ECG Waveforms

What is normal?

Normal sinus rhythm with normal QRS morphology.

The heart’s electrical activity is represented on the monitor or ECG tracing by three basic waveforms: the P wave, the QRS complex, and the T wave. A U wave is sometimes present.

Between the waveforms are the following segments and intervals: the PR interval, the PR segment, the ST segment, and the QT interval. Although the letters themselves have no special significance, each component represents a particular event in the depolarization–repolarization cycle.

The P Wave

• Atrium depolarization/activation or the spread of the impulse from the SA node throughout the atria.
• A waveform representing atrial repolarization is usually not seen on the ECG because atrial repolarization occurs during ventricular depolarization and is hidden in the QRS complex.

• Can be used to reveal right/left atrial hypertrophy or atrial arrhythmias

• Best determined in leads II and V1 during sinus rhythm

Characteristics of a normal P wave:

• The maximal height of the P wave is 2.5 mm in leads II and / or III
• The p wave is positive in II and AVF, and biphasic in V1
• The p wave duration is shorter than 0.12 second

There are 2 types of abnormal P waves: 

1.   Abnormal Sinus P Wave

• Originates in the sinus node and travels through enlarged atria. 
• Results in abnormal depolarization of atria – abnormal looking P waves. If P wave enlarged, the atria are enlarged.
• Altered P wave morphology is seen in left or right atrial enlargement.

• Impulses traveling through an enlarged right atrium (right atrial hypertrophy) result in P waves that are tall and peaked.

  • The abnormal P wave in right atrial enlargement is sometimes referred to as p pulmonale because the atrial enlargement that it signifies is common with severe pulmonary disease (for example, pulmonary stenosis and insufficiency, chronic obstructive pulmonary disease, acute pulmonary embolism, and pulmonary edema).

• Impulses traveling through an enlarged left atrium (left atrial hypertrophy) result in P waves that are wide and notched. The term p mitrale is used to describe the abnormal P waves seen in left atrial enlargement because they were first seen in patients with mitral valve stenosis and insufficiency. Left atrial enlargement can also be seen in left heart failure.

2. Ectopic P wave

“Ectopic” means away from its normal location. An ectopic P wave arises from a site other than the SA node. Abnormal sites include the atria and AV junction.

• P waves from the atria can be positive or negative, some are small, pointed, flat, wavy, or sawtooth in appearance.
• P waves from the AV junction are always negative (inverted) and may not be visible.
  • If P wave is inverted, it is most likely an ectopic atrial rhythm not originating from the sinus node.

PTa segment – used to diagnose pericarditis or atrial infarction

Elevation or depression of the PTa segment (part between p wave and beginning of QRS  complex) can result from atrial infarction or pericarditis

PR Interval

• The PR interval represents the time from the onset of atrial depolarization to the onset of ventricular depolarization.
• PR segment is a part of the PR interval, is the short isoelectric line between the end of the P wave to the beginning of the QRS complex. It is used as a baseline to evaluate elevation or depression of the ST segment.

The QRS Complex

• The QRS complex depicts ventricular depolarization, or the spread of the impulse throughout the ventricles.

ST Segment

• The ST segment represents early ventricular repolarization.
• Between end of QRS complex and onset of T wave. 
• Usually isolectric (at the same level as preceding PR interval)

Causes of an elevated ST segment:

• Acute Ischemia
• Early repolarization
• Acute pericarditis: ST elevation in all leads except aVR
• Pulmonary embolism: ST elevation in V1 and aVR 
• Hypothermia: ST elevation in V3-V6, II, III and aVF
• Hypertrophic cardiomyopathy: V3-V5 (sometimes V6)
• High potassium (hyperkalemia): V1-V2 (V3)
• During acute neurologic events: all leads, primarily V1-V6
• Acute sympathic stress: all leads, especially V1-V6
• Brugada syndrome, Cardiac aneurysm, Cardiac contusion
• Left ventricular hypertrophy*
• Idioventricular rhythm including paced rhythm

Important to diagnosis ischemia: presence of reciprocal ST segment depression

ST elevation measured at junctional or “J-point”

Possible causes of depressed ST Segment:

• Ischemia
• Reciprocal ST segment depression. If one lead shows ST segment elevation then usually the lead ‘on the other side’ shows ST segment depression. (This is usually seen in ischemia as well.
• Left ventricular hypertrophy with “strain” or depolarization abnormality
• Digoxin effect
• Low potassium / low magnesium
• Heart rate-induced changes (post tachycardia)
• During acute neurologic events.

The T Wave 

• The T wave represents ventricular repolarization.

Possible Causes of T wave changes:

• Ischemia and myocardial infarction
  
• Pericarditis, Myocarditis
• Cardiac contusion
• Acute neurologic events, such as a subarachnoid bleed.
• Mitral valve prolapse
• Digoxin effect
• Right and left ventricular hypertrophy with strain

The U Wave

• Not always present, represents late ventricular repolarization

The QT Interval

• Represents total ventricular activity (the time from the onset of ventricular depolarization to the end of ventricular repolarization).

The cardiac cycle

A cardiac cycle consists of one heartbeat or one PQRST sequence. It represents a sequence of atrial contraction and relaxation followed by ventricular contraction and relaxation. The basic cycle repeats itself again and again. Regularity of the cardiac rhythm can be assessed by measuring from one heartbeat to the next (from one R wave to the next R wave, also called the R-R interval). Between cardiac cycles, the monitor or ECG recorder returns to the isoelectric line (baseline), the flat line in the ECG during which electrical activity is absent. 

Any waveform above the isoelectric line is considered a positive (upright) deflection and any waveform below this line a negative (downward) deflection. A deflection having both a positive and negative component is called a biphasic deflection. This basic concept can be applied to the P wave, the QRS complex, and the T wave deflections.(Huff, ECG Workout) 

Waveform and current flow

A monitor lead, or ECG lead, provides a view of the heart’s electrical activity between two points or poles (a positive pole and a negative pole). The direction in which the electric current flows determines how the waveforms appear on the ECG tracing. An electric current flowing toward the positive pole will produce a positive deflection; an electric current traveling toward the negative pole produces a negative deflection. Current flowing away from the poles will produce a biphasic deflection (both positive and negative). Biphasic deflections may be equally positive and negative, more negative than positive, or more positive than negative (depending on the angle of current flow to the positive or negative pole). 

The size of the wave deflection depends on the magnitude of the electrical current flowing toward the individual pole. The magnitude of the electrical current is determined by how much voltage is generated by depolarization of a particular portion of the heart. The QRS complex is normally larger than the P wave because depolarization of the larger muscle mass of the ventricles generates more voltage than does depolarization of the smaller muscle mass of the atria.