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Cardiac Cycle Electrical Changes: Understanding ECG Interpretations

Learn about the electrical events in the cardiac cycle and how they reflect on an ECG, including P wave, QRS complex, and T wave. Explore the significance of ST segment changes and myocardial ischemia.

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Cardiac Cycle Electrical Changes: Understanding ECG Interpretations

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  1. Cardiac Cycle Electrical changes in heart tissue cause mechanical changes. Thus, changes in electrical membrane potential of specific parts of the heart tissue represent mechanical events in specific areas of the heart tissue.

  2. Electrical Events of the Cardiac Cycle Each wave or interval represents depolarization or repolarization of myocardial tissue.

  3. 1-P wave represents depolarization of atria which causes atrial contraction. 2-QRS complex reflects depolarization of ventricles which causes contraction. 3-T wave reflects repolarization of muscle fibers in ventricles.

  4. Electrophysiology 1-If an electrode is placed so that wave of depolarization spreads toward the recording electrode, the ECG records a positive (upward) deflection. 2-If wave of depolarization spreads away from recording electrode, a negative (downward) deflection occurs.

  5. Electrophysiology

  6. Cardiac Current Flow

  7. Cardiac Current Flow

  8. ECG Time & Voltage 1-ECG machines can run at 50 or 25 mm/sec. 2-Major grid lines are 5 mm apart, at standard 25 mm/s, 5 mm corresponds to .20 seconds. 3-Minor lines are 1 mm apart, at standard 25 mm/s, 1 mm corresponds to .04 seconds. 4-Voltage is measured on vertical axis. 5-Standard calibration is 0.1 mV/mm of deflection.

  9. 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.

  10. 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

  11. 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

  12. 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.

  13. ECG Limb Leads

  14. ECG Augmented Limb Leads

  15. ECG Precordial Leads

  16. 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.

  17. Exercise 12-Lead ECG

  18. 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.

  19. 12-Lead ECG Strip

  20. 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.

  21. Characteristics of the Normal ECG • Rate 50-100 bpm • Sinus rhythm • Cardiac Axis -30º to +90º • P <120 ms • PR <200 ms • QRS <120 ms • QT Male <0.43 s Female <0.45 s • ST isoelectric

  22. Interpretation of ECG

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

  24. Interpretation of ECG 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

  25. 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

  26. ST Segment Changes • Horizontal/downsloping depression  1.0 mV of ST segment, or • Upsloping/elevation  1.5 mV of ST segment, or • (Normalisation of ST segment displacement present on resting ECG) Suggest myocardial ischaemia • Chest pain (angina) accompanied by ST segment displacement generally considered to be diagnostic of myocardial ischaemia

  27. ST Segment Changes • Horizontal/downsloping depression  1.0 mV of ST segment, or • Upsloping/elevation  1.5 mV of ST segment, or • (Normalisation of ST segment displacement present on resting ECG) Usually indicate myocardial ischaemia

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