Cardiac Conduction Pathway System
SA node> AV node> Bundle of His> Left and Right bundle branches> Purkinjie fibers
Pacemakers of the heart
♥
... [Show More] Natural: SA Node = 60-100 beats/minute
♥ Backup: AV Node = 40-60 beats/minute
♥ Backup: Purkinje Fibers (ventricles) = 20-40 beats/minute
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SA Node
• Intrinsic Pacemaker of the heart
• Rate 60-100 beats/min
• RA (Right Atrium), close to SVC (Superior Vena Cava)
• Blood supply from RCA (Right Coronary Artery) & LCA (Left Coronary Artery)
Interatrial/Internodal Tracts
Transmits impulses from the SA node to the AV node through the RA & LA
AV node
• Slows conduction (40-60 beats/min)
• Physiologic delay allows atrial kick on floor of RA near tricuspid valve
Bundle of His
Bundle of cardiac muscle fibers that conducts the electrical impulses from the AV node in the right atrium to the septum between the ventricles and then to the left and right ventricles.
bundle branches
Right and Left (Left anterior & Left posterior fascicles)
Purkinjie Fibers
Fibers from Bundle Branches imbedded into the ventricle walls.
Depolarization vs Repolarization
Depolarization – electrical excitation of the cell membrane, normally followed by mechanical contraction
Repolarization – return of cell membrane to its resting state, normally followed by mechanical relaxation
Electrical and mechanical activity of the heart
♥ The heart has two activities that are performed rhythmically: electrical activity followed by mechanical activity
♥ Electrical activity always precedes mechanical activity
♥ It is possible to have electrical activity without mechanical response
♥ Always check the patient - Do Not Depend on the Machine!!!
Measurement of the boxes in the ekg
♥ A standard ECG is printed at 25mm per second or 25 small squares per second. Since one second divided by 25 small boxes, then each 1 mm box = 0.04 seconds. The larger boxes indicated by the heavier lines are equal to 0.20 seconds.
♥ Voltage is measured along the vertical axis and is expressed in millivolts (mV). The standard calibration is that a 1 mV signal produces a 10-mm deflection (0.1 mV=1mm). Simply put 10 small squares vertically is equal to 1 millivolt
EKG waveforms and intervals
♥ P wave – rounded with upright deflection in lead II, atrial depolarization
♥ PR interval- delay @ AV junction
• measure from beginning of P wave to beginning of QRS
• Normal duration 0.12 – 0.20 seconds •
S wave – Negative deflection following the R-wave
♥ QRS complex – multiple components, Ventricular depolarization, - 0.06 –0.11
• Q wave – first negative deflection after P wave R wave
• Positive deflection after the Q
♥ J point - marks where the QRS complex ends and the ST segment begins
♥ ST segment – normally isoelectric line between QRS and the beginning of the T wave
♥ QT interval – beginning of the QRS complex to the end of the T wave; changes with heart rate, usually half of the R to R interval. beginning of vent activation through Ventricular depolarization. 0.44 –0.48 (Rate dependent)
♥ T wave - follows the QRS; typically, larger than the P wave and slightly asymmetric; Ventricular repolarization
♥ U wave – not usually seen, may be due to hypokalemia or digitalis toxicity
Isoelectric Line –No perceived electrical current
indication of a flat line at any time in the duration of a series of waves
indicates no electrical activity at that particular moment
Nursing Responsibilities when theres changes in ECG rhythm
♥ Print, measure, interpret and post rhythm strips per unit standard.
♥ Get a 12 lead ECG for any rhythm changes.
♥ Get a set of vital signs for any rhythm changes.
♥ Know your unit standards and when to notify the MD.
Why is normal conduction and heart rate important?
critical for adequate filling & optimum cardiac output (CO)
Mechanism of dysrhythmias: Altered Automaticity
change in rate of pacer cells; sympathetic vs. parasympathetic
Mechanism of dysrhythmias: Abnormal automaticity
non-pacer cells fire spontaneously due to:
• injury/ischemia, hypoxia,
• stretch with volume overload
• hypokalemia hypomagnesaemia
Mechanisms of Dysrhythmias: Altered Conductivity
blocks in normal conduction
• fibrotic changes, valvular disease, acute ischemia
Mechanisms of Dysrhythmias: Re-entry
a single impulse can depolarize the same tissue more than once
Consequences of Dysrhythmias
♥ Decrease HR →decreased CO
♥ Increased HR →decreased CO
♥ Premature beats →decreased stroke volume
♥ Loss of AV synchrony →loss of atrial kick (~20%) [Show Less]