Small Bowel Obstruction (#2)

SBO 2

A Prospective, Multicenter Evaluation of Point‐of‐care Ultrasound for Small‐bowel Obstruction in the Emergency Department

Academic Emergency Medicine August 2019 - Pubmed Link


Take Home Points

1. POCUS for small bowel obstruction may not be as accurate as previously thought.

2. We still need to look at more patient centered outcomes for this application of POCUS.


Background

We have covered the use of point-of-care ultrasound for small bowel obstruction (SBO) before. The premise is simple. SBO is readily seen on US. Abdominal xrays are bad at detecting SBO. CT costs time and radiation. It sure seems like this would be a good place where US could save the day. Unfortunately, we still don’t have an answer regarding how helpful this is. There are two questions unanswered 1) is it accurate and then, more importantly 2) does it change patient centered outcomes? There is still work to be done regarding the latter, and this study does not bring much to the table there. However, it does add to our understanding of accuracy. Most of the studies to date have been small and with significant limitations (and only focusing on #1). This study (along with the other one recently published) is arguable the best evidence we have to date on the topic. Lets see what they found out.


Check out The Evidence Atlas - Bowel for a review of prior evidence on point of care ultrasound in SBO.


Question

What is the accuracy of point of care ultrasound compared to CT for the diagnosis of SBO?


Population

Three US academic sites, all had emergency medicine residencies and ultrasound fellowships


Inclusion:

  • At least 18 years old

  • Could be consented in English

  • Symptoms concerning for SBO (not explicitly defined but based on treating physician’s assessment)


Exclusion:

  • Pregnancy

  • Had undergone radiology imaging prior to ultrasound

  • Did not receive CT imaging


Design

Prospective, multicenter, observational study, convenience sample


Adult ED patients presenting between July 2014 and May 2017 with suspicion for SBO underwent goal‐directed POCUS of the abdomen for the evaluation of SBO

Data only collected if a participating physician was available.

POCUS findings were interpreted at bedside by a physician sonographer blinded to laboratory and imaging results, including CT

Each study marked as “positive”, “negative”, or “indeterminant” based on small bowel dilation or abnormal peristalsis (the other findings below served only to supplement their impression). Indeterminant was taken as a positive.

POCUS findings also interpreted retrospectively by an expert reviewer after de-identification of the images. Expert reviewers were ultrasound fellowship trained and 2 were ultrasound directors.

Reference standard was abdominal CT. If CT was “noncommittal” and considered SBO a possibility, it was treated as SBO.


This study was registered on clinicaltrial.gov although they mis-listed the number in the manuscript, here is the link. There were some deviations from this originally planned study, discussed below in limitations.


Power analysis: needed 96 patients. Seems they expanded when they decided to make it multi-site and wanted to make this study larger than prior studies.


Who did the ultrasounds?

Senior EM resident (PGY2 or 3), ultrasound fellow, or ED attending. All residents had completed an EM ultrasound rotation. A total of 41 physicians participated.

All residents and fellows received a 30 min lecture on technique and brief hands-on scanning practice on normal individuals

All residents had performed at least 50 ultrasound exams


The Scan

Curvilinear transducer

Curvilinear


Systematic evaluation of entire abdomen - acquired at least one clip in all four quadrants

Attempted to obtain longitudinal views of bowel


Assessed for:

  • Dilation (>or equal to 25 mm)

  • Abnormal peristalsis (“to and fro”)

  • Small bowel wall edema (“Keyboard sign”, no measured thickness cutoff used)

  • Intraperitoneal free fluid (seen between bowel loops, “Tanga” sign)

  • Transition point between dilated and normal bowel


Learn how to do Ultrasound for SBO from 5 Minute Sono!

5minsono


Check out Bowel/GI Pathology on the POCUS Atlas!

thePOCUSAtlas


Results

232 subjects enrolled (exclusions were only for no CT, 15 excluded)

N = 217

  • 98.7% of patients were at two of the sites, only 3 patients from third

  • Prevalence of SBO was 42.9%

  • Median BMI 25

  • Median symptoms for 2 days

  • Roughly 70% of CTs were IV contrast only


Primary Outcome - Accuracy of POCUS for SBO compared to CT

Sensitivity 88% (CI 0.80-0.94)

Specificity 54% (CI 0.45-0.63)

LR+ 1.92 (CI 1.56-2.35)

LR- 0.22 (CI 0.12-0.39)


Secondary Findings

Subgroup analysis by level of training:

  • PGY2s had highest specificity 73%, +LR 3.12 (higher than fellows or attendings)

  • Fellows (closely followed by PGY3) had highest sensitivity 93% -LR 0.25

Trainees (grouped together) were slightly more sensitive (91% vs 85%) and less specific (51% vs 61%) compared to attendings.


