The Focused Assessment with Sonography for Trauma, (FAST) procedure.

Transcription

1 The Focused Assessment with Sonography for Trauma, (FAST) procedure. ROBERT H. WRIGLEY Professor Veterinary Diagnostic Imaging University of Sydney Veterinary Teaching Hospital Professor Emeritus Colorado State University A standardized, quick ultrasound exam procedure has been developed for the evaluation of humans soon after they had been admitted to a medical facility after they had sustained blunt abdominal trauma. The FAST procedure is a simple technique what can be performed quickly and within minutes of a patient s arrival at an emergency medical facility. The technique is specifically designed for early detection of free peritoneal fluid such as hemorrhage and urine. Only a small portable ultrasound machine is needed and the simplicity of the protocol allows for the examination to be carried out by personnel with minimal sonography skills. Four areas examined for free fluid are the perihepatic, renal recesses, perisplenic space and caudal aspect of the abdomen. The study is noninvasive, rapid, repeatable and has been found in humans to have a good sensitivity (81%-98%) and a specificity (98%- 100%) for detection of free fluid in the abdominal cavity. Using the FAST protocol, the presence of fluid can also be sampled under sonographic guidance for diagnostic cytology. With post trauma hemorrhage, a positive FAST result most likely supports a hemoperitomeum which should be monitored and if severe or progresses should be followed by a laparotomy. Negative findings on a FAST study of a patient with suspected internal bleeding suggests the need to search for extra abdominal site of hemorrhage or a hemolytic syndrome Until sonography had been developed, detection of small amounts of free peritoneal fluid in dogs and cats was a difficult diagnosis on physical examination and relied on needle abdominocentesis. Sometimes it would be necessary to make multiple attempts but false-negative results are not uncommon. Plus there is the risk of perforating or aspirating from bowel or adjacent organs. Almost 20 years ago a study (1) was reported by experienced veterinary radiologist sonographers where increasing volumes of peritoneal dialysis fluid was injected into the peritoneal cavity of normal dogs. Subsequent radiographic and sonographic evaluations were made and at least 1.8 ml/kg of fluid had to be present for detection on radiographic examination. Sonography was more sensitive, detecting peritoneal effusion after approximately 1ml/kg had been administered intraperitoneally. In 2004 a study (2), Evaluation of a focused assessment with sonography for trauma protocol to detect free abdominal fluid in dogs involved in motor vehicle accidents was published. This study adapted the human FAST protocol for use by veterinary critical care specialists and residents with minimal previous ultrasonographic experience. The study of a 100 dogs confirmed that the veterinary modified FAST procedure was a useful diagnostic and monitoring procedure. Free abdominal fluid was detected in 45% of patients. Subsequent abdominocentesis of 40 dogs with positive FAST findings, revealed 38 to have hemoperitoneum and 2 with uroperitoneum. In the study the FAST protocol was adapted by first placing the dog in left lateral recumbency unless an injury precluded this positioning. Imaging was best performed,

