History • Mid 1880’s - Ludwig & Waller discovered that the heart’s rhythm could be monitored from a person’s skin • 1901 - Dr. William Einthoven invented the EKG machine • EKG - Electrocardiogram - Electrical activity of the heart (contraction of the myocardium)
Electrical Conduction • Heart muscle cells - “myocytes” • “Depolarization” moves as a wave through the myocardium • “Depolarization” stimulates the heart’s myocytes, they become + and contract • Cell-to-cell depolarization through the myocardium is carried with Na+ ions
EKG • Myocardial contraction is caused by depolarization. The recovery phase that follows depolarization is known as repolarization. • The SA Node (“Sinus Node) is the heart’s dominant pacemaker. - Sinus Rhythm • Each depolarization wave emitted through the SA Node spreads through the atria producing a P Wave.
P Wave = Atrial Depolarization • AV Node - Conducting pathway b/t the atria and the ventricles • When a wave of atrial depolarization enters the AV Node, depolarization slows, producing a brief pause, thus allowing blood to enter the ventricles. Ca++ ions
Ventricular Conduction • Depolarization shoots rapidly through the HIS Bundle, and the Left & Right Bundle Branches • The HIS Bundle and both Bundle Branches are made up of rapidly conducting Purkinje fibers. • Depolarization of the entire ventricular myocardium produces a QRS complex. • QRS represents ventricular contraction.
Ventricular Conduction • The Q Wave, when present, always occurs at the beginning of the QRS complex and is the first downward deflection. • The next upward deflection represents the RWave. • The next downward wave is the S Wave. • The horizontal segment of baseline that follows the QRS is known as the ST Segment.
Continued. • If the ST Segment is elevated or depressed beyond baseline level, this is usually a sign of problems. • The ST Segment represents the initial phase of Ventricular Repolarization. • The T Wave represents the final, “rapid” phase of ventricular repolarization. • The T Wave is usually a low, broad hump.
Repolarization is accomplished by K+ ions leaving the myocites. • Ventricular systole (contraction) begins with the QRS and persists until the end of the T Wave. • Physiologically, a cardiac cycle represents atrial contraction, and ventricular contraction followed by a resting stage. Page 29
Recording the EKG • Page 31 • Smallest divisions are 1 mm high X 1 mm wide. • 5 small squares make up one large square • The height or depth of waves is voltage. • The upward or downward deflection is called amplitude.
EKG Recordings • The amount of time represented by the distance b/t two heavy lines is .2 of a second. • Each small division represents .04 seconds. • A standard EKG has 6 limb leads and six chest leads. • Bipolar limb leads - Page 38.
Limb Leads • AVR, AVL, & AVF are unipolar limb leads. • Page 42. • Page 43. • Page 44. • 6 Chest Leads • V1 - V6
Rate • R to R intervals • 300, 150, 100, 75, 60, 50 • 6 second strip • Count cycles and add a zero to the end