Lesson # 6

1. Lesson # 6 The Heart-2 Chapter 20 Objectives: 1- Explain the events of an action potential in cardiac muscle. 2- Identify the electrical events associated with the electrocardiogram (ECG). 3- Explain the events of the cardiac cycle. 4- Defining cardiac output and how it is regulated.

2. Atrioventricular bundle or bundle of His Right and left bundle branches Sinoatrial node (SA node) Atrioventricular node (AV node) Purkinje fibers The Conducting System It connects electrically the atria to the ventricles. They conduct the impulse to the Purkinje fibers. It establishes the heart rate (pacemaker). It delays the impulses to allow the atria to finish contracting before the ventricles start to contract. They conduct the impulse to the lateral walls of the ventricles allowing the contraction to spread from the apex to the base.

3. Na+ The Sinoatrial (SA) Node K+ It is a reduction in the membrane potential because the interior of the cell becomes less negative or more positive. DEPOLARIZATION 0 mV Depolarization Depolarization is produced when gated sodium channels are open and sodium ions enter the cell. Repolarization It is an increase in the membrane potential because the interior of the cell becomes more negative. HYPERPOLARIZATION -90 mV Hyperpolarization is produced when gated potassium channels are open and potassium ions exit the cell. Hyperpolarization

4. +50 +50 +50 Action Potential Pacemaker Potential Resting Potential 0 0 Fast K+ outflow 0 Fast Ca+ and Na+ inflow -60 mV -40 -40 -40 mV -60 -60 -60 -70 -70 -70 Slow Na+ inflow SA node does not have a stable resting membrane potential. It starts at -60 mV. • It drifts upward because of a slow inflow of Na+ . • When it reaches a threshold of -40 mV, voltage-gated Ca2+ and Na+ channels open and a faster depolarization occurs peaking at 0 mV. • The K+ channels then open and K+ leaves the cell causing repolarization. Action Potentials: They are changes in the transmembrane potential that, once initiated, affect an entire excitable membrane. Each depolarization of the SA node sets off one heartbeat. At rest, fires every 0.8 seconds or 75 bpm.

5. Threshold Prepotential (spontaneous depolarization) Time (sec) Changes in the membrane potential of a pacemaker cell in the SA node that is establishing a heart rate of 72 beats per minute. Note the presence of a prepotential, a gradual spontaneous depolarization.

6. The Electrocardiogram or ECG (EKG) • An ECG is a composite of all action potentials of nodal and myocardial cells, detected, amplified and recorded by electrodes on arms, legs and chest

7. The Electrocardiogram R Waves and Segments QRS complex Depolarization of ventricles. +1 Depolarization of atria. T P Repolarization of ventricles 0 Q S PQ segment ST segment The ST segment represents the time during which the ventricles contract and eject blood. Millivolts 100 msec Atrial systole Ventricular systole

8. The Cardiac Rhythm Sinus rhythm: It is the normal heartbeat triggered by the SA node At rest, the sinus rhythm is about 70 to 80 times per minute (rates from 60 to 100 bpm). Tachycardia: It is a persistent, resting adult heart rate above 100 bpm. Bradycardia: It is a persistent, resting adult heart rate below 60 bpm. Extra-systoles: Extra heart beats produced in any region of spontaneous firing other than the SA node. If the SA node is damaged, other part of the myocardium may take over the governance of the heart rhythm. Any region of spontaneous firing other than the SA node. The most common ectopic focus is the AV node, which produces a nodal rhythm. Ectopic focus: Nodal rhythm: It is the cardiac rhythm produced by the AV node. It is a slower heartbeat of 40 to 50 bpm. If neither the SA nor AV nodes is functioning, an artificial pacemaker is required Arrhythmia: It is any abnormal cardiac rhythm. Heart block: It is the failure of any part of the of the cardiac conducting system to transmit signals.

9. Heart Sounds During ventricular systole (contraction) the two AV close at the same time and produce the first sound referred aslubb. Lubb Dupp Lubb Dupp When the ventricles relax (diastole) the two semilunar valves close at the same time and produce the second sound referred asdubb.

