Electrical Activity
of the Heart
Introduction
✦
Where does the “electro” in
electrocardiography come from?
Under this condition, the heart cell is said to
be polarized
Polarization
✦
Imagine two micro-electrodes; one
outside the cell, one inside the cell
✦
Difference between the two equals -90
mV inside
✦
The cell is said to be ‘polarized’
Action Potential
Action Potential in
Skeletal Muscle Fiber
closed
gates
opened
gates
Action Potential
Skeletal
Cardiac
Myocyte Action
Potentials
✦
Fast and Slow
✦
Fast = non-pacemaker cells
✦
Slow = pacemaker cells (SA and AV node)
Ions
Ion
Extra-
Intra-
Na
140
10
K
4
135
Ca
2
0.1
Action Potential
✦
Ion influx
✦
Na channels (fast and slow)
✦
K channels
✦
Ca channels
Inside
Outside
thevirtualheart.org/CAPindex.html
Action Potential
✦
Phase 0
✦
Stimulation of the
myocardial cell
✦
Influx of sodium
✦
The cell becomes
depolarize
✦
Inside the cell = +20 mV
Action Potential
✦
Phase 1
✦
✦
Ions
✦
Influx of sodium
✦
Efflux of potassium
Partial repolarization
Action Potential
✦
Phase 3
✦
✦
✦
Ions
✦
Efflux of potassium*
✦
Influx of calcium
Repolarization (slower process than
depolarization)
Phase 4
✦
Interval between repolarization to the
next action potential
✦
Pumps restore ionic concentrations
Ion
0
1
Na
influx
influx
K
Ca
efflux
2
3
4
pump
efflux
efflux*
pump
influx
influx
pump
Refractory Periods
✦
Absolute refractory period - phase 1 midway through phase 3
✦
Relative refractory period - midway through
phase 3 - end of phase 3
SA Node Action
Potential
✦
“Funny” currents (phase 4);
slow Na channels that initiate
spontaneous depolarization
✦
No fast sodium channels
✦
Calcium channels (slow)
✦
✦
Long-lasting, L-type
✦
Transient, T-type
Potassium channels
Action Potentials
✦
Fast and Slow
Action Potentials
It is important to note that non-pacemaker action potentials can
change into pacemaker cells under certain conditions. For
example, if a cell becomes hypoxic, the membrane depolarizes,
which closes fast Na+ channels. At a membrane potential of about
–50 mV, all the fast Na+ channels are inactivated. When this
occurs, action potentials can still be elicited; however, the inward
current are carried by Ca++ (slow inward channels) exclusively.
These action potentials resemble those found in pacemaker
cells located in the SA node,and can sometimes display
spontaneous depolarization and automaticity. This mechanism may
serve as the electrophysiological mechanism behind certain types
of ectopic beats and arrhythmias, particularly in ischemic heart
disease and following myocardial infarction.
❖
Conduction speed varies throughout the
heart
❖
Slow - AV node
❖
Fast - Purkinje fibers
Action Potential
✦
ECG records depolarization and
repolarization
✦
Atrial depolarization
✦
Ventricular depolarization
✦
✦
Atrial repolarization
Ventricular repolarization
The Body as a Conductor
This is a graphical representation of the geometry and
electrical current flow in a model of the human thorax.
The model was created from MRI images taken of an
actual patient. Shown are segments of the body surface,
the heart, and lungs. The
colored loops
represent the flow of electric
current through the thorax for a
single instant of time, computed from
voltages recorded from the surface of the heart during
open chest surgery.
Assignment
✦
Read “Non-pacemaker Action Potentials”
✦
Read “SA node action potentials”
Basic ECG
Waves
Chapter 2
ECG Complexes
ECG Complexes
Action Potential &
Mechanical Contraction
ECG Paper
✦
Small boxes = 1 mm
✦
Large boxes = 5 mm
✦
Small boxes = 0.04 seconds
✦
Large boxes = 0.20 seconds
✦
5 large boxes = 1.0 second
✦
Paper speed = 25 mm / sec
ECG Paper
✦
Horizontal measurements in seconds
✦
Example, PR interval = .14 seconds (3.5 small
boxes)
ECG Paper
✦
Standardization mark
✦
10 mm vertical deflection = 1 mVolt
ECG Paper
✦
Standardization marks
✦
Double if ECG is
too small
✦
Half is ECG is too
large
Top: Low amplitude complexes in an obese women
with hypothyroidism
Bottom: High amplitude complexes in a hypertensive
man
ECG Description
✦
ECG amplitude (voltage)
✦
recorded in mm
✦
positive or negative or biphasic
ECG Waves
✦
Upward wave is described as positive
✦
Downward wave is described as negative
✦
A flat wave is said to be isoelectric
✦
✦
Isoelectric as describes the baseline
A deflection that is partially positive and
negative is referred to as biphasic
ECG Waves
✦
P wave
✦
✦
atrial depolarization
✦
≤ 2.5 mm in amplitude
✦
< 0.12 sec in width
PR interval (0.12 - 0.20 sec.)
