Paediatric ultrasound and point of care ultrasound — formative SAQs

SAQ 1 (10 marks)

An 8-month-old presents to the emergency department with a 24-hour history of fever, grunting, and increased work of breathing. The team plans a bedside point-of-care lung ultrasound rather than an immediate chest radiograph. [1] [4]

1. Outline how you will choose the probe and set the image-quality controls before scanning this infant. (3) [1]
2. Describe the lung ultrasound artefact repertoire you will read, and the sign that would confirm a community-acquired pneumonia at the bedside. (4) [3] [4]
3. State the systematic scanning protocol you will run, and explain how you will avoid confusing artefact with pathology. (3) [2] [3]

Model answer — SAQ 1

(1) Probe and image-quality controls (3). I choose a high-frequency linear probe for this small infant, because the structures of interest are superficial and higher frequency buys resolution; the thin paediatric chest wall makes the pleura easy to reach. Before scanning I set four controls. Depth is set so the pleural line and the near lung fill the screen rather than sitting in the top quarter. Gain is adjusted so fluid is black and tissue is mid-grey, avoiding over-gain that creates false echoes. Focus is placed at the depth of the pleural line. I confirm the probe marker orientation so I know left from right on the screen, and I apply warm gel generously to exclude any air layer, which would reflect the beam and ruin the image. [1]

(2) Artefact repertoire and the pneumonia sign (4). I read five things at each zone. A-lines are horizontal, equally spaced bright lines repeating below the pleural line and indicate a dry, air-filled lung. B-lines are vertical, well-defined comet-tail streaks arising from the pleural line, spreading to the bottom of the screen without fading, and moving with sliding; three or more in a single intercostal space indicate alveolar-interstitial syndrome (a wet lung). Lung sliding is the to-and-fro shimmer of the two pleural layers and rules out a pneumothorax at that point. The lung point, where sliding meets absent sliding, is highly specific for a partial pneumothorax. A pleural effusion shows the quad sign and sinusoid sign. The sign that confirms pneumonia is a subpleural, tissue-like, hypoechoic consolidation resembling liver (hepatisation) that reaches the pleura, threaded with dynamic air bronchograms that move with respiration; a 2024 meta-analysis found lung ultrasound as accurate as or more accurate than the chest radiograph for paediatric community-acquired pneumonia, with no ionising radiation. [3] [4]

(3) Protocol and avoiding artefact-as-pathology (3). I run the eight-zone bilateral scan: each hemithorax is divided by the anterior and posterior axillary lines and a horizontal line at the nipple into anterior-upper, anterior-lower, lateral and posterior zones, and both sides are scanned in the same order. I read each zone for A-lines, B-lines, sliding, consolidation, and effusion and interpret the pattern with the BLUE protocol. To avoid artefact-as-pathology I ensure good gel contact and a perpendicular probe, because an oblique probe or a poor gel layer can generate spurious B-lines; I scan the contralateral side as an internal control because asymmetry is more informative than any single image; and I rescan a window in two planes before declaring a finding such as a lung point. [2] [3]

SAQ 2 (10 marks)

Two children arrive in quick succession. The first is a 6-year-old hemodynamically unstable victim of a high-speed motor vehicle collision, in whom a focused assessment with sonography in trauma (FAST) shows anechoic free fluid in Morison's pouch. The second is a 4-week-old with projectile non-bilious vomiting in whom the bedside ultrasound shows a thickened pylorus. [7] [9]

1. For the traumatised child, state what the FAST scan has shown, the single most important limitation of the FAST in children, and the correct next step. (4) [7]
2. For the infant, describe the sonographic finding and the conventional measurement cut-offs for hypertrophic pyloric stenosis, and the caveat about those cut-offs. (3) [9]
3. Separately, a dehydrated 3-year-old with no palpable veins needs cannulation. State the evidence for ultrasound-guided peripheral cannulation and the procedural safeguards that must still apply. (3) [10]

Model answer — SAQ 2

(1) FAST finding, limitation, and next step (4). The FAST has shown free intraperitoneal fluid in Morison's pouch — a positive scan. The single most important limitation in children is that the FAST is highly specific but has a low and variable sensitivity, because the paediatric liver and spleen frequently bleed into their own parenchyma rather than into the peritoneal cavity, so free fluid is often absent even in a significant injury. The correct next step in this haemodynamically unstable child with a positive FAST is urgent surgical or retrieval intervention, not further imaging; the child goes to theatre or to the retrieval team. I would not repeat the scan, observe, or arrange computed tomography first, because the child's circulation cannot wait, and computed tomography is reserved for the stable child. [7]

(2) Pyloric finding, cut-offs, and the caveat (3). The finding is a thickened pyloric muscle seen as a hypoechoic ring on transverse view with an elongated channel on longitudinal view. The conventional cut-offs are a pyloric muscle wall thickness around 3 to 4 mm and a channel length around 15 to 17 mm. The caveat is that these cut-offs sit at the boundary in small infants and are under active revision: an Australasian consensus argues the criteria should change for contemporary infant size, so I measure in the correct longitudinal mid-plane with a relaxed stomach and reinterpret the numbers against the infant's age and size rather than applying a single threshold blindly. [9]

(3) Ultrasound-guided cannulation evidence and safeguards (3). The EPIC superiority randomised clinical trial established higher first-attempt success for ultrasound-guided peripheral intravenous catheter insertion than standard insertion in difficult access. I would use a high-frequency linear probe to identify a patent vessel, confirm it is compressible and non-pulsatile in the transverse view, and guide the needle in real time. The safeguards that must still apply are sterile technique, visualisation of the needle tip throughout, secure fixation of the catheter, and confirmation of venous placement, because ultrasound guidance does not prevent dislodgement, extravasation, or infection on its own. [10]