Lecture 6.2 - Interpretation of ECG

ECG Tracing

• The x-axis measures time in seconds, with a small box representing 0.04s and a large box representing 0.2s.
• The y-axis measures voltage in millivolts, with a small box representing 0.1mV and a large box representing 0.5mV.

Assessing Rhythm and Rate

• Normal sinus rhythm is characterized by a heart rate between 60-100 bpm, presence of p-waves, and a constant R-R interval.
• A heart rate can be determined using the 300 method for regular rhythms or the 6 times method for irregular rhythms.
• The 300 method involves counting the number of large squares between consecutive P or R waves and dividing 300 by that number.
• The 6 times method involves counting the number of P or R waves in a 10-second strip and multiplying by 6.

Wave Characteristics

• P-waves are less than 3 small boxes (0.12s) in duration.
• PR intervals are 3-5 small boxes (0.12-0.2s) in duration.
• QRS complexes are less than 3 small boxes (0.12s) in duration.
• T-waves are typically upright, and an inverted T-wave may indicate myocardial infarction.
• ST segments are isoelectric.
• QT intervals are less than half of the preceding R-R interval.

Axis Determination

• The axis of the heart is normally between -30 to +90 degrees.
• Axis determination involves examining the QRS complexes in leads I, II, and aVF.
• A normal axis is characterized by a positive QRS in lead I and aVF.
• Right axis deviation (RAD) is indicated by a negative QRS in lead I and a positive QRS in aVF.
• Left axis deviation (LAD) is indicated by a positive QRS in lead I and a negative QRS in aVF.
• Extreme axis deviation (EAD) is indicated by a negative QRS in leads I, II, and aVF.