Mechanical and Electrical Events of the Cardiac Cycle

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Mechanical and Electrical Events of the Cardiac
Cycle

• PE 4370 Exercise Management for Special
  Populations
• PE 3510 Exercise Physiology

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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.

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Mechanical Events of the Cardiac Cycle

• Ventricular Filling Period ventricular diastole,
  atrial systole
• Isovolumetric Contraction Period ventricular
  systole
• Ventricular Ejection Period ventricular systole
• Isovolumetric Relaxation Period ventricular
  diastole

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Cardiac Cycle

• Electrical changes in heart tissue cause
  mechanical, i.e. muscle contraction, changes.
• Thus, changes in electrical membrane potential of
  specific parts of the heart tissue represent
  mechanical events in specific areas of the heart
  tissue.

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Electrical Events of the Cardiac Cycle

• Each wave or interval represents depolarization
  or repolarization of myocardial tissue.
• P wave represents depolarization of atria which
  causes atrial contraction.
• QRS complex reflects depolarization of ventricles
  which causes contraction.
• T wave reflects repolarization of muscle fibers
  in ventricles.

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Cardiac Cycle

• Coordination of
• Electrical Changes
• Pressure Changes in Left Atria, Left Ventricle
  and Aorta
• Ventricular Volume Changes
• Cardiac Valves

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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.

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Electrocardiography

• ECG - electrocardiogram
• graphic recording of electrical events
• established electrode pattern results in specific
  tracing pattern
• electrical pattern reveals blood supply problems

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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.

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Electrophysiology
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Cardiac Current Flow
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Cardiac Current Flow
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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.

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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.

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Electrophysiology

• 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

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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

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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.

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ECG Limb Leads
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ECG Augmented Limb Leads
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ECG Precordial Leads
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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.

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Exercise 12-Lead ECG
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12-Lead ECG

• Limb lead II shows large QRS amplitude because
  left ventricle vector lies parallel with
  electrode placement.
• Chest lead V1 has large S wave because left
  ventricle current vector is directed away from
  electrode.

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12-Lead ECG Strip
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Interpretation of ECG

• Rate. First measurement to calculate is heart
  rate. PQRST waves represent one complete cardiac
  cycle.
• At standard paper speed, divide 1500 by distance
  between R to R waves.
• Find R wave on heavy line. Count off 300, 150,
  100, 75, 60 for each following line. Where next R
  lands is quick estimate.
• Multiply number of cycles in 6 second marks by 10.

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Interpretation of ECG
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Interpretation of ECG

• Normal heart rhythm has consistent R-R interval.
• Mild variations due to breathing also normal.

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Interpretation of ECG

• Normal Sinus Rhythm
• Rate 60-100 b/min
• Rhythm regular
• P waves upright in leads I, II, aVF
• PR interval lt .20 s
• QRS lt .10 s

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Interpretation of ECG

• 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

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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.
• Scheidt, Stephen. 1983. Basic Electrocardiography,
  Clinical Symposia, 35 (2)