Nitrous oxide supports combustion just as oxygen does — it is not an inert diluent.
Prevention: FiO₂ <0.30 where possible, avoid nitrous, let alcohol prep dry fully, wet swabs around the field, and warn the surgeon before using diathermy near the airway.
Airway fire drill: stop gases and remove the tracheal tube, flood the field with saline, then ventilate with air, re-intubate and bronchoscope to assess damage.
Lasers
LASER = Light Amplification by Stimulated Emission of Radiation. The beam is monochromatic, coherent and collimated — the three defining properties.
Types: CO₂ (superficial, absorbed by water) · Nd:YAG (deeper penetration) · argon (absorbed by haemoglobin, used in retinal work).
Hazards: eye injury (retinal for Nd:YAG and argon, corneal for CO₂), airway fire, plume containing viable particles and viral DNA.
Precautions: wavelength-specific goggles for everyone, warning signs and covered windows, matt instruments, laser-resistant tracheal tube with a saline-filled cuff (often dyed), and smoke evacuation.
Ultrasound physics
Generated by the piezoelectric effect — a crystal deforms in an electric field and, in reverse, generates voltage when struck by a returning echo.
c = f λ. Velocity in soft tissue is about 1540 m/s. Frequencies used are 2–15 MHz.
Higher frequency = better resolution but less penetration. Linear high-frequency probes (10–15 MHz) for superficial blocks and vascular access; curvilinear low-frequency (2–5 MHz) for deep structures such as the sciatic nerve.
Acoustic impedance = density × velocity. Reflection occurs at interfaces where impedance differs — which is why air (huge mismatch) blocks the image and why gel is needed.
Artefacts: acoustic shadowing (behind bone or calcification), posterior enhancement (behind fluid), reverberation, and mirror image.
Doppler: Δf = 2 f₀ v cos θ / c. Accuracy falls as the angle of insonation increases — aim for a small angle; at 90° there is no Doppler shift at all.
Defibrillators & diathermy
Biphasic defibrillation achieves a given success rate at lower energy than monophasic, with less myocardial injury — 150–200 J first shock.
The stored energy is in a capacitor; an inductor shapes the waveform to lengthen the pulse and reduce peak current.
Synchronised cardioversion delivers the shock on the R wave, avoiding the vulnerable period of the T wave (which could induce VF). Use for organised rhythms with a pulse.
Diathermy: 300 kHz–3 MHz, above the threshold for depolarising excitable tissue. Cutting uses a continuous low-voltage waveform; coagulation uses pulsed high-voltage bursts. Bipolar passes current between the forceps tips only — no plate needed, and safe with pacemakers.
Burns occur if the return plate is small, poorly applied, or if current finds an alternative path — hence plate placement over well-perfused muscle, close to the site, away from bony prominences and metal implants.
Scavenging & MRI
Scavenging
Five components: collecting system, transfer tubing (30 mm, deliberately non-interchangeable), receiving system with reservoir, disposal, and pressure-relief valves.
Active systems use a pump with a low-pressure relief valve to prevent excessive negative pressure on the patient's circuit; passive rely on the patient's expiration.
Static magnetic field — the projectile hazard, and the dominant risk. Field strength 1.5–3 T typically.
Gradient fields — acoustic noise up to 120 dB (ear protection needed) and peripheral nerve stimulation.
Radiofrequency — tissue heating and burns from looped cables.
Zones 1–4, with zone 4 the scanner room. All equipment must be MRI-conditional. Quench — boiling off of liquid helium — risks asphyxiation and requires evacuation.
Practical issues: remote monitoring, long breathing circuits and infusion lines, and difficult access to the patient.