Basics of ECG

What does an electrocardiogram (ECG or EKG) record?

The heart's electrical activity.

What does the ECG represent?

The sum of the action potentials of cardiomyocytes.

What does the electrocardiogram (EKG) represent?

It represents the electrical events of the cardiac cycle.

What does each waveform on an EKG represent?

A specific electrical event in the cardiac cycle.

Why is studying EKG waveforms important?

It helps in understanding a patient's cardiac pathophysiology.

What is ECG paper made up of?

Horizontal and vertical lines.

What are EKG leads?

Electrodes that measure the difference in electrical potential.

What do bipolar leads measure?

The difference in electrical potential between two different points on the body.

What do unipolar leads measure?

The difference in electrical potential between one body point and a virtual reference point at the heart's center.

What are the four extremity electrodes?

• LA - left arm

• RA - right arm

• N - neutral, on the right leg

• F - foot, on the left leg

What are the six chest electrodes?

V1, V2, V3, V4, V5, V6

Where is V1 placed?

In the 4th intercostal space, right of the sternum.

Where is V2 placed?

In the 4th intercostal space, left of the sternum.

Where is V3 placed?

Between V2 and V4.

Where is V4 placed?

In the 5th intercostal space at the nipple line; recommended to place under the breast in women.

Where is V5 placed?

Between V4 and V6.

Where is V6 placed?

In the midaxillary line at the same height as V4.

What are the three standard extremity leads?

Lead I, Lead II, and Lead III.

Where does lead aVL point?

To the left arm.

Where does lead aVR point?

To the right arm.

Where does lead aVF point?

To the feet.

What types of pathology can we identify from ECG?

• Arrhythmias

• Myocardial ischemia and infarction

• Pericarditis

• Chamber hypertrophy

• Electrolyte disturbances

• Drug toxicity

How much time does 1 small square represent on EKG paper?

40 ms.

How many small squares are in 1 large square?

5 small squares.

How much time do 5 small squares represent?

200 ms.

How much time is 1 large square?

0.2 seconds or 200 ms.

How much time is 5 large squares?

1 second.

Methods for Determining the REGULAR Heart Rate?

1. Rule of 300

2. Count the number of Big Boxes

3. 1500 Method

How do you apply the Rule of 300?

Divide 300 by the number of big boxes between QRS complexes.

Count the number of big boxes formula:

1 big box = 300 bpm

2 big box = 150 bpm

3 big box = 100 bpm

4 big box = 75 bpm

5 big box = 60 bpm

6 big box = 50 bpm

7 big box = 43 bpm

8 big box = 38 bpm

9 big box = 33 bpm

What is the most accurate method to determine heart rate in an ECG?

1500 method

How do you apply the 1500 Method?

Count the small squares between two R waves and divide 1500 by that number.

Formula for Determining the IRREGULAR Heart Rate?

Count the number of cardiac cycles in 10 seconds and multiply this by 6

Where does the electrical impulse in the heart originate?

At the sinoatrial (SA) node.

Where is the SA node located?

On the posterior wall of the right atrium near the junction of the superior and inferior vena cava.

How many impulses per minute does the SA node generate?

60-100 impulses per minute.

Where is the AV node located?

In the lower interatrial septum.

How many impulses per minute does the AV node generate?

40-60 impulses per minute.

What is the path of the electrical impulse after the AV node?

Through the bundle of His, then to the Purkinje fibers.

How many impulses per minute does the bundle of His generate?

40-60 impulses per minute.

Where are the Purkinje fibers located?

In the myocardium.

How many impulses per minute do the Purkinje fibers generate?

20-40 impulses per minute.

What are the ECG Waveforms?

1. P wave

2. QRS complex

3. T wave

What does the P-wave represent?

Atrial depolarization

What does the PR interval represent?

Delay of the AV node to allow filling of the ventricles.

What does the QRS complex represent?

Ventricular depolarization (triggering main pumping contractions)

What does the T-wave represent?

Ventricular repolarization

What does the ST segment represent?

Beginning of ventricular repolarization; it should be flat.

What is depolarization?

Heart muscle contraction in response to an electrical stimulus (squeeze).

What causes depolarization?

Movement of Na, Ca, and K across the cell membrane.

What is repolarization?

Heart muscle relaxation as electrolytes move back across the membrane.

Why is repolarization important?

It prepares the cell for the next electrical impulse.