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Basics of Ultrasonography (USG) and Its Role in Anaesthesia

MD Anaesthesia PG Exam Answer

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PART 1: BASICS OF ULTRASONOGRAPHY

Definition and Principle

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Physics of Sound Waves

V = f × λ Velocity = Frequency × Wavelength

• Velocity of sound in soft tissue = 1,540 m/s (assumed constant by the machine)
• This is used to calculate depth: deeper structures have longer echo return times

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Piezoelectric Effect

• Transducers contain piezoelectric elements (PZEs) - crystals or crystalline ceramics
• When alternating current is applied, they expand and contract, generating sound waves (compression and rarefaction)
• When returning echoes hit the PZE, it generates an electrical signal - reverse piezoelectric effect
• Each PZE emits brief pulses (few microseconds) then "listens" for the reflected signal

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Interaction of Sound with Tissues

1. Reflection

• Occurs at interfaces between media of differing acoustic impedance
• Greater impedance difference → greater reflection → brighter image
• Bone, metal, calcifications (high impedance): most sound reflected back → hyperechoic (bright) with acoustic shadow beyond
• Blood, fluid (homogeneous, no impedance change): no reflection → anechoic/echolucent (black)

2. Refraction

• Bending of sound beam at tissue interfaces (like light through a lens)

3. Scattering

• Occurs when sound meets structures smaller than its wavelength
• Creates "speckle" pattern - characteristic tissue texture

4. Attenuation

• Absorption of sound energy as it travels through tissue
• Higher frequency → greater attenuation → less penetration
• Lower frequency → greater penetration but less resolution

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Types of Transducers

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Image Resolution

1. Axial Resolution

• Ability to distinguish two structures along the scan line (depth direction)
• = half the wavelength of the sound used
• A 5-MHz probe: wavelength ~0.3 mm → axial resolution ~0.3 mm
• A 10-MHz probe: wavelength ~0.15 mm → axial resolution ~0.15 mm
• Higher frequency = better axial resolution

2. Lateral Resolution

• Ability to distinguish two structures side by side (perpendicular to scan line)
• Lateral resolution ≈ depth/50
• At 5 cm depth → ~1 mm; at 10 cm depth → ~2 mm
• Worsens with depth; improved by focal zone adjustment

3. Elevational (Slice Thickness) Resolution

• The third dimension perpendicular to the sector scan
• Elevational resolution ≈ depth/30

4. Temporal Resolution

• Frame rate (frames per second, fps)
• Phased array transducers: 96-256 scan lines per frame

• 25 fps appears as smooth motion to the eye

• Determined by sector width and sector depth (narrower/shallower = higher frame rate)

Key Trade-off: High frequency = better resolution BUT less penetration

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Imaging Modes

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Doppler Principle

• Object moving toward transducer → higher reflected frequency
• Object moving away from transducer → lower reflected frequency
• Frequency shift → velocity of blood flow calculated

Types of Doppler:

1. Pulsed Wave (PW) Doppler - measures velocity at a specific point; limited by aliasing at high velocities (Nyquist limit)
2. Continuous Wave (CW) Doppler - measures all velocities along the beam; no aliasing; cannot localize
3. Colour Flow Doppler - colour map of flow velocity superimposed on 2D image (Red = toward, Blue = away - "BART": Blue Away, Red Toward)
4. Tissue Doppler Imaging (TDI) - measures myocardial wall velocity; used for diastolic function

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Ultrasound Artifacts

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PART 2: USG IN ANAESTHESIA

Overview

1. Regional anaesthesia (nerve blocks)
2. Vascular access
3. Perioperative echocardiography (TTE/TEE)
4. Point-of-care ultrasound (POCUS)
5. Airway management
6. Critical care monitoring

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1. ULTRASOUND-GUIDED REGIONAL ANAESTHESIA

• Direct visualization of nerves, vessels, and surrounding structures
• Visualization of local anaesthetic (LA) spread in real time
• Smaller volumes of LA required (reduced systemic toxicity risk)
• Higher success rates with faster onset
• Fewer needle passes and reduced patient discomfort
• Avoidance of vascular puncture
• Reduced risk of pneumothorax (for shoulder/thoracic blocks)
• Better safety in anticoagulated patients

Common Nerve Blocks with USG Guidance:

• High-frequency linear probe at C5-C6 level
• Plexus appears as hypoechoic "stoplight sign" between anterior and middle scalene muscles
• In-plane or out-of-plane needle approach
• 10-15 mL LA; volumes as low as 5 mL may avoid diaphragmatic paresis
• Use colour Doppler to identify vascular structures
• Side effects: ipsilateral phrenic nerve block (100% at C6 level) → diaphragmatic paresis

• Brachial plexus at distal trunk/proximal division level - most compact point
• High-frequency linear transducer just proximal to supraclavicular fossa
• Plexus appears as a "cluster of grapes" lateral to subclavian artery
• Subclavian artery, pleura, and first rib all visualized - safety advantage
• Continuous visualization of needle tip mandatory
• Risk: pneumothorax (reduced with USG)

• Lateral and posterior cords of brachial plexus visualized around axillary artery

• Terminal nerves (median, ulnar, radial, musculocutaneous) identified around axillary artery
• Best block for hand and forearm surgery

