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Fundamentals of ECG, Part 2 The basics: the fundamentals of ECG recording, reporting and interpretation MCQs
Fundamentals of ECG · MBBS / Medicine · 9 practice questions
- Paper speed effects
- Regular high-frequency artifact
- Muscle artifact vs true baseline abnormality
- Calibration artifacts
- Broad complex tachycardia in acute MI
- Atrial fibrillation - ECG mechanism and rate control
- Wolff-Parkinson-White syndrome - pre-excitation mechanism
- Broad complex tachycardia - differentiating VT from SVT with bundle branch block
1. An ECG obtained in a hospital ward shows a series of regular, sharp, high-frequency spikes superimposed evenly along the entire baseline, giving it a thickened appearance, while the QRS complexes themselves remain clearly identifiable and of normal morphology. Which explanation is most consistent with this finding?
- AElectrical interference from nearby mains-powered equipment
- BUnder-calibration of the ECG recorder
- CParkinsonian tremor affecting the recording
- DSkeletal muscle artifact from patient shivering
Show answer
A. Electrical interference from nearby mains-powered equipment Electrical interference classically produces regular, sharp, high-frequency spikes that thicken the baseline uniformly while leaving the underlying cardiac complexes recognizable, distinguishing it from muscle-related artifact. Shivering and Parkinsonian tremor instead produce irregular, less uniform spikes that can obscure the QRS complexes themselves, especially in the limb leads. Under-calibration would reduce the amplitude of all complexes rather than superimposing regular spikes on the baseline.
2. An ECG is recorded on a shivering patient in a cold cardiology suite. The limb leads show sharp, tightly synchronized spikes that nearly obscure the underlying waveform, while the chest leads are relatively preserved. What is the most appropriate immediate action?
- AIncrease the paper speed to 50 mm/s to better separate the artifact from the QRS complexes
- BReport the finding as atrial fibrillation with a rapid ventricular response
- CWarm and settle the patient, then repeat the recording
- DRecalibrate the machine, suspecting a faulty 1 mV calibration signal
Show answer
C. Warm and settle the patient, then repeat the recording Shivering produces synchronized skeletal muscle contraction that generates sharp spikes obscuring the true cardiac signal, particularly in the limb leads; the correct remedy is to make the patient warm, relaxed and comfortable before repeating the recording. Reporting this as atrial fibrillation would be a misinterpretation of muscle artifact as a cardiac rhythm. Changing paper speed does not address the underlying cause (muscle contraction) and would only alter spacing, not eliminate the artifact. There is no indication of a calibration problem here, since calibration issues affect amplitude of all complexes rather than causing synchronized spikes.
3. An ECG technician in a UK clinic reviews a tracing and notes what appears to be a markedly prolonged QT interval and a slow heart rate, despite the patient being asymptomatic and having a normal resting heart rate on manual pulse check. Which technical explanation best accounts for these apparent findings?
- AThe calibration signal produced a 2 cm deflection for 1 mV
- BThe patient was shivering during the recording
- CThe paper speed was set to 12.5 mm/s instead of the standard 25 mm/s
- DThe paper speed was set to 50 mm/s instead of the standard 25 mm/s
Show answer
D. The paper speed was set to 50 mm/s instead of the standard 25 mm/s Running the paper faster than the standard 25 mm/s (i.e., 50 mm/s) stretches out the tracing horizontally, creating the illusion of a longer QT interval and wider spacing between QRS complexes (apparent bradycardia), even though rate and intervals are actually normal. A slower speed (12.5 mm/s) would instead bunch complexes together, giving the impression of a fast rate, not a slow one. Over-calibration affects amplitude, not timing. Shivering produces muscle artifact spikes, not a falsely long QT or slow rate.
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