1. Introduction, Scope & Technique Standardization

1.1 Core Principles

POCUS is acquisition, interpretation, and immediate clinical integration by the treating clinician. It is question-driven and is not a substitute for formal echocardiography, CT, MRI, CTA, full duplex, specialist ultrasound, or ophthalmology when definitive assessment is required.

The reference uses a two-layer model:

  • Layer 1 — parameter: discrete numbers, scores, counts, diameters, velocities, or grades that answer “how much?”

  • Layer 2 — protocol: a structured bedside sequence that answers “why is this happening, and what should I do now?”

Basic POCUS, advanced POCUS, and ultrasound-assisted physical examination are related but not interchangeable. POCUS is a screening, triage, and management-adjunct tool. A negative POCUS does not exclude disease, and a positive POCUS does not replace definitive imaging when clinical probability remains high.

Number-based data tell you how much. Protocol integration tells you why and what to do. Thresholds are operating points, not universal laws. Interpretation depends on the clinical question, patient context, ventilation status, rhythm, valve disease, body habitus, image quality, and local protocol.

In v5, numeric cutoffs are embedded in the relevant main text. There is no separate consolidated cutoff quick table.

1.2 Universal Acquisition Rules

Probe selection

  • High-frequency linear 12–18 MHz: lung, ONSD, ophthalmic, procedural, superficial airway structures, cricothyroid membrane, tracheal air-mucosa interface.

  • Phased-array 1–5 MHz: cardiac, subcostal IVC, parasternal/suprasternal aortic views.

  • Curvilinear 2–5 MHz: abdominal, eFAST, vascular, subcostal IVC, deeper airway views, hyomental distance.

  • Airway adjunct: linear probe approximately 5–14 MHz for superficial structures; curvilinear probe approximately 4–10 MHz for deeper tongue, submandibular, and hyomental measurements.

  • Obstetric/gynaecologic POCUS: transabdominal curvilinear for free fluid, uterine size, gross adnexal pathology; transvaginal endocavitary probe when available for early pregnancy, cervical length, and placental localization.

  • Aortic POCUS: curvilinear for abdominal aorta; phased-array for parasternal, suprasternal, subxiphoid thoracic aorta.

Gain — Set mid-gain. Eliminate near-field noise and far-field dropout. Depth — Place the structure of interest in the lower third of the screen. Focus — Place the focus at the depth of the target. Pressure — Minimal probe pressure. Zero pressure when an open globe, fragile target, or procedural compression artifact is suspected.

Position

  • Cardiac: left lateral decubitus when feasible.

  • Lung: supine or seated.

  • Abdominal: supine.

  • IVC: supine, neutral head.

  • Airway: supine by default; neck neutral, ramped, or hyperextended depending on question and patient stability.

  • Obstetric: supine, lateral tilt when pregnant; transvaginal scanning with empty bladder when appropriate.

Respiratory phase

  • IVC diameter maximum, Dmax, at end-expiration.

  • IVC diameter minimum, Dmin, during quiet inspiration in spontaneous breathing.

  • Free breathing for lung B-lines.

  • Apnea for some cardiac M-mode measurements.

  • Airway measurements should be standardized for neck position, respiration, swallowing, and phonation.

Number of cycles

  • IVC: at least 3 cardiac cycles and at least 2 respiratory cycles.

  • LVOT VTI: average 3–5 beats.

  • TCD: stable acquisition over 5–10 respiratory cycles.

  • ONSD: 3–5 measurements per eye, then mean.

Laterality — Compare with the contralateral side where applicable. Image quality — Record as good, moderate, or poor. Image quality limits interpretation and should be documented.

Note on IVC probe selection — In adults, subcostal IVC assessment is not reliably performed with a high-frequency linear probe because adequate depth penetration to the IVC–right atrium junction is required. A low-frequency phased-array or curvilinear probe is preferred. A linear probe may occasionally visualize a superficial IVC segment in a very thin adult, but this is not the standard routine approach for adult IVC diameter or collapsibility/distensibility.

1.3 Minimal Practical Views by Domain

Cardiac basic/FoCUS — Subxiphoid; parasternal long-axis; parasternal short-axis at mid-papillary level (default rapid LV size/global systolic function view); apical 4-chamber; subcostal 4-chamber; optional apical 5-chamber; aortic valve short-axis added when aortic valve, LVOT, RVOT, or aortic root question applies.

Lung basic — Bilateral upper anterior windows; bilateral lower anterior windows at 4th/5th intercostal spaces midclavicular; posterior axillary PLAPS windows at 8th/9th intercostal spaces when clinically relevant.

IVC — Subxiphoid sagittal, 1–3 cm caudal to the hepatic vein–IVC junction.

Abdominal eFAST — RUQ/Morison; LUQ/splenorenal; pelvis; suprasplenic.

Abdominal extended — eFAST views plus bilateral longitudinal kidneys, abdominal aorta sagittal and transverse, spleen longitudinal.

Vascular DVT — Common femoral, superficial femoral, and popliteal veins in longitudinal and transverse compression planes.

Ophthalmic posterior B-scan — Globe → lens → vitreous → retina → optic nerve/ONSD → orbit.

Adult TCD — Bilateral temporal MCA/ACA/PCA windows; orbital ICA windows; foramen magnum VA/BA window.

Neonatal cranial US — Coronal; axial/lateral; posterior fossa via anterior fontanelle.

Airway POCUS — Suprahyoid; thyrohyoid; thyroid/tracheal; cricothyroid; suprasternal. The number of views is tailored to the clinical question. A focused ETT confirmation scan may require only thyroid/tracheal and suprasternal views. A difficult-airway scan may require multiple views. A front-of-neck access scan prioritizes the cricothyroid view.

Obstetric/gynaecologic focused POCUS — Transabdominal pelvic views for free fluid, uterine size, gross adnexal pathology; transvaginal views when feasible for early IUP, fetal cardiac activity, cervical length, and placental relationship to the internal os.

Aorta-focused POCUS — Parasternal long-axis; suprasternal notch; superior intercostal/superior parasternal; small-scale parasternal descending aorta; subxiphoid thoracoabdominal aorta; abdominal midline longitudinal and transverse.

PSAX specification note — “PSAX” is not a single view but a family of short-axis levels: aortic valve, mitral valve/basal, mid-papillary, and apical. In basic POCUS, the level must be stated. For rapid LV assessment, the mid-papillary short-axis view is the standard reference. The aortic valve PSAX is a separate problem-focused view for aortic valve, LVOT, RVOT, or aortic root questions. Pitfall (v4 §16, distributed): Using aortic valve PSAX as the default basic POCUS short-axis view for LV assessment; mid-papillary PSAX is standard for rapid LV evaluation.

1.4 Universal Pitfalls

  • POCUS is operator-dependent. Know your limits. Call for help.

  • POCUS does not replace formal imaging. Negative POCUS does not exclude disease.

  • Do not overinterpret a single parameter. Multimodal integration is required.

  • Context matters: trauma, renal failure, ascites, sepsis, post-operative state.

  • Measurements must be reproducible: consistent planes, perpendicular measurement, documented level.

  • Do not interrupt resuscitation unnecessarily. Cardiac arrest scanning should be brief and protocol-driven.

  • Basic POCUS is not advanced POCUS. Do not attempt advanced assessments without training.

  • POCUS in the wrong hands can be dangerous.

References:

(none)