Ultrasound Physics, Doppler Modalities, Echocardiography, and Nerve Locators — WACS Viva & Clinical Scenarios (Physics in Anaesthesia)

Exam-style ultrasound physics, doppler modalities, echocardiography, and nerve locators viva scenarios with examiner probes and model answers for ANAESTHESIA…

Scenarios covered

• SCENARIO 1: You are performing an ultrasound-guided internal jugular vein cannulation in an obese, critically ill patient. As you insert the needle, multiple equidistant parallel linear echoes appear beneath the anterior vessel wall, obscuring the lumen, and the needle tip is lost from view. Using ultrasound physics, explain the generation of reverberation and mirror-image artefacts, explain why ultrasound cannot penetrate bone or air, and describe how you physically adjust transducer frequency, gain, and beam angle to resolve the image.
• SCENARIO 2: A 35-year-old male requires an ultrasound-guided popliteal sciatic nerve block for ankle reconstruction. You have a high-frequency linear array transducer (10-15 MHz) and a low-frequency curvilinear transducer (2-5 MHz). Detail the physical trade-offs between transducer frequency, wave attenuation, depth of penetration, axial resolution, and lateral resolution, and justify which probe you will choose for this block.
• SCENARIO 3: During a focused transthoracic echocardiogram in a hypotensive intensive care patient, you place a pulsed-wave Doppler gate across the left ventricular outflow tract. The displayed spectral velocity waveform wraps around the baseline, displaying high velocities in the opposite direction (aliasing). State the Doppler equation, explain the Nyquist limit, and explain three physical adjustments you can make on the machine to eliminate aliasing.
• SCENARIO 4: You are performing an axillary brachial plexus block using an electrical peripheral nerve stimulator. Explain why the nerve stimulator must be a constant-current generator rather than a constant-voltage generator, define rheobase and chronaxie with reference to strength-duration curves for A-alpha and C-fibres, and explain the physical significance of eliciting a motor response at 0.3 mA versus 0.1 mA.