Electrocardiogram (ECG) | CardioSecur
The electrocardiogram (ECG or EKG) is a diagnostic tool that measures and records the A standardized system has been developed for the electrode placement for a routine ECG. information about whether the heart muscle cells are conducting electricity appropriately. . Electrocardiogram (ECG) Related Articles. This video and other related images/videos (in HD) are available for instant download licensing here. The Electrical Conduction System of the Heart. In order to be able to Somewhat confusingly a lead ECG only has 10 electrodes. These
How do electrical impulses flow throughout the heart? The heart muscle pumps blood in a specific rhythm throughout the entire body.
Clinical electrocardiography and ECG interpretation – ECG learning
In order to do this, the heart muscle must contract, which requires an electrical impulse. The electrical current is then transmitted via specific pathways throughout the heart, enabling regular contraction and relaxation. This electrical current can be detected on the surface of the body i. Pathway of the electrical impulse: From the sinus node, the electrical impulse starts by spreading throughout the atria from right to left.
When this happens, the cells lose their internal negativity, a process known as depolarization. The depolarization of the atria causes them to contract. The electrical current then spreads to the atrioventricular node AV nodefrom where it is further transmitted to the intraventricular Septum separates the left and right ventricles.
In order to depolarize the ventricles, the electrical impulse travels through the bundle of His, along the right and left bundle branches from left to rightand ends at the Purkinje fibers. This process causes depolarization of the ventricles, causing them to contract.Cardiology - Relationship of conduction system, ventricular contraction and ECG
This succession, from the sinoatrial node to the Purkinje fibers, is referred to as pacemaker hierarchy. Figure 4 displays the pacemaker hierarchy. It should be noted that the automaticity discussed above is normal automaticity, which only occurs in the sinoatrial node and the latent pacemakers. However, there is also abnormal automaticity, which can arise anywhere in the heart, including in ventricular myocardium.
This is discussed later.
Overview of the electrical impulse during a cardiac cycle left hand side and the pacemaker hierarchy right hand side. The rate of spontaneous depolarization is highest in the sinoatrial node, which is why it is the primary pacemaker. Impulse transmission conduction, propagation The cells of the conduction system have virtually no contractile function. Conduction cells are merely responsible for spreading the depolarization rapidly and synchronously to the contractile cells, so that they can contract in concert.
However, there are fewer conduction cells than contractile cells, which implies that conduction cells only communicate with a portion of the contractile myocardium. Recall that all cells in the heart are connected, both mechanically and electrically and this enables the electrical impulse to spread from one cell to the next.
Atrial impulse transmission The conduction system is vaguely defined in the atria, as compared with the ventricles which boasts with distinct conduction structures such as the bundle of His and the bundle branches. There are, however, three rather distinct fiber bundles which appear to serve as the conduction system of the atria. Refer to Figure 1 above. The atrioventricular conduction system The atrioventricular AV node is the bridge between the atria and the ventricles.
It is located in the atrial septum and normally the only connection between the atria and ventricles. The delay is due to the slow conduction through the atrioventricular node.
The purpose of the slow conduction is to give the atria adequate time to fill the ventricles with blood, before ventricular contraction commences. The bundle of His The atrioventricular node continues in the bundle of His which splits up in the left bundle branch and the right bundle branch.
These bundles successively branch into finer bundles and ultimately Purkinje fibers which sprout into the myocardium.
The Electrical Conduction System of the Heart:
Note that the left bundle branch splits into an anterior and a posterior fascicle. This means that activation of the ventricles except from the septum starts in the endocardium and spreads towards the epicardium Figure 5. The rapid impulse transmission in the Purkinje network allows virtually all ventricular myocardium to be activated simultaneously.
As noted above, when the impulse is delivered to the contractile myocardial cells, the subsequent impulse transmission takes place from one contractile cell to the next, which is much slower 0. Schematic figure of the ventricular wall. Note that the term myocardium is often used to refer to all layers. Influences of the autonomic nervous system The vagus nerve provides the heart with parasympathetic fibers. These fibers primarily innervate the sinoatrial node and the atrioventricular node.
Increased vagal tone leads to slowed automaticity in the sinoatrial node and slowed conduction through the atrioventricular node.
This leads to lower heart rate and a negligble increase in the delay in the atrioventricular node. Sympathetic fibers innervate the entire heart, both the conduction system and contractile myocardium. The fibers run along the blood vessels and are particularly dense in ventricular myocardium. Sympathetic stimulation leads to increased excitability in all cells. Definition of rhythm A rhythm is defined as three consecutive heart beats displaying more or less identical waveforms on the ECG.
Cardiac electrophysiology The formal study of the electrical conduction system of the heart is called cardiac electrophysiology EP. An electrophysiology study involves a formal study of the conduction system and can be done for various reasons. During such a study, catheters are used to access the heart and some of these catheters include electrodes that can be placed anywhere in the heart to record the electrical activity from within the heart.
Some catheters contain several electrodes and can record the propagation of electrical activity. Interpretation[ edit ] Interpretation of the ECG is fundamentally about understanding the electrical conduction system of the heart.
Normal conduction starts and propagates in a predictable pattern, and deviation from this pattern can be a normal variation or be pathological. An ECG does not equate with mechanical pumping activity of the heart, for example, pulseless electrical activity produces an ECG that should pump blood but no pulses are felt and constitutes a medical emergency and CPR should be performed. Ventricular fibrillation produces an ECG but is too dysfunctional to produce a life-sustaining cardiac output.
Certain rhythms are known to have good cardiac output and some are known to have bad cardiac output. Ultimately, an echocardiogram or other anatomical imaging modality is useful in assessing the mechanical function of the heart. Like all medical tests, what constitutes "normal" is based on population studies.
The heartrate range of between 60 and beats per minute bpm is considered normal since data shows this to be the usual resting heart rate. In order to understand the patterns found, it is helpful to understand the theory of what ECGs represent. The theory is rooted in electromagnetics and boils down to the four following points: For example, depolarizing from right to left would produce a positive deflection in lead I because the two vectors point in the same direction.
In contrast, that same depolarization would produce minimal deflection in V1 and V2 because the vectors are perpendicular and this phenomenon is called isoelectric. Normal rhythm produces four entities — a P wave, a QRS complex, a T wave, and a U wave — that each have a fairly unique pattern.