10. The Cardiac Cycle At the beginning of their contraction (systole) the ventricles contracts isovolumetrically (the pressure increases but the volume inside the ventricles does not changes). In the period of isovolumetric contraction, the ventricles contract and the pressure rises, but blood does not flow because all the valves are closed. Pressure Pressure

11. The Cardiac Cycle At the beginning of their contraction (systole) the ventricles contracts isovolumetrically (the pressure increases but the volume inside the ventricles does not changes). Once pressure in the ventricles exceeds that in the arterial trunks (pulmonary and aortic), the semilunar valves open and blood flows into the pulmonary and aortic trunks. This point marks the beginning of the period of ventricular ejection. Pressure Pressure

12. At the beginning of their relaxation (diastole) the ventricles relaxes isovolumetrically (the pressure decreases but the volume inside the ventricles does not changes). It is the period of isovolumetric relaxation, the ventricles relax and the pressure drops, but blood does not flow because all the valves are closed. Pressure Pressure

13. At the beginning of their relaxation (diastole) the ventricles relaxes isovolumetrically (the pressure decreases but the volume inside the ventricles does not changes). Once pressure in the atria the AV valves open and blood flows into the ventricles. This point marks the beginning of the ventricular filling. Pressure Pressure

14. The Cardiac Cycle The cardiac cycle consist of the events during a complete heart beat. Start Atrial systole begins: Atrial contraction forces a small amount of additional blood into relaxed ventricles. Atrial systole ends, atrial diastole begins Ventricular diastole—late: All chambers are relaxed. Ventricles fill passively. Ventricular systole— first phase: Ventricular contraction pushes AV valves closed but does not create enough pressure to open semilunar valves. Cardiac cycle 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.

15. At the start of the atrial systole, the ventricles are already filled to about 70% of their normal capacity, due to passive blood flow. (b) At the end of the atrial systole, each ventricle contains a maximum amount of130 mL of blood: End-diastolic volume (a) 1- Ventricular Filling - Passive (70%) (c) - Active (30%) In the period of isovolumetric contraction, the ventricles contract and the pressure rises, but blood does not flow because all the valves are closed. 2- Isovolumetric Contraction (d) (e) This point marks the beginning of the period of ventricular ejection. 4- Isovolumetric Relaxation 3- Ventricular Ejection

16. At the start of the atrial systole, the ventricles are already filled to about 70% of their normal capacity, due to passive blood flow. (f) (a) Atrial Systole A small amount of blood (30 %) is forced to the ventricles Fist Phase: Ventricular contraction closes the AV valves (first sound). Isometric contraction. Second Phase: Ventricular Systole Pressure increases and semilunar valves open. Ventricular ejection. Early: Pressure decreases in the ventricles and semilunar valves close (second sound). Ventricular Diastole and Atrial Diastole Late: Atria are also in diastole. Passive blood flow fills the ventricles (70%).

18. QRS complex Depolarization of ventricles Depolarization of atria Repolarization of ventricles T P PQ segment ST segment ATRIAL ATRIAL ATRIAL DIASTOLE VENTRICULAR VENTRICULAR VENTRICULAR SISTOLE DIASTOLE DIASTOLE SISTOLE DIASTOLE 1- Ventricular Filling 2- Isovolumetric Contraction 3- Ventricular Ejection 4- Isovolumetric Relaxation 1- Ventricular Filling Passive (70%) Active (30%)

19. Second sound Ventricular ejection Isovolumetric relaxation Isovolumetric contraction. First sound Ventricular filling Ventricular filling

20. End-Diastolic Volume Stroke Volume End-Sistolic Volume End-Diastolic Volume (EDV) It is the volume of blood that each ventricle contains at the end of ventricular filling (about 130 mL). Stroke Volume (SV) It is the volume of blood that each ventricle ejects during ventricular ejection (about 70 - 80 mL). End-Systolic Volume (ESV) It is the volume of blood left behind in the ventricles after ventricular ejection. EDV – SV = (ESV) Ejection fraction It is the percentage of the end-diastolic volume (EDV) that is ejected (about 54%). Cardiac Output (CO) The amount of blood pumped by the left ventricle in one minute Cardiac Output (CO) = Stroke Volume (SV) x Heart Rate (HR) 75 bpm x 80 mL/beat = 6000 mL/min (6L/min)