Mechanical and Electrical
Events of the Cardiac Cycle
PEP 3510: Exercise Physiology
PEP 4370: Exercise Management for
Special Populations
Cardiac Cycle
Cardiac Cycle: the electrical, pressure and
volume changes that occur in a functional
heart between successive heart beats.
• Phase of the cardiac cycle when
myocardium is relaxed is termed diastole.
• Phase of the cardiac cycle when the
myocardium contracts is termed systole.
– Atrial systole: when atria contract.
– Ventricular systole: when ventricles contract.
Mechanical Events of the
Cardiac Cycle
1.
2.
3.
4.
Ventricular Filling Period [ventricular
diastole, atrial systole]
Isovolumetric Contraction Period [ventricular
systole]
Ventricular Ejection Period [ventricular
systole]
Isovolumetric Relaxation Period [ventricular
diastole, atrial diastole]
Cardiac Cycle

Electrical changes in heart tissue cause
mechanical changes, i.e. muscle
contraction.
 Thus, changes in electrical membrane
potential of specific parts of the heart tissue
represent mechanical events in specific
areas of the heart tissue.
Electrocardiography

Two common abbreviations for
electrocardiogram: EKG and ECG.
 EKG comes from German language where
cardiogram is written as kardiogram.
 The ECG records the electrical activity of
the heart.
 Mechanical activity of the heart is sensed by
echocardiography.
Electrocardiography
ECG - electrocardiogram
– graphic recording of electrical events
– established electrode pattern results in specific
tracing pattern
– electrical pattern reveals blood supply problems
Electrophysiology

If an electrode is placed so that wave of
depolarization spreads toward the recording
electrode, the ECG records a positive
(upward) deflection.
 If wave of depolarization spreads away
from recording electrode, a negative
(downward) deflection occurs.
Electrophysiology
Electrophysiology
Electrophysiology
Electrophysiology

When myocardial muscle is completely
polarized or depolarized, the ECG will not
record any electrical potential but rather a
flat line, isoelectric line.
 After depolarization, myocardial cells
undergo repolarization to return to electrical
state at rest.
Electrical Events of the
Cardiac Cycle
•
•
•
Sinoatrial (SA) node is the normal pacemaker
of heart and is located in right atrium.
Depolarization spreads from SA node across
atria and results in the P wave.
Three tracts within atria conduct depolarization
to atrioventricular (AV) node.
• Conduction slows in AV node to allow atria to
empty blood into ventricles before vent. systole.
•
•
Bundle of His connects AV to bundle branches.
Purkinje fibers are terminal bundle branches.
Cardiac Cycle
Coordination of :
 Electrical Changes
 Pressure Changes in Left Atria, Left
Ventricle and Aorta
 Ventricular Volume Changes
 Cardiac Valves
ECG Time & Voltage
•
•
•
•
•
ECG machines can run at 50 or 25 mm/sec.
Major grid lines are 5 mm apart; at standard
25 mm/s, 5 mm corresponds to .20 seconds.
Minor lines are 1 mm apart; at standard 25
mm/s, 1 mm corresponds to .04 seconds.
Voltage is measured on vertical axis.
Standard calibration is 0.1 mV per mm of
deflection.
Basic Electrographic Complexes
•
P wave represents depolarization of atria which
causes atrial contraction
• Repolarization of atria not normally detectable on
an ECG
• Excitation of bundle of His and bundle branches
occur in middle of PR interval
• QRS complex reflects depolarization of
ventricles
• T wave reflects repolarization of muscle fibers in
ventricles
Electrocardiogram





Normal P wave has
amplitude of ≤ 0.25 mV
Q wave is first downward
deflection after P wave;
signals start of ventricular
depolarization
R wave is positive
deflection after Q wave
S wave is negative
deflection preceded by Q
or R waves
T wave follows QRS
Standard 12-Lead ECG

Usually performed when person is resting in
supine position.
 Composed of three bipolar limb leads: I, II,
and III; three augmented voltage leads:
aVR, aVL, aVF; and six chest or precordial
leads: V1 – V6.
 All limb leads lie in frontal plane.
 Chest leads circle heart in transverse plane.
ECG Limb Leads
ECG Augmented Limb Leads
ECG Precordial Leads
Standard 12-Lead ECG

Each lead provides a
different electrical angle
or picture of the heart.
 Anterior part of heart by
looking at V1 – V4.
 Lateral view of heart: I,
aVL, V5 and V6.
 Inferior view of heart: II,
III, and aVF.
Exercise 12-Lead ECG
12-Lead ECG

Limb lead II shows large
R amplitude because left
ventricle current vector
lies parallel with
electrode placement.
 Chest lead V1 has large S
wave because left
ventricle current vector is
directed away from
electrode.
12-Lead ECG Strip
Interpretation of ECG:
Rate
First measurement to calculate is heart rate.
PQRST waves represent one complete cardiac
cycle.
1. At standard paper speed, divide 1500 by
distance between R to R waves.
2. Find R wave on heavy line. Count off 300,
150, 100, 75, 60 for each following line.
Where next R lands is quick estimate.
3. Multiply number of cycles in 6 second marks
by 10.
Interpretation of ECG: Rate
Interpretation of ECG:
Rhythm
•
Normal heart rhythm has consistent R-R interval.
• Mild variations due to breathing also normal.
Interpretation of ECG: Rhythm
Normal Sinus Rhythm
•
Rate: 60-100 b/min
• Rhythm: regular
• P waves: upright in
leads I, II, aVF
• PR interval: < .20 s
• QRS: < .10 s
Sinus Bradycardia
•
•
Rate: < 60 bpm
Rhythm: regular
Sinus Tachycardia
•
Rate: > 100 bpm
AV Conduction Disturbances
o
Atrioventricular
conduction
disturbances refer to
blockage of electrical
impulse at AV node.
o 1st degree P waves result
in delayed QRS.
o 2nd degree some but not
all P waves have QRS.
Arrhythmias
Arrhythmia: an irregular
heartbeat.
•
•
•
Sinus arrhythmia- P
wave precedes @ QRS
but RR interval varies.
Premature Atrial
Contraction (PAC)
Premature Ventricular
Contraction (PVC)
Arrhythmias
Myocardial Ischemia
ST segment depression.
• Hallmark of myocardial ischemia.
• Reduction of oxygen-rich blood supply alters normal
cellular action causing ST segment displacement ≥ 1
mm below line.
• Upsloping, horizontal, downsloping
Illustration References

McArdle, Katch, Katch. 2000. Essentials
of Exercise Physiology Image Collection,
2nd ed. Lippincott Williams & Wilkins
 Foss and Keteyian. 1998. Physiological
Basis for Exercise and Sport, 6th ed. WCB
McGraw-Hill.
 Robergs and Keteyian. 2003.
Fundamentals of Exercise Physiology, 2nd
ed. McGraw-Hill.