Cardiac Conduction
Physiology of Cardiac Conduction
• The excitatory & electrical conduction system of
the heart is responsible for the contraction and
relaxation of the heart muscle.
• The sinoatrial node (SA node) is the pacemaker
where the electrical impulse is generated.
Cardiac conduction pathway
1.
SA node fires to the AV node through gap jxn’s
2. AV node delays 0.1 s for atrial diastole
3. AV node fires to the AV bundle (HIS)
4. AV bundle depolarizes through right & left bundle
branches
5. Bundle branches carry impulses through Purkinje fibers to
the ventricular myocardium for ventricular systole
Total time of conduction = 0.22 s
Nodal Firing Rates
•
•
•
•
Sa node = 75 b.p.m.
AV node = 50 b.p.m.
AV bundle = 30 b.p.m.
Purkinje fibers = 30 b.p.m.
What would happen if the SA node could not conduct
an impulse to the AV node?
Heart block (no gap jxn’s found between atria & ventricles)
How to Read an
Electrocardiogram Paper
• A grid system where time is measured along the
horizontal axis.
• Each small square is 1 mm in length
& represents 0.04 seconds.
• Each larger square is 5 mm in
length & represents 0.2 seconds.
ECG Voltage
• Is measured along the vertical axis.
• 10 mm is equal to 1mV.
Heart rate can be calculated from the EKG strip
• When the rhythm is regular, the heart rate
is 300 divided by the number of large
squares between the QRS complexes.
• For example, if there are 4 large squares
between regular QRS complexes, the heart
rate is 75 (300/4=75).
Heart rate can be calculated from the EKG strip
• Can be used with an irregular rhythm
to estimate the rate. Count the
number of R waves in a 6 second strip
and multiply by 10.
• For example, if there are 7 R waves in
a 6 second strip, the heart rate is 70
(7x10=70).
Quick Quiz
• True or false
• On a typical EKG grid, 5 small
squares, or 1 large square, represent
0.20 seconds of time
True
PQRST wave
Deflection waves
P wave
• Lasts 0.08 s
• Results due to depolarization from SA node
throughout atria
• Atrial systole
• Normal duration is not longer than 0.11 seconds
(less than 3 small squares)
• Amplitude (height) is no more than 3 mm
Deflection waves
QRS complex
• Lasts 0.08 s (Normally not longer than 0.10 s in duration)
• Results due to depolarization of ventricles
• Ventricular systole & atrial diastole
• R waves are deflected positively and the Q and S waves
are negative
T wave
• Results due to repolarization of ventricles
• Lasts 0.16 s
• Ventricular diastole
Normal ECG Deflection Wave Pattern
Abnormal ECG Deflection Wave Patterns
Sinus Bradycardia
Rate = 40-59 b.p.m.
Sinus Tachycardia
Rate = 101-160 b.p.m.
Causes
•CHF, hypoxia, pulmonary edema
•Increased temperature
•Stress or response to pain
Sinus Arrhythmia
Rate = 45-100 b.p.m.
Sinus Arrest
Causes
Myocarditis
MI
Digitalis toxicity
Atrial Flutter
Rate = 250-350 b.p.m.
Precipitates CHF
Atrial Fibrillation (afib)
Causes
COPD
CHF
AV block
Causes
•Digitalis toxicity
•Acute infection
•MI
•Degeneration of the conductive tissue.
Ventricular Tachycardia
(V-tac)
Rate = 100-220 b.p.m.
Causes
•CAD
•Acute MI
•Digitalis toxicity
•CHF
Ventricular Fibrillation (V-fib)
Causes
Acute MI
Asystole