ecg & heart block [doctors online]
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ECG & HEART BLOCK
Electrocardiography
• A recording of the electrical activity of the heart over time
• Gold standard for diagnosis of cardiac arrhythmias
• Helps detect electrolyte disturbances (hyper- & hypokalemia)
• Allows for detection of conduction abnormalities• Screening tool for ischemic heart disease during
stress tests• Helpful with non-cardiac diseases (e.g.
pulmonary embolism or hypothermia
Electrocardiogram (ECG/EKG)
Is a recording of electrical activity of heart conducted thru ions in body to surface
ECG Graph Paper
• Runs at a paper speed of 25 mm/sec• Each small block of ECG paper is 1 mm2
• At a paper speed of 25 mm/s, one small block equals 0.04 s
• Five small blocks make up 1 large block which translates into 0.20 s (200 msec)
• Hence, there are 5 large blocks per second• Voltage: 1 mm = 0.1 mV between each individual
block vertically
Normal conduction pathway:
SA node -> atrial muscle -> AV node -> bundle of His -> Left and Right Bundle Branches -> Ventricular muscle
Recording of the ECG:
Leads used:• Limb leads are I, II, II. So called because at one
time subjects had to literally place arms and legs in buckets of salt water.
• Each of the leads are bipolar; i.e., it requires two sensors on the skin to make a lead.
• If one connects a line between two sensors, one has a vector.
• There will be a positive end at one electrode and negative at the other.
• The positioning for leads I, II, and III were first given by Einthoven. Form the basis of Einthoven’s triangle.
Types of ECG Recordings
Bipolar leads record voltage between electrodes placed on wrists & legs (right leg is ground)
Lead I records between right arm & left arm
Lead II: right arm & left leg
Lead III: left arm & left leg
Limb Leads Placement
Lead I Connects the right arm with the left arm
Lead II Connects the right arm with the left leg
Lead III Connects the left arm with the left leg
aVR Right arm
aVL Left arm
aVF Left leg
Placement of Precordial LeadsPrecordial Leads
Placement
V1 Fourth intercostals space, just to the right of the sternum
V2 Opposite V1, over the fourth intercostals space at the left sterna border
V3 Midway between V2 and V4
V4 Over the fifth intercostals space at the left midclavicular line
V5 Over the fifth intercostals space at the left anterior axillary line
V6 Over the fifth intercostals space at the left mid axillary line
Goldberger AL. Electrocardiography. In: Kasper DL, Braunwald E, Fauci AS, Hauser SL, Longo DL, Jameson JL, editors. Harrison’s principles of internal medicine(16th ed). McGraw-Hill;2005.p.1311-1319.Electrocardiogram analysis. In: Levine J, Munden J, Schaeffer L, Thompson G,editors. Portable ECGinterpretation. Lippincott Williams & Wilkins; 2007. P.257-364.
3 distinct waves are produced during cardiac cycle
P wave caused by atrial depolarization
QRS complex caused by ventricular depolarization
T wave results from ventricular repolarization
ECG
P wave
• First half is produced largely by depolarization of the right
atrium
• Second half is produced largely by depolarization of the
left atrium
Duration : 0.06 to 0.12 second Configuration : Usually rounded
and upright Amplitude : 2 to 3 mm high
Basic concept and the normal electrocardiogram. In: Weinberg RW, Miller KC, Somers J, editors. Rapid analysis Of Electrocardiogram: a self study programme. Lippincott Williams And Wilkins; 1999.p.3-17.
P-R Interval Includes the P wave and P-R segment0.12-2.0 sec Represents the time of transmission of the electrical
impulse from the atria to ventricle
Location : From the beginning of the
P wave to the beginning of the QRS
complex
• QRS complex:
• Represents ventricular depolarization
• Larger than P wave because of greater muscle mass of ventricles
• Normal duration = 0.08-0.12 seconds
• Its duration, amplitude, and morphology are useful in diagnosing cardiac arrhythmias, ventricular hypertrophy, MI, electrolyte derangement, etc.
• Q wave greater than 1/3 the height of the R wave, greater than 0.04 sec are abnormal and may represent MI
Basic concept and the normal electrocardiogram. In: Weinberg RW, Miller KC, Somers J, editors. Rapid analysis Of Electrocardiogram: a self study programme. Lippincott Williams And Wilkins; 1999.p.3-17.
