Electrical Injuries & Drowning Pathophysiology

TL;DR

Electrical Injuries & Drowning Pathophysiology are core MRCP Part 1 topics that test applied physiology—particularly cardiac arrhythmias, hypoxia, and tissue injury mechanisms. Electrical injury severity depends on voltage, current type, and pathway, while drowning leads to hypoxaemia via aspiration, surfactant loss, and V/Q mismatch. The exam focuses on mechanisms, delayed complications, and key management priorities. Understanding physiology—not memorisation—is essential.

Why this matters

Environmental injuries such as electrical trauma and drowning are high-yield in MRCP Part 1 because they integrate multiple systems—cardiovascular, respiratory, renal, and neurological physiology. Questions often test your ability to link mechanism → clinical consequence → management, rather than recall isolated facts.

To build a structured foundation, review the MRCP Part 1 overview and consolidate learning with exam-style practice using Free MRCP MCQs.

Core sections

1. Electrical injuries: fundamental mechanisms

Electrical injury severity is determined by:

1. Voltage (high vs low)

2. Type of current (AC vs DC)

3. Pathway through the body

4. Duration of exposure

Mechanisms of damage:

• Thermal injury (Joule heating): Causes deep tissue necrosis

• Electrical disruption: Affects cardiac conduction and neural pathways

• Mechanical injury: Tetanic contractions → fractures or dislocations

Exam insight: External burns often underestimate internal tissue destruction, particularly muscle necrosis.

2. Alternating current vs direct current (classic MRCP concept)

Feature	Alternating Current (AC)	Direct Current (DC)
Source	Domestic electricity	Lightning, batteries
Muscle effect	Sustained tetany (“can’t let go”)	Single contraction
Cardiac effect	Ventricular fibrillation	Asystole
Exposure	Prolonged	Brief

Key takeaway: Low-voltage AC (e.g. household current) is particularly dangerous because tetany prolongs exposure.

3. Cardiac complications of electrical injury

• Ventricular fibrillation → most common cause of death

• Asystole → more common in high-voltage or lightning injury

• Delayed arrhythmias → may occur hours later

Pathophysiology:

• Direct disruption of myocardial depolarisation

• Autonomic instability

• Myocardial thermal injury

Exam rule: Any electrical injury involving the thorax → consider cardiac monitoring for 24 hours

4. Rhabdomyolysis and acute kidney injury

Electrical current preferentially damages muscle:

• Muscle necrosis → myoglobin release

• Myoglobin → renal tubular obstruction → acute kidney injury

Clinical clues:

• Dark (“tea-coloured”) urine

• Elevated creatine kinase

• Hyperkalaemia

Management principle:

• Aggressive IV fluids ± urine alkalinisation

5. Neurological complications

• Immediate: confusion, seizures, loss of consciousness

• Delayed: neuropathy, cognitive dysfunction

Important: Neurological deficits may be delayed and progressive, a common exam trap.

6. Drowning: definition and phases

Drowning is defined as respiratory impairment due to submersion or immersion in liquid.

Phases:

1. Breath-holding

2. Laryngospasm

3. Aspiration

4. Hypoxia → cardiac arrest

7. Pathophysiology of drowning

The central mechanism is hypoxaemia.

Key processes:

• Aspiration of water → alveolar epithelial damage

• Surfactant washout → alveolar collapse

• Ventilation-perfusion mismatch → severe hypoxia

• Pulmonary oedema → ARDS

Exam pearl: Death occurs due to hypoxia, not fluid overload.

8. Freshwater vs seawater drowning (low-yield nuance)

Feature	Freshwater	Seawater
Osmolality	Hypotonic	Hypertonic
Theoretical effect	Haemolysis	Fluid shift into alveoli
Clinical importance	Minimal	Minimal

MRCP tip: These differences are not clinically significant—avoid overemphasis.

9. Hypothermia in drowning

• Cold water reduces metabolic rate

• May delay brain injury

Principle: “Not dead until warm and dead”

Mechanism:

• Reduced oxygen consumption → neuroprotection

10. Post-rescue complications

• Acute respiratory distress syndrome (ARDS) (most tested)

• Aspiration pneumonia

• Electrolyte abnormalities (less significant than expected)

Practical examples / mini-cases

MCQ:

A 30-year-old electrician sustains a low-voltage AC shock. He has entry and exit wounds but is haemodynamically stable with a normal ECG. What is the most appropriate management?

A. Immediate dischargeB. Cardiac monitoring for 24 hoursC. Prophylactic antibioticsD. CT brain scan

Answer: B. Cardiac monitoring for 24 hours

Explanation: Even with a normal ECG, electrical injuries—especially AC—carry a risk of delayed arrhythmias, including ventricular fibrillation. Observation is required if current may have passed through the thorax.

Common pitfalls (5 bullets)

• Assuming superficial burns reflect injury severity

• Forgetting delayed arrhythmias after electrical exposure

• Overemphasising freshwater vs seawater differences

• Missing rhabdomyolysis as a cause of AKI

• Misidentifying cause of death in drowning (hypoxia, not fluid volume)

FAQs

1. Why is alternating current more dangerous than direct current?

AC causes sustained muscle contraction (tetany), prolonging contact and increasing the risk of ventricular fibrillation.

2. What is the most common fatal arrhythmia in electrical injury?

Ventricular fibrillation is the leading cause of death, particularly with low-voltage AC exposure.

3. What causes hypoxia in drowning?

Aspiration leads to surfactant loss, alveolar collapse, and V/Q mismatch, resulting in severe hypoxaemia.

4. Are freshwater and seawater drowning clinically different?

No. While physiological differences exist, they have minimal clinical impact on management.

5. Why can hypothermia be protective in drowning?

Cold temperatures reduce metabolic demand, potentially delaying hypoxic brain injury.

Ready to start?

To master high-yield physiology topics like this, practise regularly with exam-style questions using Free MRCP MCQs or simulate exam conditions with a Start a mock test. For a complete roadmap, revisit the MRCP Part 1 overview.

Sources

• MRCP(UK) official website: https://www.mrcpuk.org/

• Resuscitation Council UK Guidelines: https://www.resus.org.uk

• BMJ Best Practice (Drowning & Electrical Injuries): https://bestpractice.bmj.com

• ATLS Student Course Manual (American College of Surgeons)