Abdominal FAST Ultrasound: Gall Bladder Wall Oedema in the Collapsed Patient, Abdominal Effusion and Pneumoperitoneum
S. Chalhoub; S. Boysen
Faculty of Veterinary Medicine, Veterinary Clinical and Diagnostic Sciences, University of Calgary, Calgary, AB, Canada
The FAST abdominal exam, described in 2004 (Boysen et al. 2004), was the first VPOCUS exam to be validated in small animals. The goal was to detect free peritoneal fluid following blunt abdominal trauma, and therefor concentrated on 4 key sites of the abdomen; sites where target organs were most likely to be injured following trauma; liver, spleen, kidneys and urinary bladder, and where fluid is most likely to accumulate based on patient positioning and gravitational forces. The study demonstrated that this FAST abdominal protocol was sensitive and specific for the detection of free abdominal fluid. The study also demonstrated that abdominal FAST can be performed during resuscitation, was rapid (<5 minutes), required minimal experience, was repeatable, and was noninvasive. Abdominal VPOCUS has now been demonstrated to be sensitive for finding effusion in non-trauma patients.
Abdominal VPOCUS Patient Position, Probe and Coupling Agent
- Patients are placed in either left or right lateral recumbency. In some instances, abdominal VPOCUS can be performed in a sternal or standing position (consider the effects of gravity and patient positioning when looking for pathology). Minimal restraint is required.
- A microconvex/curvilinear probe is used for all abdominal VPOCUS scanning, with a frequency generally between 5 MHz (patients >15 kg) and 7.5 MHz (patients <15 kg).
- Gain is adjusted to maximize detection of anechoic fluid using either bile in the gall bladder or urine in the urinary bladder as a reference echogenicity for fluid.
- Depth is adjusted as needed during the abdominal VPOCUS with the greatest depth setting generally at the subxiphoid location which allows evaluation of the pleural and pericardial spaces.
- It is not wrong to shave the patient, but shaving is not required unless the patient’s fur coat is too thick to allow good image resolution (e.g., Husky and Northern breeds with thick undercoats).
- Alcohol is used but it is important to part the fur before or after applying the alcohol. Gel is not necessary (but can be used with shaving if higher resolution is desired or can be applied after the fur is parted and alcohol is applied). Hand sanitizers that combine alcohol and gel can be used.
- Patients should not be placed in dorsal recumbency as this can compromise the patient (increased work of breathing, decreased venous return and cardiovascular collapse).
Abdominal VPOCUS (Abdominal FAST) Protocol
The probe is placed on 4 regions of the abdomen in a consistent systematic approach. At each site, the probe is fanned and rocked through an angle of 45 degrees.
Note a 5th umbilical region view can be performed (5th site, see description below) prior to the gravity dependent renal view as it might detect smaller quantities of fluid that would be displaced when the ultrasound probe is slid under the patient when trying to find the gravity dependent kidney (the authors prefer to examine all 4 original sites as well as the umbilical site essentially making the exam a rapid 5-point evaluation).
The original 4 sites:
Subxiphoid or Diaphragmatico-Hepatic (DH) Site: Just Caudal to the Xiphoid Process
Key structures to identify include the diaphragm, liver, gallbladder, caudal vena cava, pleural and pericardial spaces (see later sections on volume status for more detail on the vena cava evaluation, and the respective sections on pleural and pericardial space evaluation).
Mirror image artifact distal to the diaphragm can only occur when there is air distal to the diaphragm, and therefore can be used to rule out pleural effusion at that location if it is noted.
It is important to consider patient positioning and the effects of gravity when evaluating any VPOCUS sites, including the subxiphoid location. Be sure to fan the probe through all liver planes to ensure a thorough evaluation of the liver is complete, and to rock the probe to assess the most ventral and cranial parts of the liver, where small accumulations of fluid may gather between the liver and diaphragm.
