Volume Conductor Principles and ECG Rules of Interpretation

• A wave of depolarization traveling toward a positive electrode results in a positive deflection in the ECG trace.

• A wave of depolarization traveling away from a positive electrode results in a negative deflection.

• A wave of repolarization traveling toward a positive electrode results in a negative deflection.

• A wave of repolarization traveling away from a positive electrode results in a positive deflection.

• A wave of depolarization or repolarization traveling perpendicular to an electrode axis results in a biphasic deflection of equal positive and negative voltages (i.e., no net deflection).

• The instantaneous amplitude of the measured potentials depends upon the orientation of the positive electrode relative to the mean electrical vector.

• The voltage amplitude is directly related to the mass of tissue undergoing depolarization or repolarization

Volume Conductor Principles and ECG Rules of Interpretation

the last cells in the ventricle to depolarize are the first to repolarize.

ATRIUM

VENTRICLE

American College of Cardiology (ACC)/AmericanHeart Association (AHA) guidelines that: “Recording theresting 12-lead ECG continues to be the most commonlyused laboratory procedure for the diagnosis of heart disease.”In addition, “The procedure is safe, simple, and reproducible;the ECG record lends itself to serial studies; and the relativecost is minimal.

CHANNELCHARACTERISTICS

Sodium Channels 

     Fast Na+Phase 0 depolarization of non-pacemaker cardiac action potentials

     Slow Na+"Funny" pacemaker current (If) in cardiac nodal tissue

Potassium Channels 

     Inward rectifier (Iir or IK1)Maintains phase 4 negative potential in cardiac cells

     Transient outward (Ito)Contributes to phase 1 of non-pacemaker cardiac action potentials

     Delayed rectifier (IKr)Phase 3 repolarization of cardiac action potentials

     ATP-sensitive (IK, ATP)KATP channels; inhibited by ATP; therefore, open when ATP decreases

during hypoxia; in vascular smooth muscle, adenosine removes the ATP inhibition and opens these channels, producing vasodilation

     Acetylcholine-activated (IK, ACh)

Activated by acetylcholine; Gi-protein coupled

     Calcium-activated (IK, Ca

or BKCa)Open in response to Ca++ influx in vascular smooth muscle

Calcium Channels 

     L-type (ICa-L)Slow inward, long-lasting current; phase 2 non-pacemaker cardiac action potentials and phases 4 and 0 of SA and AV nodal cells; important in vascular smooth muscle contraction

     T-type (ICa-T)Transient current that contributes to phase 4 pacemaker currents in SA and AV nodal cells

Electrocardiographic criteria of right ventricular enlargement

 Example #1: (note RAD +120 degrees; RAE; R in V1 > 6 mm; R in aVR > 5 mm)

 Example #2: (more subtle RVH: note RAD +100 degrees; RAE; Qr complex in V1 rather than qR is atypical

 I. Left Ventricular Hypertrophy (LVH)General ECG features include:QRS amplitude (voltage criteria; i.e., tall R-waves in LV leads, deep S-waves in RV leads) Delayed Intrinsicoid deflection in V6 (i.e., time from QRS onset to peak R is 0.05 sec) Widened QRS/T angle (i.e., left ventricular strain pattern, or ST-T oriented opposite to QRS direction). This pattern is more common with LVH due to pressure overload (e.g., aortic stenosis, systemic hypertension) rather than volume overload.Leftward shift in frontal plane QRS axis Evidence for left atrial enlargement (LAE)ESTES Criteria for LVH ("diagnostic", 5 points; "probable", 4 points)

+ECG CriteriaPoints

Voltage Criteria (any of):1.R or S in limb leads 20 mm2.S in V1 or V2 30 mm 3.R in V5 or V6 30 mm

3 points

ST-T Abnormalities:Without digitalisWith digitalis

 3 points1 point

Left Atrial Enlargement in V13 points

Left axis deviation2 points

QRS duration 0.09 sec1 point

Delayed intrinsicoid deflection in V5 or V6 (0.05 sec)

1 point

 CORNELL Voltage Criteria for LVH (sensitivity = 22%, specificity = 95%)1.S in V3 + R in aVL > 24 mm (men) 2.S in V3 + R in aVL > 20 mm (women)Other Voltage Criteria for LVH1.Limb-lead voltage criteria:2.R in aVL 11 mm or, if left axis deviation, R in aVL 13 mm plus S in III 15 mm3.R in I + S in III >25 mm4.Chest-lead voltage criteria:5.S in V1 + R in V5 or V6 35 mm

Example 1: (Limb-lead Voltage Criteria; e.g., R in aVL >11 mm; note wide QRS/T angle)

Example 2: (ESTES Criteria: 3 points for voltage in V5, 3 points for ST-T changes; also LAE and LAD of -40 degrees; note also the PVC)