Cardiac Axis in Right and Left Bundle Branch Block

Understanding the ECG: A Physiological Approach to Cardiac Axis and Conduction Abnormalities

Reading an ECG (electrocardiogram) can initially seem challenging. However, ECG interpretation becomes much easier when it is approached systematically and, most importantly, when the underlying cardiac physiology is understood.

An ECG is a graphical representation of the electrical activity of the heart. It provides information about the heart rate, rhythm, cardiac axis, atrioventricular conduction, ventricular depolarisation and repolarisation. It can also provide important clues about underlying structural heart disease, myocardial ischaemia, electrolyte abnormalities and conduction-system disorders. A 12-lead ECG is an essential part of the initial assessment of patients with suspected rhythm or conduction abnormalities. [1]

What does an ECG actually represent?

The heart generates electrical impulses that are conducted through a specialised conduction system. Under normal circumstances, the impulse originates in the sinoatrial (SA) node, spreads through the atria, passes through the atrioventricular (AV) node, and then travels through the His–Purkinje system to activate the ventricles.

The normal conduction pathway can be simplified as:

SA node → atria → AV node → His bundle → right and left bundle branches → Purkinje fibres → ventricular myocardium

The ECG records the electrical consequences of this process from different anatomical perspectives.

Therefore, an ECG is not simply a collection of waves and intervals to be memorised. Each ECG finding reflects an underlying physiological or pathological process.

For example:

• P waves represent atrial depolarisation.
• PR interval reflects conduction from the atria through the AV node and His–Purkinje system to the ventricles.
• QRS complex represents ventricular depolarisation.
• ST segment and T wave reflect ventricular repolarisation.
• The QRS axis represents the overall direction of ventricular depolarisation in the frontal plane.

Understanding these principles makes ECG interpretation much more intuitive.

What does an abnormal ECG mean?

An abnormal ECG does not necessarily mean that the myocardium is damaged.

ECG abnormalities can occur because of abnormalities in:

• impulse formation;
• conduction through the cardiac conduction system;
• myocardial structure;
• ventricular or atrial size;
• electrolyte balance;
• medications;
• myocardial ischaemia or infarction; or
• congenital or physiological variations.

For example, a bundle branch block occurs because ventricular depolarisation is conducted abnormally through the His–Purkinje system. In contrast, ST-segment changes may indicate myocardial ischaemia, while changes in the QRS complex may suggest ventricular hypertrophy or conduction disease.

Thus, the ECG should be regarded as a reflection of cardiac electrical physiology, rather than simply a test for myocardial damage.

Understanding the cardiac axis

The cardiac axis, usually referring to the QRS axis, describes the predominant direction of ventricular depolarisation in the frontal plane.

The ventricles do not depolarise in a single straight line. Instead, millions of myocardial cells depolarise in a coordinated sequence, producing a resultant electrical vector. The ECG allows us to estimate the direction of this resultant vector.

In adults, the normal QRS axis is generally considered to lie approximately between −30° and +90°, although definitions can vary slightly between sources.

The axis may therefore be described broadly as:

• Normal axis
• Left axis deviation (LAD)
• Right axis deviation (RAD)
• Extreme or indeterminate axis

The axis is influenced by several factors, including ventricular mass, conduction pathways, heart position and underlying cardiac pathology.

Why does the axis change?

Think of the cardiac axis as the overall direction of ventricular electrical activation.

If the relative contribution of one ventricle changes, or if ventricular activation occurs through an abnormal conduction pathway, the direction of the resultant electrical vector may change.

For example, conditions associated with right ventricular pressure or volume overload can produce rightward displacement of the QRS axis. Conversely, left ventricular hypertrophy or fascicular conduction abnormalities may produce a more leftward axis.

However, axis deviation should never be interpreted in isolation. The patient's age, clinical presentation, other ECG findings and previous ECGs are all important.

Bundle branch block and cardiac axis

One important point when learning ECG interpretation is that bundle branch block and axis deviation are related but are not the same thing.

In a right bundle branch block (RBBB), conduction through the right bundle is delayed or blocked, resulting in delayed activation of the right ventricle. Importantly, isolated RBBB does not necessarily produce right axis deviation.

Similarly, an isolated left bundle branch block (LBBB) does not simply mean that the patient will have left axis deviation. The QRS axis depends on the overall pattern of ventricular activation and any associated fascicular or structural abnormalities.

