Faradic, Galvanic & SD Curve: Exam Masterclass
💡 The Core Concept: Low-frequency currents are used to stimulate muscles. The key distinction is the State of Innervation.
- Faradic (Short Pulse): Stimulates the Nerve → Muscle contracts. Used for Innervated muscles.
- Galvanic (Long Pulse): Stimulates the Muscle membrane directly. Used for Denervated muscles.
1. The Technical Arsenal (Faradic vs. Galvanic)
Understanding these parameters is crucial for answering "Case-Based" questions on muscle stimulation.
| Parameter | Faradic Current | Galvanic (Interrupted DC) |
|---|---|---|
| Pulse Duration | Short (0.1 - 1 ms) | Long (>100 ms up to 300-600 ms) |
| Frequency | 50 - 100 Hz | 30 per min (Manual interruption) |
| Target | Innervated Muscle (Via Motor Nerve) | Denervated Muscle (Direct Muscle Fiber) |
| Sensation | Prickling / Tetanic Contraction | Stabbing / Sluggish Contraction |
| Chemical Effect | Negligible (Short pulse) | High Risk (Chemical burn under electrodes) |
2. Strength-Duration (SD) Curve
The SD Curve plots the relationship between Intensity (Strength) and Pulse Width (Duration) required to produce a minimal palpable contraction.
A. Key Definitions (Exam Favorites)
- Rheobase: The minimum intensity (current amplitude) required to produce a contraction with a pulse of infinite duration (>100ms or 300ms).
- Chronaxie: The minimum duration (pulse width) required to produce a contraction at an intensity of 2x Rheobase.
- Significance: Chronaxie is the standard index of excitability.
B. Curve Characteristics
| Condition | Chronaxie Value | Curve Shape |
|---|---|---|
| Normal (Innervated) | < 1 ms (0.05 - 1 ms) | Smooth, sits to the left. |
| Partial Denervation | Variable | "Kink" in the curve (Wait for the kink to disappear to confirm re-innervation). |
| Complete Denervation | > 10 ms (Often 30-50 ms) | Shifted Up & Right. Steep hyperbola. |
3. Special Techniques & Tests
A. Faradic-Galvanic (FG) Test
Used to assess the integrity of the neuromuscular system.
- Normal Response: Brisk contraction to Faradic; Brisk contraction to Galvanic.
- Denervated Response (Reaction of Degeneration): NO response to Faradic; Sluggish/Worm-like response to Galvanic.
B. Iontophoresis
Uses Continuous DC (Galvanic) current to drive ionized drugs through the skin.
- Principle: Like repels Like. (Positive drugs under Anode, Negative under Cathode).
- Safety: High risk of Chemical Burn. The Cathode (Negative pole) produces an Alkaline reaction (NaOH), which is more caustic/damaging than the Acidic reaction at the Anode.
🏆 AIIMS "Golden Points"
- Nerve Accommodation: Nerves adapt to slowly rising currents. Muscles do not. This is why we need sharp rising pulses (Faradic) for nerves, but can use slow rising pulses (trapezoidal) for denervated muscle to avoid stimulating sensory nerves.
- Active Electrode: In Monopolar stimulation, the Cathode (Negative) is usually the active electrode because it depolarizes the membrane more effectively (Cathodal Closing Circuit > Anodal Closing Circuit).
- Progression: As a nerve regenerates, the Chronaxie value will decrease (move from >10ms towards 1ms).
📝 20 High-Yield MCQs (Practice Now)
Click an option to check your answer immediately.
Q1. What is the normal Chronaxie value for an innervated muscle?
Rationale: Innervated muscles respond to short pulses via the motor nerve. The chronaxie is typically very low, usually between 0.05 and 0.5 ms (definitely < 1 ms).
Q2. Chronaxie is defined as the pulse duration required to produce a response at an intensity of:
Rationale: This is the standard definition. First, determine Rheobase (min intensity). Then double that intensity (2x) and find the minimum time required. That time is Chronaxie.
Q3. Which current is specifically indicated for stimulating Denervated Muscle?
Rationale: Denervated muscle fibers have lost their nerve supply and cannot accommodate. They require long pulse durations (>100ms) found in Interrupted Galvanic current to depolarize the muscle membrane directly.
