ecg

Action Potentials = Change in membrane potential occurring in nerve, muscle, heart and other cells

The ECG is not an action potential butreflects their cumulative effect at the level of the skin where the recordingelectrodes are located.

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• Electrocardiograph: instrument used to record the electrical activity of the heart

• Electrocardiogram (ECG): graphic representation of the electrical activity of the heart

Electrocardiogram (ECG/EKG)

• Is a recording of electrical activity of heart conducted thru ions in body to surface

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Normal conduction pathway:

SA node -> atrial muscle -> AV node -> bundle of His -> Left and Right Bundle Branches -> Ventricular muscle

• 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

Elements of the ECG:• P wave: Depolarization of both atria;

• Relationship between P and QRS helps distinguish various cardiac arrhythmias

• Shape and duration of P may indicate atrial enlargement• PR interval: from onset of P wave to onset of QRS

• Normal duration = 0.12-2.0 sec (120-200 ms) (3-4 horizontal boxes)

• Represents atria to ventricular conduction time (through His bundle)

• Prolonged PR interval may indicate a 1st degree heart block

• QRS complex: 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:• Connects the QRS complex and T wave• Duration of 0.08-0.12 sec (80-120 msec

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

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

Elements of the ECG:

• P wave• Depolarization of both atria;• Relationship between P and QRS helps distinguish various cardiac arrhythmias• Shape and duration of P may indicate atrial enlargement

PQRST wave

Deflection waves

P wave

• Lasts 0.08 s• Results due to depolarization from SA node

throughout atria • Atrial systole• Normal duration is not longer than 0.11

seconds (less than 3 small squares) • Amplitude (height) is no more than 3 mm

•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

• PR interval: • From onset of P wave to onset of QRS

• Normal duration = 0.12-2.0 sec (120-200 ms) (3-4 horizontal boxes)

• Represents atria to ventricular conduction time (through His bundle)

• Prolonged PR interval may indicate a 1st degree heart block

Deflection waves

QRS complex• Lasts 0.08 s (Normally not longer than 0.10 s in duration) • Results due to depolarization of ventricles• Ventricular systole & atrial diastole• R waves are deflected positively and the Q and S waves are negative

T wave• Results due to repolarization of ventricles• Lasts 0.16 s• Ventricular diastole

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

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

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Producing the ECG Waveform

• The 12-lead ECG produces a complete picture of the heart’s electrical activity

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12-Lead ECG• Utilizes 10 lead wires• Wires are color coded• Six chest leads• Four limb leads• Each lead is attached to an electrode

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Types of Leads• 10 lead wires produce 12 different lead

circuits– Three standard leads (bipolar)– Three augmented leads (unipolar)– Six chest leads (unipolar)

• Lead wires produce 12 different views of the heart

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Einthoven Triangle

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Waveform Types• Isoelectric – flat, no current flowing• Positive – upright, current flows to positive

electrode• Negative – downward, current flows away

from positive electrode

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Standard Limb Leads• Lead I – records tracing from right arm (-) to

left arm (+) and produces positive deflection• Lead II – records tracing from right arm (-) to

left leg (+) and produces positive deflection• Lead III – records electrical activity from left

arm (-) to left leg (+)

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Standard Limb Leads

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Augmented Leads• aVR• aVL• aVF• Known as augmented leads because their

tracings are increased in size by the ECG machine

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Augmented Leads• aVR– Measures in the direction of the right arm– Records activity from midway between left arm

and left leg to right arm

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Augmented Leads• aVL– Measures in the direction of the left arm– Records activity from the midpoint between right

arm and left leg to left arm

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Augmented Leads• aVF– Measures in the direction of the left foot– Records activity from the midpoint between right

arm and left arm to left leg

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Augmented Leads

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Chest Leads

• Also known as the precordial leads• Measure in one direction only

(unipolar)• Placed on specific sites on the chest• Numbered V1 – V6

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Chest Leads

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V1: Fourth intercostal space at the right sternal border. (First palpable intercostals space, below the clavicle is the 2nd intercostal space. V2: Fourth intercostal space at the left sternal border V3: Midway between V2 and V4 V4: Fifth intercostal space in the midclavicular line V5: Anterior axillary line at the same horizontal level as V4 V6: Mid-axillary line at the same horizontal plane as V4 and V5

Electrocardiogram Paper

• A grid system where time is measured along the horizontal axis.