• Femoral nerve lateral to femoral artery, beneath fascia iliaca
• High value for hip fracture analgesia

• Multiple approaches: popliteal, subgluteal
• Large hypoechoic oval nerve

• Visualized with linear or curvilinear probe

• TAP (Transversus Abdominis Plane), Rectus sheath, Serratus anterior plane

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2. VASCULAR ACCESS

• USG guidance for internal jugular vein (IJV) cannulation is now standard of care
• Identifies IJV (compressible, oval) vs. carotid artery (pulsatile, round, non-compressible)
• Reduces arterial puncture, pneumothorax, and number of attempts
• Short-axis (transverse) view: best for confirming needle tip in vessel lumen
• Long-axis (longitudinal) view: better for in-plane needle visualization

• Guides cannulation of difficult veins (obese, oedematous patients)

• Visualizes radial or femoral artery
• Reduces haematoma formation

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3. PERIOPERATIVE ECHOCARDIOGRAPHY

a) Transoesophageal Echocardiography (TEE)

• Cardiac and major vascular surgery
• Unexplained haemodynamic instability during any surgery
• Assessment of volume status
• Evaluation of new wall motion abnormalities (ischaemia)
• Assessment of valvular function

• Comprehensive TEE: 28 views
• Basic TEE: 11 views
• Views organized by: midesophageal (ME), transgastric (TG), and upper esophageal (UE) positions

• Advance/withdraw (depth)
• Rotate left/right (anteflexion/retroflexion)
• Rotate multiplane (0° to 180°)
• Turn left/right

b) Transthoracic Echocardiography (TTE)

• Simplified, point-of-care examination
• Qualitative assessment (yes/no format):
  • Is there pericardial effusion/tamponade?
  • Is LV function severely reduced?
  • Is there right heart strain?
  • Is the patient hypovolaemic?
• Not a substitute for comprehensive echo

• FoCUS: specific questions, qualitative, yes/no format, decision support
• Limited TTE: broader scope, quantitative, requires advanced training

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4. POINT-OF-CARE ULTRASOUND (POCUS)

Cardiac POCUS (FoCUS)

• LV/RV function, pericardial effusion, volume status, tamponade

Pulmonary POCUS

• Pneumothorax: Loss of lung sliding + absent B-lines + "stratosphere sign" on M-mode
• Pleural effusion: Anechoic space above diaphragm
• Pulmonary oedema/ARDS: B-lines (comet tails from thickened interlobular septa)
• Consolidation: Tissue-like echogenicity with air bronchograms

Abdominal POCUS

• FAST (Focused Assessment with Sonography in Trauma): free fluid in peritoneum
• Gastric content assessment (gastric POCUS) - important in anaesthesia
  • Empty stomach: flat antrum
  • Fluid content: fluid-filled antrum (volume estimated)
  • Solid content: "frosted glass" appearance

Airway POCUS

• Confirm endotracheal intubation: tracheal ring visualization, bilateral lung sliding
• Cricothyroid membrane identification for emergency airway
• Assessment of subglottic diameter in paediatric patients

Ocular POCUS

• Optic nerve sheath diameter (ONSD) >5 mm suggests raised intracranial pressure

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5. AIRWAY MANAGEMENT

• Identify cricothyroid membrane before difficult airway management
• Confirm ETT position: bilateral lung sliding (vs. endobronchial - unilateral)
• Detect oesophageal intubation: bilateral lung sliding absent, "double trachea sign"
• Subglottic stenosis assessment

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6. FLUID ASSESSMENT & HAEMODYNAMIC MONITORING

• IVC diameter and collapsibility: IVC <2 cm with >50% collapse suggests hypovolaemia
• Stroke volume and cardiac output via Doppler VTI (velocity time integral) at LVOT
• Passive Leg Raise (PLR) + echo: dynamic preload assessment
• Left ventricular function: Visual estimation (eyeballing) of EF

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Summary Table: USG Applications in Anaesthesia

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Key Exam Points (High-Yield)

1. Frequency trade-off: High freq (>10 MHz) = better resolution, less penetration (superficial); Low freq (<5 MHz) = deep structures, less resolution
2. Axial resolution = half the pulse wavelength; improved by higher frequency
3. Lateral resolution worsens with depth = depth/50
4. Acoustic impedance difference determines reflection (metal/bone = hyperechoic + shadow; fluid = anechoic)
5. Doppler alignment must be <20° to flow direction
6. BART mnemonic for colour Doppler: Blue Away, Red Toward
7. FoCUS vs comprehensive echo: FoCUS is qualitative, bedside, yes/no; comprehensive is quantitative, full training required
8. Nyquist limit: PW Doppler aliases at high velocities; CW Doppler has no alias limit
9. Nerve appearance on USG: nerves appear as honeycomb/fascicular pattern (hypoechoic fascicles in hyperechoic connective tissue) - "US appearance of nerve = bunch of grapes or honeycomb"
10. Lung sliding (seashore sign on M-mode) confirms pleural apposition; absent in pneumothorax

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• Barash's Clinical Anesthesia, 9e, Chapter 27 - Perioperative Echocardiography and POCUS
• Miller's Anesthesia, 10e, Chapter 42 - Peripheral Nerve Blocks