Negatively charged resting cardiac cell is said to be?
Polarized
Depolarization
Discharge of electricity-occurs when cardiac cell becomes positively charged. ( MUSCLE CONTRACTION)
Repolarization
Return of the cardiac cell to its RESTING negative charge.
Electrical stimulus precedes mechanical response
Cannot have myocardial contraction without first having had depolarization.
Depolarization & Repolarization
are electrical events
Action Potenial
Phase 4- Cardiac cell at rest. Corresponds with isoelectric line of EKG.
Phase 0-Depolarization. Cell becomes positively charged. Corresponds with QRS complex.
Phase 1 & 2- Early repolarization. Calcium is released. Muscle contraction begins. Corresponds
Refactory Periods-
Cardiac cell resists responding to/depolarizing from an impulse.
Absolute refectory period-
Cardiac cell cannot respond to another impulse, no matter how strong.
Relative refractory period-
Cell can respond only to very strong impulse.
Supernormal period-
Cardiac cell is "HYPER," will respond to very weak stimulus.
Each P-QRS-T sequence represents one heartbeat.
True
P wave-
Artial depolarization
Ta wave-
atrial reploarization- usually not seen, asit occurs simultaneous with QRS.
QRS complex-
Ventricular depolarization. A series of spiky waves.
T wave-
Ventricular repolarization
U wave-
Late ventricular repolarization - not usually seen.
PR segment-
Flat line between the P wave and the QRS complex.
ST segment-
Flat line between the QRS complex and the T wave.
Cardiac conduction system-
Pathway of specialized cells-job is to create and conduct impulses to tell the heart when to beat.
Conduction Pathway-
Sinus node-interatrial tracts-atrium-internodal tracts-AV node-bundle of His-bundle branches-Perkinje fibers-ventricle.
Characteristics of cardiac cells:
Automaticity-Electrical-ability to create an impulse.
Conductivity-Electrical-ability to pass that impulse along to neighboring cells.
Excitability-Electrical-ability to respond to that impulse by depolarizing.
Contractility-Mechanical-ability to contract
Inherent rates of pacemaker cells:
sinus node-60-100 beats per min
Av junction-40-60 beats per min
Ventricle-20-40 beats per min
Sinus Node
Normal pacemaker of the heart. (fastest pacemaker at any given moment is the one in control)
Describe Escape:
Predominant pacemaker slows down; lower pacemaker takes over at its slower inherent rate-results in a slower heart rate then before.
Escape beat:
Beat that comes in after a pause longer than normal R-R interval.
Escape rhythm:
Series of escape beats.
Usurpation (irritability)
Lower pacemaker becomes "hyper"; fires in at an accelerated rate, stealing control away from slower predominant pacemaker-results in a faster heart rate than before.
Heart rhythm
Pattern of successive heart beats.
Arrhythmia
Abnormal heart rhythm.
12-lead EKG
Printout of heart's electrical activity from 12 different angles
Lead
Electrocardiographic picture of the hearts electricity.
Rhythm Strip
Printout of 1 to 2 leads on small roll of graph paper.
Intervals
Measurements of time between EKG waves and complexes.
PR interval-
Measures time it takes impulse to get from atrium to ventricle. Normal PR interval 0.12-0.20 sec.
QRS interval-
Measures time it takes to depolarize the ventricle. Normal QRS interval is less than 0.12 sec.
QT interval-
Measures depolarization/repolarization time in ventricle.