• Each small square is 1 mm in length & represents 0.04 seconds.

• Each larger square is 5 mm in length & represents 0.2 seconds.

ECG Graph PaperDot matrix– Requires less ink– Easier to read– Makes sharper

photocopies

Standard grid Less expensive

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ECG Graph Paper• Graph paper may be heat/pressure sensitive and may

be erased by:– Alcohol– Plastic– Sunlight– X-ray film

• Some graph paper requires no special handling/storage and is guaranteed for 50 years

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Graph Paper Measurements• Horizontal readings– Represent time– Measured in millimeters

• Vertical readings– Represent voltage– 1 cm = 1 millivolt

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Graph Paper Measurements• Each large box is 5mm x 5mm • Each large box or vertical heavy line is 0.2

second in time• Each horizontal heavy line is 5mV in voltage• Each small box is 0.04 second in time and 1mV

in voltage

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Graph Paper Measurements

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Heart rate can be calculated from the EKG strip

• When the rhythm is regular, the heart rate is 300 divided by the number of large squares between the QRS complexes.

• For example, if there are 4 large squares between regular QRS complexes, the heart rate is 75 (300/4=75).

Heart rate can be calculated from the EKG strip

• Can be used with an irregular rhythm to estimate the rate. Count the number of R waves in a 6 second strip and multiply by 10.

• For example, if there are 7 R waves in a 6 second strip, the heart rate is 70 (7x10=70).

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• Perceptible• output• Output

• display

• Control• And• feedback

• Signal• processing

• Data• transmission

• Data• storage

• Variable• Conversion• element

• Sensor

• Primary• Sensing• element

• Measurand

• Calibration• signal

• Radiation,• electric current,• or other applied• energy

• Power• source

Generalized Instrumentation System

Types of ECG Machines• Single-channel recorders• Multi-channel recorders

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Single Channel Recorder• Monitors leads individually• Produces strips six feet long• Tracing strip must be cut and mounted or

placed on a card for interpretation

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Multi-channel Recorder

• Monitors leads three to six leads at a time• Recording time is approximately 10

seconds

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ECG Machine Functions• Input – Impulse from electrodes

• Signal processing – Amplifies electrical impulse inside the machine

• Output display – Either a printed report or an oscilloscope

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ECG Machine Functions (Cont’d)

• Computerized measurements and analysis• Storage• Communication• Interpretation

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ECG Machine Controls• Speed– Regulates speed of paper– Normally 25 mm/sec unless directed by physician– Changes must be noted on ECG report– Troubleshooting – Very fast heart rates may

require faster paper speed.

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ECG Machine Controls• Gain– Controls height of waveform– Normal setting is 10mm/mV– Changes must be noted on ECG report

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ECG Machine Controls

• Artifact filter– Normal setting between 40 and 150 Hz– Can be used to reduce artifact or abnormal marks

on tracing– Computer interpretation will be non-filtered.

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ECG Machine Controls• LCD display

– Liquid crystal diode

– Area where patient info can be viewed

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ECG Machine Controls• Heart rate limits– Machines may allow user to set heart rate limits

which activate an audio alarm and marking on the tracing.

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ECG Machine Controls• Stylus and standardization control– Stylus control found on older machines sets the

intensity of print and must be standardized before use.

