EKG AUTORHYTHMICITY

Figure 21.11 The Cardiac Cycle Start Atrial systole begins: Atrial contraction forces a small amount of additional blood into relaxed ventricles. Atrial systole ends, atrial diastole begins 0 msec 800 msec 100 msec Atr ial sy sto le Ven tri cul ar dia sto le Ven tri cul ar sy sto le Cardiac cycle Ventricular systole— first phase: Ventricular contraction pushes AV valves closed but does not create enough pressure to open semilunar valves. Ventricular diastole—late: All chambers are relaxed. Ventricles fill passively. Atr ial dia sto le 370 msec Ventricular systole— second phase: As ventricular pressure rises and exceeds pressure in the arteries, the semilunar valves open and blood is ejected. Ventricular diastole—early: As ventricles relax, pressure in ventricles drops; blood flows back against cusps of semilunar valves and forces them closed. Blood flows into the relaxed atria. 2

EKG Electrokardiogram- device to record the action potential of cardiac muscle summation Cannot detect force of contraction Cannot detect Blood Pressure Can detect abnormal heart rates/ rhythems Can detect abnormal conduction pathways Can detect hypertrophy and atrophy and relative position of damage P Wave- Action potential depolarization of atrial myocardium Causes Atrial contraction QRS Complex- Ventricular depolarization Causes onset of ventricular contraction Also atrial repolarization masked by QRS signal T Wave- repolarization before ventricular relaxation U wave – repolarization of the purkinje fibers PQ/PR Interval- .16 sec- atria contract and relax QT Interval- .3 sec- ventricles contract and relax 1 Cardiac Cycle- from onset muscle contraction to next

Autorhythmicity of Cardiac Muscle Action potentials in heart without external stimuli After each action potential the membrane potential returns to its resting membrane potential Unstable slow ion channels open and cause depolarization This causes fast channels to open and increase depolarization When depolarization reaches threshold--action potential happens more often in SA Node because more slow channels Plateau Phase- Prolonged period of depolarization, separates contractions in the heart Heart has long action potential so the heart will rest between contractions and not tetanic contractions Absolute refractory period – CM insensitive to further stimuli

Figure 21.11 The Cardiac Cycle Start Atrial systole begins: Atrial contraction forces a small amount of additional blood into relaxed ventricles. Atrial systole ends, atrial diastole begins 0 msec 800 msec 100 msec Atr ial sy sto le Ven tri cul ar dia sto le Ven tri cul ar sy sto le Cardiac cycle Ventricular systole— first phase: Ventricular contraction pushes AV valves closed but does not create enough pressure to open semilunar valves. Ventricular diastole—late: All chambers are relaxed. Ventricles fill passively. Atr ial dia sto le 370 msec Ventricular systole— second phase: As ventricular pressure rises and exceeds pressure in the arteries, the semilunar valves open and blood is ejected. Ventricular diastole—early: As ventricles relax, pressure in ventricles drops; blood flows back against cusps of semilunar valves and forces them closed. Blood flows into the relaxed atria. 7

The Cardiac Cycle ANIMATION The Heart: Cardiac Cycle ANIMATION The Cardiac Cycle: Part 1 ANIMATION The Cardiac Cycle: Part 2 ANIMATION The Cardiac Cycle: Part 3 ANIMATION The Cardiac Cycle: Part 4 ANIMATION The Cardiac Cycle: Part 5 8