Electrocardiography Questions and Answers

Answer: C

The key gives pulsed waveform of micro volt.

An ECG is the time-varying potential difference picked up by electrodes on the body as the heart beats. It is a repeating waveform (P wave, QRS complex, T wave), so it is not a steady dc level, which rules out option B, and its energy lies at low frequencies, roughly 0.05 to 100 Hz, which rules out the "high frequency signal" of option D. The quantity recorded is a voltage, not a current, so option A is out.

Option C is the only one describing a repeating waveform of a voltage. Its "micro volt" is loose: surface ECG amplitudes are about 0.1 to 2 mV, with the P wave near 100 to 250 µV. The item is a best-answer one, and C is the best of the four.

Answer: B

An ECG is recorded with surface metal electrodes.

The electrocardiogram picks up the heart's electrical activity at the skin surface. Metal electrodes (commonly silver-silver chloride) are placed on the chest and limbs with a conducting gel to reduce skin contact impedance.

Penetrating (needle) electrodes are used for recordings such as EMG from muscle, not routine ECG. Non-contact probes and optical electrodes are not the standard way of recording the ECG.

Answer: B

The ECG is based on the electrical activity of the heart.

As the action potential spreads from the SA node through the atria, the AV node and the ventricles, the depolarising and repolarising heart muscle creates currents that spread through the body. Electrodes on the skin pick up the resulting potential differences, a few millivolts at most, and record them as the P wave, the QRS complex and the T wave.

Blood pressure changes are recorded by pressure transducers, mechanical contraction by methods such as echocardiography, and blood flow in arteries by Doppler ultrasound. The ECG records none of these.

Answer: A

Einthoven's triangle is formed by the limb electrodes.

Einthoven treated the right arm, left arm and left leg as the corners of an equilateral triangle with the heart at its centre. The three bipolar limb leads form its sides: lead I (LA - RA), lead II (LL - RA) and lead III (LL - LA). Einthoven's law, I + III = II, follows from this. The right-leg electrode is a reference or ground and is not part of the triangle.

The chest electrodes form the unipolar precordial leads V1 to V6 in the horizontal plane. Head electrodes are used for EEG, and a ground electrode is not a lead.

Answer: A

ECG electrodes are generally made of silver-silver chloride.

An Ag/AgCl electrode is nearly non-polarisable: charge crosses the interface freely through the reversible reaction between silver and silver chloride. This gives a low, stable half-cell potential, low noise and little motion artefact, which is why disposable ECG electrodes use an Ag/AgCl sensor with chloride gel.

Platinum is a polarisable metal whose half-cell potential drifts, so it is used more for stimulation and some implanted electrodes. Copper and nickel corrode, are noisy and can irritate the skin.