ST Segment
• Represents the earlier phase of repolarization of both the ventricles
Basic concept and the normal electrocardiogram. In: Weinberg RW, Miller KC, Somers J, editors. Rapid analysis Of Electrocardiogram: a self study programme. Lippincott Williams And Wilkins; 1999.p.3-17.
Contd..
• Extends from the end of QRS complex to the beginning of T
wave
• Usually isoelectric or on the baseline
• Neither elevated (positive) nor depressed (negative)
• The point at which the ST segment joints the QRS complex
is known as the J (junction) point
T wave: • Represents repolarization or recovery of
ventricles• Interval from beginning of QRS to apex of T is
referred to as the absolute refractory period
ST segment:• Connects the QRS complex and T wave• Duration of 0.08-0.12 sec (80-120 msec
QT Interval• Measured from beginning of QRS to the end of
the T wave• Normal QT is usually about 0.40 sec• QT interval varies based on heart rate
AV Nodal Blocks (heart blocks)
Disturbances of the conduction through the heart, occurring at the AV Node
AV Node – damaged/diseased – delay or total block of impulses at the AV Node
This conduction defect can be seen on the ECG
Increased vagal tone (parasympathetic nervous system)
IHD (MI) Endocarditis Degeneration (age) Sclerosis (Aortic) Cardiac surgery trauma
Causes
AV Node
AV nodal conduction time is represented on the ECG as the PR segment.
But - we always measure the PR interval.
First Degree Heart Block (1º)
SA Node – normal Normal P wave
AV Node conducts more slowly than normal
Prolonged PR Interval Rest of conduction is normal
Normal QRS
First Degree Heart Block (1º)
PR Interval > 0.2 seconds (5 small sq)
Note – the PR Interval is constant
Significance
Clinical significance None
Treatment None
Note – this can progress to 2º or 3º heart block
Second Degree Heart Block (2º)
Mobitz Type I (Wenkebach)
Mobitz Type II
2 : 1
Second Degree Heart Block (2º)
Mobitz Type I(Wenkebach)
Conduction through the AV Node – progressively delayed until a drop beat is seen
Second Degree Heart Block (2º)
Mobitz Type I(Wenkebach)
PR Interval prolongs with each beat until a dropped beat is seen
The PR Interval is NOT constant After each dropped beat, the PR
interval is normal and the cycle starts again
Second Degree Heart Block (2º)
Mobitz Type I(Wenkebach)
PR PR PR DROPPED BEAT
2nd Degree AV block Mobitz 1
Significance
Clinical Significance Slight symptoms e.g.. Lethargy, Confusion
Treatment Pacemaker if during day &/or symptoms
No treatment if at night Note – this can progress to 3º Heart
Block
Second Degree Heart Block (2º)Mobitz Type II
Conduction through the AV node is constant.
PR interval is normal and constant Occasionally a dropped beat is seen
Second Degree Heart Block (2º)
Mobitz Type II
PR PR DROPPED BEAT PR
Significance
Clinical significance – this is more significant disease
Treatment – pacemaker
Note – this can progress to 3º Heart Block
Second Degree Heart Block (2º) 2 : 1
Unable to strictly classify as Mobitz Type I or II
Particular type of second degree Heart Block
Ratio 2 P waves : 1 QRS
Second Degree Heart Block (2º) 2 : 1
Significance
Clinical significance – unable to classify as Mobitz type I or II Will be associated with symptoms,
dizziness, lethargy etc.
Treatment – pacemaker
Note – this can deteriorate to 3º Heart Block
Third Degree Heart Block (3º)
(Complete) Complete failure of the AV Node
No impulses from Sinus Node will pass through to the ventricles
Some part if the conducting system will take over as pacemaker of the heart (even a myocardial cell 10-15 bpm)
Third Degree Heart Block (3º)
(Complete) P wave rate – normal
Ventricular rate – slow
Ventricular complex may be broad Idioventricular rhythm
Complete dissociation between P waves & QRS
Third Degree Heart Block (3º)
(Complete)
P P P P P
QRS QRS
3rd degree AV block
Significance
clinical significance Symptoms LOC, Confusion, Dizziness, Low BP
Can lead to standstill, VT or VF (stokes Adams)
Treatment - pacemaker
Summary
1º - prolongation of PR Interval ALL
2º - Mobitz I – Increasing PR Interval until dropped beat is seen SOME
Mobitz II – Constant PR Interval with more P waves to QRS
2 : 1 – Constant PR Interval with more P waves to QRS
3º - Complete dissociation between P waves & QRS
NONE
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