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)/American Heart Association (AHA) guidelines that: “Recording the resting 12-lead ECG continues to be the most commonly used 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 relative cost is minimal.
Transient current that contributes to phase 4 pacemaker currents in SA and AV nodal cells T - type ( I Ca - T ) 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 L - type ( I Ca - L ) Calcium Channels Open in response to Ca ++ influx in vascular smooth muscle Calcium - activated ( I K, Ca or BK Ca ) Activated by acetylcholine; Gi - protein coupled Acetylcholine - activated ( I K, ACh ) K ATP 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 ATP - sensitive ( I K, ATP ) Phase 3 repolarization of cardiac action potentials Delayed rectifier ( I Kr ) Contributes to phase 1 of non - pacemaker cardiac action potentials Transient outward ( I to ) Maintains phase 4 negative potential in cardiac cells Inward rectifier ( I ir or I K1 ) Potassium Channels " Funny " pacemaker current ( I f ) in cardiac nodal tissue Slow Na + Phase 0 depolarization of non - pacemaker cardiac action potentials Fast Na + Sodium Channels CHARACTERISTICS CHANNEL
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)
CORNELL Voltage Criteria for LVH (sensitivity = 22%, specificity = 95%)
S in V3 + R in aVL > 24 mm (men)
S in V3 + R in aVL > 20 mm (women)
Other Voltage Criteria for LVH
Limb-lead voltage criteria:
R in aVL 11 mm or , if left axis deviation , R in aVL 13 mm plus S in III 15 mm
R in I + S in III >25 mm
Chest-lead voltage criteria:
S in V1 + R in V5 or V6 35 mm
1 point Delayed intrinsicoid deflection in V5 or V6 ( 0.05 sec) 1 point QRS duration 0.09 sec 2 points Left axis deviation 3 points Left Atrial Enlargement in V1 3 points 1 point ST-T Abnormalities : Without digitalis With digitalis 3 points
Voltage Criteria (any of) :
R or S in limb leads 20 mm
S in V1 or V2 30 mm
R in V5 or V6 30 mm
Points + ECG Criteria
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)