High-Frequency Currents: SWD & MWD | Practice MCQ

High-Frequency Currents: SWD & MWD Masterclass

💡 The Core Concept: Diathermy literally means "heating through." Unlike hot packs that heat the skin via conduction, Diathermy uses high-frequency electromagnetic waves to generate heat deep inside the tissues via conversion.

1. The Technical Arsenal (Parameters)

Memorizing these numbers is non-negotiable for competitive exams like AIIMS and JIPMER.

Parameter	Short Wave (SWD)	Microwave (MWD)
Frequency	27.12 MHz (Standard)	2450 MHz (Standard)
Wavelength	11 meters	12.25 cm
Mechanism	Electric/Magnetic Fields	Dipole Rotation
Penetration	Deep (3-5 cm)	Semi-superficial

2. Application Methods & Physics

A. Short Wave Diathermy (SWD)

There are two methods, and they heat different tissues. This is a frequent exam question.

• Capacitor Field (Electric Field):
  • Uses 2 plates (pads). Patient is part of the circuit.
  • Exam Trap: It heats FAT > Muscle. Because fat offers resistance, heat concentrates there.
  • Contra-planar: Deepest heating (plates on opposite sides).
• Inductor Field (Magnetic Field):
  • Uses a Drum (Monode) or Cable.
  • Produces Eddy Currents.
  • Exam Trap: It heats MUSCLE > Fat. Best for vascular tissues.

B. Microwave Diathermy (MWD)

• Mechanism: Dipole Rotation (Water molecules rotate causing friction).
• The "Hot Spot" Risk: Microwaves reflect off bone surfaces. The incoming wave and reflected wave collide to form a Standing Wave. This causes intense heat at the bone-muscle interface (Periosteal Burn).

⚠️ Absolute Contraindications

• Metal Implants: (Steel/Titanium) concentrates the field → Severe Internal Burns.
• Pacemakers: Can cause cardiac arrest. Maintain >3 meters distance.
• Pregnancy: Teratogenic risks.
• Eyes (MWD Specific): Causes Cataracts because the lens is avascular and cannot cool down.
• Wet Skin/Sweat: Water concentrates the current → Scalding burns.

🏆 AIIMS Exam "Golden Points"

• Tuning: The patient circuit must be in Resonance with the machine circuit for treatment to work.
• Inverse Square Law: If you double the distance of the lamp/emitter, intensity drops to 1/4th.
• Spacing: In Capacitor SWD, proper air gap allows the field to diverge for uniform heating. Too close = Fat burn.

📝 20 Important MCQs (Practice Now)

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Q1. The standard frequency and wavelength for Short Wave Diathermy (SWD) is:

A. 2450 MHz, 12.25 cm B. 27.12 MHz, 11 meters C. 13.56 MHz, 22 cm D. 40.68 MHz, 7.5 meters

Rationale: 27.12 MHz is the standard frequency allocated for SWD, which corresponds to an 11-meter wavelength. 2450 MHz is for Microwave.

Q2. Which tissue is at greatest risk of overheating when using the Capacitor Field (Pad) method?

A. Muscle B. Bone C. Subcutaneous Fat D. Ligaments

Rationale: In the capacitor field, tissues are in a series circuit. Fat acts as a resistor (insulator). According to Joule's law, heat is produced where resistance is highest.

Q3. What creates "Eddy Currents" in the tissue during Inductothermy?

A. Electric Field B. Magnetic Field C. Sound Waves D. Radiation

Rationale: The cable or drum applicator creates a magnetic field perpendicular to the coil, which induces circular "Eddy currents" in conductive tissues like muscle and blood.

Q4. Why is MWD absolutely contraindicated near the eyes?

A. Causes Glaucoma B. Causes Cataracts C. Causes Myopia D. Causes Dry Eye

Rationale: The lens of the eye is avascular (no blood supply) and cannot dissipate heat. Heating leads to protein coagulation and opacity (Cataracts).

Q5. Metal implants are a contraindication for SWD because:

A. Metal blocks the waves B. Metal rusts inside the body C. Field concentrates causing burns D. Metal becomes magnetic

Rationale: Electromagnetic lines of force concentrate on highly conductive materials like metal, leading to extreme focal heating and severe internal burns.

