October 15, 2019 (5th week – Winter Semester 2019/2020)

Electrophysiology of the heart

Cardiovascular System

What is a 12 lead ECG?

Records the electrical activity of the heart

(depolarisation and repolarisation of the

myocardium)

Views the surfaces of the left ventricle

from 12 different angles

Willem Einthoven

Why do a 12 lead ECG?

Monitor patients heart rate and rhythm

Evaluate the effects of disease or injury on

heart function

Detect presence of ischaemia / damage

Evaluate response to medications, e.g anti

dysrhythmics

Obtain baseline recordings before, during

and after surgical procedures

Recording an ECG - Procedure

1. Explain procedure to patient, obtain consent, check for allergies

2. Check cables are connected

3. Ensure surface is clean, dry

4. Ensure electrodes are in good contact with skin

5. Enter patient data

6. Wait until the tracing is free from artefact

7. Request that patient lies still.

8. Push button to start tracing

Procedure (cont.)

Before disconecting the leads ensure the

recording is -

Free from artifact

Paper speed is 25 or 50 mm/sec

Normal standardisation of 1mv, 10mm

Lead placement is correct

ECG is labelled correctly

Anatomy and Physiology

Review

A good basic knowledge of the heart and

cardiac function is essential in order to

understand the 12 lead ECG

Anatomical position of the heart

Coronary Artery Circulation

Conduction System

Resting Membrane Potential

Na+Na+

K+ K+

MICP ECP

KA K+ + A-

A-

A-

A-

A-

Na+

Na+

Na+

Na+

K+

+_ - 90 mV

Na+

K+ATP

Fig.: AP of Contractile Cardiac cells

Rapid depolarization

Rapid, partial early

repolarization,

prolonged period of

slow repolarization -

plateau phase

Rapid final

repolarization phase

AP of Contractile Cardiac cells

APs of cardiac contractile cells exhibit prolonged positive

phase (plateau) accompanied by prolonged period of

contraction

– Ensures adequate ejection time

– Plateau primarily due to activation of slow L-type Ca2+

channels

Autorhythmic cells:

– Initiate action potentials

– Have resting potentials called pacemaker potentials

– Pacemaker potential - membrane slowly depolarizes

“drifts” to threshold, initiates action potential, membrane

repolarizes to -60 mV.

– Use Ca influx (rather than Na) for rising phase of the AP

Figure

Fig.: Pacemaker Potential

• the pacemaker potential gradually becomes less

negative until it reaches threshold, triggering an AP

• ion movments during an action and pacemaker

potential

• states of various ion channels

Fig.: Refractory period

Long refractory period in a cardiac muscle prevents tetanus.

Fig. Membrane Potentials in SA Node vs.

Ventricle

Action Potentials

Fig./Table: Comparison of APs in cardiac and skeletal muscle

Excitation-Contraction Coupling in Cardiac

Contractile Cells

Ca2+ entry through L-type channels in T tubules

triggers larger release of Ca2+ from sarcoplasmic

reticulum

– Ca2+ induced Ca2+ release leads to cross-bridge cycling and

contraction

– Figure: EC Coupling in cardiac contractile cells

Electrocardiographic leads

An electrocardiographic lead is the pair of electrical

conductors used to detect cardiac potential differences.

ECG leads give the voltages measured between

different sites on the body.

A standard 12-lead ECG includes 6 limb leads and 6

chest leads.

The ECG is calibrated so that two dark horizontal lines

(1 cm) represent 1 mV, and five dark vertical lines

represent 1 second. This means that one light vertical

line represents 0.04 sec.

The standard 12 Lead ECG

6 Limb Leads 6 Chest Leads (Precordial leads)avR, avL, avF, I, II, III V1, V2, V3, V4, V5 and V6

Limb leads

Unipolar limb leads - aVL, aVR, aVF

aVL (yellow) – left arm

aVR (red) – right arm

aVF (green) – left leg

Bipolar limb leads - I, II, III

I – between aVL and aVR

II - between aVR and aVF

III - between aVL and aVF

I

II III

Bipolar limb leads (I,II,III)

Bipolar limb leads (I,II,III) give the potential difference

between two electrodes placed at different sites.

Electrodes of the bipolar limb leads are placed on the left

arm, right arm, and left leg.

