I.  Cardiac cycle

A. Consists of one beat, or one P-QRS-T sequence.

1. The sequence: Atrial contraction (systole) and relaxation (diastole), followed by ventricular contraction and relaxation

2. The cardiac cycle is measured on ECG from one R wave to the next R wave

3. Between cycles the ECG recorder returns to baseline or isoelectric line – the flat line between the T wave and the next P wave

B. Waveform deflections

1. Any waveform above the isoelectric line is positive (upright)

2. Any waveform below the isoelectric line is negative (downward)

3. Any waveform having both a positive and a negative component is called a biphasic deflection

II. Waveforms and current flow

A. An ECG lead provides a particular view of the hearts electrical activity between two points or poles

B. Each lead consists of a positive (+) pole and a negative (–) pole

C. The configuration of the waveform seen in each lead will depend on the placement of that lead relative to the path of cardiac electrical activity

D. The wave will be positive (deflected upward) if the electrical activity of the heart approaches the positive lead

E. Conversely, the wave will be negative (deflected downward) if the electrical activity of the heart move away from the positive lead

III. ECG paper

A. The P-QRS-T sequence is recorded on special graph paper

B. The vertical lines measure amplitude or voltage

1. Each small box represents 0.1 mV

2. Each large block (made up of 5 small boxes) represents 0.5 mV

3. To determine the amplitude of a wave, segment or interval, count the number of small boxes from the baseline to the highest or lowest point

C. The horizontal lines measure time

1. Each small box equals 0.04 seconds

2. Each large block (made up of 5 small boxes) equals 0.2 seconds (multiply 0.04 x 5 = 0.2)

3. The width of complexes and intervals are determined by the number of small boxes they extend over, then multiply by the time 0.04

4. When measuring or analyzing a heart rate or rhythm, a minimum of 6 seconds (30 large blocks) should be used

IV. Waves, Intervals, Segments, and Complexes

A. P wave

1. The first deflection of the cardiac cycle

2. Represents depolarization of the atria

3. Begins as the waveform begins to leave the baseline, and ends when it returns to baseline

4. Normal: Location - precedes every QRS complex
b) Amplitude - 0.5 to 2.5 mm high Rounded
c) Duration - 0.06 to 0.12 seconds
d) Configuration - small, rounded, similar in size and shape
e) Deflection - positive (upright) in lead II

5. Normal P waves indicate:

a) the electrical impulse originated in the SA node

b) that normal depolarization of the atria occurred

6. Abnormal P waves:

a) Inverted - indicates reverse (retrograde) conduction from the AV junction backward toward the atria

b) Peaked, notched or enlarged - the sinus impulse traveled through altered or damaged atria

c) Varying - the impulse may be coming from different sites

d) Absent - conduction by a route other that the SA node

e) Not preceding a QRS - heart block

B. PR interval

1. Represents the time required for the electrical impulse to leave the SA node and travel through the atria, AV node, bundle branches, and Purkinje fibers

2. Normal:

a) Location - from the beginning of the P wave to the beginning of the QRS complex, includes the P wave and the short flat (isoelectric) line that follows it

b) Amplitude - not measured

c) Duration - 0.12 to 0.20 seconds

3. Normal PR interval indicates that the electrical impulse leaving the SA node was conducted through the entire electrical system within a normal amount of time

4. Abnormal PR interval:

a) Short

(1) Electrical impulse was conducted through an abnormal pathway that bypasses the AV node, or

(2) The impulse originated in an ectopic site close to the AV junction

b) Long - indicates the electrical impulse is delayed traveling through the AV node and bundle of His

C. QRS complex

1. Represents ventricular depolarization

2. Measured from the beginning of the QRS as the first wave leaves the baseline to the J point

3. J point

a) The junction between the QRS and the ST segment

b) Where the last wave of the complex begins to flatten out at, above, or below the baseline

c) Elevation or depression of the ST segment can make finding the J point difficult

4. Normal:

a) Location - follows the PR interval

b) Amplitude  - varies greatly

c) Duration - 0.06 to 0.10

d) Configuration (use figure 3-6, p18, Huff)

(1) consists of the Q wave (the negative deflection before the R wave), the R wave (the first positive deflection after the Q wave), the S wave (the first negative deflection after the R wave)

(2) May not always see all three waves.

(3) The second R or S waves are labeled R'(R prime) or S'(S prime)

e) Deflection - different in each lead

5. Normal QRS indicates that ventricular depolarization occurred within a normal amount of time

6. Abnormal (wide) QRS indicates that abnormal depolarization has occurred due to

a) Bundle branch block

b) Conduction through an abnormal pathway

c) The impulse originated in an ectopic site in the ventricles

D. ST segment

1. Represents the end of ventricular conduction

2. Normal:

a) Location - extends from the S wave to the beginning of the T wave

b) Deflection - usually isoelectric

3. Normal indicates the end of ventricular depolarization and the beginning of repolarization

4. Abnormal

a) Elevation is a sign of myocardial injury

b) Depression is most often associated with myocardial ischemia

E. T wave

1. Represents ventricular recovery or repolarization

2. Normal:

a) Location - follows the S wave

b) Amplitude - 0.5 mm, greater in MCL

c) Duration - duration 0.10 to 0.25 seconds

d) Configuration - rounded, slightly asymmetrical (ascends more slowly than the descent)

