202.3.3 science - cardiac, ecg physiology

8/6/2019 202.3.3 Science - Cardiac, ECG Physiology

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NMIH202

Science: The Physiology of ECGs,2011.

Peter Thomas.

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Introduction.

This lecture builds on the assumed knowledgefrom Lecture 3 (Week 3) Science:Electrocardiology (eLearning Exercise).

There are practice MCQs on this topic underAssessments in the eLearning site. TheseMCQs are similar too but not the same as theMCQs that may appear in the class quiz in Week13.

Students require information from this lecture tounderstand Lecture 5 (Week 8) ECGMonitoring: Basic Rhythms & Interpretations.

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ECG 1.

When muscles contract there is a change in theelectrical potential across the membranes ofmuscle fibres.

Body fluids & tissues are good conductors ofelectricity,

& the electrical changes which occur in thecontracting myocardium can be detected byattaching electrodes to the skin.

An electrocardiogram (ECG or EKG) can beseen on a monitor (continuous cardiacmonitoring) or as a intermittent tracing on paper.


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ECG 2.

Each deflection on the ECG indicates anelectrical event in the heart (which has asubsequent mechanical event).

A 12 lead ECG uses 9 electrodes (plus an earth)to take 12 tracings.

A cardiac monitor uses Lead II of an ECGbecause it gives a good cross-section throughthe heart.

ECG tracings look very different for differentleads.

Textbook ECG tracings are of Lead II.


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ECG 3. Bipolar Limb Leads: (standard leads) Lead I measures the current between the right &

left arms. Lead II between the right arm & left foot. Lead III between the left arm & left foot. Unipolar Limb Leads: (augmented leads) aVR is right arm. aVL is left arm. aVF is left leg.

These 6 leads examine the frontal (vertical)plane of the heart.


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ECG 4.

Precordial Unipolar Chest Leads:

V1, V2, V3, V4, V5, V6.

They are placed over the heart on theanterior chest.

These leads provide a horizontal view ofthe heart.

Put all 12 leads together & you get goodcross-sections through the heart.


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ECG 5.

V1V

6Placements:

V1 4th intercostal space, right of sternum.

V2 4th ICS, left of sternum.

V3 midway between V2 & V4.

V4

5th ICS, midclavicular line.

V5 5th ICS, anterior axillary line.

V6 5th ICS, midaxillary line.

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ECG 6.

An ECG is recorded on a box grid.

Each small horizontal box is 0.04 sec.

Each small vertical box is 0.1 mV.

(The larger boxes are 0.2 sec x 0.5 mV).

We are looking for:

1. Abnormal pattern of impulses.

2. Abnormal duration of phases of electricalstimuli.

3. Abnormal strength of impulses.

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ECG 7.

The horizontal line is the isoelectric line. There is no movement of electrical impulse in

the myocardium with regards to the recordingelectrode.

An upward deflection on the ECG trace refersto a depolarisation of cardiac muscles towardsthe positive electrode,

ie, the electrical impulse is moving towards theelectrode.

A downward deflection refers to depolarisationmoving away from the positive electrode.


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ECG 8.

A normal Lead II ECG tracing has a P wave,QRS complex & T wave.

P Wave: The P wave begins when the SA node

discharges. The P wave is caused by the passage of the

electrical impulse (depolarisation) through both

atria. Atrial contraction begins about the middle of the

P wave & continues through the PR segment. It is atrial systole.

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ECG 9. QRS Complex: It is ventricular depolarisation. It involves the movement of the impulse through

the AV bundle & the Purkinje fibres. The various QRS deflections reflect this & the

differences between the 2 ventricles. Ventricular contraction begins about way

through the QRS complex. It is ventricular systole. It is 0.06-0.10 sec (1.5-2.5 boxes). (The beginning of atrial repolarisation (diastole)

is hidden within the large ventricular electricaldischarge).


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ECG 10.

T Wave:

It is rapid ventricular repolarisation.

It represents ventricular (& atrial) diastole.

(U Wave:

Occasionally a U wave occurs.

It is late repolarisation of some Purkinjefibres).


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ECG 11.

PR Interval:

It is occasionally called the PQ interval,

but because the Q wave is often verysmall, is commonly known as the PRinterval.

It is the time it takes the atria to contract &begin to relax.

It is normally 0.12-0.20 sec (3-5 boxes).


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ECG 12.

QT Interval:

It is from the beginning of the QRScomplex to the end of the T wave.

It is the time required for the ventricles tocontract & begin to relax.

It is 0.30-0.38 sec.

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ECG 13.

PR Segment:

It is from the end of the P wave to the beginningof the R wave.

It is the movement of the impulse through the AVnode, bundle of His & Purkinje fibres.

It allows the ventricles to complete filling withblood.

ST Segment:

The entire ventricular myocardium is depolarised& contracting.

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ECG 14.

The following are some examples of ECG

tracings.

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References.

http://sprojects.mmi.mcgill.ca/cardiophysio

/default.htm

http://ecg.bidmc.harvard.edu/maven/mave

nmain.asp

http://library.med.utah.edu/kw/ecg/

http://www.rnceus.com/ekg/ekgframe.html

http://www.ecglibrary.com/ecghome.html

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Revision Questions.

1. The atrioventricular node:

a. is the cardiac pacemaker

b. causes ventricular diastole

c. initiates atrial systole

d. is found in the right atrium

2. On an ECG the P wave is:

a. atrial depolarisation

b. atrial diastole

c. ventricular repolarisation

d. ventricular systole

Answers: 1d, 2a.

202.3.3 science - cardiac, ecg physiology

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