Focused abdominal ultrasound assessment is increasingly being adapted for use in non-traumatised emergency patients as well as in the critical hospitalised patient. In people, the role of abdominal point-of-care ultrasonography (POCUS) in emergency and critical care settings has evolved to expedite management of non-traumatic surgical conditions and has potential to be equally beneficial in our veterinary patients. This lecture will focus on advanced veterinary abdominal POCUS techniques as diagnostic, monitoring, and procedural aids.

The original veterinary abdominal POCUS technique (abdominal focused assessment with sonography for trauma [AFAST]) involved placing the ultrasound transducer at four different anatomic landmarks; the diaphragmatic-hepatic view, the splenorenal view, the hepatorenal view and the cystocolic view. Gross abnormalities of organs such as the liver, kidneys, spleen, bladder, and uterus would also be assessed at these sites. These views can be adapted in order to individually tailor the POCUS technique to the clinical situation and your clinical question. For example, documentation of overt renal pelvic dilatation in a cat with renal colic and a newly documented azotaemia would raise the index of suspicion for ureteric obstruction, expediting the decision to pursue diagnostic abdominal ultrasonography.

In animals with ‘acute abdomens,’ particularly in the early stages, there may be no evidence of peritoneal fluid on initial inspection. Where index of suspicion for a surgical condition exists, alongside routine diagnostic imaging, repeat abdominal POCUS even within 1–2 hours would be recommended as free fluid may unveil itself. The ultrasound protocol may even be adapted to assess for the presence of a pneumoperitoneum as evidence of gastrointestinal tract perforation.

Pneumoperitoneum is characterised by an enhanced peritoneal stripe sign; a reverberation artefact that originates from the interface between the parietal peritoneum and adjacent peritoneal gas. This reverberation artefact extends distally, obliterating normally visible intra-abdominal structures. Accurate detection of pneumoperitoneum requires practice, however, and there may be a steep learning curve. Common pitfalls include mistaking reverberation artefacts from normal luminal intestinal gas, especially where intestinal walls are thin (e.g., colon), or misinterpretation of a thickened linea alba as a peritoneal stripe sign. Detection of the peritoneal stripe sign must also be interpreted in light of the patient’s history and physical examination findings; this is particularly important in animals that have had recent abdominal surgery. Pneumoperitoneum can persist post laparotomy for approximately 3 weeks but will tend to resolve within 2 weeks.

Abdominal POCUS may be useful for the early detection of small intestinal obstructions. Abdominal pain is a common complaint associated with gastrointestinal obstructions but abdominal palpation in such cases may be unrewarding. Placement of the transducer in the area of focal pain may identify findings compatible with small intestinal obstruction including segmental dilation of small intestines, and to-and-fro or whirling motions of luminal content. In people, abdominal POCUS performed by emergency clinicians has been found to be specific for the diagnosis of small intestinal obstruction but intestinal obstructions were commonly missed; when utilised, it should not be considered a diagnostic test, but a useful adjuvant to the physical examination.

Intussusceptions are commonly palpated as firm tubular structures. However, by palpation alone it may be difficult to differentiate them from faeces or foreign material. Ultrasonographically, however, intussusceptions have a characteristic cross-sectional appearance; a target-like lesion comprising of multiple hyperechoic and hypoechoic concentric rings with a hyperechoic centre, created by juxtaposition of the wall layers of the intussusceptum and intussuscipiens. Recent literature describes advanced ultrasound techniques being used to facilitate external manual reduction of the intussusception. This technique involves application of pressure to the intestine aborally to the intussusception with one hand to stabilise, and pushing of the intussusception orally with the other. Ultrasound can subsequently be used to confirm resolution of the intussusception.

Abdominal POCUS can also be used to assess gastrointestinal motility; aiding the detection of ileus, the monitoring of the prokinetic interventions you make, and the assessment of the patient’s tolerance of enteral nutrition.

Early enteral nutrition is recommended in many critical inpatients. This is often delivered via naso-oesophageal or nasogastric feeding tubes. The benefit of the latter is that gastric decompression can also be performed. Point-of care ultrasound can be used as a non-invasive method to confirm correct tube placement. The tube can be directly identified in the oesophageal lumen or stomach. Ultrasound of the cervical oesophagus and stomach performed by emergency veterinarian has high specificity for correct feeding tube placement and has the advantage of being possible at the patient side. However, this high specificity is associated with low sensitivity for tube detection, meaning alternative tests may be required to confirm placement if the tube is not readily identified.

Proficiency in POCUS is also of crucial importance for sampling and instrumentation purposes. It is common that a small volume of peritoneal effusion carries huge diagnostic potential, and sampling of that effusion should be ultrasound guided.

Sampling can be performed both in- and out-of-plane, but is perhaps most frequently performed in-plane. These ultrasound guided techniques are also extremely useful for ultrasound guided abdominal drain placement; for example, when harvesting haemorrhagic effusions for autotransfusion purposes, or securing urinary diversion in cases with urogenital tract rupture.

Point-of-care abdominal ultrasound beyond the four-point scan shows promise in ensuring prompt diagnosis and management of abdominal emergencies and in improving the care of critical patients. Although POCUS plays an important role assisting the detection of life-threatening abnormalities, it does not replace a complete abdominal ultrasound examination. Further research is required to determine the extent of the utility of POCUS in the management of small animal emergency and critical patients.

References

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