Point-of-Care Ultrasound
in Nephrology

POCUS in nephrology is built on three habits: match the probe to the question, optimize the image before interpreting it, and read artifact as information, not noise. Skipping any of these turns a confident bedside read into a confidently wrong one.^1,3

1.1 Physics in plain terms

Ultrasound is the trade between frequency (higher = better axial resolution, worse penetration) and depth. A 2–5 MHz curvilinear beam reaches the deep retroperitoneum and IVC; a 7–12 MHz linear beam resolves a 2 mm fistula vessel wall but stops in the subcutaneous tissue. Axial resolution is the smallest distance the probe can separate along the beam (limited by wavelength); lateral resolution is across the beam (limited by beam width — sharper at the focal zone). Attenuation is the loss of signal with depth; gain (and time-gain compensation, TGC) is how we restore it without amplifying noise.

1.2 Probe selection — three transducers, three jobs

1.3 Knobology — the five dials that matter at the bedside

• Depth — set so the structure of interest fills two-thirds of the screen.
• Gain & TGC — gain raises the whole image; TGC tunes near vs far. Aim for a fluid-filled vessel that is truly anechoic, not "dirty black."
• Focal zone — place at the depth of the target (sharpest lateral resolution there).
• Presets — abdominal for kidney/IVC; cardiac for focused echo (reverses default orientation marker); vascular for fistula.
• Doppler — color box small and tilted ≤ 60° to flow; pulsed-wave gate inside the vessel; lower PRF for slow venous signals, higher for arterial.

1.4 Artifact literacy — read what isn't really there

• Acoustic shadowing — hyperechoic surface (stone, calcification, rib) + black "void" deep to it. Diagnostic for a calculus or rib edge.
• Posterior enhancement — bright signal deep to a fluid-filled structure (cyst, full bladder). Distinguishes simple cyst from solid mass.
• Reverberation — repeating horizontal lines below a strong reflector. In the lung this is the normal A-line.
• Ring-down (B-line) — vertical laser-like line from the pleural line to the screen's bottom edge, erasing A-lines, moving with sliding. Marker of extravascular lung water (§4).
• Mirror artifact — a duplicate of liver/spleen across the diaphragm — a common pitfall when judging pleural effusion.

1.5 Governance — image archiving, infection control, scope

Every bedside study should be archivable: a representative still or short clip per window, labeled with patient identifier and indication. Probes are semi-critical when they contact intact mucosa (e.g. transvaginal — not used here) and non-critical for intact skin: low-level disinfectant wipe (manufacturer-approved) between every patient; high-level disinfection for sterile-field use (e.g. internal jugular cannulation with sterile sheath). Scope of practice is institutional — POCUS findings should be documented as focused (not "complete US"), and any uncertain finding triggers formal imaging.²

2.1 The four bedside questions

• Is there obstruction? (hydronephrosis — and is the bladder full?)
• Is this acute or chronic kidney disease? (size, cortex thickness, echogenicity)
• Is the bladder the problem? (post-void residual)
• Is a mass cystic or solid? (simple vs complex; when to escalate)

2.2 Acquisition

• Right kidney via the hepatic acoustic window: probe at the mid-axillary line, coronal orientation, marker cephalad. Use the liver as the sonographic "lens." Sweep through the kidney in long axis, then rotate 90° for short axis.
• Left kidney via the splenic window: more posterior and cephalad than the right (the spleen is smaller; bowel gas often obscures). Have the patient take a deep breath; roll into right lateral decubitus if needed.
• Bladder: suprapubic, transverse first to find the maximal AP and transverse dimensions, then sagittal for craniocaudal. Volume ≈ 0.52 × W × H × D (mL).

2.3 Interpretation & thresholds

2.4 Pitfalls & mimics

• Extrarenal pelvis — a pelvis that lies outside the renal sinus is normally a few mL of fluid and mimics mild hydronephrosis. Reassess after voiding.
• Parapelvic cysts — clustered, round, do not communicate with the ureter; look for the cyst wall vs branched calyceal anatomy.
• Full bladder / reflux — a distended bladder back-pressures the upper tract and falsely "creates" mild bilateral dilation. Rescan after catheter drainage.
• Decompressed obstruction — early or volume-depleted obstruction may show minimal dilation; bowel gas commonly limits the left kidney.
• Renal cell carcinoma — a solid renal lesion is never a POCUS diagnosis; refer for contrast-enhanced CT/MRI.

