Electrical Function of the Heart

What is the pacemaker of the heart

SA node (crista terminalis) → Automaticity of the heart

What is automaticity

Ability for heart cells to depolarize and create AP WITHOUT external stimuli

AP of pacemaker cells

Phase 4 → Na, K current through HCN channel, Phase 0 → Ca center cell (depolarization), Phase 3 → K flow out of cell

What is the action of ANS for heart beat

Heart does NOT need ANS to beat; ANS modulate HCN channel by cAMP

What is cAMP relationship with HCN channel

High cAMP = high f channel activity

What happens in phase 4 of pacemaker AP

Depolarization occurs from Na INFLUX through HCN channel → T type Ca channel open

What happens in phase 0 of pacemaker AP

L type Ca channel open → Ca enter cell (depolarization)

What happens in phase 3 of pacemaker AP

K+ channel open, L type Ca channel close → K+ efflux out of cell → Repolarization

What node replaces SA node if it fails

AV node (second fastest conductor)

Why is the SA node the pacemaker

It discharges the most rapidly → Depolarization spreads to other regions the fastest

What is the expressway of the conduction system

SA → Internodal pathway → AV node → His bundle → Bundle branches (left and right) → Purkinje 

What activates the left atrium

Bachmann bundle from the SA node

What cause atrial-ventricular contraction

AV node conduction delay

What activates the septum and apex

Purkinje fiber → Ventricle

What conducts electrical impulse from atrium to ventricle

Bundle of His; fibrous tissue separates A-V

What is P wave in ECG

Depolarization of ATRIA

What is QRS complex in ECG

VENTRICULAR depolarization

What is T wave in ECG

Ventricular REpolarization

What are the differences in distribution between the SA and AV nodes

Right vagus → SA node; Left vagus → AV node; right sympathetic → SA node; left sympathetic → AV node

What is function of NorE (autonomic) in contractility

NorE increase cAMP → Ca influx → Faster depolarization and contraction; NorE increase SR Ca uptake → Repolarization period shorten → Higher HR

What is function of Ach (autonomic) in contractility

Reduce cAMP → HR reduce; Reduce Ca influx → Reduce contractility

In what direction does ventricular excitation start

Apex to the base; inside to outside

What is the PR interval in ECG

Beginning of P wave to beginning of QRS complex → Long PR interval = AV node conduction delay

What is Einthoven triangle

Lead I-III formed by placing leads on Rt Arm, Lt arm and Lt leg

What is augmented limb leads

Unipolar where one limb is positive charge and negative charge is the average between the two remaining limbs

What is lead I

Rt arm neg, Lt arm pos → Rt arm to Lt arm

What is lead II

Rt arm neg, Lt leg pos → Rt arm to Lt leg

What is lead III

Lt arm neg, Lt leg pos → Lt arm to Lt leg

What is aVF

Lt leg pos, both arms are negative

If both lead 1 and aVF are POSITIVE what is the axis of rotation

Between 0-90 degrees = NORMAL

If lead I positive, aVF negative what is the axis of rotation

Left atrial deviation between 0 to -90 degrees

If lead I negative, aVF positive what is the axis of rotation

Right atrial deviation between 90-180 degrees

If both lead 1 and aVF are NEGATIVE what is the axis of rotation

Extreme axis between -90 and 180 degrees

What areas are each chest lead placed

V1= at 4th IC space left to sternum; V2 = 4th IC space right to sternum; V3 between V2 and 4; V4 at 5th IC space midclavicular; V5 horizontal from V4 lt anterior axillary; V6 horizontal midaxillary

What is normal transition phase in ECG

Between V3 and V4

What would happen if transition phase was between V2 and V3

COUNTERclockwise rotation → LV voltage dominate → LV hypertrophy

What would happen if transition phase was between V4 and V5

CLOCKWISE rotation → RV voltage dominate → RV hypertrophy or heart rotation