores were 3.6 ± 0.4 and 3.56 ± 0.44 for the supine-first group and 3.5 ± 0.35 and 3.6 ± 0.38 for the prone-first group. The mean numbers of colonic segments were 1.59 ± 0.3 and 1.48 ± 0.25 for the supine-first group and 1.51 ± 0.3 and 1.52 ± 0.35 for the prone-first group.
Pain
In the supine-first group, none of the patients experienced pain before
CTC. The presence of pain was noted in 88 patients (49.7%) in the first
position and 72 patients (40.7%) in the second position (p < 0.01)
and correlated with degree of pressure (p < 0.001)
(Table 2). In the prone-first
group, none of the patients experienced pain before CTC. The presence of pain
was noted in 100 patients (49.5%) in the first position and in 36 patients
(17.8%) in the second position. The presence of pain correlated with the
degree of pressure in the second position (p < 0.001)
(Table 3).
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Multivariate analysis showed that pain was associated with increased pressure regardless of position or group (p < 0.001). There was no correlation between pain and retention of feces or fluid (p = 0.19). Air reflux to the small bowel was found in 66.8% of the patients in the supine-first group and in 60.7% of the patients in the prone-first group (not significant). No statistically significant relation was found between pain and reflux to the small bowel. The incidence of diverticulosis (Table 4) did not differ between study groups. Diverticulosis was most common in the sigmoid section of the colon and was associated with pain (p = 0.03).
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We found no perforations in this series, and the highest rectal pressure recorded was 48.1 mm Hg. This pressure is lower than that at conventional colonoscopy. Sustained intraluminal colonic pressure with a maxi mum of 58 mm Hg was recorded in 34 patients in one study of conventional colon oscopy [26]. The lower pressure may partly explain the lower incidence of perforation at CTC than at nontherapeutic conventional colonoscopy [4]. The pressures we recorded at CTC also were less than those observed in barium enema examinations and intus susception reduction, the recommendation for which is less than 120 mm Hg [11].
We found higher pressure in the prone than in the supine position regardless of which position was used first. We assume this finding may be related to compression and flattening of the abdomen against the CT table, resulting in a decrease in the anteroposterior diameter of the abdominal cavity and relative compression of the colon between the abdominal wall and the spine. In our study, the pressures measured in the first position did not differ significantly between the groups. In the second position, however, pressure increased in the supine-first group and decreased in the prone-first group.
The disadvantages of the widespread use of colonic screening are colonic preparation and study-related patient discomfort. In CTC, insufflation is a prerequisite for polyp detection but also causes pain. In our study, the proportions of patients reporting pain in the initial position were the same; approximately 50% of patients in each group reported pain. There was a significant difference, however, between the two groups in proportion of patients reporting pain in the second position. Only 18% of the patients in the prone-first group reported pain, compared with 50% in the supine-first group. At the beginning of air insufflation, pain may be due to tension of the bowel wall and corresponding distention. Later, when patients are more accustomed to colonic distention, pain may be related to factors such as redistribution and movement of air in the colon, especially with compression and flattening of the abdomen against the CT table in the prone position [19].
Colonic pressure drops considerably when the supine position is used later, and patients report less pain because less pressure is exerted on the colonic wall. The impressive decrease in pressure and pain also can be attributed to a rapid increase in anteroposterior diameter of the abdomen and possible repositioning of bowel gas. Other authors may argue for an approach in which the patient is more comfortable at the beginning of the examination (the supine position), adjusts to the sensation of colonic distention, and then endures the more uncomfortable prone position at the end of the examination.
Measurements of insufflated air include intracolonic air volume, refluxed small-bowel air, and air that the patient cannot hold and thus expels. We found that the total amount of insufflated air was approximately 2,300 mL in the supine-first group and 2,000 mL in the prone-first group with no difference in distention score in the two positions. The distention measures we used were a combination of objective parameters evaluated by the observer and data automatically generated by the software.
It has been suggested [22, 24] that diverticulosis with colonic wall deformity and stiffness can result in relative obstruction and pain. Our findings show that diverticulosis is an independent factor associated with pain. However, retention of feces or fluid and reflux were not related to pain, possibly because retained fecal particles and fluid do not influence wall deformity as diverticulosis does. The presence of reflux of air to the distal small bowel was not statistically related to the patients' notion of pain. This interesting phenomenon may be explained by the greater diameter of the colon and the greater tension on its wall. Laplace's law states that wall tension increases as pressure and radius increase [12]. Therefore, because the diameter of the colon is larger than that of the small bowel, the tension on its wall also is greater and probably contributes more to the notion of pain.
The intracolonic pressure limit (25 mm Hg) is low when commercial CO2 pumps are used. The recommended volume for CO2 insufflation (3,000 mL) is substantially lower than the volume reported in clinical practice (4,200 mL) [22] but higher than the volume used in our study (2,000–2,300 mL). Rapid CO2 absorption can explain the need for the increased volume of gas needed to achieve optimal colonic distention.
Our study had several limitations. Because our goal was to evaluate technical parameters of CTC, we did not prospectively aim at evaluating polyp detection rates. Because we measured sustained pressure at the end of but not during manual air insufflation, we might not have documented pressure peaks during insufflation. Although CO2 pumps are used at some centers, we did not use a CO2 pump. We believe, however, that our study results may be important and certainly applicable to many centers performing CTC examinations, even those performing the examination with a CO2 pump. We did not use a pain scoring system; doing so might have increased the detail of analysis of perceived pain. We believe, however, that the pain assessment by a single experienced physician performing insufflation using objective and subjective signs of pain was representative of the patients' sensations. There was, however, inherent bias in assessing pain.
Because a single reader interpreted CTC studies, we have no indication of variability between readers in these examinations. However, the technical aspects of the examination that we evaluated, such as distention, retention of feces or fluid, reflux, and diverticulosis, are relatively easy to assess, and the reader was highly experienced and blinded to all technical measurements at CTC. Thus we believe that our results can be reproduced at experienced CTC centers. We assumed that rectal pressure is representative of colonic pressure, as has been proved [13]. We tried to portray a real-life situation. Although invasive techniques of pressure measurement in various parts of the colon have been performed on animals, we did not intend to perform such invasive methods on humans.
Insufflation was a dynamic process with sustained pressure higher in the prone than in the supine position. Imaging in the prone position first resulted in a significant decrease in pressure in the supine phase of the study, less pain, and a corresponding decrease in the amount of insufflated air needed to achieve adequate distention. Pain was asso ciated with pressure and with diverticulosis but not with reflux or retention of feces or fluid.
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