3.1 IVC — right atrial pressure estimate

The IVC is a low-pressure venous capacitance vessel that mirrors right atrial pressure (RAP) through its diameter and respiratory variation. Image the IVC 2–3 cm caudal to the cavo-atrial junction (or just caudal to the IVC–hepatic vein confluence) in long axis, with the probe in the subxiphoid window. Measure diameter at end-expiration and assess collapse with a sharp sniff.

Confounders. Mechanical ventilation (positive pleural pressure inverts the normal respiratory variation), raised intra-abdominal pressure (transmits to IVC), pulmonary hypertension, athletic physiology, and severe tricuspid regurgitation all distort the diameter–pressure relationship. Read the IVC as a gate, not the whole story.⁴

3.2 VEXUS — Venous Excess Ultrasound grading

VEXUS converts IVC plethora into a graded venous-congestion score by adding pulsed-wave Doppler of the hepatic vein, portal vein, and intrarenal vein. It tracks dynamically across hemodialysis ultrafiltration (ACUVEX cohort)⁵ and predicts worsening renal function in acute heart failure⁶ and the ICU AKI prevalence cohort.⁷

Component Doppler severity

Grade assignment

3.3 Cross-organ link

VEXUS is the sonographic face of cardiorenal syndrome (CRS) types 1 & 2, and of hepatic congestion in the same patient. The unifying frame: the kidney is an early victim of systemic venous congestion. When VEXUS is paired with lung ultrasound (§4) and a focused echo, the volume status of the AHF or HD patient is mapped end-to-end at the bedside.

4.1 Lung ultrasound — A-lines vs B-lines

Set the curvilinear (or phased) probe perpendicular to the chest wall in a longitudinal orientation so two rib shadows flank a bright pleural line ("bat sign"). The lung sits deep to the pleura.

• A-lines — horizontal lines parallel to and equidistant below the pleural line. Reverberation artifact = normal aerated lung or pneumothorax (if sliding is also absent).
• B-lines — vertical, hyperechoic, laser-like lines arising from the pleural line, extending to the bottom of the screen, erasing A-lines, moving with sliding. They mark interstitial fluid (extravascular lung water, EVLW).
• Quantification. ≥ 3 B-lines in a single intercostal field = positive zone. Sum positive zones across an 8-zone (4 per hemithorax) or 28-zone protocol for a semi-quantitative congestion score.

Also screen for

• Pleural effusion — anechoic stripe above the diaphragm with the "spine sign" (vertebrae visible above the diaphragm because of the fluid acoustic window).
• Pneumothorax — absent pleural sliding + absent B-lines + a "lung point" where sliding resumes. (Uncommon in pure nephrology workflow but important after access / biopsy.)

4.2 Focused cardiac POCUS

Four windows, four questions — none of them an echocardiographer's exam.

Pericardial effusion / tamponade physiology

Anechoic stripe surrounding the heart inside the bright pericardium. Tamponade is a clinical diagnosis, but POCUS supports it: a circumferential, often large effusion, with right-atrial systolic collapse, right-ventricular early-diastolic collapse, and a plethoric, non-collapsing IVC. In a uremic patient with hypotension, this is a stat-call sign.

5.1 AV access surveillance (linear probe)

Use the high-frequency linear probe across the fistula/graft in short axis first (round, anechoic lumen with thin echogenic wall) to locate the anastomosis, then long axis for length and flow direction. Add color Doppler (low PRF for venous outflow, high PRF for arterial inflow) and PW Doppler for peak velocities.

What to detect

• Juxta-anastomotic / outflow stenosis — > 50% diameter reduction or a focal > 2:1 peak-velocity ratio across the narrowing; visible aliasing on color Doppler.
• Thrombosis — non-compressible lumen, no color or PW signal, possibly echogenic intraluminal material.
• Aneurysm / pseudoaneurysm — focal saccular dilation; yin-yang color flow inside a pseudoaneurysm with a feeding "neck."
• Steal — high-volume access with distal arterial flow reversal, particularly in diabetic / proximal accesses (clinical + perfusion findings still primary).
• Pre-creation vascular mapping — arterial inflow (radial / brachial) calibre & patency; venous outflow (cephalic / basilic) calibre, patency, and depth.

