conduction system & electrocardiography (EKG)

heart contraction

results from series of depolarization waves that travel through heart before each beat; entire myocardium behaves like a single unit

functional syncytium

cardiac muscle cells are mechanically, chemically, & electrically connected to one another; entire tissue resembles a single, enormous muscle cell

intrinsic conduction

heart does NOT depend on impulses from the nervous system to initiate contraction; will contract even if all nerves are severed

2 types of controlling systems affect activity

1. intrinsic conduction system (nodal system)

2. autonomic nervous system (SNS & PNS)

intrinsic conduction system (nodal system)

specialized non-contractile myocardial cells, pacemaker/autorhythmic cells; ensures heart muscle depolarizes in orderly/sequential manner

autonomic nervous system (SNS & PNS)

modifies rate set by conduction system.. increases OR decreases HR

intrinsic conduction system steps

1. sinoatrial (SA) node (pacemaker) generates impulses

2. impulses pause (0.1s) at atrioventricular (AV) node

3. Atrioventricular (AV) bundle connects the atria to ventricles

4. bundle branches conduct the impulses through the interventricular septum

5. subendocardial conducting network (purkinje fibers) depolarizes the contractile cells of both ventricles

Sinoatrial (SA) node location

within the right atrium, inferior to superior vena cava opening

atrioventricular (AV) node

base of right atrium

AV bundles (Bundle of His)

interventricular septum

right & left bundles

interventricular septum

purkinje fibers

ventricular walls

electrocardiography

conduction of impulses through heart generates electrical currents (throughout body); impulses can be detected on body's surface & recorded

electrocardiogram (ECG or EKG)

graphical recording of electrical changes through the myocardium

p wave

start of P deflection to return to baseline

P-R interval

start of P deflection to start of Q deflection

ORS complex

start of Q deflection to S return to baseline

S-T segment

end of S deflection to start of T wave

Q-T interval

start of Q deflection to end of T wave

T wave

start of T deflection to retuen to baseline

T-P segment

end of T wave to start of next P wave

R-R interval

peak of R wave to peak of next R wave

EKG image

EKG segments & intervals

Heart depolarization & repolarization steps

1. atrial depolarization, initiated by SA node, causes P wave

2. atrial depolarization complete, impulse is delayed at AV node

3. ventricular depolarization begins at apex, causing QRS complex. atrial repolarization occurs

4. ventricular depolarization complete

5. ventricular repoarization begins at apex, causing t wave

6. ventricular repolarization complete

tachycardia

heart rate over 100 beats/min

bradycardia

heart rate below 60 beats/min

1 tiny square = 1 mm

= 0.04 sec

2 ways to calculate heart rate

1. 60/ duration of R-R interval in seconds

2. 1500/ # of mm making up R-R interval (# of tiny square)