Electrical Events of the Cardiac Cycle Electrophysiology Dr.s.a.moezzi

Cardiac Cycle Cardiac Cycle: the electrical, pressure and volume changes that occur in a functional heart between successive heart beats. Phase of the cardiac cycle when myocardium is relaxed is termed diastole. Phase of the cardiac cycle when the myocardium contracts is termed systole. Atrial systole: when atria contract. Ventricular systole: when ventricles contract.

Mechanical Events of the Cardiac Cycle Ventricular Filling Period [ventricular diastole, atrial systole] Isovolumetric Contraction Period [ventricular systole] Ventricular Ejection Period [ventricular systole] Isovolumetric Relaxation Period [ventricular diastole, atrial diastole]

Cardiac Cycle Electrical changes in heart tissue cause mechanical changes, i.e. muscle contraction. Thus, changes in electrical membrane potential of specific parts of the heart tissue represent mechanical events in specific areas of the heart tissue.

Electrical System of Heart

Electrocardiography Two common abbreviations for electrocardiogram: EKG and ECG. EKG comes from German language where cardiogram is written as kardiogram. The ECG records the electrical activity of the heart. Mechanical activity of the heart is sensed by echocardiography.

Electrophysiology If an electrode is placed so that wave of depolarization spreads toward the recording electrode, the ECG records a positive (upward) deflection. If wave of depolarization spreads away from recording electrode, a negative (downward) deflection occurs.

Electrophysiology

Electrophysiology

Electrophysiology

Electrophysiology When myocardial muscle is completely polarized or depolarized, the ECG will not record any electrical potential but rather a flat line, isoelectric line. After depolarization, myocardial cells undergo repolarization to return to electrical state at rest.

Electrical Events of the Cardiac Cycle Sinoatrial (SA) node is the normal pacemaker of heart and is located in right atrium. Depolarization spreads from SA node across atria and results in the P wave. Three tracts within atria conduct depolarization to atrioventricular (AV) node. Conduction slows in AV node to allow atria to empty blood into ventricles before vent. systole. Bundle of His connects AV to bundle branches. Purkinje fibers are terminal bundle branches.

Electrical Conduction System Sinoatrial node (SA node) Intra-atrial Pathways Atrioventricular node (AV node) Bundle of His Left and Right Bundle Branches Purkinjie Fibers

The P wave The first wave form is called a P wave: it represents Atrial depolarization. It is gently rounded, and not larger than 2-3 mm. It is usually positive (above the isoelectric line) It should not be large, notched, or peaked

The QRS Complex The QRS complex represents ventricular depolarization. The Q wave is the first negative deflection. It should not be greater than 1mm wide or larger than 1/3 height of R wave. The R wave is the first positive deflection after the Q wave. The S wave is the first negative deflection after the R wave The normal QRS complex should be < .12 sec.

The T wave The T wave represents ventricular repolarization. It is usually positive, but can be negative or biphasic. It is usually the same polarity as the QRS complex.

The PR Interval The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex. The PR interal measures the beginning of atrial depolarization through the beginning of ventricular depolarization. The normal PR interval is from .12-.20 seconds.

The QT Interval The QT interval is measured from the beginning of the Q wave to the end of the T wave. This measures ventricular depolarization and repolarization. Any QT longer than .50 seconds can predispose certain dangerous arrhythmias.

ST Segment The ST segment is measured from the end of the S wave to the beginning of the T wave. The ST segment is normally isoelectric, or curves slightly upwards into the T wave. Horizontal or downsloping ST depression of 2 mm or more is abnormal, indicating ishemia. ST segment elevation > 1mm indicates myocardial infarction

ECG Time & Voltage ECG machines can run at 50 or 25 mm/sec. Major grid lines are 5 mm apart; at standard 25 mm/s, 5 mm corresponds to .20 seconds. Minor lines are 1 mm apart; at standard 25 mm/s, 1 mm corresponds to .04 seconds. Voltage is measured on vertical axis. Standard calibration is 0.1 mV per mm of deflection.

