HEMODYNAMICS With intact electromechanical coupling, there is a close temporal correlation between the ECG and the mechanical activity of the heart. Using the left ventricular volume curve, the systolic and diastolic phases are demonstrated. With ventricular depolarization (QRS), contraction against closed heart valves (isovolumetric contraction phase-1) occurs until the intraventricular pressure exceeds the aortic pressure (80 mmHg). Then the aortic valve opens, and the stroke volume is ejected with a further increase in pressure reaching up to 120 mmHg (ejection phase-2). With the conclusion of the T wave the systole is ended, and the aortic valve closes as soon as the intraventricular pressure is less than the aortic pressure. Diastole begins with a short relaxation phase (3) and the subsequent ventricular filling. Through the valve level mechanism, the ventricle is filled quickly (4). The slow filling (diastasis-5) is caused by venous return flow. With atrial excitation (P wave) the atria contract and contribute approx. 15% of the ventricular filling (6). The ejection fraction (EF) can be computed from the ratio of stroke volume to maximal filling of the left ventricle. The EF is an important value in evaluating left-ventricular performance.
HEMODYNAMICS DURING TACHYCARDIA With a rate increase, the temporal ratio between systole and diastole changes mainly by a reduction in the diastole. This example shows dilative cardiomyopathy (enlarged, dilated heart with severely reduced contraction force) with chronic tachycardia. Due to the cardiac disease and the unfavorable hemodynamic situation (preload, afterload) the stroke volume is reduced. An end-diastolic volume of 200 ml (left-ventricular filling volume) points to an enlargement of the left heart. An ejection fraction of 20% is a poor reserve for cardiac performance.
We use the term Conduction System to describe the electrical pathway of an electrical impulse that causes a heart beat.
The Conduction System in a normal heart is begins with the Sinus Node or SA Node. The Sinus Node (SA Node): Located in the upper right atrium Known as the heart ’ s ‘ Natural Pacemaker ’ Produces resting rates between 60-100 BPM The SA Node has ‘ automaticity ’ , which will be discussed later in this Module. It ’ s rate of automaticity is normally faster than all other parts of the heart, and therefore, dictates the rate at which the heart beats. This is known as “ Sinus Rate ” .
The Atrioventricular Node or AV Node: Located between the Atrium and the Ventricles in the interatrial septum close to the tricuspid valve Receives the impulse from the SA Node and delivers it through the Bundle of His (the forefront of the His-Purkinje network) Produces rates at 40-60 BPM if the SA Node fails to fire Conduction through the AV Node is slow, allowing appropriate fill time for the ventricles prior to ventricular contraction. If the SA Node fails to deliver an impulse to the AV Node, the AV Junctional Tissue will deliver an impulse to the Bundle of His at rates between 40-60 BPM.
Bundle of His: Together with the AV Node make up the AV Junctional Tissue Begins conduction to the ventricles Junctional tissue produces rates between 40-60 BPM
Bundle Branches & Purkinje Fibers (make up the Purkinje Network): Distribute the electrical impulse to the cardiac muscle allowing for depolarization (contraction) of the ventricle Together with the Purkinje Fibers make up the Ventricular Conduction System Can deliver impulses at rates between 20-40 BPM, known as an ‘ escape ’ rhythm
We will start by discussing normal impulse formation and then move into common conduction disturbances.
Initiation of the cardiac cycle normally begins with initiation of the impulse at the SA (sinoatrial) node.
After the SA node fires, the resulting depolarization wave passes through the right and left atria, which produces the P-wave on the surface EKG and stimulates atrial contraction.
Following activation of the atria, the impulse proceeds to the atrioventricular (AV) node, which is the only normal conduction pathway between the atria and the ventricles. The AV node slows impulse conduction, which allows time for the atria to contract and for blood to be pumped from the atria to the ventricles prior to ventricular contraction. Conduction time through the AV node accounts for most of the duration of the PR interval. Just below the AV node, the impulse passes through the bundle of His. A small portion of the last part of the PR interval is represented by the conduction time through the bundle of His.
After the impulse passes through the bundle of His, it proceeds through the left and right bundle branches. A small portion of the last part of the PR interval is represented by the conduction time through the bundle branches.
Next the impulse passes through the Purkinje fibers (interlacing fibers of modified cardiac muscle). Conduction time through the Purkinje system is represented by a small portion of the last part of the PR interval.
The impulse passes quickly through the bundle of His, the left and right bundle branches, and the Purkinje fibers, leading to depolarization and contraction of the ventricles. The QRS complex on the EKG represents the depolarization of the ventricular muscle mass.
The Plateau Phase lasts up to several hundred milliseconds.
Repolarization of the ventricles generates a current in the body fluids and produces a T-wave. This takes place slowly, and generates a wide wave.
Here is another graphical view with each EKG wave represented with respect to the heart function associated with it.
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