5.2 Procedural guidance

• Central venous access — real-time ultrasound-guided cannulation of the internal jugular and femoral veins is now standard of care: pre-scan for patency, thrombus, and a compressible vein, then keep the needle tip in view through the wall. (Subclavian is anatomic, not US-guided.)
• Kidney biopsy — patient prone, breath-held expiration; target the lower-pole cortex; confirm no overlying bowel loop, large vessel, or pleural reflection. Real-time guidance (out-of-plane or in-plane) is now preferred to a single pre-scan "mark and stab."
• PD catheter siting — ultrasound-marked entry to avoid epigastric vessels and adhesions; intra-procedural check for the catheter tip in the pelvis.
• Difficult bladder catheterization — sonographic confirmation that the bladder is actually full before another pass; rules out the empty-bladder false-failure.

6.1 The five-point scan

In any undifferentiated AKI, suspected congestion, or pre-dialysis assessment, scan five sites in this order and record one finding per site:

1. Kidneys / bladder — hydronephrosis? PVR?
2. IVC — diameter and collapse → RAP tier.
3. Lungs — 8-zone B-line count.
4. Focused cardiac — LV "eyeball" EF, pericardial effusion, RV size.
5. VEXUS Doppler — hepatic, portal, intrarenal vein (only if IVC ≥ 2 cm).

6.2 Phenotype → action

6.3 An integrated example

A 68-year-old with HFrEF and CKD-G3b is admitted with worsening creatinine. Reflex instinct: "pre-renal — give fluids." Five-point scan: no hydronephrosis, IVC 2.6 cm with < 20% collapse, 4 B-lines per zone bilaterally in lower zones, mildly reduced LV with a moderately dilated RV, VEXUS Grade 3 (S-reversal in hepatic, portal pulsatility 60%, monophasic intrarenal). The action changes entirely: decongestion, not volume; SGLT2i continued; falling VEXUS at 48 h becomes the objective endpoint.

Documented training gap: in published surveys, only ~38% of nephrology fellows received POCUS education and only ~33% of those felt competent to perform it independently.¹⁰ This guide is a self-study scaffold; it does not replace supervised acquisition.

7.1 Tiered competency

1. Tier 1 — Image acquisition. Can the trainee obtain a diagnostic-quality image at each window? Direct-observation checklist per application.
2. Tier 2 — Interpretation. Can the trainee distinguish normal from abnormal and label findings against published thresholds (e.g. VEXUS grades, RAP tiers, rule of 6s)?
3. Tier 3 — Clinical integration. Can the trainee combine multi-organ findings into a phenotype and a guideline-aligned action — and articulate what would change it?

7.2 Image logbook

• Suggested minimum supervised studies per domain before independent use (institutional thresholds vary; ASN precourse offers a reasonable target).²
• Archive representative still images and short clips (with patient identifier, indication, and a one-line interpretation) for every supervised case.
• Quarterly portfolio review with a designated POCUS faculty mentor.

7.3 Assessment

• Entrustable tasks — directly observed acquisition + interpretation.
• Image-review quizzes — graded with annotated answer keys.
• Integrated case OSCEs — the trainee performs a five-point scan and proposes a phenotype + action.

7.4 Governance — a departmental POCUS committee

• Owns credentialing criteria and the privileging pathway.
• Runs QA image review (random audits of archived studies).
• Maintains the curriculum and the institutional protocol pack (probe disinfection, archiving, reporting language).
• Mirrors published ASN recommendations for nephrology POCUS programs.²

8.1 Quick-reference card

8.2 Figure manifest

9.1 Acronyms — physiology, syndromes & programs

9.2 VEXUS, IVC & venous Doppler vocabulary

9.3 Renal, bladder & obstruction terms

9.4 Sonographic physics & artifact terms

9.5 Lung & cardiac POCUS vocabulary

9.6 Access & procedural terms