Action Potentials and ECG

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4 CHARACTERISTICS OF HEART TISSUE: automaticity, conductivity, refractatory, excitability

TWO TYPES OF MUSCLE CELLS:

Contractile Cells (Ventricular Action Potential):

repolarization: going downwards on the graph

depolarization: increase in the graph (going upwards)

This is ventricular because it takes longer to repolarize

Autorhythmic Cells:

pacemaker potential: influxes (depolarization and repolarization)

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Refractory period: cardiac muscle occurs immediately following an action potential

the cardiac muscle membrane is unresponsive to electrical excitation

cardiac muscle's refractory period is longer

Autonomic Stimulation:

SNS: bathes the heart in NE and increases the leakiness of the Na+ channel to increase heart rate

PNS: stimulation decreases heart rate, vagus nerve to the SA and AV node

Pacing:

Normally, SA is supposed to be the fastest, AV second, and purkinje fibers last

Scenarios

if SA fails, AV would lead, making heart rate slower 50 mph

If AV fails, SA would lead which results in ventricles assuming their own in a slower rhythm

If Purkinje is too fast (faster than SA), the whole heart will be faster

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ECGs

12 lead ECG provides: standardized diagnostic tool for visualizing the sum of cardiac electrical activity

6 limb: I, II, III, aVR, aVL, and a aVF

6 chest: V1 to V6

Interpretations:

PQRST waves:

P wave: atrial depolarization: allows the sodium to enter past the channels (around 0.1 seconds delay)

QRS: Ventricular depolarization (stronger) (less than 0.2 seconds)

T wave: ventricular repolarization (when there are more K+ ions) normal at zero deflection

Abnormalities:

heart block (PR segment is too long)

P waves occur independently (inconsistent)

treatment: defibrillator

Wolff-Parkinson-White Syndrome:

sloped delta wave and shortened PR

treatment: catheter ablation

Ventricular Tachycardia

when only the SA node is in control

treatment: defibrillation

Ventricular Fibrillation

when there is ineffective pumping (quivering)

treatment: defibrillation

Heart Attack

abnormalities in electrical conduction