✦
time of stimulus through atria and AV
node
✦
e.g. prolonged interval = first-degree heart
block
ECG Waves
✦
QRS wave
✦ Ventricle depolarization
✦ Q wave: when initial deflection is negative
✦ R wave: first positive deflection
✦ S wave: negative deflection after the R
wave
✦
QRS
✦
✦
✦
✦
ECG Waves
May contain R wave only
May contain QS wave only
Small waves indicated with small letters (q, r,
s)
Repeated waves are indicated as ‘prime’
ECG Waves
✦
QRS
✦
width usually 0.10 second or
less
ECG Waves
✦
RR interval
✦
interval between two consecutive QRS
complexes
ECG Waves
✦
✦
J point
✦
end of QRS wave and...
✦
...beginning of ST segment
ST segment
✦
beginning of ventricular repolarization
✦
normally isoelectric (flat)
✦
changes-elevation or depression-may indicate
a pathological condition
ECG Waves
ECG Waves
✦
T wave
✦
part of ventricular repolarization
✦
asymmetrical shape
✦
usually not measured
ECG Waves
✦
QT interval
✦
from beginning of QRS to the end of the T
wave
✦
ventricular depolarization & repolarization
✦
length varies with heart rate (table 2.1)
✦
long QT intervals occur with ischemia,
infarction, and hemorrhage
✦
short QT intervals occur with certain
medications and hypercalcemia
ECG Waves
✦
QT interval should be less than half the R-R interval
✦
If not, use Rate Corrected QT Interval
✦
normal ≤ 0.44 sec.
ECG Waves
✦
✦
Long QT interval
✦ certain drugs
✦ electrolyte distrubances
✦ hypothermia
✦ ischemia
✦ infarction
✦ subarachnoid hemorrhage
Short QT interval
✦ drugs or hypercalcemia
ECG Waves
✦
U Wave
✦
last phase of repolarization
✦
small wave after the T wave
✦
not always seen
✦
significance is not known
✦
prominent U waves are seen with
hypokalemia
Heart Rate Calculation
1. 1500 divided by the number
of small boxes between two R
waves
• most accurate
• take time to calculate
• only use with regular rhythms
2. 300 divided by the number of • quick
• not too accurate
large boxes between two R
• only use with regular rhythm
waves
3. Number of large squares w/i RR interval
1 lg sq = 300 bpm
2 lg sq = 150 bpm
3 lg sq = 100 bpm
4 lg sq =
5 lg sq =
6 lg sq =
75 bpm
60 bpm
50 bpm
Heart Rate Calculation
✦
For regular rhythm...
✦
Count the number of large boxes
between two consecutive QRS
complexes. Divide 300 by that number
✦
✦
300 ÷ 4 = 75
Count the small boxes. Divide 1500 by
that number
✦
1500 ÷ 20 = 75
Heart Rate Calculation
✦
✦
For irregular rhythms…
Count the number of cardiac cycles in 6
seconds and multiple this by 10. (Figure
2.15)
The ECG as a Combination of
Atrial and Ventricular Parts
✦
Atrial ECG = P wave
✦
Ventricular ECG = QRS-T waves
✦
Normally, sinus node paces the heart and P
wave precedes QRS
✦
✦
P-QRS-T
Sometimes, atria and ventricles paced
separately (e.g. complete heart block)
ECG in Perspective
1. ECG recording of electrical activity not the
mechanical function
2. ECG is not a direct depiction of
abnormalities
3. ECG does not record all the heart’s
electrical activity
Questions
✦
End of chapter 2, questions 1-5 and 7.