Diagnostic Features of ECGs Flashcards Preview

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Flashcards in Diagnostic Features of ECGs Deck (19):

Steps of cardiac conduction/depolarization

  • sinoatrial (SA) node = pacemaker
  • Electrical impulses initiated @ SA node --> internodal tracts --> wave of depolarization in the atrium --> atrioventricular (AV) node.
  • @ AV node: brief delay --> impulses to Bundle of His and activate the ventricles through the right and left bundles
  • bundles --> Purkinje fibers --> activate ventricular myocardial cell depolarization and contraction.


Main clinically relevant components of EKG (6)

  • P wave
  • PR interval
  • QRS complex
  • QT interval
  • T wave
  • U wave


P wave fxnl action

atrial depolarization


PR interval fxnl significance

  • measure of AV node conduction time
  • normal = 0.12 - 0.20 secs


QRS fxnl significance

  • ventricular depolarization
  • normal duration = 0.06 - 0.10 secs
  • Q: negative
  • R: positive
  • S: late negative deflection


QT interval fxnl significance

  • lasts from begin of Q to end of T
  • total duration of depolarization and repolarization


T wave fxnl significance

ventricular repolarization


U wave fxnl significance

  • not always seen
  • follows T wave
  • possibly repolarization of purkinje fibers or papillary muscles


Line measurements/speed of EKG paper

  • paper speed = 25mm/sec
  • vertical lines = 0.04 sec
  • thick vertical lines = 0.2 sec


Calculation of heart rate from EKG

HR=300/# of heavy lines between P waves


EKG changes produced by ventricular hypertropy

  • both left and right ventricular hypertrophy result in greater muscle mass --> greater voltage associated with depolarization and repolarization of the myocardium.
    • ecg ventricular hypertrophy is seen as a R wave with greater amplitude.
  • Left ventricular hypertrophy: large positive deflections (R waves) in V5 and V6 and large negative deflections (S waves) in V1.

  • Right ventricular hypertrophy: high voltage in V1 and V2.



EKG changes caused by myocardial ischemia

  • Ischemic changes alter ventricular repolarization and affect the ST segment and the T wave.
  • Ischemia due to sudden high oxygen demand in the presence of a fixed coronary obstruction causes depression of the ST segment. 

    • In some patients a resting ekg is normal, but ST depression is only visible during exercise due to transient ischemia. 

  • Ischemia due to acute coronary artery obstruction during low oxygen demand can cause T wave inversion.

  • Normally, T waves are in the same direction of the QRS complex.

    Inversion of a T wave→myocardial ischemia



EKG changes in acute myocardial infarction

  • ST elevation is a sign of transmural injury in an acute coronary syndrome, usually with a clot due to platelet aggregation obstructing a coronary artery. 

  • Sizeable (>0.04 s) Q waves can be a sign of transmural necrosis. Infarcts usually involve only the left ventricle.

    • Inferior leads (II, III, aVF): inferior infarcts 

    • V1-V4: anterior wall infarct 

    • I, aVL and V5, V6: lateral wall infarcts.




Evolution of transmural acute myocardial infarct

  • Giant upright “hyperacute” T wave 

  • T wave inverts and ST segment rises. 

  • Sometimes, ST elevation precedes of occurs simultaneously with T inversion.

  • Q waves are usually the last to develop.



Transmural vs. subendocardial acute myocardial infarction

  • Transmural—involves the entire thickness of the LV 

  • Subendocardial—localized to the inner layer of the LV wall. 

    • Subendocardial infarcts do not have Q waves or ST elevation.

    • They do have persistent ST depression.



EKG changes in hypercalcemia

  • shortened QT interval 

  • often associated with hyperparathyroidism. 


EKG changes in hypocalcemia

  • lengthened QT interval 

  • may be associated with life threatening ventricular arrhythmias



EKG changes associated with hyperkalemia

  • increased T wave voltages with a distinctive peaked, symmetrical appearance
  • At higher levels, the P waves may be flattened and the QRS and T waves widened.
  • broad S wave often appears.
  • At very high levels, a sinusoidal pattern appears without P or R waves.


EKG changes associated with hypokalemia

  • QT interval is generally prolonged
  • prominent U waves are frequent
  • T waves may be inverted