Electrical EventsElectrical Events of the Cardiac Cycleof the Cardiac Cycle

ElectrophysiologyElectrophysiology

Dr.s.a.moezzi

Cardiac CycleCardiac 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 Mechanical Events of the Cardiac CycleCardiac Cycle

1. Ventricular Filling Period [ventricular diastole, atrial systole]

2. Isovolumetric Contraction Period [ventricular systole]

3. Ventricular Ejection Period [ventricular systole]

4. Isovolumetric Relaxation Period [ventricular diastole, atrial diastole]

Cardiac CycleCardiac 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 HeartElectrical System of Heart

ElectrocardiographyElectrocardiography

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.

ElectrophysiologyElectrophysiology

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.

ElectrophysiologyElectrophysiology

ElectrophysiologyElectrophysiology

ElectrophysiologyElectrophysiology

ElectrophysiologyElectrophysiology

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 Electrical Events of the Cardiac CycleCardiac 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 SystemElectrical Conduction System

Sinoatrial node (SA node)Intra-atrial PathwaysAtrioventricular node (AV node)Bundle of HisLeft and Right Bundle BranchesPurkinjie Fibers

The P waveThe 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 ComplexThe 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 waveThe 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 IntervalThe 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 IntervalThe 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 SegmentST 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 & VoltageECG 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 NSRComponents of a NSR

Standard 12-Lead ECGStandard 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 LeadsECG Limb Leads

ECG Augmented Limb LeadsECG Augmented Limb Leads

ECG Precordial LeadsECG Precordial Leads

Standard 12-Lead ECGStandard 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 ECGExercise 12-Lead ECG

12-Lead ECG12-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 Strip12-Lead ECG Strip

What types of pathology can we What types of pathology can we identify and study from EKGs?identify and study from EKGs?

ArrhythmiasMyocardial ischemia and infarctionPericarditisChamber hypertrophyElectrolyte disturbances (i.e. hyperkalemia,

hypokalemia)Drug toxicity (i.e. digoxin and drugs which

prolong the QT interval)

InterpretationInterpretation of ECG: of ECG:RateRate

First measurement to calculate is heart rate. PQRST waves represent one complete cardiac cycle.

1. At standard paper speed, divide 1500 by distance between R to R waves.

2. Find R wave on heavy line. Count off 300, 150, 100, 75, 60 for each following line. Where next R lands is quick estimate.

3. Multiply number of cycles in 6 second marks by 10.

The Rule of 300The Rule of 300It may be easiest to memorize the following table:

# of big # of big boxesboxes

RateRate

11 300300

22 150150

33 100100

44 7575

55 6060

66 5050

Interpretation of ECG: RateInterpretation of ECG: Rate

10 Second Rule10 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?What is the heart rate?

33 x 6 = 198 bpm

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

Precordial LeadsPrecordial Leads

Summary of LeadsSummary of Leads

Limb LeadsLimb Leads Precordial LeadsPrecordial Leads

BipolarBipolar I, II, IIII, II, III(standard limb leads)(standard limb leads)

--

UnipolarUnipolar aVR, aVL, aVF aVR, aVL, aVF (augmented limb leads)(augmented limb leads)

VV11-V-V66

Arrangement of Leads on the EKGArrangement of Leads on the EKG

Anatomic GroupsAnatomic Groups(Septum)(Septum)

Anatomic GroupsAnatomic Groups(Anterior Wall)(Anterior Wall)

Anatomic GroupsAnatomic Groups(Lateral Wall)(Lateral Wall)

Anatomic GroupsAnatomic Groups(Inferior Wall)(Inferior Wall)

Anatomic GroupsAnatomic Groups(Summary)(Summary)

The QRS AxisThe 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 AxisThe QRS AxisBy 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 1Quadrant Approach: Example 1

Quadrant Approach: Example 2Quadrant Approach: Example 2

Equiphasic Approach: Example 2Equiphasic Approach: Example 2

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

Interpretation of ECG:Interpretation of ECG:RhythmRhythm

• Normal heart rhythm has consistent R-R interval.• Mild variations due to breathing also normal.

Interpretation of ECG: RhythmInterpretation of ECG: RhythmNormal 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• Rhythm: regular

Sinus Tachycardia• Rate: > 100 bpm

AV Conduction DisturbancesAV Conduction Disturbances

o Atrioventricular conduction disturbances refer to blockage of electrical impulse at AV node.o 1st degree P waves result

in delayed QRS.o 2nd degree some but not

all P waves have QRS.

ArrhythmiasArrhythmias

Arrhythmia: an irregular heartbeat.

• Sinus arrhythmia- P wave precedes @ QRS but RR interval varies.

• Premature Atrial Contraction (PAC)

• Premature Ventricular Contraction (PVC)

ArrhythmiasArrhythmias

Where is the Conduction Where is the Conduction Problem?Problem?

11stst Degree AV block Degree AV block

22ndnd Degree AV block Degree AV block

22ndnd Degree AV block Degree AV block

22ndnd Degree AV block Degree AV block

33rdrd Degree AV Block Degree AV Block

Atrial FibrillationAtrial FibrillationPredominantly of left atrial originHigh 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 FibrillationAtrial Fibrillation

Atrial FibrillationAtrial Fibrillation

Atrial FlutterAtrial Flutter

Narrow Complex tachycardiaNarrow Complex tachycardia

Broad Complex TachycardiaBroad Complex Tachycardia

Left Atrial Abnormality Right Atrial AbnormalityProlonged 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

 

Parameter CriteriaSokolow-Lyon index SV1 + (RV5 or RV6) > 3.5 mV

RaVL > 1.1 mVRomhilt-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

LV hypertrophyLV hypertrophy

RV hypertrophyRV hypertrophy

RESTREST

Action potential & IschemiaAction potential & Ischemia

IschemiaIschemia

Ant . STEMIAnt . STEMI

ANT .& INF. STEMIANT .& INF. STEMI

ACUTE PERICARDITISACUTE PERICARDITIS

Problems with ECG recordingProblems with ECG recording

Patient identityLead positionPaper speed and amplificationArtifact

Misinterpretation is much more common than poor recording technique.

Patient IdentityPatient Identity

Lead PositionLead Position

Paper SpeedPaper Speed

Signal AmplificationSignal Amplification

ArtifactArtifact

ArtifactArtifact