More Electrical Conduction in the Heart
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5 More Electrical Conduction; The Electrocardiogram Spreading Excitation Atrial From the SA node, excitation spreads across the two atria, passing from cell to cell at a rate of approx 1 m/sec and initiating atrial systole. After passing down the atrial septum, the electrical impulse reaches the AV node. The impulse is delayed in the node approx .1 sec (at resting heart rates). The resulting delay is important because it allows the atria sufficient time to contract before the ventricals are activated. AN, N, NH Regions. AV nodal cells have relative refractory periods that extend long beyond complete repolarization time. If atria are depolarized at very high frequencies, not all impulses will get through because of this. Another protection against tetanus. Vagal activity hyperpolarizes cells and fibers in AV node, which may prolong AV conduction time or even block some impulses. Sympathetic nerves decrease conduction time in the AV node. NE increases amp and slope of AV nodal APs. Ventricular The bundle of His is a bundle of fast-conducting muscle fibers transmits excitation from the A/V node to the ventricular septum and on to the Purkinge fibers (large diameter fibers that transmit very quickly because of their large caliber, and thereby spread excitation throughout the ventricles very quickly). Bundle of His splits into right and left bundle branches, which proceed down the septum. Left much thicker and breaks into anterior and posterior divisions. Block of either bundle branch is called bundle branch block, or block of one division of left is called hemiblock (i.e. left anterior hemiblock). Purkinge Fibers--The broadest cells in the heart, large diameter accounting for greater conduction velocity. Conduction in Purkinge cells can be as fast as 4 m/sec, so entire ventricular myocardium activated very quickly. AP of Purkinge cell looks like that of any other ventricular myocardial cell. Long refractory periods Premature Atrial Contraction--premature activation of the atria causes premature atrial contraction, impulse passes through AV node blocked by Purkinge fibers, so not followed by ventricular contraction The septum and Papillary muscles are activated first, but spread to other parts very rapid. Reentry Re-excitation of an area. Caused by unidirectional block (caused by ischemia or other injury). Electrocardiogram Reflects only electrical activity in the heart. Heart may or may not be contracting properly. Recording Issues Electrode Placement--Einthoven Triangle Standard 12-lead ECG quick and dirty ECG P-Wave Excitation of Atria cell mass (depolarization of the SA node involves too few cells to cause a visible wave); Corresponds with the upstroke of the atrial action potentials, so precedes slightly the atrial contraction. Atrial contraction ocurs during the PR interval. PR interval Reflects the time taken for excitation to spread over the atria and through the conduction system to reach the ventricular septum. Atrial repolarization is too slow and diffuse to register on the ECG. QRS Complex Excitation of Ventricles. A large mass of muscle is involved, resulting in the huge spike. Q and S waves may or may not be present based on the placement of electrodes. ST segment Plateau of the ventricular action potential. Rapid ejection occurs during this period T-Wave Ventricular Repolarization--usually positive indicating that repolarization occurs in the opposite direction of depolarization. Sometimes a U--any wave between a T and the next P. Arrhythmias All cardiac tissue has the potential of becoming (temporarily or permanently) a pacemaker for the heart and usurp the role of the normal pacemaker. Such ectopic foci are responsible for most cardiac arrhythmias. Premature Atrial Contraction--Results from a non-nodal atrial pacemaker; P wave may look different depending on the location of the pacemaker responsible. QRS complex usually normal. Premature Ventricular Contraction--Caused by an ectopic focus in the ventricle. QRS complex and T are completely different because the activation origninates at an unusual place on the myocardium. Compensatory pause because atrium and SA node are still functioning normally. Ventricular impulse does not conduct in retrograde direction, and/or atrial didn't get there quickly. Missed SA nodal impulse did not affect ventricle because it was still refractory. Heart Block--not all p-waves may be followed by QRS complexes, because a block in the myocardial tissue keeps some atrial impulses from passing on the ventricles. 1st degree heart block--prolonged P-R interval (over .2 seconds in adult) 2nd degree heart block--not all P waves are followed by QRS complexes 3rd degree heart block--complete heart block--impulses unable to pass from atria to ventricles. Atrial and ventricular rhythms are completely independent. Because of slow ventricular rhythms, circulation is usually inadequate. Artificial pacemakers often inserted for this. Paroxysmal tachycardia Both may be caused by metabolic or electrolyte disturbances or ischemic injury or cardiomyopathy. Supraventricular tachycardia--originates in atrium or AV node. Ectopic focus maintains a very high rate. QRS complexes usually normal. May be caused by a circular wave of excitation traveling around a damaged area. Vagus stimulation (i.e., valsalva maneuver, diving reflex) often will abort such episodes. Ventricular tachycardia--ectopic focus in the ventricles. Repeated bizarre QRS complexes Fibrillation-- Atrial Fibrillation--fairly normal but irregular QRS complexes. No P's. May be reverted by drugs that prolong the refractory period. Ventricular Fibrillation--leads to loss of consciousness within a few seconds. Uncoordinated twitchings of ventricular muscle fibers pumps no blood. Rarely reverts to normal spontaneously. Caused by multiple reentrant waves, which continue to propagate over the ventricles. Each time a cell completes its refractory period, it is activated again. Self sustaining. A brief, strong electric current leaves all cells refractory at the same time, breaking the cycle. Then, hopefully, the SA node will take back over as pacemaker.
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