e) Deflection - usually positive

3. Abnormal T waves indicate abnormal repolarization has occurred

F. QT interval

1. Measures ventricular depolarization and repolarization

2. Normal:

a) Location - from the beginning of the QRS to the end of the T wave

b) Duration

(1) Varies according to heart rate, influence of medications, and other factors

(2) Should not be greater than half the distance between two regular R waves

3. Abnormal:

a) Long - indicates that ventricular repolarization time has slowed, which means that the relative refractory period (the vulnerable period) of the cardiac cycle is longer, predisposing patients to potentially lethal ventricular dysrhythmias

b) Short - may result from digitalis toxicity or hypercalcemia

G. U wave

1. Represents the recovery period of the Purkinje fibers

2. Is not present of every rhythm strip, best seen with slow heart rates

3. Configuration is the most important characteristic

4. Normal:

a) Location - after the T wave and before the next P wave

b) Configuration - upright and rounded

c) Deflection - upright

5. Abnormal - prominent U waves may be due to hypercalcemia, hypokalemia or digitalis toxicity

V. Cardiac monitors – 2 types of recordings

A. Standard 12 lead (additional material regarding 12 lead recordings is outside the scope of this class)

1. Uses 10 electrodes to obtain a more complete view the heart

2. Records 12 different views of the heart, at different angles

3. Provides a complete picture of the hearts electrical activity

1. Also known as rhythm monitoring

2. Provides continuous information about the hearts electrical activity

3. Chest leads pick up the electrical activity and displays it on a monitor

4. Electrodes are conductive gel discs that are placed on the patient's chest, these discs should be changed every 24 hours to avoid drying of the discs. Drying of the disc can cause poor tracings

VI. Types of single lead monitoring

A. Hard wire monitoring

1. Electrodes are attached to a cable which feed into a cardiac monitor

2. The monitor can be permanently attached in a patient room or can be portable

3. 3 leadwire system (used with portable monitors)

(1) white – right arm (RA)

(2) black – left arm (LA)

(3) red – left leg (LL)

(1) RA is placed right below the clavicle (2nd intercostal space) right midclavicular line

(2) LA is placed right below the clavicle (2nd intercostal space) left midclavicular line

(3) LL is placed on the left lower rib cage (8th intercostal space, left midclavicular line)

c) Leads placed in this position will allow the monitoring in leads I, II, or III using the lead select on the monitor

d) To monitor MCL (V1) – reposition the LL lead to the V1 position (4th intercostal space, right sternal border), and change the lead select on the monitor

4. 5 leadwire system (used mostly in the Critical Care Units)

(1) white – right arm (RA)

(2) black – left arm (LA)

(3) red – left leg (LL)

(4) green – right leg (RL)

(5) brown – chest lead (C)

(1) RA is placed right below the clavicle (2nd intercostal space) right midclavicular line

(2) LA is placed right below the clavicle (2nd intercostal space) left midclavicular line

(3) LL is placed on the left lower rib cage (8th intercostal space, left midclavicular line)

(4) RL is placed on the right lower rib cage ( 8th intercostal space, right midclavicular line)

(5) C is usually placed in the MCL or V1 position (4th intercostal space, right sternal border), but can be moved to any of the 6 chest lead positions

c) This configuration allows you to monitor any of the of the standard 12 leads using the lead selector on the monitor

B. Telemetry

1. The patient is attached to a small transmitter box

2. Radio waves transmit the tracing to a central monitor (this is when cell phones can interfere)

3. Telemetry systems

(1) negative – usually white

(2) positive – usually red

(3) ground – usually black

b) The lead that will be monitored depends on the placement of the electrodes

(1) lead II – NEGATIVE (white) @ 2nd intercostal space, right midclavicular line, POSITIVE (red) @ 8th intercostal space, left midclavicular line, GROUND (black) @ (2nd intercostal space, left midclavicular line

(a) best lead for viewing the atrial activity –  P wave

(b) lead II tracing shows a positive or upward deflection since the electrical activity of the heart approaches the positive lead

(2) MCL – NEGATIVE (white) @ 2nd intercostal space, left midclavicular line, POSITIVE (red) @ 4th intercostal space, right sternal border, GROUND (black) @ 2nd intercostal space, right midclavicular line

(a) best lead for differentiating ventricular arrhythmia

(b) MCL tracing shows a negative or downward deflection since the electrical activity of the heart move away from the positive lead

C. Applying electrodes

1. Prepare the skin

a) Shave the skin if necessary – hair interferes with the contact between the electrode disc and the skin

b) Remove skin oil – use a dry washcloth or soap and water

2. Attach electrode disc

a) Avoid placing over bony areas

b) Check to make sure electrode dics are not dry

VII. Criteria for identification of rhythm strips

A. Use calipers

B. 6 step method of interpretation

1. Check rhythm for regularity

a) Is the rhythm regular

b) Is the rhythm irregular

c) Is the rhythm regularly irregular

2. Calculate the ventricular rate (use one of the three methods)

a) If regular: divide 1500 by the number of small boxes between two R waves

b) If irregular: count the number of R waves on a 6 second strip and multiply by 10

c) Rapid method: start with an R wave that falls on a heavy black line and stop at the closest R wave counting 300, 150, 100, 75, 60, 50, 43, 37

3. Measure the QRS complex

a) Are all the complexes the same size and shape

b) Is the QRS complex normal, less than 0.10 seconds

4. Identify and examine the P waves

a) Do the P waves have normal configuration

b) Do all the P waves have the same size and shape

c) Is there only one P wave for each QRS

d) If more than one P wave for each QRS, what is the ratio

5. Measure the PR interval

a) Is the PR interval normal, 0.12 - 0.20 seconds

b) Is the PR interval constant

6. Identify the rhythm or rhythms