Misplacement of limb leads

ECG:  There is a reversal of the electrodes of both arms; lead I is inverted (negative P-wave, QRS-complex and T-wave); right axis deviation; normal R wave progression in precordial leads;

Later, a new ECG was made with correct placement of the electrodes.

ECG: P-waves, QRS-complexes and T-waves in lead I have become positive by correciton of the arm electrodes. We see a normal ECG with sinus rhythm of 60 beats per minute with an intermediate heart axis.

Comments: Normally, lead I represents the difference in potential between the right arm (negative pole) and the left arm (positive pole), lead II represents the difference between the right arm (negative pole) and the left leg (positive pole), lead III the difference between the left arm (negative pole) and the left leg (positive pole); aVR is determined by the right arm, aVL by the left arm and aVF by the left leg; the right leg electrode is neutral and is used for grounding. The limb electrodes can be indiscriminately placed at various sites on the limb, either near the hips/shoulders or distally without any change in electrocardiographic pattern.

A reversal of electrodes occurs when an electrode is not connected to the proper wire, with the electrocardiographic pattern differing significantly depending on the inverted electrodes:

•  reversal of the 2 upper limb electrodes (right arm/left arm): this is the most common  error and the most easily recognizable owing to its characteristic electrocardiographic pattern; lead I exhibits reciprocal images (-I) with negative P-waves, QRS-complexes and T-waves; there may therefore be a prominent right axis deviation; leads II and lead III are switched; similarly, lead aVL is interchanged with aVR which explains why the P-wave is negative in aVL and positive in aVR; lead aVF is unchanged; a negative P-wave in lead I together with an equally negative QRS-complex should in principle evoke a reversal of electrodes, the differential diagnosis being dextrocardia; analysis of R wave progression in the precordial leads allows differentiating between the two: the progression is normal when the electrodes are inverted (progressive increase in R wave amplitude from V1 to V5) and inverted for dextrocardia (progressive decrease from V1 to V5);
• reversal of the left arm/left leg electrodes: lead III is inverted and becomes – III (the left arm becomes the positive pole and the left leg, the negative pole); lead I becomes lead II and vice versa; lead aVR remains unchanged while aVF and aVL are switched; the changes are more subtle and more difficult to demonstrate in the absence of a reference tracing; the P-wave is taller in lead I than in lead II (normally, P-wave amplitude is maximal in lead II); unless there is careful analysis, the majority of these reversals go unnoticed;
• reversal of the right arm/right leg electrodes: there is virtually no difference in potential between the left and right leg electrodes; this explains why lead II (measured herein between both legs) displays a pseudo-asystole pattern (flat line, very low voltages); leads aVR and aVF become identical (one is directed toward the right leg, the other toward the left leg); these two abnormalities are strongly evocative of this type of electrode reversal; lead I becomes an inverted lead III (-III); lead III remains unchanged; the placement of the neutral electrode having been modified (from the right leg to the right arm), there may be moderate changes in the pattern of the precordial leads;
• reversal of the right arm/left leg electrodes: leads I, II and III are inverted; lead aVL is unchanged; leads aVR and aVF are switched;
• reversal of the left arm/right leg electrodes: lead III (measured between both legs) displays a pseudo-asystole pattern (flat line, very low voltages); leads aVL and aVF become identical (one is directed toward the right leg, the other toward the left leg); similarly, leads I and II become identical; these three abnormalities are strongly evocative of this type of electrode reversal; there are also other signs, although more difficult to identify: lead II is unchanged; lead aVR corresponds to an inverted lead II; the placement of the neutral electrode having been modified (from the right leg to the left arm), there may be moderate changes in the pattern of the precordial leads;
• reversal of electrodes of both arms and both legs: the electrodes of each arm record the same potential (those of both legs) and lead I which measures their difference in potential is almost isoelectric (flat line, pseudo-asystole); this represents the most characteristic sign of this type of electrode reversal; leads II, III and aVF all become identical; leads aVR and aVL also become identical;
• reversal of left leg/right leg electrodes: the electrical signals from both legs being identical, the ECG pattern is unchanged; it is therefore impossible to highlight this reversal which, in any event, will not affect the interpretation of the ECG;

The presence of a negative P-wave and negative QRS in lead I should lead to suspect a reversal of left and right arm electrodes; a flat-line (pseudo-systole) pattern in leads I, II or III should evoke a reversal between the right leg electrode and one of the other 3 limb electrodes.

Below we present a few more ECGs with electrode switches. Proceed to the bottom of this page to continue to the next case.

ECG 2:

ECG: Switch of left arm and left leg. Lead III is mirrored, lead I and II are switched.

ECG 3: 

ECG: Switch of right arm and leg. Pseudo-asystole in lead II, aVR and aVF become identical. Lead I is mirrored.

ECG 4: 

ECG: Swtich of right arm and right leg. Lead I, II and III are mirrrored.

ECG 5: ECG: Switch of right arm and leg. Pseudo-asystole in lead II, leads aVR and aVF become identical, leads I and II are identical.