2 along the gravity dependent side (i.e. left flank in left lateral recumbency). If the patient status allowed then it was rolled to right recumbency to repeat the imaging especially in the region of the right flank. The FAST exam consisted of performing transverse and longitudinal view at 4 sites (Fig A).For examination of the Location 1, the perihepatic region, ~ 4 cm square of hair was clipped just caudal to the xiphoid process. For examination of Location 2, the bladder region, it may not be necessary to clip away hair as often the hair coat is thin on the caudal midline of the abdomen. For examination of Location 3, the right renal recess ~ 4 cm square of hair was clipped just caudal ~4 cm square of hair was clipped just caudal dorsally over the right ribs. at the most gravity-dependent location of the abdominal wall. Similarly an area was clipped on the left side of the flank for Location 4, the perisplenic region Alcohol or other liquids approved by the ultrasound vendor or ultrasound scanning gel is needed and the examination can best be performed with 5 to 7 MHz sector or curvilinear transducers. Transverse and longitudinal sonograms should be made at each location while fanning and rotating the transducer attempting to locate the abnormal presence of anoechoic/hyperechoic fluid (FF). If no fluid was detected then consideration should be given to rolling the dog to the opposite recumbency and the examination of the dependent side be repeated. The aim is to determine the presence of abnormal anoechoic/hypoechoic fluid pockets and record the size of the fluid accumulation so that it can be re-evaluated by serial FAST exams over time. At location 1, abnormal fluid (FF) will accumulate between the diaphagram and liver lobes ( Figure1). At location 2 fluid will be detected against the wall of the urinary bladder (Figure 2). At location 3, abnormal fluid will locate in the renal fossa between the right kidney and the caudate lobe of the liver (Figure 3). At location 4, abnormal fluid will outline the borders of the left kidney and spleen (Figure 4). If the fluid is somewhat echogenic, such as having a swirling appearance, this tends to support the presence of a hemoabdomen or peritonitis, where as a uroabdomen tends to result in anechoic fluid without echogenic content. If the fluid pocket is large, the direct abdominocentesis should enable a sample to be obtained. If the fluid pocket is small then with practice, ultrasound guided fluid sampling should be possible. Collection of a volume of nonclotting blood is indicative of a hemoperitoneum.. Estimation of the volume of the hemoabdomen should be made and if only a minor volume is detected at only one or 2 sites then follow-up FAST studies should be considered while the patient is monitored. Recheck of the locations where fluid was previously detected will help to determine if the degree of hemorrhage is progressing and could become life threatening. If the volume of the hemoabdomen is present at all 4 locations and progresses, a thorough abdominal sonogram should be considered and even if no obvious organ pathology is detected, then preparation for exploratory laparotomy should be considered. Alternately if the abdominal fluid sample does not appear likely to be blood, then creatine concentration should be measured and compared to serum creatine levels as a uroabdomen will result in the abdominal fluid sample have >2 times creatine levels of circulating blood. If the location of rupture of the urinary track is not apparent on sonography, radiographic contrast studies of the urinary tract a can be used to locate the source of the uroperitoneum. The veterinary study of FAST reported that the procedure could be performed rapidly (median study time was 6 minutes). Free fluid (when present) was most commonly observed when the lateral recumbency was the same side as being examined. Even though the more serious injury from the motor vehicle trauma was

3 frequently outside the abdomen, surprisingly FAST lead to detection of hemoabdomen in more than 1/3 rd of the dogs. This incidence is much higher than had been reported previously. Most of the dogs in the study received IV administration of crystalloid and/or colloid solutions as part of the medical stabilization. So that when the FAST findings in the 38 dogs with hemoabdomen were combined with clinical follow-up, blood transfusion was warranted in only 10 dogs. Excluding the 2 dogs who underwent laparotomy to repair rupture of the urinary system, 8 of the transfused dogs recovered without need for abdominal surgery. The FAST ultrasound study is a simple quick ultrasound examination, requiring minimal sonography skills. While it is not designed to replace a through abdominal sonogram, the FAST findings provide useful information to guide further diagnostic procedures and treatment for animals who have substained abdominal significant trauma. Figure A. Illustration of transducer placement for the FAST ultrasound examination of the abdomen of a dog. The transducer is initially centered at each location to make transverse and longitudinal images. Then the transducer should be moved at least 4cm and fanned through at least 45 degrees in a cranial, caudal and to the left and right around each region illustrated. Location 1, is centred at the subxiphoid region. Location 2 is along midline cranial to the pubis at the level of the urinary bladder. Location 3 and 4 are in the region of the cranial aspect of the left and right flanks. Location 3 is at the level of the right kidney and adjacent caudate lobe of the liver is best performed in right lateral recumbency. Location 4 is at the level of the left kidney/spleen and is best performed in left lateral recumbency.

4 Fig 1 At location 1, abnormal fluid (FF) will accumulate between the diaphagram and liver lobes Fig 2 At location 2 fluid will be detected against the wall of the urinary bladder

5 . Fig 3 Abnormal fluid will (FF) located in the renal fossa between the right kidney and the caudate lobe of the liver Fig 4 At location 4, abnormal fluid will outline the borders of the left kidney and spleen 1 Henley, Hager and Ackerman A comparison of 2D ultrasonography and radiography for detection of small amounts of free peritoneal fluid in the dog. (1989) Veterinary Radiology and Ultrasound 30(3): BOYSEN S R. ; ROZANSKI E A. ; TIDWELL A S. ; HOLM J L. ; SHAW S P. ; RUSH J E. Evaluation of a focused assessment with sonography for trauma protocol to detect free abdominal fluid in dogs involved in motor vehicle accidents. J Am Vet Med Assoc 2004; 225: