A Case of Dizziness
A 68 year old female arrives at the emergency room in an ambulance. That evening
she had been feeling “weak and dizzy” after ingesting a handful of her “heart pills” and
later passed out. Her heart rate was irregular but near 33 beats per minute.
Her patient records and talks with her family revealed that she is being treated for
poorly controlled hypertension and congestive heart failure. Her records indicate she
has been prescribed the following medications:
Doxazosin
Avapro
Tiazac
Toprol
Lasix
Potassium supplements
Digoxin
Zyrtec, celebrex
Her EKG records displayed several arrhythmias and while efforts at treatment were
being made, she went into ventricular fibrillation.
The EKG
Physiology in action
Objectives
understand basic cardiac anatomy
 understand how cellular action potentials
give rise to a signal that can be recorded
with extracellular electrodes
 understand the path for action potential
propagation through the heart
 understand the origin of the main phases of
electrocardiogram (EKG)

The Heart
is a pump
has electrical activity
(action potentials)
generates electrical
current that can be measured
on the skin surface (the EKG)
Currents and Voltages




At rest, Vm is constant
No current flowing
Inside of cell is at
constant potential
Outside of cell is at
constant potential
A piece of cardiac muscle
inside
-----------------------------++++++++++++++++++
outside
-
+
0 mV
Currents and Voltages
A piece of cardiac muscle




During AP upstroke,
Vm is NOT constant
Current IS flowing
Inside of cell is NOT
at constant potential
Outside of cell is NOT
at constant potential
An action potential propagating
toward the positive ECG lead
produces a positive signal
AP
inside
++++-----------------------------++++++++++++++
outside
current
-
+
Some positive
potential
More Currents and Voltages
A piece of cardiac muscle
inside
++++-----------------------------++++++++++++++
An action potential propagating
Away from the positive ECG lead
produces a negative signal
outside
current
-
+
A negative voltage reading
More Currents and Voltages
During Repolarization
A piece of cardiac muscle
A piece of totally depolarized
cardiac muscle
inside
------------+++++++++++
inside
+++++++++++++++++++
+++++++------------------outside
------------------------------outside
Vm not changing
No current
No ECG signal
current
Repolarization spreading toward
the positive ECG lead produces
a negative response
Some negative potential
-
+
The EKG
Can record a reflection of cardiac electrical
activity on the skin- EKG
 The magnitude and polarity of the signal
depends on

– what the heart is doing electrically
depolarizing
 repolarizing
 whatever

– the position and orientation of the recording
electrodes
Cardiac Anatomy
Superior
vena cava
Pulmonary
veins
Sinoatrial (SA)A node
Atrial muscle
Atrioventricular (AV) node
Left atrium
Mitral valve
Internodal
conducting
tissue
Tricuspid valve
Ventricluar
muscle
Inferior
vena cava
Purkinje
fibers
Descending aorta
Flow of Cardiac Electrical
Activity
SA node
Internodal
conducting
fibers
Atrial muscle
Atrial muscle
AV node (slow)
Purkinje fiber
conducting system
Ventricular muscle
Conduction in the Heart
0.12-0.2 s
approx. 0.44 s
Superior
vena cava
SA
node
Pulmonary
veins
SA node
Atrial muscle
Atria
AV
node
Ventricle
Left atrium
Mitral valve
Specialized
conducting
tissue
Tricuspid valve
Purkinje
AV node
Ventricluar
muscle
Inferior
vena cava
Purkinje
fibers
Descending aorta
The Normal EKG
Right Arm
“Lead II”
approx. 0.44 s
0.12-0.2 s
QT
PR
Left Leg
Atrial muscle
depolarization
R
T
P
Q
S
Ventricular muscle
depolarization
Ventricular
muscle
repolarization
Action Potentials in the Heart
0.12-0.2 s
approx. 0.44 s
PR
QT
Superior
vena cava
ECG
Pulmonary artery
SA
Atria
AV
Pulmonary
veins
Ventricle
AV node
SA node
Left atrium
Atrial muscle
Mitral valve
Specialized
conducting
tissue
Tricuspid valve
Purkinje
Aortic artery
Ventricluar
muscle
Inferior
vena cava
Interventricular
septum
Purkinje
fibers
Descending aorta
Start of EKG Cycle
Early P Wave
Later in P Wave
Early QRS
Later in QRS
S-T Segment
Early T Wave
Later in T-Wave
Back to where we started
A Case of Sudden Death
A 68 year old female arrives at the emergency room in an ambulance. That evening she had
been feeling “weak and dizzy” after ingesting a handful of her “heart pills” and later passed
out. Her heart rate was irregular but near 33 beats per minute.
Her patient records and talks with her family revealed that she is being treated for poorly
controlled hypertension and congestive heart failure. Her records indicate she has been
prescribed the following medications:
Doxazosin
Avapro
Tiazac
Toprol
Lasix
Potassium supplements
Digoxin
Zyrtec, celebrex
Her EKG records displayed several arrhythmias and while efforts at treatment were being
made, she went into ventricular fibrillation.
A Case of Sudden Death
As noted, the patient’s heart rate was irregular and so were her
EKG records. The figures below show two types of patterns seen: