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BIOC33/BIOC34 Lec 3

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Biological Sciences
Stephen Reid

BIOC34 Lec 3 - Jan. 13/14  Last week:  ECG: Standard Limb Leads o Have Pwave: atrial depolarization, o QRS complex: ventricular contraction, o T wave - repolarization of ventricles o PR distance = time it takes to conduct electrical activity from SAnode toAV node o Arrhythmias - wandering atrial pacemaker - get electrical activity of pacemaker potentials from other areas of atria  Arrhythmias: heart beat originating from the ventricle o Can come from other areas than SAnode, or can come from SAnode and be faster or slower o Can also have abnormal rhythms from the ventricles  Litmus test of looking at pacemaker activity is whether the QRS complex is narrow or wide  Narrow (but normal) QRS complex = electrical activity is probably arising from the atria  Abnormally wide/m-tooth/saw-tooth shape = electrical activity that caused this is from the ventricle • Problematic; requires artificial pacemakers  Atrial and ventricular ectopic beats o Can arise from atria or ventricle o Ectopic beat = electrical activity forming a pacemaker potential is coming from somewhere other than the SAnode o Atrial ectopic beat - looks normal but it is coming too quickly; premature beat; originating from certain foci  Overdrive: increased pump activity - hyperpolarization -- suppression  Termed ectopic pacemaker because it is being generated not at SA node  Beat comes prematurely because that foci location is closer toAV node; therefore, it takes less time for electrical current to reachAV node o Ventricular ectopic beat - abnormal, premature QRS complex with saw-tooth, M shape. It is being generated at apex of ventricle functioning as a pacemaker potential  Case of advanced degree heart block  No electrical activity coming from atria into ventricle  Ventricle is creating its own pacemaker potentials  Arrhythmias: premature and late beats o Ectopic beats show up as early beats or late beats in atria or ventricle o In upper diagram of premature beat of atrial origin, can tell it is atrial because it has a narrow QRS complex - looks like the other ones  Ectopic location o All narrow complexes are generated in the atria and produce a normal, stable heartbeat  NOT a saw-tooth shape o In bottom case, have a premature contraction originating in ventricle  Wider QRS complex indicates it is happening in the ventricles o If it is a normal shape but kind of wide, it is not as bad as a saw-tooth shape  Arrhythmias: Impaired conductance o Common type: heart block  Start to get abnormal or absent electrical activity through theAV node o Normal pacemaker activity produced in SAnode but at some point, transmission toAV node is slowed or abolished o In a normal case, see P wave, QRS, T wave  P-R distance = red lines = indicative of time it takes for electrical activity in SAnode to get toAV node and into ventricles  First-degree heart block o In most benign sense, conduction throughAV node is slowed or delayed o Key to identifying first degree heart block: look at P-R distance  Prolonged PR interval (>0.2 sec) o Top diagram - first degree heart block  P-R distance is longer than in a normal rhythm  Normally shaped P wave - not an ectopic pacemaker - still generated at SA node, just taking longer to get toAV node  Not a very dangerous situation - happens under normal conditions • Enhanced vagal tone - parasympathetic innervation of the heart; impinges on SAnode andAV node and can slow down conduction to theAV node • AV node disease • Electrolyte imbalance  Benign situation; no treatment  Second degree heart block o In this case, there is still some transmission from atria intoAV node into ventricles, but sometimes it is completely blocked o Missing QRS complexes  Not all are missing, but have missing ones o Every now and then, electrical activity that has generated the P wave is not generated toAV node and do not get a QRS complex o Starting to become problematic o Progressive increase in delay until a beat is skipped (Type I) o 2-4 P waves for every QRS complex (Type II)  Third degree heart block o Dangerous situation o Complete inability ofAV node to conduct electrical activity from atria to ventricles o Normal pacemaker activity is not being transmitted down into ventricles o Some region of the ventricles has to serve as a pacemaker to generate ventricular contraction o Begin to see saw-tooth or m-type pattern. These patterns of activity are indicative of blockade o “Escape QRS Complex”: Generated in the ventricle  Narrow QRS complex (stable heart) • Generated near the Bundle of His  Wide QRS complex (unstable heart) • Anywhere below the Bundle of His, get wide QRS complexes that are almost always jagged and m-shaped  Requires medical intervention and implantation of an artificial pacemaker  Heart block o First degree = slowing of conduction through