Expert reviewer interpretation compared to CT:

  • Sensitivity 89% (CI 0.81-0.95)

  • Specificity 82% (CI 0.74-0.88)

  • LR+ 4.99 (CI 3.39-7.33)

  • LR- 0.13 (CI 0.07-0.24)

Agreement between expert and POC Sonographers K = 0.468


15.2% of POCUS were indeterminant

They reanalyzed their data classifying indeterminant as negative:

  • POC Sonographers (everyone): Sens 77%, Spec 73% (so sensitivity worse, specificity better)

  • Expert Reviewers: Sens 82%, Spec 87%

Also reanalyzed removing all patients with indeterminate CT

  • POC Sonographers (everyone): Sens 90%, Spec 54%

  • Expert Reviewers: Sens 95%, Spec 82%


Best case scenario analysis: excluding all indeterminant POCUS, excluded all indeterminant CTs and assessed the expert reviewer accuracy:

Sensitivity 94%

Specificity 86%


Looked at all the individual findings to see which were more accurate:

  • Bowel dilation was most sensitive (87%) but not that specific (60%)

  • Free fluid and transition point were both most specific (82%)

These trends were pretty similar to the expert reviewer interpretation


Compared POCUS to discharge diagnosis → Not much difference, a little more sensitive

CT compared to discharge diagnosis:

  • 94% sensitive

  • 90% specific

  • K = 0.829


11 false negatives

  • 2 of these had discharge diagnosis of ischemic colitis and inflammatory bowel disease

57 false positives

  • Some common CT findings in these cases were hepatic masses, ileus, large bowel obstruction, colitis/diverticulitis


Limitations

This was a wide range of learners. That is beneficial because it applies to a broad audience. It is limiting in that we don’t know if it could have been more accurate with more training or more experience.

Indeterminant was taken as a positive. More positives means it could inflate the sensitivity.


Compared to the clinical trial registration there were a few deviations from their original proposal:

  • Criterion standard originally included OR report, discharge dx, not just CT

  • Originally going to look at length of stay as well

  • Originally not multi-center

  • Planned for longer didactic training than what happened ultimately

It is generally poor practice to change things up without explanation; however, perhaps there was explanation for these changes. Most of these are not “deal-breakers”. It would have been nice to have length of stay. Adding extra sites likely strengthened the study.


Even with expert reviewers, not optimal accuracy. This is surprising given that this is not a particularly hard diagnosis to see. This makes me think that the problem may be in acquiring the images.

Have to consider if this has the potential to change a patient centered outcome. The second study (below) mentioned did assess for time to diagnosis. That theoretically can help, but only if it means you are going to do something more quickly. Can it get the patient to a room faster? To the OR faster? Perhaps it just would help the diagnosing physician have diagnostic closure more quickly. This is something to consider going forward.


Discussion

This is the second similar article - Boniface et al Annal of EM 2019.

Mini Summary:

Prospective observational study, single center. Enrolled if there were suggestive symptoms and a CT was being ordered. Similar training (20 minutes didactics, 5 non-pathologic scans). Same diagnostic criteria - 25mm and abnormal peristalsis but no indeterminant option. Use research associates for enrollment and data collection. They had 125 patients, 25.6% had SBO. Sensitivity 87.5%, Specificity 75.3%, LR+ 3.5, LR- 0.2. Overall, more specific but who cares, still not great. Results did not vary with resident vs attending/fellow. Ordering CT took 3 hrs 42 minutes before radiologist prelim interpretation (read the CT yourself, it's not rocket science). POCUS took 11 minutes to perform. Note that in this study, POCUS saved no one any time since everyone got CT anyway. This is just introducing the potential for improvement. Interestingly, they found 30mm was better cutoff for diameter. Dilated bowel loop was more sensitive (94.2%) and specific (93.8%) than in the other study.


Why do both of these studies show worse accuracy than previous evidence? Two metaanalyses (Taylor 2013 and Gottlieb 2018) also found it is much more accurate than in the reviewed studies. Maybe it was because these included non-ED based studies? The main emergency department prospective studies Jang 2011 and Unluer 2010 had sensitivity 91-98% and specificity 84-93%. The authors of the current paper think this discrepancy may be due to more training of sonologists in other studies. That is certainly possible. Alternately maybe its just not that good. It would have been great to have more evidence of how the practitioners were fooled. I still have a suspicion that someone who has seen a lot of SBO on ultrasound could diagnosis it with a high accuracy; but I think that based on the evidence we have to be careful.


Take Home Points

1. POCUS for small bowel obstruction may not be as accurate as previously thought.

2. We still need to look at more patient centered outcomes for this application of POCUS.


More Great FOAMed on this Topic

The Breach - Ultrasound for Small Bowel Obstruction

EM Docs - Ultrasound for Small Bowel Obstruction

EM Cases - POCUS Cases Small Bowel Obstruction

ALIEM - Small Bowel Obstruction: Diagnosis by Ultrasonography


Our score

3 Probes


Expert Reviewer for this Post

Au

Arthur Au, MD @arthurkau

Arthur is the Associate Ultrasound Director and Ultrasound Fellowship Director for the Department of Emergency Medicine at Thomas Jefferson University in Philadelphia, PA.


Reviewer's Comments

This is a well done study which includes sonographers with minimal experience in performing SBO ultrasounds. Not surprisingly this is reflected in the results that ultrasound is not quite as sensitive or specific for diagnosing SBO as previously reported. Looking forward, we need more patient centered outcomes to see how the use of ultrasound in cases of possible SBO actually affects patient care. Can an ultrasound first model decrease CT scanning, length of stay, determine necessity or timing of NG tube placement or guide operative vs supportive management?


Cite this post as

Michael Prats. Small Bowel Obstruction (#2). Ultrasound G.E.L. Podcast Blog. Published on October 15, 2019. Accessed on November 13, 2019. Available at https://www.ultrasoundgel.org/78.
Published on 10/15/19 06:00 AM
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