Components of a NSR

Standard 12-Lead ECG Usually performed when person is resting in supine position. three bipolar limb leads: I, II, and III; three augmented voltage leads: aVR, aVL, aVF; six precordial leads: V1 – V6. All limb leads lie in frontal plane. Chest leads circle heart in transverse plane.

ECG Limb Leads

ECG Augmented Limb Leads

ECG Precordial Leads

Standard 12-Lead ECG Each lead provides a different electrical angle or picture of the heart. Anterior part of heart by looking at V1 – V4. Lateral view of heart: I, aVL, V5 and V6. Inferior view of heart: II, III, and aVF.

Exercise 12-Lead ECG

12-Lead ECG Limb lead II shows large R amplitude because left ventricle current vector lies parallel with electrode placement. Chest lead V1 has large S wave because left ventricle current vector is directed away from electrode.

12-Lead ECG Strip

What types of pathology can we identify and study from EKGs? Arrhythmias Myocardial ischemia and infarction Pericarditis Chamber hypertrophy Electrolyte disturbances (i.e. hyperkalemia, hypokalemia) Drug toxicity (i.e. digoxin and drugs which prolong the QT interval)

Interpretation of ECG: Rate First measurement to calculate is heart rate. PQRST waves represent one complete cardiac cycle. At standard paper speed, divide 1500 by distance between R to R waves. Find R wave on heavy line. Count off 300, 150, 100, 75, 60 for each following line. Where next R lands is quick estimate. Multiply number of cycles in 6 second marks by 10.

The Rule of 300 It may be easiest to memorize the following table: # of big boxes Rate 1 300 2 150 3 100 4 75 5 60 6 50

Interpretation of ECG: Rate

10 Second Rule As most EKGs record 10 seconds of rhythm per page, one can simply count the number of beats present on the EKG and multiply by 6 to get the number of beats per 60 seconds. This method works well for irregular rhythms.

What is the heart rate? 33 x 6 = 198 bpm The Alan E. Lindsay ECG Learning Center ; http://medstat.med.utah.edu/kw/ecg/ 33 x 6 = 198 bpm

Precordial Leads

aVR, aVL, aVF (augmented limb leads) Summary of Leads Limb Leads Precordial Leads Bipolar I, II, III (standard limb leads) - Unipolar aVR, aVL, aVF (augmented limb leads) V1-V6

Arrangement of Leads on the EKG

Anatomic Groups (Septum)

Anatomic Groups (Anterior Wall)

Anatomic Groups (Lateral Wall)

Anatomic Groups (Inferior Wall)

Anatomic Groups (Summary)

The QRS Axis The QRS axis represents the net overall direction of the heart’s electrical activity. Abnormalities of axis can hint at: Ventricular enlargement Conduction blocks (i.e. hemiblocks)

The QRS Axis By near-consensus, the normal QRS axis is defined as ranging from -30° to +90°. -30° to -90° is referred to as a left axis deviation (LAD) +90° to +180° is referred to as a right axis deviation (RAD)

Quadrant Approach: Example 1

Quadrant Approach: Example 2

Equiphasic Approach: Example 2 The Alan E. Lindsay ECG Learning Center ; http://medstat.med.utah.edu/kw/ecg/

Interpretation of ECG: Rhythm Normal heart rhythm has consistent R-R interval. Mild variations due to breathing also normal.

Interpretation of ECG: Rhythm Normal Sinus Rhythm Rate: 60-100 b/min Rhythm: regular P waves: upright in leads I, II, aVF PR interval: < .20 s QRS: < .10 s Sinus Bradycardia Rate: < 60 bpm Sinus Tachycardia Rate: > 100 bpm

AV Conduction Disturbances Atrioventricular conduction disturbances refer to blockage of electrical impulse at AV node. 1st degree P waves result in delayed QRS. 2nd degree some but not all P waves have QRS.