theAV node; indicated by a prolonged PR interval o Second-degree = drop some QRS complexes, therefore, not all electrical activity from atria is going to ventricle o Third-degree = no transmission through theAV node and we now get QRS complexes that are wide, m-shaped and are generated by cells in the ventricle  Arrhythmias: impaired conductance Branch bundle block o Recall - from theAV node, have the Bundle of His, then two branch bundles going to right and left ventricles which move up through Purkinje fibres o In branch bundle block - Have one branch bundle not conducting properly; the other is working fine but one is not o If the left one is blocked, and transmission is slow or not at all, then if wave of depolarization coming from the right branch bundle is normal, then the right ventricle will contract for the left ventricle o This is where the M-shaped QRS complex come from, have both atria contracting at different times  Get an R wave from one ventricle and one from another; are not synchronized and therefore get the M-shape  Arrhythmias: Flutter and fibrillation o Final type o Continuum of one another o Very rapid rates of electrical excitation and contraction in either the atria or the ventricles can produce flutter or fibrillation o Situation where there is rapid electrical activity in the atria or ventricle and flutter is a slower level of activity than fibrillation o If flutter and fibrillation are occurring in the atria; it is not very dangerous - atria only pumps 10% of blood into ventricles  Not that dangerous o Once get fibrillation in ventricles, this is a life threatening situation o Occurs because of recycling of electrical activity on the heart o Atrial Flutter = rapid levels of depolarization (200-300 contractions/minute)  Get a P wave, QRS, T, P, etc.  No isoelectric line - they blend into one another • Heart rate is so fast that the isoelectric interval between the end of the T wave and the beginning of the P wave disappears  Some electrical activity goes intoAV node and produces QRS complex • AV-node and ventricles are activated by every second or third atrial impulse  Normal rhythm in terms of ventricular contraction  Not overly dangerous  Arrhythmias: Fibrillation Atrial fibrillation o If staying in atria and moving from flutter to fibrillation, there is no cut off point of depolarizations per minute o Like a continuation of flutter to fibrillation o Once it is fibrillation, electrical activity is much more chaotic; do not get standard shaped P, T waves - blend into each other o Get electrical activity going into ventricles and normal QRS waves o Rapid depolarizations in atria are benign  Ventricular fibrillation o Dangerous o It is a very uncoordinated, chaotic, rapid electrical activity occurring in the ventricle o When it happens in the ventricle, blocks coordinated activity o Have little regions of the ventricles contracting at different times; have the muscle depolarizing and contracting all over the place, do not get normal bottom-to-top contraction and ejection of blood o The reason there are little regions of the heart depolarizing on their own is that electrical activity is stuck in re-entry circuits (circuit waves)  Caused by continuous recycling of electrical activity through the myocardium  Recycling is normally prevented due to the myocardium refractory period (post-contraction) o If some cells emerge from their refractory period before others, electrical waves can be continuously regenerated and conducted leading to uncoordinated contraction and impotent pumping o Get waves and waves of depolarization that go round and round causing these little regions to depolarize and contract on their own o In fibrillation, get different regions of ventricle depolarizing and contracting o Arrows represent circular motion of waves of depolarization relapping each other in circular, re-entry circuits  Re-entry circuits o Can think of them as conductive pathways or flow of electrical activity in the heart. o In normal situation, have a wave of depolarization go through contractile cells and branch into 2 different directions. Wave of depolarization going in a normal direction. When excitable cells depolarize they become excited, depolarize and go into refractory period. Get a wave of depolarization going down branch 1 and heads towards the middle because the cells at 3 have been stimulated and are now in refractory period. Same thing happens when wave goes down branch 2 - cells at branch 3 are in refractory period., don’t get a lapping around of current, or circular motion of current because cells hit refractory cells o In fibrillation, there is blockage of conduction. Blockage of conduction leads to a recycling of electrical activity. Down branch 1 have a normal depolarization. Down branch 2, there is region of cells/tissues that have a unidirectional block - the wave of depolarization cannot
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