The potential differences between each combination of

two of these electrodes give leads I, II, and III.

By convention, the left arm in lead I is the positive pole,

and the left leg is the positive pole in leads II and III.

Limb Leads

3 Bipolar Leads

form (Einthovens Triangle)

Lead I - measures electrical potential

between right arm (-) and left arm (+)

Lead II - measures electrical

potential between right arm (-) and

left leg (+)

Lead III - measures electrical

potential between left arm (-) and

left leg (+)

Unipolar augmented limb leads

The exploratory electrode for an augmented limb lead

(Goldberg) is an electrode on a single limb.

Lead aVR gives the potential difference between the right arm

(exploring electrode) and the combination of the left arm and

the left leg (reference).

Lead aVL gives the potential difference between the left arm

and the combination of the right arm and left leg.

Lead aVF gives the potential difference between the left leg

and the combination of the left arm and right arm.

Unipolar chest leads

Unipolar chest leads (Wilson) are designated V1 to V6 and

are placed over the areas of the chest.

V1 is just to the right of the sternum in the fourth intercostal

space.

V2 is just to the left of the sternum in the fourth interspace.

V4 is in the fifth interspace in the midclavicular line.

V3 is midway between V2 and V4.

V5 is in the fifth interspace in the anterior axillary line.

V6 is in the fifth interspace in the midaxillary line.

The three limb leads are combined to give the reference

voltage (zero) for the unipolar chest lead (V).

V1

V2

V3

V4

V5

V6

Unipolar chest leads

Inferior

II, III, AVF

Antero-Septal

V1,V2, V3,V4

Lateral

I, AVL, V5,

V6

Posterior

V1, V2, V3

RIGHT LEFT

ECG Waveforms

a positive deflection

a negative deflection

• The P wave represents atrial depolarisation

• the PR interval is the time from onset of atrial activation to

onset of ventricular activation

• The QRS complex represents ventricular depolarisation

• The S-T segment should be iso-electric, representing the

ventricles before repolarisation

• The T-wave represents ventricular repolarisation

• The QT interval is the duration of ventr. activation and recovery.

ECG waves, spikes, segments, intervals

P

Q

R

S

T

P-R

interval

Q-T interval

QRS length

Normal: PR interval: 0.12-0.2 sec

QRS length: <0.10 sec

QT interval: 0.3-0.4 sec

Abnormalities in:

QRS – ventricular depolarizaton

problems

P-R interval – A/V conduction

problems

ECG waves, spikes, segments, intervals

Different parts of ECG record can be

correlated to specific cardiac events

Portions of the ECG Are Associated With Electrical

Activity in Specific Cardiac Regions

EKG Axis Determination

Atrial

Depolarization

Septal

Depolarization

Apical

Depolarization

Late

Ventricular

Depolarization

Repolarization

Lead I:

Determining Mean Electrical Axis

Use 2 different leads and measure the sum of the height

and the negative depth of the QRS complex.

Measure that value in mm onto the axis of the lead and

draw perpendicular lines.

The intersection is at the angle of the mean axis.

The mean QRS electrical axis is influenced by:

a) the position of the heart in the chest,

b) the properties of the cardiac conduction system,

c) the excitation and repolarization properties of the

ventricular myocardium.

The mean QRS electrical axis can provide valuable

information about a variety of cardiac diseases.

Mean Electrical Axis

The ECG provides three types of information about

the ventricular myocardium:

The Pattern of Ventricular Excitation (→ an abnormality in

the QRS complex)

Changes in the Mass of Electrically Active Ventricular

Myocardium.

Abnormal Dipoles Resulting From Ventricular Myocardial

Injury (ST segment).

How to read the ECG

– Look at the whole tracing.

−quality of the recordingRhythm: Is there a P wave before each QRS complex? − Yes: sinus rhythm No: AV junctional or heart block

Rate: Count boxes; use caliper, ruler

PR interval: Normal - 0.20 sec. or less

QRS complex: Skinny (0.10 sec. or less) or broad (BBB or ventricular)

ST segment: Isoelectric (normal), elevated or depressed

T wave: Upright, flat or inverted

electrical axis, transition zone

Interpretation: Normal or abnormal.

− Is the rhythm dangerous?

Heart Excitation Related to ECG

Figure: Electrical events of the cardiac cycle