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normal heart rate of an adult with a healthy heart:
72 bpm
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Excitatory and Electrical conduction systems of the heart is responsible for:
contraction and relaxation of heart muscles
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SA node:
- pacemaker
- electricle impulse is generated
- located: along posterior wall of right atrium; beneath opening of superior vena cava
- crescent shaped
- impulse from SA node -> AV node
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AV node:
- responsible for conduction of the impulse from atria -> ventricles
- impulse delayed slightly to allow complete emptying of atria before ventricles contract
- impulse -> AV bundles -> bundles of Purkinje fibers
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Purkinje Fibers:
conduct the impulse to all parts of the ventricles; causing contraction
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Vagal Stimulation:
- parasympathetic
- decrease in rate at SA node
- decrease excitability of AV junction fibers
- slows heart rate
- severe cases: blockage of impulse through AV junction
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Sympathetic Stimulation:
- increase rate at SA node
- increase rate of conduction and excitability
- increases force of myocardial contraction
- overall workload on heart is increased
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Ectopic Foci:
- small areas of heart become more excitable and cause abnormal heart beats
- extra or skipped heartbeats
- caused by ischemia, stimulants (nicotine/ caffine), lack of sleep
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Aberrant Conduction:
- cause arrhythmias by providing a block or alternate route for the wave of depolarization
- form short circuit bridge across the nonconductive tissue; tissue can depolarize prematurely or out of sync
- conduction within the AV node or bundle of His can be blocked; AV nodal blocks
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Channelopathy:
- inherited or aquired
- predipose individual to dangerous arrhythmias
- faulty ion channels result in prolonged repolarization; lengthening QT interval
- heart is most vulnerable during repolarization
- can be due to leaky or slow closing Na+ channels, ineffective K+ channels (inherited) or drugs and toxins which can affect the transport of ions across channels (aquired)
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EKG paper:
- time: horizontal axis; small square = 0.04 sec; large square = 0.2 sec
- voltage: vertical axis; 10mm = 1mV
- heart rate can be easily estimated from EKG strip
- count R waves in 6 sec strip; multiply by 10 = heart rate
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P wave:
- atrial depolarization (followed by atria contraction)
- normal duration: not longer than 0.11 sec (less 3 small squares)
- height: no more than 3 mm
- no notching or peaking
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QRS complex:
- ventricular depolarization (followed by ventricles contraction)
- normal duration: not longer than .10 sec
- height: not less than 5 mm
- R = positively deflected; Q and S = negatively deflected
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T wave:
- ventricular repolarization
- height: not more than 5 mm
- rounded and asymmetrical
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ST segment:
- early ventricular repolarization
- Depressed: no more than 0.5 mm
- may be elevated: no more than 1 mm
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PR interval:
- AV conduction time
- normal duration: 0.12 to 0.20 sec
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QT interval:
- ventricular depolarization and repolarization
- varies with rate
- Prolonged QT may be inherited or aquired
- inherited: defective Na+ or K+ channels
- aquired: drugs (at least 50 drugs are know to affect QT; quinidine), electrolyte imbalance, or MI
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