Electrical Events of the Cardiac Cycle Electrophysiology Electrical Events of the Cardiac Cycle...
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Transcript of Electrical Events of the Cardiac Cycle Electrophysiology Electrical Events of the Cardiac Cycle...
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Electrical EventsElectrical Events of the Cardiac Cycleof the Cardiac Cycle
ElectrophysiologyElectrophysiology
Dr.s.a.moezzi
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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.
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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]
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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.
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Electrical System of HeartElectrical System of Heart
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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.
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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.
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ElectrophysiologyElectrophysiology
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ElectrophysiologyElectrophysiology
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ElectrophysiologyElectrophysiology
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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.
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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.
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Electrical Conduction SystemElectrical Conduction System
Sinoatrial node (SA node)Intra-atrial PathwaysAtrioventricular node (AV node)Bundle of HisLeft and Right Bundle BranchesPurkinjie Fibers
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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
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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.
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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.
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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.
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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.
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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
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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.
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Components of a NSRComponents of a NSR
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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.
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ECG Limb LeadsECG Limb Leads
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ECG Augmented Limb LeadsECG Augmented Limb Leads
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ECG Precordial LeadsECG Precordial Leads
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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.
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Exercise 12-Lead ECGExercise 12-Lead ECG
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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.
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12-Lead ECG Strip12-Lead ECG Strip
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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)
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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.
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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
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Interpretation of ECG: RateInterpretation of ECG: Rate
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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.
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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/
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Precordial LeadsPrecordial Leads
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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
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Arrangement of Leads on the EKGArrangement of Leads on the EKG
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Anatomic GroupsAnatomic Groups(Septum)(Septum)
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Anatomic GroupsAnatomic Groups(Anterior Wall)(Anterior Wall)
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Anatomic GroupsAnatomic Groups(Lateral Wall)(Lateral Wall)
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Anatomic GroupsAnatomic Groups(Inferior Wall)(Inferior Wall)
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Anatomic GroupsAnatomic Groups(Summary)(Summary)
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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)
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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)
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Quadrant Approach: Example 1Quadrant Approach: Example 1
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Quadrant Approach: Example 2Quadrant Approach: Example 2
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Equiphasic Approach: Example 2Equiphasic Approach: Example 2
The Alan E. Lindsay ECG Learning Center ; http://medstat.med.utah.edu/kw/ecg/
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Interpretation of ECG:Interpretation of ECG:RhythmRhythm
• Normal heart rhythm has consistent R-R interval.• Mild variations due to breathing also normal.
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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
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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.
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ArrhythmiasArrhythmias
Arrhythmia: an irregular heartbeat.
• Sinus arrhythmia- P wave precedes @ QRS but RR interval varies.
• Premature Atrial Contraction (PAC)
• Premature Ventricular Contraction (PVC)
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ArrhythmiasArrhythmias
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Where is the Conduction Where is the Conduction Problem?Problem?
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11stst Degree AV block Degree AV block
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22ndnd Degree AV block Degree AV block
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22ndnd Degree AV block Degree AV block
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22ndnd Degree AV block Degree AV block
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33rdrd Degree AV Block Degree AV Block
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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
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Atrial FibrillationAtrial Fibrillation
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Atrial FibrillationAtrial Fibrillation
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Atrial FlutterAtrial Flutter
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Narrow Complex tachycardiaNarrow Complex tachycardia
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Broad Complex TachycardiaBroad Complex Tachycardia
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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
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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
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LV hypertrophyLV hypertrophy
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RV hypertrophyRV hypertrophy
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RESTREST
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Action potential & IschemiaAction potential & Ischemia
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IschemiaIschemia
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Ant . STEMIAnt . STEMI
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ANT .& INF. STEMIANT .& INF. STEMI
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ACUTE PERICARDITISACUTE PERICARDITIS
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Problems with ECG recordingProblems with ECG recording
Patient identityLead positionPaper speed and amplificationArtifact
Misinterpretation is much more common than poor recording technique.
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Patient IdentityPatient Identity
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Lead PositionLead Position
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Paper SpeedPaper Speed
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Signal AmplificationSignal Amplification
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ArtifactArtifact
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ArtifactArtifact