normal heart rate of an adult with a healthy heart:
72 bpm
Excitatory and Electrical conduction systems of the heart is responsible for:
contraction and relaxation of heart muscles
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
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
Purkinje Fibers:
conduct the impulse to all parts of the ventricles; causing contraction
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
Sympathetic Stimulation:
increase rate at SA node
increase rate of conduction and excitability
increases force of myocardial contraction
overall workload on heart is increased
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
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
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)
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
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
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
T wave:
ventricular repolarization
height: not more than 5 mm
rounded and asymmetrical
ST segment:
early ventricular repolarization
Depressed: no more than 0.5 mm
may be elevated: no more than 1 mm
PR interval:
AV conduction time
normal duration: 0.12 to 0.20 sec
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