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Lecture 3

BIOC33-34 Lecture 3.docx

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Zachariah Campbell

Lecture 3 – The Electrocardiogram (pt. II), Electrical Axis of the Heart, the Cardiac Cycle 1. Heart Block and Branch Bundle Block: • Branch bundles can be blocked o The 2 branch bundles (left and right) take the waves of depolarization down from the AV node, through the bundle of His and then down to either side of the heart until it reaches the Purkinje fibers o Therefore it is possible for conduction to be slowed in either one of the branches, creating either a right or left branch bundle block  The ECG traces created by this condition are strange; the lines are saw-tooth or ‘m’ shaped o One of the consequences of a branch bundle block is that the ventricles will no longer contract simultaneously  Instead, the ventricles that has the block on its side will contract slightly after the ventricle that does not 2. Flutter and Fibrillation: • A common form of arrhythmia is heart flutter, and a less common one is heart fibrillation but more serious o Flutter and fibrillation can occur in the atria, ventricles or both.  Rapid rates of electrical excitation and contraction in either of these areas will lead to these conditions o Flutter is more benign (gentle), while fibrillation is more dangerous, particularly in the ventricles • In atrial flutter, the rapid rate of electrical excitation leads to an enhanced number of P-waves o If these P-waves were to trigger a QRS complex, then heart rate would be highly elevated (between 200-300 beats per minute)  The appearance of P-waves can become so fast that the isoelectric interval between the end of the T-wave and the beginning of the P-wave disappears o However, in reality, the AV node and ventricles are only activated by every second or third atrial impulse (P-wave)  Thus, actual heart (ventricular) function is not affected to a huge extent by atrial flutter • In atrial fibrillation, the electrical activity in the atria becomes extremely chaotic and rapid o This can result in different parts of the atria depolarizing and contracting at different times  However, the function of the heart is not dependent upon the contraction of the atria so as long as the QRS complex remains intact, then the ventricles will continue to contract properly • Heart rate, however will be irregular • Ventricular fibrillation, the electrical activity in the ventricles becomes chaotic and extremely rapid which can become dangerous and lead to uncoordinated and inefficient pumping o Generally caused by the recycling of electrical activity in the ventricular myocardium (muscle)  This recycling of electrical activity produces circus waves o Normally, waves of depolarization travel in one direction along a relatively defined path, and once an area of the heart depolarizes, it goes into a refractory period.  This refractory period of the myocardial muscle prevents recycling of electrical activity (normal) o However, if some cardiac cells emerge from their refractory period before others (abnormal), the waves of depolarization can be continuously regenerated and conducted through the heart (circus waves)  This causes the contraction of different areas of muscle at different times making coordinated pumping action impossible  In this situation of ventricular fibrillation, electrical defibrillators are brought into play to reset the heart’s natural rhythm o The recycling of electrical activity in the ventricle, leading to ventricular fibrillation, takes the form of what we call a re-entry circuit  Under normal conditions, electrical activity moves up the Purkinje fibers into the myocardial muscle causing it to contract • Imagine an upwards, equilateral triangle made up of 3 branches, with each intersection of branches leading off into another pathway (look at page 4 for picture) o In the normal pathway, a single wave of electrical activity moving downward splits and moves down both of the two downward branches  When these 2 waves of depolarization reach the bottom two corners of the triangle, they can either move off to the right or left from the triangle or travel through the bottom branch toward the middle  If the activity moves through the bottom branch, the two currents will collide in the middle of the bottom branch, cancelling each other out and the tissue will go into a refractory period • Thus, the wave of depolarization can never (normally) travel around the triangle, only through it o In a heart with ventricular fibrillation, there tends to be a unidirectional block in the conduction pathway, meaning that electrical activity can only travel in one direction through this block  There are no opposing waves of electricity to cause cells to become refractory and electricity begins to cycle around and around • This happens through the ventricle, causing fibrillation  Numerous waves of depolarization (circus waves) move in circles through various parts of the ventricle causing these regions of ventricular muscle to contract independently of other regions of the ventricle • Ventricular fibrillation can lead to death within minutes but it can be stopped, or reset with a defibrillator o An electrical discharge is placed across the chest and the electrical activity generated will reset the heart o The overwhelming burst of electricity wipes out all of the re-entry circuits and puts the heart back onto a normal cycle in which a normal rhythm is established by an endogenous pacemaker 3. The Electrical Axis of the Heart: a) What is the Electrical Axis of the Heart? • It is a vectoral analysis of the direction and magnitude of current flow at various instances during the heart’s cycle o Defined as the average of all the instantaneous mean electrical vectors occurring sequentially during depolarization of the ventricles • Atrial depolarization and repolarization does not contribute a lot to the mean electrical axis since they are much less muscular than the ventricles • As waves of depolarization are flowing through the conduction system in the ventricles and the myocardium, there is a general direction (vector) in which electrical activity is flowing o If we take all the directional vectors that electrical activity is flowing in, during the time that the ventricle is depolarizating, we can get the mean axis (or direction) of electrical flow during the ventricular contraction phase • During the first phase of the contraction of the ventricles, the wave of depolarization is travelling downward through the branch bundles and the movement of electrical activity is left to right o This is because as the waves go down the branch bundles, the left bundle depolarizes slightly faster than the right, shown on an ECG as a slight dip (Q-wave) • As the ventricle continues to depolarize, the primary movement of electrical activity is downward toward the apex of the heart o This accounts for the large upswing of the R-wave on the ECG trace • The large downswing of the R-wave is caused by the third phase of the contraction of the ventricles, when the primary movement in electricity shifts from flowing downwards to flowing right to left across the frontal plane of the body o This is due to the full depolarization of the ventricles • Lastly, the electrical activity flows backward (into the body) toward the left, causing the negative S-wave • If we look at all of these instantaneous vectors and sum them in a vectoral analysis over the entire QRS complex, we find that the average direction of electrical flow in the heart is oriented at about 60 degrees o Almost exactly the same as the orientation of lead II (standard limb) b) Diagnostic Uses of the Electrical Axis • The electrical axis of the heart is normally oriented around 60 degrees but it can shift depending upon physiological conditions or disease states o It can shift left or right (during inspiration and expiration), which is normal but a serious shift is considered to be about 60 degrees in either direction (axis at 0 or 120 degrees) o Deviations to the left or the right are usually the result of increased tissue mass in the ventricles  If something increases the mass of the right ventricle, it means there’s more muscle in the right ventricle to depolarize and the mean electrical axis shifts to the right • Looking at the circulatory system and the relationship of the heart to the pulmonary
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