Right axis deviation | |
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Hexaxial reference system |
The electrical axis of the heart is the net direction in which the wave of depolarization travels. It is measured using an electrocardiogram (ECG). Normally, this begins at the sinoatrial node (SA node); from here the wave of depolarisation travels down to the apex of the heart. The hexaxial reference system can be used to visualise the directions in which the depolarisation wave may travel.
On a hexaxial diagram (see figure 1):
RAD is an ECG finding that arises either as an anatomically normal variant or an indicator of underlying pathology.
There are often no symptoms for RAD and it is usually found by chance during an ECG. Many of the symptoms exhibited by patients with RAD are associated with its different causes. The table below displays the four most common causes and the signs, symptoms and risk factors associated with it.[ citation needed ]
Signs and symptoms | Risk factors | |
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Lateral myocardial infarction | Anxiety Chest pain Fatigue Shortness of breath [1] | Smoking or Tobacco Obesity Gender Hypertension Diabetes Physical inactivity Age Alcohol |
Right ventricular hypertrophy | Shortness of breath Dizziness Fainting | Chronic lung disease (COPD) [2] Pulmonary hypertension Mitral stenosis Pulmonary embolism Congenital heart disease Arrythmogenic right ventricle cardiomyopathy |
Fascicular Block | Light-headedness Fainting Palpitations [3] | Hereditary bundle branch block |
Pre-excitation Syndromes | Asymptomatic | Wolff-Parkinson-White [4] |
Other causes | Variable | Ectopic ventricular beats Drug toxicity (e.g. tricyclic antidepressants [5] ) Hyperkalaemia |
Blockage of the left posterior fascicle would lead to activation of the anterior portion of the left ventricle followed by activation of the rest of the ventricle in a superior to inferior direction and directed towards the right. This would lead to right axis deviation findings on an ECG. [6] Bifascicular block is a combination of right bundle branch block and either left anterior fascicular block or left posterior fascicular block. Conduction to the ventricle would therefore be via the remaining fascicle. The ECG will show typical features of RBBB plus either left or right axis deviation. [7] [8]
The lateral wall of the left ventricle is supplied by branches of the left anterior descending (LAD) and left circumflex (LCx) arteries. [8] Infarction of the lateral wall will thus lead to deviation of the axis away from the site of infarction. [9]
Increased thickness of the right ventricle leads to right axis deviation [ citation needed ]
Pre-excitation refers to early activation of the ventricles due to impulses bypassing the AV node via an accessory pathway. [10] Accessory pathways are abnormal conduction pathways formed during cardiac development. An example of pre-excitation syndromes is Wolff Parkinson White syndrome. Here, the presence of a left lateral accessory pathway leads to right-axis deviation. [11]
Fascicular tachycardia usually arises from the posterior fascicle of the left bundle branch. They produce QRS complexes of relatively short durations with a right bundle branch block pattern. Tachycardias originating in the anterior left fascicle would lead to right axis deviation.[ citation needed ]
Right ventricular outflow tract tachycardia originates from the outflow tract of the right ventricle or the tricuspid annulus. As it arises from the right ventricle, the impulse spreads inferiorly from beneath the pulmonary valve, and there right axis deviation. [12]
Ventricular ectopy is when the heartbeat is an abnormal heartbeat where the QRS complex is significantly wider. When the origin of the ectopic heartbeat is in the anterior fascicule then there is right axis deviation. [13]
The pathophysiology depends on the specific cause of right axis deviation. Most causes can be attributed to one of four main mechanisms. [14] [15] These include right ventricular hypertrophy, reduced muscle mass of left ventricle, altered conduction pathways and change in the position of the heart in the chest.[ citation needed ]
Enlargement of right ventricular myocardial mass can result in right axis deviation. There are 2 main reasons for this mechanism. [15] Firstly, more muscle mass will result in greater amplitude of depolarisation of that side of the heart. [15] Secondly, depolarisation of the heart will be slower through the right ventricle relative to the left, and therefore the effects of the right ventricle on the axis of the heart will be dominant. [15]
Decrease in myocardial mass of the left ventricle will shift the balance of depolarisation towards the right. For example, scarring and atrophy caused by ischaemia of the left ventricle will cause depolarisation of the left side of the heart to be less forceful. [15] Hence, depolarisation of the right ventricle will be greater in amplitude than left, shifting the axis to the right.[ citation needed ]
Changes in the conduction pathways of the heart can result in right axis deviation. For example, an accessory pathway from the left atrium to the left ventricle, as in Wolff-Parkinson-White Syndrome, will result in the left ventricle finishing depolarisation earlier than the right. [16] Hence, the right ventricle will have more of an effect on the axis of the heart.[ citation needed ]
The apex of the heart is normally orientated towards the left. A more vertical orientation of the heart, shifts the axis to the right. Physiologically, this can occur in tall and thin individuals. [16] Pathologically, conditions such as a left-sided pneumothorax and lung hyperinflation (e.g. COPD) [17] can cause rightwards displacement of the heart. The congenital condition of dextrocardia results in right axis deviation.
