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A 33-year-old woman sustained an electric shock to the dorsum of her right hand from a domestic power supply. The only symptom she reported was a transient numbness in her right arm. There was no loss of consciousness. On arrival at the accident and emergency department she was well perfused and alert. Examination of her skin revealed no superficial burns. She had a regular tachycardia at a rate of 150 beats/min, and her blood pressure was 120/70 mmHg. The remainder of the examination was normal. Her admission electrocardiogram (ECG) is shown in figure 1. The patient was given 3 mg and then 6 mg of adenosine intravenously without effect: the outcome of 12 mg of adenosine is shown in figure 2.
Further investigations included serial creatinine kinase (myocardial bound fraction) and troponin I levels to exclude myocardial damage. These were within normal limits. The lung fields were clear on chest X-ray. An echocardiogram showed normal ventricular and atrial dimensions and function and no valvular abnormality.
- Describe the features shown in the ECG (figure 1).
- What does the rhythm strip in figure 2 show, and what information does this give us about the mechanism of the tachycardia?
The ECG shows a narrow complex tachycardia with retrograde P wave activity. The P wave axis indicates that is a junctional tachycardia.
The rhythm strip shows normal P wave activity followed by a few sinus beats before the junctional rhythm returns. As the tachycardia was temporarily abolished by adenosine, it must involve the AV node.
The following day, as the patient remained in the junctional tachycardia, she was given 25 mg of atenolol which led to a restoration of sinus rhythm and she was discharged home.
Electrocution can be subdivided into low-tension shocks and high-tension shocks. It is well recognised that electrical injury may cause immediate cardiac arrhythmias. High-tension shocks (>1000 Volts) from lightening strikes and overhead power cables cause severe skin burns and may cause cardiac arrest usually due to asystole.1 High-voltage injuries lead to a high incidence of cardiac abnormalities, up to 30% of those with cardiac arrhythmias on presentation have long-term reduction in cardiac function measured by echocardiography.2 Changes in cardiac conduction including complete heart block, and prolongation of the QT interval suggest that electric shocks can significantly modify intracardiac conduction.3
Low-tension shocks which occur from domestic appliances (<300 Volts at 50Hz) can cause ventricular fibrillation,1 but the incidence of cardiac arrhythmias following low-tension electrocution is not known. One retrospective study demonstrated that those patients with a normal ECG on presentation did not subsequently develop a serious arrhythmia.4 However, there is an appreciable risk of late ventricular fibrillation in injuries involving the passage of current through the thorax. In these cases, myocardial biopsies have shown patchy myocyte necrosis and fibrosis.5
The likelihood of tissue injury and cardiac arrhythmias following electrocution is dependent on the size of the current flow. For example, an alternating current of approximately 70 mA is required to induce ventricular fibrillation. Current flow is predominantly influenced by the voltage of the shock and resistance of the tissues involved. These principles are expressed by Ohms law: Current = Voltage
Bone has the highest resistance of the internal structures of the body, followed by fat. As electrical wounds enter through the skin, the resistance of the skin at the time of the injury contributes significantly to the total body resistance. Therefore, an electrical shock on wet skin has potential to cause more serious damage to tissues.6 7
electrocution can be divided into high-tension (>1000 V) or low-tension injury (<350 V)
there is a high incidence of cardiac damage with high-tension injuries
low-tension injuries may cause ventricular fibrillation
patients with high-tension injuries or low-tension injuries with an abnormal ECG or passage of current through the thorax should be admitted to hospital for cardiac monitoring
Patients should be admitted to hospital and monitored in all cases of high-tension electrocution because of the high incidence of myocardial damage. In low-tension injuries, admission and monitoring of cardiac rhythm is necessary if the admission ECG is abnormal or if there is any suggestion that the passage of electrical current was through the thorax. This may be inferred from any superficial burns at the entry and exit site or from the clinical history. In our patient, the initial ECG precipitated admission for further monitoring.
Junctional tachycardia following low-tension electrocution.