Note: Lead II equals Lead I plus Lead III

Lead Access System and Lead Placements

  • The discussion introduces the lead access (lead axis) system used in ECG interpretation, with emphasis on how the mean vector of polarization relates to deflections in the leads.
  • A mnemonic reference appears: “Three little ones ran too far” to remind that certain leads correspond to different regions of the heart (lipid: limb leads I, II, III). The intent is to remember lead orientations in the frontal plane.
  • Precordial (chest) leads are placed to view different heart regions:
    • V1 and V2 view the septal region of the heart.
    • V3 and V4 view the anterior region.
    • V5 and V6 view the lateral region.
  • Additional limb leads help form the frontal plane view:
    • Lead I, Lead II, Lead III (Einthoven leads) and augmented leads (AVL, AVF, AVR).
    • AVL is commonly referred to as low lateral; AVL and AVF terminology may be used interchangeably by some texts.
  • AVR (augmented vector right) is positive when the mean vector points toward the right arm; a positive AVR is described as indicating something “screwy” or abnormal going on in the heart.
  • If AVR is positive, it suggests the heart’s electrical activity is oriented away from AVR’s positive electrode, implying abnormal orientation or pathology in some cases.
  • The placement terms mentioned in the transcript include a note that AVL can be called low lateral and V5/V6 are high lateral in some terminology, but the standard is to refer to them simply as lateral leads.
  • Basic principle: when the mean vector of polarization points toward a positive electrode, the corresponding QRS deflection in that lead is positive; when it points away, the deflection is negative.

Mean Electrical Axis (MEA) and Vector Concepts

  • The mean electrical axis (MEA) is the overall direction of the heart’s depolarization vector in the horizontal plane.
  • If the MEA points toward the positive electrode of a lead, that lead’s QRS is positively deflected; if away, the QRS is negatively deflected.
  • The MEA is a single vector summarizing the direction of electrical activity during ventricular depolarization.
  • The basic physics explained: as muscle mass increases (hypertrophy) or as conduction changes occur, the MEA can shift.
  • The speaker emphasizes oxygen and energy as background physiology: more muscle growth (hypertrophy) means more electrical activity, which can influence the MEA.
  • If coronary arteries become blocked and an area of the heart becomes infarcted, those cells lose electrical activity. The MEA then points away from the infarcted area toward regions with preserved activity. This is used to infer infarct location from axis direction.
  • Example given: if AVR is positive (abnormal orientation), the inferred infarct location discussed is the right atrium in that scenario (the vector would point away from the infarct toward the non-infarcted tissue).
  • The practical implication: axis orientation helps identify regions of the heart with electrical activity or with infarction, and a positive AVR indicates an abnormal pattern that merits further interpretation.

How to Determine the MEA in a Diagram (Quadrants and Normal Axis)

  • The MEA can be visualized in a horizontal plane with a mean axis line; the diagram uses quadrants to categorize the axis location.
  • There is a normal quadrant, and other quadrants correspond to left axis deviation and right axis deviation (as described in the lecture).
  • The instructor directs students to look at the QRS polarity in Lead I and another lead (e.g., aVF) to determine whether the axis is more positive or more negative, which then places the MEA in one of the quadrants.
  • If the QRS is more positive in Lead I, the MEA is toward the left; if more positive in some inferior lead (like aVF), it helps define the quadrant as normal or deviated.
  • The MEA is compared against a coordinate system defined by Lead I (horizontal axis) and the inferior axis (aVF) to assign the axis to a quadrant.
  • The quadrant labeling in the notes includes:
    • Normal quadrant (consistent with a normal MEA).
    • Leftward axis (left axis deviation).
    • Rightward axis (right axis deviation).
  • The speaker demonstrates this by pointing to a sample ECG image and asking students to decide in which quadrant the MEA lies based on which QRS complexes are most positively deflected (e.g., Lead I and AVF polarity).

How to Read QRS Deflections and Lead Changes Across the Chest (V1–V6)

  • The progression of QRS amplitude across the precordial leads is described:
    • The R wave becomes progressively taller from V1 toward V4 (sometimes extending to V5 or V6).
  • The QRS complexes can appear in various configurations in terms of q, r, and s waves; they can be predominantly q, predominantly r, or predominantly s, or any combination thereof.
  • Definitions for R and Q waves (assumed knowledge for the course):
    • R wave is the first upward deflection after the P wave.
    • Q wave is the first downward deflection after the P wave.
    • The QRS complex can be any combination of Q, R, and S waves.
  • The QRS complex morphology can vary with lead placement; this is expected and part of normal variability.
  • Calibration and verification of leads:
    • The calibration mark on EKGs is 10 mm in height, which is used for amplitude measurement and standardization.
    • The commonly used rule for checking leads is that the net deflection of a particular lead should be approximately the expected relationship (e.g., one positive and three negative deflections summing to a positive deflection in the overall display).
    • A mnemonic tied to lead alignment is described as: one plus three equals two, referring to the relationship among leads (Lead II is the vector sum of Lead I and Lead III in the Einthoven triangle: $$ ext{Lead II} = ext{Lead I} + ext{Lead III} \