The document outlines the mechanical and electrical events of the cardiac cycle, detailing phases such as diastole and systole, while also explaining the roles of electrocardiography (ECG) in monitoring heart activity. It emphasizes the significance of electrical changes in heart tissue and their correlation with mechanical events, as well as providing insights into ECG interpretation and the detection of arrhythmias and myocardial ischemia. The document includes detailed information about the standard 12-lead ECG setup and normal versus abnormal heart rhythms.

1. Mechanical and Electrical
Events of the Cardiac Cycle
PEP 3510: Exercise Physiology
PEP 4370: Exercise Management for
Special Populations

2. 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.

3. Mechanical Events of the
Cardiac Cycle
1. Ventricular Filling Period [ventricular
diastole, atrial systole]
2. Isovolumetric Contraction Period [ventricular
systole]
3. Ventricular Ejection Period [ventricular
systole]
4. Isovolumetric Relaxation Period [ventricular
diastole, atrial diastole]

5. 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.

6. 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.

7. Electrocardiography
ECG - electrocardiogram
– graphic recording of electrical events
– established electrode pattern results in specific
tracing pattern
– electrical pattern reveals blood supply problems

8. 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.

9. Electrophysiology

10. Electrophysiology

11. Electrophysiology

12. 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.

13. 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.

15. Cardiac Cycle
Coordination of :
 Electrical Changes
 Pressure Changes in Left Atria, Left
Ventricle and Aorta
 Ventricular Volume Changes
 Cardiac Valves

17. 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.

19. 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

20. 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

22. 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.

23. ECG Limb Leads

24. ECG Augmented Limb Leads

26. ECG Precordial Leads

28. 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.

29. Exercise 12-Lead ECG

30. 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.

31. 12-Lead ECG Strip

32. 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.

33. Interpretation of ECG: Rate

34. Interpretation of ECG:
Rhythm
• Normal heart rhythm has consistent R-R interval.
• Mild variations due to breathing also normal.

35. 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

37. 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.

38. Arrhythmias
Arrhythmia: an irregular
heartbeat.
• Sinus arrhythmia- P
wave precedes @ QRS
but RR interval varies.
• Premature Atrial
Contraction (PAC)
• Premature Ventricular
Contraction (PVC)

39. Arrhythmias

40. 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

41. 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.