Medical Lab Procedures (3) chapter 45

electrocardiogram

a noninvasive and painless procedure frequently used in the diagnosis of heart disease and heart dysfunction; a recording of the electrical impulses of the heart muscle

An ECG is performed to…

detect damage from previous heart attacks, detect enlargement of the heart muscle, detect disturbances in the rhythm of other abnormal conditions, determine a cardiac baseline

What does the Q, R, and S wave represent?

ventricular contraction or systole

Q wave

a negative deflection/wave

R wave

positive deflection/wave

S wave

negative wave

T wave

positive wave and represents ventricular repolarization or diastole; ventricles relaxing

U wave

a positive deflection associated with repolarization; occurs in some patients not all

P wave

contraction (systole); atrial depolarization

electrocardiogram tracing

shows the electrical activity of the heart by way of waves; each wave represents what is happening at each stage of the cardiac conduction pathway

depolarization

electrical impulse that causes the heart to contract; positive ions (sodium) rush into the heart cells creating a signal that triggers muscle contraction

repolarization

subsequent electrical reset that allows the heart to relax and prepare for the next beat; when the ions return to their original resting state allowing muscle to relax (potassium)

QRS complex

ventricular depolarization

baseline or isoelectric line

represents the period of time in the cardiac cycle when there is no electrical activity

Electrical impulses originate in the …

SA node; sinoatrial node

The electrical impulse from the SA node causes the atria to contract; which is known as ___________.

atrial depolarization

What does the P wave represent?

the first part of the cardiac cycle; atrial depolarization

QRS complex of waves represents…

an electrical impulse traveling to the ventricles of the heart

multichannel machines

record the heart from 12 views using 12 leads

single channel machines

only measures 1 view of the heart

ECG tracing (or print out) is generated in about…

1 minute on a single sheet of “8×10” paper

ECG paper is marked with…

light and dark dots with lines that form squares

vertical axis (up and down)

records voltage

horizontal axis (side to side)

records the time

The paper moves through the machine at a rate of…

25 mm per second

Each small square on an ECG tracing paper represents ___________.

1mm or 0.04 seconds

Each large square on an ECG tracing paper is…

5mmx5mm and represents 0.2 seconds

5 large squares on an ECG tracing paper equals…

1 second

PR segment or interval

the duration from the start of the P wave to the beginning of the QRS complex; measuring the time it takes for the electrical signal to travel from the atria to the ventricles

How long is a normal PR interval?

between 0.12 and 0.20 seconds

slowed conduction (first degree heart block) results in…

a prolonged PR interval

What causes a short PR interval?

pre-excitation syndrome (like Wolff-Parkinson-White Syndrome)

What is a ECG deflection?

an upward or downward wave movement; represents electrical activity (depolarization and repolarization)

Positive deflection can show…

ventricular depolarization; seen in R wave

Negative deflection can be seen in…

Q or S waves