33.4e Relationship of the ECG to Electrical Events in the Heart

electrical changes

The ECG tracing’s components reflect (?) in the heart (Figure 33-20).

P wave

The first component of the ECG, the (?) corresponds to atrial depolarization.

positive, rounded wave

On lead II, it is a (?) before the QRS complex (Figures 33-21 to 33-25).

QRS complex

The (?) reflects ventricular depolarization.

Q wave, R wave, S wave

The (?) is the first negative deflection after the P wave; the (?) is the first positive deflection after the P wave; and the (?) is the first negative deflection after the R wave.

All three waves are not always present

(?), and the shape of the QRS complex can vary among individuals (Figure 33-26).

T wave

The (?) reflects repolarization of the ventricles.

Normally positive in lead II

(?), it is rounded and usually moves in the same direction as the QRS complex (Figure 33-27).

U wave

Occasionally, a (?) appears.

T waves

U waves follow (?) and are usually positive; they can be associated with electrolyte abnormalities, or they can be a normal finding.

PR interval

In addition to the waveforms described, the ECG tracing reflects these important time intervals (Figure 33-28): The (?) is the distance from the beginning of the P wave to the beginning of the QRS complex.

time the impulse takes to travel from the atria to the ventricles

It represents the (?) .

PQ interval

Occasionally, the R wave is absent, in which case this interval is called the (?).

“PR interval” and “PQ interval”

The terms “(?)” and “(?)” can be used interchangeably.

QRS interval

The (?) is the distance from the first deflection of the QRS complex to the last.

time necessary for ventricular depolarization

It represents the (?) .

ST segment

The (?) is the distance from the S wave to the beginning of the T wave.

isoelectric line

Usually, it is an (?); however, it can be elevated or depressed in certain disease states such as ischemia.

0.12 to 0.20 second

A normal PR interval is (?).

short PRI, prolonged PRI

A (?) lasts less than 0.12 second; a (?) lasts longer than 0.20 second.

delay in the AV node

A prolonged PRI indicates a (?) .

0.04 and 0.12 second

A normal QRS complex lasts between (?) .

less than 0.12 second

A value of (?) means that the ventricles depolarized in a normal length of time.

QT interval

The (?) represents the total duration of ventricular depolarization.

0.33 to 0.42 second

A normal QT interval is (?).

inverse relationship

QT intervals and heart rate have an (?) : Increases in heart rate usually decrease the QT interval whereas decreases in heart rate usually prolong it.

corrected QT (QTc)

Generally, the QT interval is expressed as a (?) by taking the QT interval and dividing it by the square root of the RR interval (interval between ventricular depolarizations).

independent of heart rate

This allows an assessment of the QT interval that is (?).

less than 0.44 second

Normal corrected QTc intervals are (?).

increased risk of certain ventricular arrhythmias and sudden death

A prolonged QT interval is thought to be related to an (?) .

antipsychotic medications

Numerous medications, particularly some of the (?), have been associated with prolongation of the QT interval.

interval when the heart cannot be restimulated to depolarize

The all-or-none nature of myocardial depolarization results in an (?) .

myocardial cells

have not yet repolarized and cannot be stimulated again

absolute refractory period and relative refractory period

This refractory period has two parts: an (?) and a (?).

absolute refractory period

During the (?) , stimulation produces no depolarization whatsoever.

beginning of the QRS complex to the apex of the T wave

This usually lasts from the (?) .

relative refractory period

During the (?) , a sufficiently strong stimulus can produce depolarization.

T wave’s downslope

This usually corresponds to the (?).

isoelectric (flat) line

As noted, the ST segment is usually an (?).

Myocardial infarction (MI)

A (?) , which is caused by lack of blood flow to a part of the heart, produces changes in this line.

electrically dead

The area affected by the MI is (?) and cannot conduct electrical impulses.

Ischemia, Injury, Necrosis

MIs usually follow this sequence: (1) (?) (2) (?) (3) (?).

distinct ST segment changes

Each of these stages results in (?).

ST segment depression or an inverted T wave

Ischemia causes (?).

symmetrical

The inversion is usually (?).

ST segment

Injury elevates the (?) , most often in the early phases of an MI.

significant Q wave

As the tissue dies, a (?) appears.

small, insignificant Q waves

As noted earlier, (?) can appear in normal ECG tracings.

at least one small square box wide, lasting 0.04 second, more than one-third the height of the QRS complex

A significant Q wave is (?) , or is (?).

extensive transient ischemia

Q waves can also indicate (?).

different ECG leads

Using the various ECG leads is comparable to waiting for a train to pass by at a railroad crossing.

how long the train is

You would like to know (?) , but when it first comes into view, you can see only the front of the train.

cameras at other viewpoints along the tracks

If you had (?) , you could see how long the train actually is.

different parts of the heart

Similarly, by combining the different ECG leads, you can view (?).

V1-V4

Leads (?) view the anterior surface of the heart.

I and aVL

Leads (?) view the lateral surface of the heart.

inferior surface

The heart’s (?) can be visualized in leads II, III, and aVF.

ischemia, injury, and necrotic changes

These leads can show (?) and can provide information about the corresponding heart surface (Table 33-3).

V1-V4

For example, significant ST elevation in (?) can indicate anterior involvement whereas elevation in leads II, III, and aVF can indicate inferior involvement.

Part 3

(?) of this chapter on 12-lead ECG analysis discusses these leads in more detail.

advanced or acute stage

When EMS arrival at the scene of an MI, the MI has usually reached an (?) in which irreversible necrosis has taken place, which can be termed acute myocardial infarction (AMI).

(The terms AMI and MI are often used interchangeably.)

(?).

Medical procedures (percutaneous coronary intervention) and drugs (fibrinolytics)

(?) can treat AMI.

earlier they are initiated, the better the patient’s potential outcome

The (?) .

Earlier identification in the field of patients with AMI

Earlier identification in the field of patients with AMI allows for earlier interventions, but the 12-lead ECG’s role in prehospital care remains unresolved.

not appropriate in many EMS settings

Its use is (?) .

Individual EMS medical directors

(?) determine the application and use of the 12-lead ECG in their specific EMS settings.