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Lesson 1 of 21 · Fundamentals

What is an EKG?

An electrocardiogram records the heart's electrical activity from the body surface. Every bump on the tracing is a wave of depolarization or repolarization moving toward or away from an electrode.

01The one rule that explains everything

A wave of depolarization moving TOWARD a positive electrode makes an upward (positive) deflection. Moving AWAY makes a downward deflection. Moving perpendicular makes a small biphasic or flat deflection.

The labeled EKG complex
PRQSTisoelectric linePR intervalQRSST segmentQT interval

02What each wave means

  • P wave: atrial depolarization
  • QRS complex: ventricular depolarization (atrial repolarization is hidden inside it)
  • T wave: ventricular repolarization
  • U wave: late repolarization (Purkinje fibers / papillary muscles), small and often absent
II
Normal sinus rhythm in lead II: P, QRS, and T in order, every beat.

03The 12 leads

Ten electrodes produce twelve 'views' of the heart. Six limb leads (I, II, III, aVR, aVL, aVF) look in the frontal plane; six precordial leads (V1–V6) look in the horizontal plane.

12-lead electrode placement
V1V2V3V4V5V6RALARLLLviewer facing the patientV14th ICS, right sternal borderV24th ICS, left sternal borderV3Midway between V2 and V4V45th ICS, midclavicular lineV5Anterior axillary line, level of V4V6Mid-axillary line, level of V4Limb electrodes: RA, LA, LLRL is the ground

Clinical pearl. Limb lead reversal (LA/RA swap) makes lead I negative and aVR positive. Suspect it before diagnosing dextrocardia.

Key takeaways

  • Toward = up, away = down
  • P = atria, QRS = ventricles, T = ventricular repolarization
  • 12 leads = 6 frontal + 6 horizontal views