ECG and the Heart’s Internal Conduction System Guided Notes
Name:________________________
Do Now
What is an action potential and where do they occur?
Intrinsic conduction system

_______________ is the heart’s natural pacemaker.
◦

Impulse travels throughout the ____________ and to
the ________________
◦
_______________________________________
_______________________________to allow atria to
finish contracting

The ______________ (aka bundle of His) transmits
the impulse to the _______________________ and
______________________________________.
Interestingly, all the cells of the conduction system are
_______________________________.
That is, all will depolarize at a certain rate.
The __________________ has the fastest rate of
depolarization, though, so it sets the pace for the entire heart.
What is the function of
-
AV node
-
Gap junctions
Travels cell-to-cell through __________
_________________ in intercalated discs –
another unique feature of cardiac muscle

◦
Quick Review – Intrinsic Conduction System
It depolarizes ~75 times / min, to start each
heartbeat.
◦
These bundles and fibers speed the
transmission of the impulse throughout the
ventricles, to coordinate ventricular
contraction
-
SA node
-
Purkinje fibers
Name 3 differences between cardiac and skeletal muscle contraction & explain why each is important to cardiac
function.
ECG

An electrocardiogram (ECG or EKG) records the electrical current
through the heart.

A normal ECG has three distinct waves:
◦
P wave – atrial depolarization
◦
QRS complex – ventricular depolarization AND atrial
repolarization
◦
T wave – ventricular repolarization
ECG Questions
What is responsible for the delay between P wave and QRS complex?
What is responsible for the delay between T wave and next P wave?
Examine this normal ECG.
 When would the ventricles be in systole?
 When would they be in diastole?
 When would the heart sounds occur, and what do the heart sounds correspond to?
ECG abnormalities
Examine these ECG readouts, and identify how they differ from a normal ECG.
a)
b)
c)
d)
Descriptors –
Name
AV block
Atrial fibrillation
Tachycardia
Bradycardia
Characteristic ECG
In severe cases, there is NO
relationship between P waves and
QRS complex. QRS still occurs
due autorhythmic cells in AV
node, but at a slower rate.
Wavy baseline, no regular p waves
QRS normal shape but occurs
randomly
Looks messy
Heart beats faster than normal
(more than 100 bpm)
Heart beats slower than normal
(less than 60 bpm)
Cause(s)
Damage to AV node, such as from
reduced blood flow (ischemia) or
fibrosis.
Treatment: dual chamber artificial
pacemaker- one that ‘listens’ to the
SA node and sends a signal to the
AV node
Heart is doing uncoordinated,
ineffective contraction due to lack
of blood, excess alcohol, or
infection
Exercise, drugs
Many conditions, including heart
damage, medications, etc,
Regulation of Heart Rate
Earlier, we said that the SA node depolarizes at a rate of ~75 beats per minute, and that this acts as a pace maker
for heart contraction.
Does this mean our hearts always beat at ~75 bpm?
What are the two branches of the autonomic nervous system, and how do they affect heart rate?
Other factors that influence heart rate
 Age (fastest in fetus, young children)
 Gender (faster in females)
 Temperature (faster in heat)
 Exercise (faster with exercise)
 Ions imbalances & medicines (faster or slower)
 Weak / damaged heart (can be either faster / slower)