In general, a positive (upwards) deflection of an ECG trace demonstrates an electrical activity that moves towards the measuring electrode, whereas a negative (downwards) deflection of an ECG trace demonstrates an electrical activity that moves away from the measuring electrode. The electrical heart axis can be estimated from the ECG by using the quadrant method or degree method. [18]
A simple tool to quickly identify axis deviation (Figure 3) is the popular mnemonic; Reaching for Right Axis Deviation and Leaving for Left Axis Deviation. This refers to the appearance of leads I and III. If the QRS complex is negative in lead I and positive in lead III, the QRS complexes appear to be "reaching" to touch each other. This signifies right axis deviation. Conversely, if the QRS complex is positive in lead I and negative in lead III. the leads have the appearance of "leaving" each other. If the QRS complex in lead II is also negative, this confirms a left axis deviation.[ citation needed ]
Electrocardiography is the process of producing an electrocardiogram, a recording of the heart's electrical activity through repeated cardiac cycles. It is an electrogram of the heart which is a graph of voltage versus time of the electrical activity of the heart using electrodes placed on the skin. These electrodes detect the small electrical changes that are a consequence of cardiac muscle depolarization followed by repolarization during each cardiac cycle (heartbeat). Changes in the normal ECG pattern occur in numerous cardiac abnormalities, including:
Ventricular fibrillation is an abnormal heart rhythm in which the ventricles of the heart quiver. It is due to disorganized electrical activity. Ventricular fibrillation results in cardiac arrest with loss of consciousness and no pulse. This is followed by sudden cardiac death in the absence of treatment. Ventricular fibrillation is initially found in about 10% of people with cardiac arrest.
Wolff–Parkinson–White syndrome (WPWS) is a disorder due to a specific type of problem with the electrical system of the heart involving an accessory pathway able to conduct electrical current between the atria and the ventricles, thus bypassing the atrioventricular node. About 60% of people with the electrical problem developed symptoms, which may include an abnormally fast heartbeat, palpitations, shortness of breath, lightheadedness, or syncope. Rarely, cardiac arrest may occur. The most common type of irregular heartbeat that occurs is known as paroxysmal supraventricular tachycardia.
The bundle of His (BH) or His bundle (HB) ( "hiss") is a collection of heart muscle cells specialized for electrical conduction. As part of the electrical conduction system of the heart, it transmits the electrical impulses from the atrioventricular node to the point of the apex of the fascicular branches via the bundle branches. The fascicular branches then lead to the Purkinje fibers, which provide electrical conduction to the ventricles, causing the cardiac muscle of the ventricles to contract at a paced interval.
Third-degree atrioventricular block is a medical condition in which the electrical impulse generated in the sinoatrial node in the atrium of the heart can not propagate to the ventricles.
The cardiac conduction system transmits the signals generated by the sinoatrial node – the heart's pacemaker, to cause the heart muscle to contract, and pump blood through the body's circulatory system. The pacemaking signal travels through the right atrium to the atrioventricular node, along the bundle of His, and through the bundle branches to Purkinje fibers in the walls of the ventricles. The Purkinje fibers transmit the signals more rapidly to stimulate contraction of the ventricles.
