Presentation on theme: "FAST but out of Focus? The focused ultrasound for trauma; assessing accuracy and techniques. Margaux Snider MS4 September 2007."— Presentation transcript:
FAST but out of Focus? The focused ultrasound for trauma; assessing accuracy and techniques. Margaux Snider MS4 September 2007
What is the FAST? Focused Assessment with Sonography for Trauma: A bedside ultrasound exam done during trauma to evaluate for intra-abdominal injury May include many views (up to 12) but always includes 4 main views: 1. Morrison’s pouch – RUQ, hepato-renal recess 2. Pericardium – subxiphoid, or long-axis parasternal 3. LUQ, Spleno-renal recess 4. Pouch of Douglas – suprapubic, between rectum/uterus and bladder
FAST in the Emergency Room Why do a FAST in the first place? Represents a quick method to assess for hemorrhage or abdominal injury without interruption of resuscitation (unlike CT) Poses low to no risk of further injury to already potentially unstable patient (1- 2% risk of bowel perforation with diagnostic peritoneal lavage)
The Literature Study in 1970 by Goldberg, established ascites as anechoic by ultrasound 1. Several studies following this began to characterize various fluids from ascites to clotted blood and hematomas via ultrasound 2,4-8. North American radiologists began studying US for use in trauma starting in ,12 Emergency medicine physicians began prospective studies using ultrasound to find free fluid in a trauma setting starting in
Literature Cont. Jehle and subsequent studies cited Goldberg’s identification of ascites as anechoic and inaccurately used blood and “free fluid” (including ascites) as interchangeable. Ultrasound in trauma became known as the FAST acronym in 1996, has been added to most level 1 trauma center ATLS algorithms 11,15. However the training is still focused on assessing solely for the presence of an anechoic stripe. Several studies have attempted to address the variable sensitivity of the FAST, examining such variables as use of portable US 20, retroperitoneal bleeding 21-23, and size of anechoic stripe 29.
A positive exam in the EM literature is indicated by the presence of an anechoic stripe/region in one of the four main views. Anechoic stripe A positive exam in the early Radiology literature includes assessing for heterogeneous echogenicity and parenchymal echo abnormalities Normal …The Images
The Anechoic Stripe An anechoic stripe on the FAST is thought to be the definitive sign of a positive exam and represent the presence of hemoperitoneum 14,15 What is actually anechoic on ultrasound? 1.Fluid – such as ascites 1,5 (not static blood) 2,4,7 2.Active flow of blood – as in blood vessels 8,9 3.Older hematoma – representing separation of plasma and clot 4,6,7
Blood on Ultrasound by Radiologists Fig. above Ultrasound of phantom containing, from left to right: water (H20), packed red cells (PRC), hemolysate (H), whole blood (WB), serum (S) and water. Note that both serum and water created the most hypo- echoic regions, whereas whole blood and PRC appear relatively heterogeneous to the surrounding tissue; not anechoic
So what’s the problem? Blood from a newly formed hematoma, as would be developing in a trauma patient, is not anechoic initially. It appears as a collection of heterogeneous internal echoes, similar to the echogenicity seen in bowel loops and gradually becomes anechoic over time 4,7,10 Although FAST examiners are frequently finding anechoic regions on exam, it more likely represents increased time of bleeding, and subsequent separation of serum out from plasma. However, many additional bleeds and early hematomas are potentially being missed.
Summary of Concerns Lack of consistency: Multiple ED studies on the FAST have shown a very wide range of sensitivity and specificity values, from 28%-92%; and % respectively 11,12, Limited Criteria: Almost all of these studies (those that did specify their criteria) used solely the presence of an anechoic stripe to delineate positivity.
Faulty Assumptions: 1.These studies routinely cite a study assessing ascites as evidence for the appearance of blood on ultrasound. They routinely confuse “free fluid,” which could be from a variety of causes, with hemoperitoneum 2.The study documenting increased sensitivity of the FAST with serial exams postulated the newly found anechoic stripe represented increased bleeding over time 24 This study, however, failed to specify the time from the actual trauma to the FAST exam, documenting only the time from initial to follow up FAST exam and did not consider changing blood character as a confound rather than increasing amounts of blood.
