Accurate EKG interpretations hinge on the correct placement of electrodes, or leads. Misplaced leads can produce tracings that mimic serious cardiac abnormalities, leading to misdiagnosis and inappropriate treatment. This comprehensive guide delves into the mechanisms of lead reversal, how to identify them, and steps to correct misplacements to ensure precise cardiac assessments.
Overview
The Critical Role of Proper Lead Placement
Each EKG lead provides a unique perspective of the heart’s electrical activity. Correct placement allows clinicians to assess the heart’s function accurately. Misplaced leads alter the electrical vectors, resulting in waveforms that may suggest pathological conditions such as myocardial infarction, arrhythmias, or axis deviations. Understanding lead reversal patterns is essential to differentiate true cardiac events from technical errors.
Lead reversal alters the recorded electrical activity by changing the orientation of the leads relative to the heart. Each limb lead records the difference in electrical potential between two points. Swapping leads changes this difference, leading to characteristic changes in the EKG tracing.
Identifying Lead Reversals
A systematic approach is vital for recognizing lead misplacements:
- Compare with Previous EKGs: Discrepancies without clinical correlation may indicate lead issues.
- Assess for Global Changes: Widespread inversions or unexpected axis deviations suggest misplacement.
- Examine Individual Leads: Isolated abnormalities, such as a flat line or inversion in a single lead, can point to specific swaps.
The Mechanism
Electrodes
- LA = left arm
- RA = right arm
- LL = left leg
- RL/N = right leg (neutral electrode)
- Electrodes for V1-V6
Leads
- Bipolar leads: I, II, III
- Augmented unipolar leads: aVL, aVF, aVR
- Unipolar Leads: V1-V6 (precordial leads)
- Wilson’s central terminus (WCT)
Whenever the electrodes are misplaced, the leads no longer measure what they are supposed to measure. For instance, lead I is generated by ensuring right arm is negative and the left arm is positive. However, if the connections are switched, i.e., if the right arm is positive and the left arm is negative, lead I records the opposite of what it was normally supposed to record!
Analysis of precordial lead reversal
Check for unusual patterns in precordial leads: For precordial lead reversal, you would see an unexplained decrease in R-wave amplitude in two consecutive leads followed by a return to the normal pattern. A couple of examples follow:
V1 and V2 Reversal
- EKG Findings:
- Reversed R-wave Progression: Unusual patterns in early precordial leads.
- Possible Misdiagnosis: May mimic posterior myocardial infarction.
- Explanation:
- Swapping V1 and V2 alters the septal view, changing the expected progression of depolarization.
Swapping V3 and V4
- EKG Findings:
- Unexpected Decrease in R-wave Amplitude: Between swapped leads.
- Abnormal Transition Zone: Disruption in normal R-wave progression.
- Explanation:
- The anatomical perspective is altered, affecting the ventricular depolarization pattern.
Detailed Analysis of Specific Limb Lead Reversals
Limb leads include the right arm (RA), left arm (LA), left leg (LL), and right leg (RL, which serves as a ground). The bolded text is usually a shortcut to suspecting lead reversals.
RA/LA Reversal
- Inverted Lead I
- Lead II becomes lead III
- Lead III becomes lead II
- aVR becomes aVL
- aVL becomes aVR (so aVR is positive and there will be a right axis deviation)
- aVF is unchanged
RA/LL Reversal
- Leads I becomes inverted lead III
- Lead II is inverted
- Lead III becomes inverted lead I
- aVR becomes aVF
- aVF becomes aVR
- aVL is unchanged.
- So, leads I,II, III, aVF that are normally positive become negative and aVR that is normally negative becomes positive!
LA/LL Reversal
- Leads I becomes lead II
- Lead II becomes lead I
- Lead III becomes inverted
- Lead aVR is unchanged
- Lead aVL becomes aVF
- Lead aVF becomes aVL
- Lead aVR remains unchanged
RL/RA
- Lead I becomes an inverted lead III
- Lead II records a flat line (zero potential)
- Lead III is unchanged
- Lead aVL approximates an inverted lead III
- Leads aVR and aVF become identical
RL/LA
- Lead I becomes identical to lead II
- Lead II is unchanged
- Lead III records a flat line (zero potential)
- Lead aVR approximates to an inverted lead II
- Leads aVL and aVF become identical
RL/LL
- No significant change from baseline EKG.
Multiple Limb Lead Reversals
RA→LA→LL→RA
- Lead I becomes lead III
- Lead II becomes an inverted lead I
- Lead III becomes an inverted lead II
- aVR, aVL, and aVF are unchanged.
RA→LL→LA→RA
- Lead I becomes inverted lead II
- Lead II becomes inverted lead III
- Lead III becomes lead I
- aVR becomes aVF
- aVL becomes aVR
- aVF becomes aVR
RA/RL + LA/LL
- Lead I becomes a flat line
- Lead II and lead III become inverted lead III
- aVR and aVL are similar
- aVF looks like a inverted lead III
RA/LA + LL/RL
- Lead I becomes inverted
- Leads II becomes Lead III
- Lead III is unchanged
- aVR becomes aVL
- aVL becomes aVR
- aVF is unchanged.