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10 Cards in this Set
- Front
- Back
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What happens in the different phases of AP of working myocardium?
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Depolarization (0):
-caused by transmission of AP from SA Node -activation of Na+ channels --> transient inward current -dv/dt (rate of slope) correlates with size of inward Na+ current Early repolarization (1): -cessation of Na+ influx (inactivation of Na+ channels) -increase permeability for K+ initiated repolarization Plaateau Phase (2): -membrane remains depolarized for long period of time -balance of 2 currents: opening of slow L-type Ca gates and small but sustained outward K+ current preventing myocytes from repolarizing rapidly Repolarization (3): -closing of slow L-type Ca channels and increase K+ influx Resting Membrane Potential (4): -plasma membrane polarized -stable due to conduction of Na+, Ca++, and K the important unique characteristics of working myocardium: -long duration -stable RMP -plateau phase |
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What are characteristics of the contraction of cardiac muscle?
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heart contracts at syncytium
contraction lasts 300 msec (ventricles) excitation-contraction coupling: -link between electrical excitation and muscle contraction -provided by Ca++ ions 2 sources of Ca: -SR (less developed than in skeletal muscles) -influx from ECF during AP generation (plateau phase) |
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What are the different periods during an AP?
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Absolute (effective) refractory period:
-cannot be excited even by suprathreshold stimuli -corresponds to phases 0,1,2 and half of 3 Relative refractory period: -AP CAN be elicited by suprathreshhold stimuli -half of phase 3 Importance of long refractory period: -no muscle tetany, cant get reexcited during AP generation and contraction until it has relaxed -Tetany of heart would be FATAL |
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What is a Electrocardiogram?
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ECG or EKG
measurement of tiny electrical potential differences in parts of the heart from surface of body electrical potential differences in the heart (depolarized and polarized) result from sequence and timing of spread of depolarization and repolarization in the myocardium ECG provides info on: -size of chambers and anatomy of heart -HR, rhythm and origin, spread, and decay of excitation -abnormalities of heart muscle and effect of drugs NO info on contraction, efficiency and blood flow |
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What is the Lead of the ECG?
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Lead - the placing of electrodes to record ECG
Bipolar standard limb leads (classical): -between 2 active electrodes RA; LA; LL + RL - ground conductor Lead I: LA-RA Lead II: RA-LL Lead III: LA-LL Unipolar chest (precordial) leads: -Active electrode RA, LA, and LL connected together forming indifferent electrode |
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What are the different ECG waves?
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P wave:
-upward deflection -reflects DEPOLARIZATION of atrial muscle (SA node --> contraction fibers of both atria) QRS complex: -ventricular depolarizaiton (+atrial repolarization masked by large ventricular waves Q wave: -depolarization of interventricular septum R wave: -depolarization of LV S wave: -depolarization of RV T wave: -ventricular repolarization U wave: -slow repolarization of papillary muscles |
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What are abnormalities in size of waves?
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Larger P wave - enlargement of atrium
Enlarged Q wave - Myocardial infarction Enlarged R wave - enlarged ventricles Flatter T wave - insufficient oxygen supply in coronary heart disease Elevated T wave - hyperkalemia |
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What are the different ECG segments?
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segment - period between 2 waves
PR segment (PQ) - connects P wave and QRS complex. Electrical conduction from AV node --> bundle of His --> bundle branches --> Purkinje Fibers ST segment: -end of S to beginning of T -ST segment is elevated in acute myocardial infarction and depressed with heart muscle receives insufficient oxygen |
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What are the different ECG intervals?
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Interval = wave + segment
PR (PQ) interval: -beginning of P wave to beginning of QRS complex -time from sinus node --> AV node --> ventricles -good estimate of AV node function QT interval: -onset of QRS complex to end of T wave -electrical systole of heart (ventricular depolarization + repolarization) R-R interval: -QRS to QRS -reflects duration of cardiac cycle -HR = 60 sec/R-R |
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What is normal sinus rhythm and Arrhythmias?
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Normal sinus rhythm:
-60-100 beats/min -P wave before each QRS complex and QRS after each P wave Arrhythmia: Classification based on speed: -Tachycardia - above 100 beats/min -Bradycardia - below 50 beats/min Classification based on origin: -supraventricular (atrial) -ventricular Sinus tachycardia: -Normal ECG + HR faster than 100 beats/min -Physiological response to exercise, stress, fever, acute drop in BP -pathology = hyperthyroidism Sinus Bradycardia: -HR less than 50 beats/min + normal sinus mechanisms (index of fitness in athletes) |