Urinary Bladder or Cysto-Colic (CC) Site
Key organs and structures to identify include the urinary bladder, the gravity dependent body wall and the apex of the bladder. Fluid tend to accumulate between the body wall and the bladder, at the apex of the bladder and between the bladder and the body wall. The probe is placed in long axis to the body between the pelvic limbs.
Once the bladder is found, it is important to manipulate the depth to see both dorsal and ventral walls of the bladder, and then to slide the probe to find the apex. Once at the apex, fanning, rocking, and then rotating the probe to short axis (and fanning, rocking in short axis) will allow visualization of abdominal effusion. The probe should also be placed on the non-gravity dependent lateral side of patient and the ultrasound bean angled through bladder and fanned to catch fluid in deeper gravity-dependent sites at the body wall. Pushing too hard will compress and can displace the bladder making it a challenge to identify.
Right Paralumbar or Hepato-Renal (HR) Site
Key organs/structures to identify include the liver, right kidney, body wall, and intestines. This view can be difficult to obtain as often it is necessary to go between ribs to visualize the normal structures. It may sometimes be necessary to start in short axis to the body so that the probe can be placed within an intercostal space between ribs. In smaller dogs and in cats, the probe can be placed in long axis to the body caudal to the 13th and final rib. In dogs, if the liver is visualized in the right paralumbar region, or between ribs, the probe can be slid caudally until the kidney is visualized. The right kidney is located quite lateral relative to midline.
Left Paralumbar or Spleno-Renal (SR) Site
Key organs and structures to identify include the spleen and left kidney, and to evaluate regions between the kidney, body wall, spleen and intestines. The probe has to be placed quite lateral to midline to find the left kidney and spleen. The spleen is located cranial and often lateral to the left kidney. The probe is placed in long axis to the body often mid abdomen and lateral to start. It is often easier to find the spleen at first, and then to slide the probe caudally until the left kidney is found. Fanning and rocking the probe helps to find the organs of interest.
Free fluid in the abdomen typically appears as dark (anechoic or hypoechoic) triangles between organs, commonly visualized at the apex of the bladder, between the bladder and the body wall, at the poles of the kidneys, between the spleen and left kidney, between liver lobes, between the liver and diaphragm, between the liver and right kidney, and/or surrounding small intestinal loops.
Is There Free Abdominal Air in the Abdomen Y/N?
Free abdominal air can be detected in many sites of the abdomen; however, it is most commonly identified at the left and right paralumbar locations with the patient in right or left lateral recumbency.
Again, it is important to consider patient positioning and where free air will accumulate when searching for free abdominal air. The author prefers to have the patient remain in lateral recumbency for a few minutes to allow air to track to the non-gravity dependent locations before trying to identify pneumoperitoneum.
- The peritoneal lining must be identified. This is essential so as not to confuse free air within the GI tract for free air in the abdomen.
- Identify the presence of reverberation artifact that originates at the peritoneal lining. This is very important to differentiate from reverberation artifact contained within the GI tract, which again, emphasizes the importance of clearly identifying the peritoneal lining.
- Identify the enhanced peritoneal stripe sign. This sonographic finding occurs when free abdominal air comes in contact with the peritoneal lining. At the point where free abdominal air comes in contact with the peritoneal lining it will cause the peritoneal lining to become more hyper-echoic.
Does the Patient Have a Gall Bladder Halo Sign Y/N?
A study by Quantz et al. 2009 demonstrated that patients with acute anaphylaxis often have a halo (double rimmed gall bladder wall) sign (the gall bladder wall is normally very thin or not easily visualized on ultrasound), and this can be seen during abdominal VPOCUS. A thicker gallbladder wall (often due to edema with or without surrounding fluid) can be seen with a “halo” effect. However, this is not specific for anaphylaxis and can be seen in patients with a number of conditions (anything that causes edema). However, with unstable patients presenting for collapse, the finding of a “halo” sign should prompt consideration of anaphylaxis, right-sided heart failure, pericardial effusion, fluid overload or changes to vascular permeability and sepsis.