Negative P and QRS in lead I
A 34-year-old man attends a pre-employment medical examination and is otherwise healthy. He takes no medicines and has no cardiac symptoms. A 12-lead ECG is recorded by a newly appointed technician. The tracing shows a negative P wave, a predominantly negative QRS complex, and a negative T wave in lead I. Lead aVR is predominantly positive. The precordial leads V1–V6 show normal R-wave progression and normal P waves. The automated report states 'abnormal ECG: possible limb lead reversal; consider right-axis deviation.' The technician is unsure whether to repeat the ECG, change the paper, or send the report with the automated interpretation attached. The clinic doctor is seeing another patient and has asked for the ECG to be ready in 10 minutes. The technician has not yet been assessed on limb lead placement. There is no previous ECG for comparison.
1. What is the most likely technical error, and which features support it?
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The most likely error is right arm/left arm lead reversal. In correct placement, lead I has an upright P wave, a predominantly positive QRS, and an upright T wave because the left arm is positive relative to the right arm. When the arm leads are swapped, lead I becomes inverted: P, QRS, and T all become negative. Lead aVR becomes positive because it is now looking from the left arm toward the heart. The precordial leads remain normal because chest lead placement is unaffected. This combination is much more consistent with arm lead reversal than with true right-axis deviation, which would not usually invert the P and T waves in lead I.
2. What should the technician do before the report is sent?
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Check the limb electrode positions and repeat the ECG with correct placement. Confirm that the right-arm electrode is on the right arm and the left-arm electrode on the left arm, and that the leg electrodes are correctly placed. Compare the repeated tracing with the first: if lead I becomes upright with a positive P wave and aVR becomes negative, the reversal is confirmed. Label the first tracing as 'probable right arm/left arm lead reversal; repeat tracing normal' and send the corrected ECG with a clear technical note. Do not send the automated abnormal report as the final report, and do not change the paper or machine settings as a substitute for correcting lead placement.
3. Why is right-axis deviation not the best explanation?
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True right-axis deviation can produce a negative QRS in lead I, but it does not usually produce a negative P wave and negative T wave in lead I together with a positive aVR. Those findings point to an electrical reversal of the arm leads. The normal precordial R-wave progression also shows that the chest leads are correctly placed, so the abnormality is confined to the limb leads. The automated report lists both possibilities, but the technician should use the pattern to choose the correct action: repeat the ECG after checking lead placement. Sending 'right-axis deviation' would be clinically misleading and could trigger unnecessary investigation.