Q6. The formation of "Standing Waves" leading to periosteal burns is a specific risk of:

A. Ultrasound B. Inductive SWD C. Microwave Diathermy D. IRR

Rationale: Microwaves are reflected at the bone-tissue interface. The superimposition of incident and reflected waves creates a hot spot (standing wave) at the periosteum.

Q7. Which method is best for treating Pelvic Inflammatory Disease (PID)?

A. Co-planar SWD B. Cross-fire SWD C. Monode D. Ultrasound

Rationale: The Cross-fire technique (treating half time in one direction, half in another) concentrates heat in deep pelvic organs while minimizing heat in superficial tissues.

Q8. What is the effect of Pulsed SWD (PSWD)?

A. Vigorous heating B. Muscle contraction C. Non-thermal / Cellular repair D. Nerve block

Rationale: PSWD delivers energy in bursts with long rest periods, allowing heat to dissipate. It is used for acute conditions to promote healing without heating.

Q9. To test if the SWD machine is outputting energy, which tool is used?

A. Thermometer B. Galvanometer C. Neon Tube D. Voltmeter

Rationale: A small neon gas tube glows when brought into the electromagnetic field of the SWD cables, confirming output.

Q10. The "Dipole Rotation" mechanism is associated with:

A. MWD B. SWD Inductive C. Ultrasound D. IFT

Rationale: Microwaves cause polar molecules (like water) to rotate millions of times a second to align with the oscillating field, generating frictional heat.

Q11. Which is the deepest heating modality among the following?

A. Short Wave Diathermy B. Microwave Diathermy C. Hydrocollator Pack D. Infrared Radiation

Rationale: SWD has the longest wavelength (11m) among electromagnetic heating agents, allowing the deepest penetration (up to 5cm).

Q12. In SWD, if the electrodes are placed too close to the skin, what happens?

A. Superficial heating increases (Burn risk) B. Deep heating increases C. Field cancels out D. Machine turns off

Rationale: Proper spacing allows the electric field to diverge. If too close, the field lines are concentrated superficially, causing excessive heating of skin and fat.

Q13. What is the minimum distance to maintain from a Pacemaker during SWD?

A. 1 meter B. 3 meters (approx 10 feet) C. 30 cm D. No restriction

Rationale: SWD creates strong electromagnetic interference that can disrupt the pacing signal or damage the pacemaker circuit. 3 meters is the standard safe distance.

Q14. The "Edge Effect" in Capacitor SWD is caused by:

A. Electrodes too far apart B. Electrodes too close together C. Using unequal electrode sizes D. High frequency

Rationale: If plates are too close edge-to-edge, the current finds the path of least resistance through the air between the edges rather than going through the patient.

Q15. If you double the distance of the MWD emitter from the skin, intensity becomes:

A. 1/2 B. 1/4 C. Double D. 1/8

Rationale: Inverse Square Law: Intensity is inversely proportional to the square of distance (1/d^2).

Q16. Which frequency is used for home-use or portable SWD devices (less common)?

A. 40.68 MHz B. 27.12 MHz C. 915 MHz D. 1 MHz

Rationale: 27.12 MHz is the standard medical frequency for almost all clinical and portable SWD devices.

Q17. Which SWD applicator is best for treating the quadriceps muscle belly?

A. Capacitor Plates (Co-planar) B. Inductive Drum/Cable C. MWD Emitter D. IR Lamp

Rationale: Inductive applicators heat low-resistance tissues (high water/electrolyte) like muscle and blood preferentially over fat.

Q18. Why must the treatment couch be wooden/plastic during SWD?

A. To prevent earthing/shock B. To keep patient warm C. It is cheaper D. No specific reason

Rationale: Metal beds attract the electromagnetic field and can ground the current through the patient, causing shocks or burns.

Q19. When applying SWD to the knee using contra-planar method, how should the electrodes be sized?

A. Slightly larger than the body part B. Much smaller than the body part C. Exactly the same size D. Shape does not matter

Rationale: Electrodes should be slightly larger than the treated area to ensure the uniform part of the electric field covers the tissues and edge effects are avoided.

Q20. Which frequency is an alternative for MWD?

A. 27.12 MHz B. 13.56 MHz C. 915 MHz D. 3 MHz

Rationale: While 2450 MHz is standard, 915 MHz is an alternative frequency used for MWD.