ECG & Heart block [doctors online]

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


ST Segment

• Represents the earlier phase of repolarization of both the ventricles


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


Mobitz Type I(Wenkebach)

Conduction through the AV Node – progressively delayed until a drop beat is seen



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



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


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)


(Complete) P wave rate – normal

Ventricular rate – slow

Ventricular complex may be broad Idioventricular rhythm

Complete dissociation between P waves & QRS


(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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ECG & Heart block [doctors online]