Lecture 6 - Introduction to Cardiac Physiology, Electrophysiology, and ECGs PDF

Introduction to Cardiovascul ar Physiology Sherwood: Chapter 9-10 Objectives Anatomy of the Heart Electrical Activity of the Heart 12-Lead ECG 2 Anatomy of the Heart Components of the Cardiovascular System nutrients...

Introduction to Cardiovascul ar Physiology Sherwood: Chapter 9-10 Objectives Anatomy of the Heart Electrical Activity of the Heart 12-Lead ECG 2 Anatomy of the Heart Components of the Cardiovascular System nutrients CO2 Main function of pH cardiovascular system Homeostasis: Maintain appropriate O2 environment around cells in body hormones cell temperature Heart metabolic waste ions Dual muscular pump that provides driving force for circulation Blood Vessels Carry blood to organs and back to heart Blood Transport medium for O2, CO2, nutrients, wastes, hormones, plasma, blood cells, and heat Anatomy of the Heart Dual pump Right side Pumps blood to pulmonary circuit Left side Pumps blood to systemic circuit Blood Flow Pathway Flow: Vena cava  RA  RV  Pulmonary Arteries lungs  Pulmonary Veins LA  LV  aorta Pressure Operated Valves 4 one way valves exist in heart Ensure blood has unidirectional flow pattern Open and close passively due to pressure changes Atrioventricular valves Between atrium and ventricle Right = Tricuspid valve Left = Mitral valve/Bicuspid Valve Also be named left and right atrioventricular valves Open when pressure in atria exceeds pressure in ventricle Chordae tendinae fasten edges of valve shut Ensure that valve doesn’t open in atria Semilunar Valves Between major arteries leaving heart and ventricles Left = aortic valve Right = pulmonary valve Also called left and right semilunar valves Open when pressure in ventricle exceeds pressure in the periphery Blood Supply to Myocardium Coronary Vessels Fill during diastole Two reasons: 1) Position of coronary orifices from Mechanism of aorta Coronary Filling 2) Contraction of heart compresses coronary vessels Myocardium Individual cardiac myocytes connected to form branched fibers In spiral arrangement around ventricle  How is Cardiac Muscle Different from Skeletal Muscle? Unique properties of myocardium: 1. No motor neuron 2. Innervated by ANS 3. Sparse innervation 4. Intrinsic regulation of contractile force 5. Intercalated discs 6. Communication via gap junctions 7. Sarcolemmal Ca2+ channels Electrical Activity of the Heart Electrical Activity of the Heart The heart exhibits autorhythmicity Generates action potentials on its own Two types of cells: 1. Autorhythmic cells = generate action potentials 2. Contractile cells = Cardiac Autorhythmic 1) I Cells channels (funny channels) f Na+ and K+ both move into cell No defined resting membrane Positive charge causes a slow potential depolarization Instead have pacemaker potential Calcium channels also contribute Membrane slowly depolarizes and drifts toward threshold 2) ICa,T = T-type Calcium (3 Channels Ion channel opens before threshold ) is reached It is open briefly think “T” = transient Once threshold is reached, different type of calcium channel opens: 3) ICa,L = L-type Calcium (2 Channels (1 ) Open at threshold and stays open for ) action potential Think “L” = Longer-lasting  Autorhythmic cells exist in the following areas: Non-SA nodal autorhythmic cells cannot maintain as fast a heart rate If SA node does not work, others can take over, but at slower pace Myocardial Action Potentials Cardiac Conduction System  Sequence of Cardiac Activation Why can’t the signal just pass directly from e atria to the cardiac myocytes in the ventricles? Ventricles would contract from top to bottom Blood would get trapped in base of ventricles Outflow tracks (pulmonary artery and aorta) are at top of heart Heart wouldn’t pump any blood out to the Excitation-Contraction Coupling Action potential enters from adjacent cell. Ca2+ 2 K+ 3 Na+ Ca2+ ECF ATP NCX Voltage-gated Ca2+ channels open. Ca2+ ICF 3 Na+ enters cell. RyR Ca2+ Ca2+ induces Ca2+ release through ryanodine receptor-channels (RyR). SR L-type Sarcoplasmic Ca2+ reticulum (SR) Ca2+ Local release causes channel Ca2+ stores Ca2+ spark. ATP Summed Ca2+ sparks T-tubule create a Ca2+ signal. Ca2+ sparks Ca2+ ions bind to troponin to initiate contraction. Ca2+ signal Ca2+ Ca2+ Relaxation occurs when Ca2+ unbinds from Actin troponin. Ca2+ is pumped back into the sarcoplasmic reticulum for storage. Contraction Relaxation Myosin Ca2+ is exchanged with Na+ by the NCX antiporter. Na+ gradient is maintained by the Na+-K+-ATPase. Refractory Period in Cardiac Contractile Cells Action potential (red curve) and contraction of cell (blue curve) end almost simultaneously Ensures heart muscle can relax Heart muscle relaxation is mandatory so ventricles can fill with blood Major factor contributing to refractory period = Na+ channel inactivation Length-Tension Relationship of the Heart Net tension Passive tension Tension Active tension Volume List the ways the heart muscle is different than skeletal muscle. 12-Lead ECG Introduction to the ECG How to read a 12-Lead ECG: 1)Rate 2)Rhythm 3)Axis 4)Rotation 5)Hypertrophy 6)Infarction Basic ECG Concepts-1 0 1 -2 2 + + + + - - - - - + -1 0 1 -1 0 1 -2 2 -2 2 + + - - - - + + - + - + - - + + + + - - * Amplitude dependent on size of tissue depolarized and proximity to dipole ECG Lead Placement Dipole Vectors and the ECG - + - G +  - G +  - G +  - G +  - Ventricular repolarization G + Now change leads… + - G  + - G  + - G  + - G Normal 12 Lead ECG: Each lead is a different picture ECG Leads 3 limb leads (Lead I, II, III) 3 augmented leads (aVR, aVL, aVF) 6 chest leads (V1-V6) Does anything look off? Conclusions: The heart shares structural properties with skeletal muscle Unlike skeletal muscle the heart has gap junctions and contraction is elicited by extracellular calcium Heart contraction force is largely controlled by the length-tension relationship of heart muscle ECG is a 3-D television for heart electrical activity