First-degree atrioventricular block is a disease of the electrical conduction system of the heart in which electrical impulses conduct from the cardiac atria to the ventricles through the atrioventricular node more slowly than normal. First degree AV block does not generally cause any symptoms, but may progress to more severe forms of heart block such as second- and third-degree atrioventricular block. It is diagnosed using an electrocardiogram, and is defined as a PR interval greater than 200 milliseconds. First degree AV block affects 0.65-1.1% of the population with 0.13 new cases per 1000 persons each year.
Ventricular tachycardia is a cardiovascular disorder in which fast heart rate occurs in the ventricles of the heart. Although a few seconds of VT may not result in permanent problems, longer periods are dangerous; and multiple episodes over a short period of time are referred to as an electrical storm. Short periods may occur without symptoms, or present with lightheadedness, palpitations, shortness of breath, chest pain, and decreased level of consciousness. Ventricular tachycardia may lead to coma and persistent vegetative state due to lack of blood and oxygen to the brain. Ventricular tachycardia may result in ventricular fibrillation (VF) and turn into cardiac arrest. This conversion of the VT into VF is called the degeneration of the VT. It is found initially in about 7% of people in cardiac arrest.
AV-nodal reentrant tachycardia (AVNRT) is a type of abnormal fast heart rhythm. It is a type of supraventricular tachycardia (SVT), meaning that it originates from a location within the heart above the bundle of His. AV nodal reentrant tachycardia is the most common regular supraventricular tachycardia. It is more common in women than men. The main symptom is palpitations. Treatment may be with specific physical maneuvers, medications, or, rarely, synchronized cardioversion. Frequent attacks may require radiofrequency ablation, in which the abnormally conducting tissue in the heart is destroyed.
In electrocardiography, the T wave represents the repolarization of the ventricles. The interval from the beginning of the QRS complex to the apex of the T wave is referred to as the absolute refractory period. The last half of the T wave is referred to as the relative refractory period or vulnerable period. The T wave contains more information than the QT interval. The T wave can be described by its symmetry, skewness, slope of ascending and descending limbs, amplitude and subintervals like the Tpeak–Tend interval.
A bundle branch block is a partial or complete interruption in the flow of electrical impulses in either of the bundle branches of the heart's electrical system.
The QRS complex is the combination of three of the graphical deflections seen on a typical electrocardiogram. It is usually the central and most visually obvious part of the tracing. It corresponds to the depolarization of the right and left ventricles of the heart and contraction of the large ventricular muscles.
A right bundle branch block (RBBB) is a heart block in the right bundle branch of the electrical conduction system.
Left bundle branch block (LBBB) is a conduction abnormality in the heart that can be seen on an electrocardiogram (ECG). In this condition, activation of the left ventricle of the heart is delayed, which causes the left ventricle to contract later than the right ventricle.
An accessory pathway is an additional electrical connection between two parts of the heart. These pathways can lead to abnormal heart rhythms or arrhythmias associated with symptoms of palpitations. Some pathways may activate a region of ventricular muscle earlier than would normally occur, referred to as pre-excitation, and this may be seen on an electrocardiogram. The combination of an accessory pathway that causes pre-excitation with arrhythmias is known as Wolff-Parkinson-White syndrome.
Left anterior fascicular block (LAFB) is an abnormal condition of the left ventricle of the heart, related to, but distinguished from, left bundle branch block (LBBB).
A left posterior fascicular block (LPFB), also known as left posterior hemiblock (LPH), is a condition where the left posterior fascicle, which travels to the inferior and posterior portion of the left ventricle, does not conduct the electrical impulses from the atrioventricular node. The wave-front instead moves more quickly through the left anterior fascicle and right bundle branch, leading to a right axis deviation seen on the ECG.
An intraventricular block is a heart conduction disorder — heart block of the ventricles of the heart. An example is a right bundle branch block, right fascicular block, bifascicular block, trifascicular block.
In electrocardiography, left axis deviation (LAD) is a condition wherein the mean electrical axis of ventricular contraction of the heart lies in a frontal plane direction between −30° and −90°. This is reflected by a QRS complex positive in lead I and negative in leads aVF and II.
The bundle branches, or Tawara branches, are offshoots of the bundle of His in the heart's ventricle. They play an integral role in the electrical conduction system of the heart by transmitting cardiac action potentials from the bundle of His to the Purkinje fibers.