The ACC/AHA/HRS guideline recognises bundle branch blocks and fascicular blocks as distinct forms of conduction-system disease.

Therefore, it is useful to remember:

Do not assume that every RBBB causes right axis deviation or that every LBBB causes left axis deviation.

Instead, assess the QRS morphology and axis separately.

Right axis deviation: is it always abnormal?

No.

Right axis deviation can be seen in pathological conditions, but a relatively rightward axis can also occur as a normal variant, particularly in younger individuals and in certain body habitus or physiological circumstances.

Pathological causes of right axis deviation include conditions such as:

• right ventricular hypertrophy;
• pulmonary hypertension;
• chronic lung disease;
• acute right ventricular strain, such as pulmonary embolism;
• left posterior fascicular block;
• some congenital heart diseases; and
• certain conduction abnormalities.

Therefore, when right axis deviation is identified, the appropriate question is not simply:

"Is this abnormal?"

Instead, ask:

"Does this axis make sense in the context of the patient's age, clinical presentation and other ECG findings?"

Left axis deviation: is it always pathological?

Again, the answer is no.

Left axis deviation is more commonly associated with an underlying conduction abnormality or structural heart disease, but it can also occur as a relatively benign or incidental finding in some individuals.

Important causes include:

• left anterior fascicular block;
• left ventricular hypertrophy;
• inferior myocardial infarction;
• certain cardiomyopathies;
• ventricular conduction abnormalities; and
• changes in cardiac position.

Therefore, it is better to avoid the statement:

"Left axis deviation is always pathological."

A more accurate approach is:

"Left axis deviation should prompt consideration of an underlying cause, particularly when it is new, marked, or associated with other ECG abnormalities or clinical features."

This distinction is important because ECG interpretation should combine pattern recognition with clinical reasoning.

A practical way to approach the cardiac axis

When assessing the QRS axis, first determine whether the QRS complexes are predominantly positive or negative in lead I and lead aVF.

A simple four-quadrant approach can provide a rapid estimate:

Lead I	Lead aVF	Approximate interpretation
Positive	Positive	Normal axis
Positive	Negative	Possible left axis deviation
Negative	Positive	Right axis deviation
Negative	Negative	Extreme axis

This method provides a quick estimate, but it has limitations. When the axis is borderline or the ECG is complex, a more precise calculation or assessment using additional leads may be necessary.

The key principle

The most important lesson in ECG interpretation is that the ECG should be interpreted as a physiological story rather than a collection of isolated abnormalities.

When you see an abnormal axis, ask:

1. Is the axis genuinely abnormal?
2. Is this a new finding?
3. Is there evidence of ventricular hypertrophy?
4. Is there a bundle branch or fascicular block?
5. Are there other abnormalities on the ECG?
6. Does the clinical presentation support an underlying cardiac or systemic pathology?

The ECG is an extremely powerful diagnostic tool, but its findings become meaningful only when they are interpreted alongside the patient's history, examination and other investigations.

Take-home messages

• The ECG is a graphical representation of the heart's electrical activity.
• Understanding cardiac physiology makes ECG interpretation considerably easier.
• An abnormal ECG does not automatically indicate myocardial damage.
• The QRS axis represents the predominant direction of ventricular depolarisation in the frontal plane.
• RBBB does not necessarily cause right axis deviation, and LBBB does not necessarily cause left axis deviation.
• Right axis deviation may be a normal variant or may indicate underlying pathology.
• Left axis deviation is more often associated with an underlying abnormality, but it is not invariably pathological.
• Always interpret the cardiac axis in conjunction with the QRS morphology, other ECG findings and the clinical context.

References

1. Kusumoto FM, Schoenfeld MH, Barrett C, et al. 2018 ACC/AHA/HRS Guideline on the Evaluation and Management of Patients With Bradycardia and Cardiac Conduction Delay. Circulation. 2019;140–e482.
2. Surawicz B, Childers R, Deal BJ, Gettes LS, eds. AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram: Part III: Intraventricular Conduction Disturbances. Circulation. 2009;119–e240.
3. Wagner GS, Strauss DG. Marriott's Practical Electrocardiography. 13th ed. Philadelphia: Wolters Kluwer; 2020.
4. Hampton JR. The ECG Made Easy. 9th ed. Elsevier; 2019.
5. Goldberger AL, Goldberger ZD, Shvilkin A. Goldberger's Clinical Electrocardiography: A Simplified Approach. 10th ed. Elsevier; 2023.