Practical Electrocardiography - Introduction - Scott E. Ewing, D.O. Cardiology Fellow Lecture #1
Introduction Electrophysiology Depolarization Anatomy ECG Paper Lead Placement Normal ECG Waves / Intervals / Segments
Cardiac Action Potential
Cardiac Action Potential Phase 0: Rapid depolarization Sodium moves rapidly into the cell Calcium moves slowly into the cell Phase 1: Early repolarization Sodium channels close Phase 2: Plateau Calcium continues to flow in Potassium continues to flow out Phase 3: Rapid repolarization Calcium channels close Potassium flows out rapidly Active transport Na/k pump Phase 4: Resting Cell membrane is impermeable to sodium Potassium moves out
Sequence of Excitation Sinoatrial (SA) node generates impulses about 75 times/minute Atrioventricular (AV) node delays impulse approximately 0.1 second Impulse passes from atria to ventricles via atrioventricular bundle (bundle of His)
Depolarization
Heart Physiology: Sequence of Excitation Figure 17.14a
ECG Paper Time intervals indicated for the thick and thin vertical grid lines Amplitudes indicated for the thick and thin horizontal grid lines Each small square 0.04 s × 0.1 mV and each large square 0.20 s × 0.5 mV
ECG Limb Leads
ECG Limb Leads
Willem Einthoven (1860 – 1927) Dutch doctor and physiologist Invented the first practical electrocardiogram in 1903 Nobel Prize Medicine in 1924 Died in Leiden in the Netherlands and is buried in the graveyard of the Reformed Church at 6 Haarlemmerstraatweg in Oegstgeest
ECG Augmented Leads
ECG Frontal Plane
ECG Frontal Plane
ECG Precordial Leads
ECG Precordial Leads
Normal ECG
Normal Findings Tall R waves Prominent U waves ST segment elevation (hightake off, benign early repolarization) Exaggerated sinus arrhythmia Sinus bradycardia Wandering atrial pacemaker Wenckebach phenomenon Junctional rhythm 1st degree heart block
Waveform Review
P Wave
P Wave
P Wave Atrial activation begins in the SA node Normal amplitude Spreads in radial fashion to depolarize the right atrium, interatrial septum, then the left atrium Last area of the left atrium to be activated is the tip of the left atrial appendage Normal amplitude Seldom exceeds 0.25 mV normally in limb leads In precordial leads, positive component is normally less than 0.15 mV
P Wave Characteristics Positive in leads I and II Best seen in leads II and V1 Commonly biphasic in lead V1 < 3 small squares in duration < 2.5 small squares in amplitude
P Wave – Lead II
P Wave – Lead V1
PR Interval / Segment
PR Interval / Segment
PR Interval Beginning P wave to beginning QRS complex Interval between onset atrial depolarization and onset ventricular depolarization Time required for the activation impulse to advance from atria through the AV node, bundle of His, bundle branches, and the Purkinje fibers until ventricular myocardium begins to depolarize Normal PR Interval 0.12-0.20 seconds (adults) Should be taken from lead with the largest and widest P wave and longest QRS duration
PR Segment Horizontal line between the end of the P wave and the beginning of the QRS complex Duration depends on the duration of the P wave as well as the impulse conduction through the AV junction Usually isoelectric, however it is often displaced in a direction opposite to the polarity of the P wave Depressed in most of the conventional leads except aVR Displacement is mainly due to atrial repolarization
PR Interval – Lead II
QRS Complex
QRS Complex
QRS Complex Q wave – Any initial negative deflection R wave – Any positive deflection S wave – Any negative deflection after an R wave
QRS Complex
QRS Complex Resultant electrical forces generated from ventricular depolarization Begins at middle third of left interventricular septal surface Spreads in a rightward direction RV begins to depolarize shortly after initiation of LV activation Basal portion of septum and posterobasal portion of LV free wall are last areas of depolarization LV contributes most of the QRS forces due to larger muscle mass
Ventricular Depolarization
QRS Complex QRS duration represents duration of ventricular activation Should be measured from lead with widest QRS complex Traditionally measured from the limb leads, but V1 or V2 may have the widest complex Normal QRS varies between 0.06-0.11 second
QRS Complex Lead I: usually records a dominant R wave Lead II: invariably has prominent R wave since mean vector is always toward II if QRS axis is normal Lead aVR: always records negative deflection Lead III: variable
QRS Complex – Lead V3
ST Segment
ST Segment Segment between end of QRS complex (J point) and beginning of T wave Represents state of unchanged polarization between end of depolarization and beginning of repolarization Stage when terminal depolarization and starting repolarization are superimposed and cancel each other
ST Segment Most important information regarding ST segment is presence or absence and degree of displacement from isoelectric line TP segment is used as reference baseline Limb leads – elevation or depression < 1mm Precordial leads- elevation sometimes seen and normal in V2-V3 (< 2mm), rarely > 1mm in V5-V6 Any ST depression in precordial leads is abnormal (normal vector in horizontal plane is anterior and leftward)
ST Segment – Lead aVF
T Wave
T Wave Represents potential for ventricular repolarization Proceeds in general direction of ventricular excitation Polarity of resultant T wave is similar to that of the QRS vector Upright in I, II, V5-V6 Inverted in aVR When inverted in 2 or more of right precordial leads, referred to as persistent juvenile pattern
T Wave Limb leads Precordial leads: Tallest in lead II Normally < 6mm in all limb leads Should never be < 0.5mm Precordial leads: Tallest in V2-V3 (average 6mm) Smaller in left precordial leads
T Wave – Lead aVF
QT Interval
QT Interval Represents duration of ventricular electrical systole Measured from beginning of QRS complex to end of the T wave Lead with a large T wave and distinct termination is used Leads V2-V3 are usually best for this specific measurement
QT Interval Varies with heart rate Increases slightly with age Lengthens as heart rate decreases Shortens as heart rate increases Increases slightly with age Diurnal variation of QT has been documented Longer during sleep than during waking hours
QT Interval Normal = 0.35 – 0.45 second for HR=70 Bazett's correction For every 10-beat increase or decrease of the rate, 0.02 second is deducted or added to the QT Bazett's correction QTc = measured QT divided by square root of RR interval Obvious abnormality if QT > ½ RR interval
QT Interval – Lead aVF
U Wave
U wave Small, low-frequency deflection that appears after the T wave Genesis is controversial Afterpotentials of ventricular myocardium Repolarization of the Perkinje fibers Amplitude is proportional to T wave Usually 5-25% of T wave voltage Largest in leads V2-V3 Prominent during slower heart rates Initial portion is normally steeper than terminal portion
8-Step Method ECG Interpretation Rate Rhythm Axis P wave PR interval QRS complex QT interval ST segment and T wave
Questions?
His-Purkinje Conduction System