Q4. In an SD Curve, a "Kink" (discontinuity) indicates:
Rationale: A kink appears because the muscle contains a mix of innervated fibers (responding to short pulses) and denervated fibers (responding to long pulses), creating a composite curve.
Q5. What happens to the SD Curve in a case of Complete Denervation?
Rationale: Denervation requires higher intensity and longer pulse durations to elicit a response. On the graph (X-axis = Duration, Y-axis = Intensity), this shifts the curve to the Right and Up.
Q6. Faradic current typically uses a pulse duration of:
Rationale: Faradic current is designed to stimulate nerves, which have a short chronaxie. Therefore, a short pulse duration of 0.1 to 1 ms is sufficient.
Q7. Rheobase is defined as the minimal intensity required at:
Rationale: Rheobase is the baseline excitability threshold. It is measured using the longest pulse duration available (effectively infinite duration) to find the lowest current needed.
Q8. In the Faradic-Galvanic Test, a muscle that responds to neither Faradic nor Galvanic stimulation indicates:
Rationale: Even denervated muscle should respond to Galvanic. If there is NO response to Galvanic, the muscle tissue itself has likely degenerated or become fibrotic (no contractile tissue left).
Q9. Iontophoresis utilizes which type of current?
Rationale: Iontophoresis requires a unidirectional, continuous flow of current to physically repel ions and drive them through the skin. Interrupted or AC currents would just move ions back and forth.
Q10. Which electrode polarity carries the highest risk of chemical burn during Iontophoresis/Galvanism?
Rationale: The Cathode attracts positive ions, forming Sodium Hydroxide (NaOH), a strong alkali. Alkaline burns are more severe and liquefy tissue (liquefactive necrosis), whereas Anodes form acid which coagulates tissue (coagulative necrosis).
Q11. What is "Reaction of Degeneration" (RD)?
Rationale: RD describes the altered electrical behavior (sluggish contraction, no response to short pulses) seen in LMN lesions.
Q12. To stimulate a muscle motor point using Monopolar technique, the active electrode should ideally be:
Rationale: Pfluger's Law states CCC > ACC (Cathodal Closing Contraction is stronger than Anodal). The negative pole depolarizes the membrane more effectively.
Q13. Faradic current is typically surged to produce:
Rationale: Surging (gradually increasing and decreasing intensity) produces a smooth, tetanic contraction that mimics voluntary movement and prevents muscle fatigue.
Q14. If Chronaxie is > 10 ms, the muscle is likely:
Rationale: A high chronaxie indicates that the tissue has low excitability (requires long pulses), which is the hallmark of denervated muscle.
Q15. Which property allows Nerves to adapt to slow-rising currents, but Muscle fibers cannot?
Rationale: Nerves have a high capacity for accommodation (raising threshold) if the stimulus rises slowly. Denervated muscle loses this ability. This principle allows selective stimulation of denervated muscle using slowly rising pulses.
Q16. The typical frequency of Faradic current is:
Rationale: 50 Hz is the standard frequency to produce tetanic contractions in innervated skeletal muscle.
Q17. Which ion is used in Iontophoresis for inflammation (e.g., Tennis Elbow)?
Rationale: Dexamethasone is a potent anti-inflammatory corticosteroid. It is negatively charged and must be placed under the Cathode (Negative pole).
Q18. The "Utilization Time" on the SD Curve corresponds to:
Rationale: Utilization time is the latent period or the minimum duration a rheobase intensity must be applied to elicit a response. It's the point where the curve flattens out on the right.
Q19. Which statement is TRUE regarding the Faradic-Galvanic Test for a Denervated Muscle?
Rationale: Denervated muscle cannot respond to the short pulses of Faradic current (Pulse duration < Chronaxie). It only responds to the long pulses of Galvanic current, but the contraction is slow and worm-like (sluggish) due to direct muscle conduction.
Q20. The main use of Faradic Current in physiotherapy is:
Rationale: Faradic is the gold standard for stimulating innervated muscles to re-educate action, prevent disuse atrophy, and strengthen weak muscles. Denervated muscles require Galvanic.
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