– Standardization is required on some machines• Thermal technology is used on digital

machines

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ECG Machine Controls

• Lead selector– Can be used if one or more leads needs to be

repeated

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ECG Electrodes • Sensors that pick up electrical activity, conducting

to the ECG machine• Ten electrodes are used for the 12-lead ECG• Most electrodes are of the disposable type

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Chapter Summary (Cont’d)• Input, signal processing, and output display

are functions of the ECG• ECG machines contain lead wires, LCD display,

and paper for output• Various types of ECG electrodes are used for

ECG machines

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Ischemic Heart Disease

• Is most commonly due to atherosclerosis in coronary arteries

• Ischemia occurs when blood supply to tissue is deficient– Causes increased lactic acid from anaerobic metabolism

• Often accompanied by angina pectoris (chest pain)

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Ischemic Heart Disease

• Detectable by changes in S-T segment of ECG • Myocardial infarction (MI) is a heart attack – Diagnosed by high levels of creatine phosphate (CPK) & lactate

dehydrogenase (LDH)

Fig 13.34

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Arrhythmias Detected on ECG

• Arrhythmias are abnormal heart rhythms• Heart rate <60/min is bradycardia; >100/min is

tachycardia

Fig 13.35

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AV Transmission Blocks

• Impulse transmission through conduction tissue blocked.

• His Bundle Electrogram may be used to localize block.

His Bundle Electrogram

A H

VAtrial WaveHis BundleWave Ventricular wave

Prolongation of eitherthe A-H or H-V intervalindicates block above or below the Bundle of His

Arrhythmias Detected on ECG continued

• In flutter contraction rates can be 200-300/min• In fibrillation contraction of myocardial cells is

uncoordinated & pumping ineffective– Ventricular fibrillation is life-threatening

• Electrical defibrillation resynchronizes heart by depolarizing all cells at same time

Fig 13.35

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• AV node block occur when node is damaged• First–degree AV node block is when conduction through AV node >

0.2 sec– Causes long P-R interval

• Second-degree AV node block is when only 1 out of 2-4 atrial APs can pass to ventricles– Causes P waves with no QRS

• In third-degree or complete AV node block no atrial activity passes to ventricles– Ventricles driven slowly by bundle of His or Purkinjes

Arrhythmias Detected on ECG continued

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• AV node block occurs when node is damaged• First–degree AV node block is when conduction thru

AV node > 0.2 sec– Causes long P-R interval

Arrhythmias Detected on ECG continued

Fig 13.36

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• Second-degree AV node block is when only 1 out of 2-4 atrial APs can pass to ventricles– Causes P waves with no QRS

Arrhythmias Detected on ECG continued

Fig 13.36

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• In third-degree or complete AV node block, no atrial activity passes to ventricles– Ventricles are driven slowly by bundle of His or Purkinjes

Arrhythmias Detected on ECG continued

Fig 13.36

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Fibrillation

• Arrhythmia that is ineffectual in pumping blood.

• Atria or Ventricles may be involved.• Is due to fragmentation of reentry loop into

multiple irregular circuits.

Atrial Fibrillation

Atria do not contract and relax sequentially.No contribution to ventricular filling.No P waves. Irregular fluctuations or f waves.Normal QRS complexes but irregular rhythm.Compatible with life and full physical activity.20-30% reduction in ventricular pumping.

Ventricular Fibrillation

•Irregular continuous twitching of the ventricular muscle.•No pumping of blood possible.•Loss of consciousness occurs rapidly•Irregular fluctuations in the EKG•Often initiated by a premature impulse arriving in the vulnerable phase.

Atrial Flutter

F wave

Normal EKG

Wolf Parkinson-White SyndromeNormal

Wolf Parkinson White

Alternate Conduction PathwayBundle of Kent

Left Atrial EnlargementLeft atrial enlargement is best observed in the P waves of Leads II and V1. In V1 P is biphasic due to the position of the lead over the heart. Remember that the right side of the P wave represents the right atrial component and the left side the left atrial component. In figure B you can see that in both P waves the left atrial component has increased. In Lead I a notch has appeared in the left atrial component and in Lead V1 the left atrial component which is negative in more prominent.

Right Atrial Enlargement

In figure A the normal P waves of Leads II and V1 are shown. Figure B shows that the right atrial component of the P waves are more prominent in both leads. This is an indication of right atrial enlargement also called P pulmonale due to its relation to pulmonary disease.