Hypotheses for proposed study H1: Poor sensitivity and high false negative values may represent failure to consider increased echogenicity of surrounding organs and poorly visualized organ edges (parenchymal echo abnormalities) as a positive exam. H2: Presence of anechoic stripe on FAST with repeat exam may represent increased time to exam from trauma, not increased bleeding amount
Study Proposal Prospective Study Participants: Patients arriving at OHSU emergency department for whom the trauma system has been activated. Patients whom the trauma team deems appropriate to receive a FAST exam, regardless of documented blunt abdominal trauma. Patients enrolled in study must have received per team discretion both a FAST and abdominal CT or abdominal surgery (to allow for verification of blood/fluid presence, or organ injury) Patients may be men or women years old. Reasonable approximation of time of trauma must be attainable
Study Proposal Methods – Part 1 4 view frames (RUQ, pericardium, LUQ, suprapubic) of participant FAST (ideally a video recording of entire FAST exam), saved, identifying data removed. Approximate time of trauma and time of FAST recorded at arrival Exam is assessed by ED physician or resident certified for FAST exam at time of trauma. Classified as: Positive – based on presence of anechoic stripe or anechoic fluid collection in any 1 or more of 4 views Negative – based on absence of anechoic region in all 4 views
Study Proposal Methods – Part 2 4 views which have been saved are then reinterpreted by blinded sonographers or ultrasound trained radiologists (unaware of final presence of fluid or ED read of FAST) They are asked to classify the exam as follows: Positive 1: presence of anechoic stripe/collection in any 1 or more of 4 views Positive 2: absence of anechoic stripe/collection but presence of increased heterogeneous echogenicity or poorly visualized organs (parenchymal echo abnormalities) Negative: absence of either 1 or 2
Study Proposal Confirmation of Findings All subjects are documented as +/- free intra- peritoneal fluid or blood based on CT or abdominal surgery findings. **Would be ideal to be able to quantify fluid – but not sure how to do this radiographically** Intra-parenchymal injury observed on CT scan or during surgery without peritoneal fluid/blood will be excluded as solid organ injury is not specifically being assessed other than edge/echo abnormalities as it contributes to identifying peritoneal fluid collections on ultrasound. Any fluid visible on CT scan as read by radiologist, or >50ml fluid visualized during surgery will be considered +.
Study Flow Chart Initial FAST obtained ED Physician assessment – real time Positive Negative Sonographer assessment Positive – group 1 Positive – group 2 Negative Confirmation of fluid presence CT ScanAbdominal Surgery +/- Time from trauma to FAST obtained Quantification of fluid?
Outcomes Accuracy, Sensitivity, Specificity, of ED readers and blinded sonographers Inter-rater reliability among ED and sonography assessors NPV and PPV of FAST for both groups Concordance of categorization: Are the same exams being categorized correctly or incorrectly among both ED physicians and sonographers
Outcomes cont. Assess how sonographer positive 2’s are recorded by ED physicians – are they always recorded as negative, or are they deemed positive? Or indeterminate? Assess time to FAST to determine: 1)whether longer time = more positive FAST exams among both groups 2) whether longer time = significantly more anechoic positives If possible, could examine the relationship of time to FAST exam and quantity(?) of actual fluid observed, comparison with presence/absence of anechoic stripe
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References cont. 28. Melniker LA, Leibner E, et al. Randomized controlled clinical trial of point-of-care, limited ultrasonography for trauma in the emergency department: the first sonography outcomes assessment program trial. Ann Emerg Med. 48(3): ; Ma OJ, Gaddis G. Anechoic stripe size influences accuracy of FAST examination interpretation. Acad Emerg Med. 13: ; Friese RS, Malekzadeh S, et al. Abdominal ultrasound is an unreliable modality for the detection of hemoperitoneum in patients with pelvic fracture. J Trauma. 63(1):97-102; 2007.