Chapter 5
###### Figure 5-1. EKG Waves and Segments

Waves, Intervals, and Common Pathologies
 Time and voltage scale When the ECG machine rate is 25 mm/s, each small box represents 0.04 s horizontally and 0.1 mV vertically P wave Represents: atrial depolarizationDuration: 80–120 msAmplitude: 2.5 mV in leads II and IIIMorphology (see Figure 5-2):Biphasic in the right precordial leads (V1 and V2)Upright in the lateral leads (I, aVL, V5, V6), reflects right to left spread of the activation frontAxis: 0–90°Pathology: atrial enlargement—P wave height greater than 2.5 mV PR (or PQ) interval Represents: impulse propagation from the atria throughout the AV node, bundle of His, bundle branches, and Purkinje fibers until the ventricular myocardium begins depolarizationPR duration: 120–200 msPathology:Long PR interval—slow AV nodal conduction (see section “First-Degree Heart Block”)Short PR with a delta wave—Wolff–Parkinson–White syndrome or preexcitationNote:PR interval: beginning of P to the beginning of the QRSPR segment: end of P to the beginning of the QRS Q wave Represents the negative deflection of the QRS before the R wave present in one or more inferior leads in more than 50% of normal adults, but is present in leads I and aVL in less than 50% of normal adultsDuration: less than 30 ms in limb leads, less than 50 ms in lead III, less than 30–40 ms in leads I, aVL, V5, and V6Amplitude:Less than 0.4 mV in all limb leads except the amplitude may be up to 0.5 mV in lead IIIDepth of the Q wave should be less than 25% of the R wave height except in lead III QRS Represents: ventricular activationDuration: 70–110 ms (measured in the lead with the widest QRS complex)Amplitude: increases through V5 (commonly referred to as “R wave progression”); see LVHMorphology: usually upwardly deflected in leads I and II, may be negative in aVL, usually downwardly deflected in aVR and V1; chest leads V2–V3 are transition leads and are usually isoelectric in appearance, variable in lead IIIAxis: −30 to +90:More negative than −30°: left-axis deviationGreater than 90°: right-axis deviationPathology: wide QRS—increased duration of ventricular depolarization: bundle branch block, LVH, ventricle depolarized by an ectopic focus QT interval Represents: duration of ventricular systoleDuration: less than 460 ms for men and less than 470 ms for women; decreases with increasing heart rate, necessitating use of the QT corrected for heart rate, QTcBazett formula for the ...

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