ECG • stands for Electrocardiography • the record of electrical signal of the heart • Can be used to analyze the mechanical activity of the heart
The intrinsic conduction system Components: • SA node (highest rate of discharge, sets the rate of depolarization for the heart; aka pacemaker) atria • AV node atrial contraction (delayed until complete) • AV bundle (Bundle of His) • Right & left bundle branches • Purkinje fibers ventricular contraction Repolarization of the SA node is spread throughout the atria while the ventricles depolarize. Then the ventricles repolarize ventricular diastole
Components of the ECG • Isoelectric line- a straight line • P wave- atrial depolarization ~ atrial contraction • QRS complex- ventricular depolarization ~ ventricular contraction • T wave- ventricular repolarization ~ ventricular relaxation • Segments- period of time from the end of one wave to the beginning of the next wave ex. ST segment • Intervals- one wave + a segment of the isoelectric line between waves ex. PR interval
Question? • Is it normal for athletes to have low resting heart beat ~ 50 BPM & left ventricular hypertrophy? • What about same condition seen in sedentary individuals?
Mean electrical axis • tells us the net direction the depolarization or repolarization is heading (-ve to +ve) • Einthoven’s Triangle: configuration of 3 leads = standard bipolar limb lead • Einthoven’s Law: Lead I + Lead II = Lead III (QRS) Q + R + S = net QRS complex
p188 • Find the Net QRS amplitude for lead I (0.1mv gradations). Draw a perpendicular line to lead I. (Note +ve or –ve direction) • Find the Net QRS amplitude for lead III. Draw a perpendicular line to lead III. (Note +ve or –ve direction) • Determine the point of intersection of these 2 perpendicular lines. • Draw a new vector from the center point (0,0) to the point of intersection. • Approximate mean electrical axis (5º gradations)