Arrhythmias Arrhythmia: an irregular heartbeat. Sinus arrhythmia- P wave precedes @ QRS but RR interval varies. Premature Atrial Contraction (PAC) Premature Ventricular Contraction (PVC)

Arrhythmias

Where is the Conduction Problem?

1st Degree AV block

2nd Degree AV block

2nd Degree AV block

2nd Degree AV block

3rd Degree AV Block

Atrial Fibrillation Predominantly of left atrial origin High rate of atrial activation (>300bpm) IRREGULAR VENTRICULAR RESPONSE Filtering effect of the AV node Protects the ventricle from high atrial rates Explains why we “rate control” AF

Atrial Fibrillation

Atrial Fibrillation

Atrial Flutter

Narrow Complex tachycardia

Broad Complex Tachycardia

Left Atrial Abnormality Right Atrial Abnormality Prolonged P wave duration >120 msec in lead II Peaked P waves with amplitudes in lead II >0.25 mV (P pulmonale) Prominent notching of the P wave, usually most obvious in lead II, with an interval between the notches of >40 (P mitrale) Rightward shift of the mean P wave axis to above +75 degrees Ratio between the duration of the P wave in lead II and the duration of the PR segment >1.6 Increased area under the initial positive portion of the P wave in lead V1 to >0.06 mm-sec Increased duration and depth of the terminal negative portion of the P wave in lead V1 (the P terminal force) so that the area subtended by it exceeds 0.04 mm-sec   Leftward shift of the mean P wave axis to between -30 and -45 degrees

LV hypertrophy Parameter Criteria Sokolow-Lyon index SV1 + (RV5 or RV6) > 3.5 mV RaVL > 1.1 mV Romhilt-Estes point score system (points)[*] Any limb lead R wave or S wave ≥2.0 mV (3) or SV1 or SV2 ≥ 3.0 mV (3) or RV5 to RV6 ≥ 3.0 mV (3) ST-T wave abnormality (no digitalis therapy) (3) ST-T wave abnormality (digitalis therapy) (1) Left atrial abnormality (3) Left axis deviation ≤ -30 degrees (2) QRS duration >90 msec (1) Intrinsicoid deflection in V5 or V6 > 50 msec (1) Cornell voltage criteria SV3 + SaVL ≥ 2.8 mV (for men) SV3 + SaVL ≥ 2.0 mV (for women) Cornell voltage-duration measurement QRS duration × Cornell voltage >2436 QRS duration × sum of voltages in all leads >17,472

LV hypertrophy

RV hypertrophy

REST

Action potential & Ischemia

Ischemia

Ant . STEMI

ANT .& INF. STEMI

ACUTE PERICARDITIS

Problems with ECG recording Patient identity Lead position Paper speed and amplification Artifact Misinterpretation is much more common than poor recording technique.

Patient Identity

Lead Position There are three of these leads which are usually designated as I, II and III. They are all bipolar (i.e., they detect a change in electric potential between two points) and detect an electrical potential change in the frontal plane. Lead I is between the right arm and left arm electrodes, the left arm being positive. Lead II is between the right arm and left leg electrodes, the left leg being positive. Lead III is between the left arm and left leg electrodes, the left leg again being positive. The same three leads that form the standard leads also form the three unipolar leads known as the augmented leads. These three leads are referred to as aVR (right arm), aVL (left arm) and aVF (left leg) and also record a change in electric potential in the frontal plane. These leads are unipolar in that they measure the electric potential at one point with respect to a null point (one which doesn't register any significant variation in electric potential during contraction of the heart). This null point is obtained for each lead by adding the potential from the other two leads. For example, in lead aVR, the electric potential of the right arm is compared to a null point which is obtained by adding together the potential of lead aVL and lead aVF.

Paper Speed

Signal Amplification

Artifact

Artifact