ECG Electrophysiology Conduction Pathway Flashcards by Hayley Little

Sinoatrial (SA) Node

Dominant pacemaker

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Sinoatrial (SA) Node location

Right atrium, near the inlet of the superior vena cava

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Sinoatrial (SA) Node function

Receives blood from the right coronary artery

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Sinoatrial (SA) Node intrinsic rate

Fasted pacemaker in the heart; 60-100 bpm

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What is the beating of the heart regulated by?

An electrical impulse that causes the characteristic reading of an ECG

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Secondary pacemakers

If the SA becomes damaged or is suppressed, the farther away from the SA node, the slower its intrinsic rate of firing

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Pacemaker rates

SA node: 60-100 beats/minute
AV junction: 40-60 beats/minute
Purkinje fibres: 20-40

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Atrioventricular (AV) node location

Posterior septal wall, right atrium

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Atrioventricular (AV) node function

Delays impulse conduction, allows time for atria to empty blood into ventricles

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Atrioventricular (AV) node intrinsic rate

40-60 beats/minute

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Bundle branches - ventricles location

Interventricular septum

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Bundle branches - ventricles function

Relays impulse to Purkinje fibres

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Bundle branches - ventricles intrinsic rate

20-40 beats/minute

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Purkinje fibres - ventricles location

Ventricular myocardium

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Purkinje fibres - ventricles function

Relays impulse to myocardium

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Purkinje fibres - ventricles intrinsic rate

20-40 beats/minute

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Dysrhythmias result from?

Disorders of impulse formation
Disorders of impulse conduction

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Impulse formation

Non-pacemaker sites firing

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Impulse conduction

Blocks, reentry

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Enhanced automaticity is an abnormal condition in which one of the following occurs

-Cardiac cells that are not normally associated with a pacemaker function begin to depolarize spontaneously
-a pacemaker site other than the SA node increases its firing rate beyond that which is considered normal

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What are some causes of enhanced automaticity

Ischemia, hypoxia, electrolyte abnormalities, exposure to other chemicals/toxic substances

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Abnormal electrical impulses

Occur during repolarization (after depolarization) when cells are normally quiet
-requires a stimulus to initiate depolarization

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Types of Conduction blocks

Partial, complete

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Partial conduction block

Slowed; all impulses are conducted, takes longer than normal to do so
Intermittent; some impulses are conducted

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Complete conduction blocks

No impulses are conducted through the affected area

Reentry

An impulse returns to stimulate tissue that was previously depolarized

4 lead ECG placement

Placed on the limbs / limb leads

12 lead ECG placement

Chest and limbs / precordial leads

What is the wave of depolarization moving across the myocardium?

Positive

Positive deflection

Towards positive electrode at left leg results in a positive deflection on the ECG

Negative deflection

Away from negative electrode at ;eft leg results in a positive deflection on the ECG

What does an ECG measure?

Electrical events in the heart

What is artifact caused by?

Muscle tremors, patient movement, loose electrodes / poor contact, pacemaker activity, ambulance moving, 60-cycle interference

Lead II

Views the inferior surface of the left ventricle

One large box (5 squares)

5mm or 0.2 seconds

One small square

1mm or 0.04 seconds

Standard recording speed

25 mm/sec

Vertical axis measures?

Voltage/amplitude of the impulse

Standard calibration of vertical axis

1mm = 0.1 millivolt (mV) 1mV = 10mm or 2 large boxes

Horizontal axis = time

5 large boxes = 1 second 15 large boxes = 3 seconds 30 large boxes = 6 seconds

Baseline

Isoelectric line; a straight line recorded when electrical activity is not detected

Waveform

Movement away from the baseline in either a positive or negative direction

Segment

A line between waveforms; named by the waveform the precedes or follows it

Complex

Several waveforms

In an isoelectric line, what does ST elevation/depression indicate?

A cardiac event; measures deviations above and below isoelectric line

P wave

First waveform preceding QRS complex, normally upright and rounded in lead II

What does a P wave indicate

Atrial depolarization

Abnormal P waves

Notched, peaked, inverted

PR interval length

0.12 - 0.20 seconds (3-5 small boxes) > 0.20 means slowed conduction through AV junction

What is the PR interval measured from?

Start of the P wave to the first deflection or Q wave

PR segment

Part of PR interval, horizontal line between the end of the P wave and the beginning of the QRS complex

Abnormal PR interval - long

Greater than 0.20 second; indicates the impulse was delayed as it passed through the atria, AV node or AV bundle

Abnormal PR interval - short

Less than 0.12 second; may be seen when the impulse originates in the atria close to the AV node or the AV bundle

QRS complex

Q wave, R wave, S wave, J point

Q wave

First downward deflection after P wave; often not present

R wave

First upward deflection after Q or P wave; always positive

S wave

First downward deflection after R wave; always negative

J point

Small notch at the start of the ST segment

QRS complex length

Normally narrow <0.12 sec (3 boxes), measured from beginning of Q or R wave to end of S wave

What does QRS complex indicate

Ventricular depolarization, atrial repolarization

Normal Q wave length

Less than 0.3 second

Abnormal Q wave length

More than 0.3 second in duration or more than 30% of the following R wave height in that lead

Normal T waves

Slightly asymmetric, usually 0.5 mm or more in height in leads I and II

What happens to the T wave following an abnormal QRS complex?

Moves in a direction opposite that of the QRS

U wave

Represent late polarization of Purkinje fibres Normally small, round, and symmetric Most easily seen when the heart rate is slow

What is ST segment measured from?

The end of the QRS complex to the beginning of the T wave

What is ST elevation or depression indicative of?

Myocardial ischemia or injury

What does an ST segment indicate?

Early part of repolarization of right and left ventricles

Where is the T wave located?

First waveform following QRS complex , normally upright and slightly rounded in lead II

What does the T wave indicate?

Ventricular repolarization

QT interval

Portion of the ECG tracing from the beginning of the QRS complex to the end of the T wave; represents total ventricular activity

R-R interval

Used to determine ventricular rate and regularity