The Heart Beat

Lecture Outline

  • Electrical conduction in the heart

  • Cardiac action potentials

  • Electrocardiogram (EKG/ECG)

  • Cardiac pumping

  • Cardiac Cycle

Aims I

  • Define Sinoatrial node, Atrioventricular node, and Purkinje Fibres.

  • Describe the conduction of electrical signals (action potentials) through the heart.

  • Define automaticity, ectopic beat, and chronotropic.

  • Describe the origin of the heartbeat and the actions of autonomic neurotransmitters on heart rate.

  • Define EKG/ECG, and list the main waves.

  • Draw and label a normal EKG.

Aims II

  • Explain the cause of the main waves present on a normal EKG.

  • Predict how simple changes in the electrical activity of the heart will alter the EKG.

  • Define atrioventricular and semilunar valves.

  • Describe the function of the heart valves and the origin of the heart sounds.

  • Describe the changes in ventricular pressure and volume that occur during the cardiac cycle.

  • Draw and label a diagram showing the EKG, heart sounds, and ventricular pressure and volume.

Contraction is Signaled Electrically

  • Cells are electrically charged.

  • Nerves and muscle use changes in charge to send electrical signals called Action Potentials.

  • Cardiac Muscle cells use action potentials to coordinate contraction across the heart and activate contraction.

Electrical Conductivity in the Heart

  • Myocardial cells are connected by gap junctions, allowing the cardiac action potential to propagate from cell to cell through a low resistance pathway.

  • Specialized electrically active cells form the Sinoatrial (SA) node, Atrioventricular (AV) node, Bundle of His, and Purkinje Fibres.

  • Electrical activity normally originates in the SA node.

  • The AV node forms the only site of electrical connection between the atria and ventricles.

Propagation of the Cardiac AP*

  • Action potential (AP) starts at the SA node.

  • AP conducted through atrial muscle.

  • AP delayed at the AV node before entering the Bundle of His.

  • Conduction through the Bundle of His and Purkinje fibres is extremely rapid.

  • Ventricles depolarize from endo to epicardium and from apex to base.

Autorhythmicity

  • Some heart cells (SA, AV node, and Purkinje network) show automaticity, the ability to generate a heartbeat.

  • These cells have an intrinsic rhythmicity which generates a pacemaker potential.

  • The heart does not require nerve or hormonal input to beat.

  • In heart transplant patients, the nerves are severed but the heart beats on.

Cardiac Pacemakers

  • Sinoatrial node has the fastest pacemaker potential (~90-100 beats/min) and is the normal pacemaker.

  • Atrioventricular node is the next fastest (~40-60 beats/min) followed by cells in the bundle of His (15-30 beats/min).

  • The fastest pacemaker normally drives the heart and suppresses other pacemakers (overdrive suppression).

  • A beat generated outside the normal pacemaker is an ectopic beat.

  • The site that generates an ectopic beat is known as an ectopic focus (foci pl.) or ectopic pacemaker.

Neural Control of Heart Rate

  • Agents that alter heart rate are chronotropic.

  • Positive chronotropic agents increase heart rate (e.g., Adrenaline and Noradrenaline acting on β-adrenergic receptors).

  • Negative chronotropic agents slow the heart (e.g., Acetylcholine acting on M-cholinergic receptors).

  • At rest, the heart is under parasympathetic tone which slows the natural rhythm of the heart.

Electrical Activity in the Heart

  • Uses of the EKG:

    • Heart Rate

    • Conduction in the heart

    • Arrhythmias

    • Direction of the cardiac vector

    • Damage to the heart muscle

    • Provides NO information about pumping or mechanical events in the heart.

Heart Valves

  • The Heart has four valves:

    • Atrioventricular (AV) valves: Between the atria and ventricles.

    • Semilunar (SL) valves: Between the ventricles and arteries.

  • Heart valves prevent backflow of blood.

  • Forward flow of blood is not impeded.

  • Closing of the valves produces sounds audible by stethoscope.

  • Abrupt closure of the AV valves produces S1, the loudest heart sound.

  • Closure of SL valves produces the second sound, S2.

Ventricular Pressure and Volume

  • After the P wave, atrial contraction slightly increases ventricular pressure.

  • Following the QRS complex, ventricular contraction greatly increases ventricular pressure.

  • S1 occurs during ventricular pressure rise.

  • S2 occurs during the fall in ventricular pressure.

  • During diastole, the ventricle fills with blood.

  • Atrial systole adds more blood.

  • Isovolumic contraction of the ventricles occurs when all heart valves are closed at the start of systole.

  • During relaxation, there is an isovolumic phase when all heart valves are closed.

Summary

  • Electrical action potentials initiate the heartbeat.

  • Action potentials start in the SA node and spread to the ventricles through the AV node.

  • The ECG/EKG shows electrical events in the heart: P, QRS, and T waves from atrial depolarization, ventricular depolarization, and ventricular repolarization.

  • Closure of the heart valves produces the heart sounds and prevents reverse flow of blood.

  • The Cardiac cycle shows the ECG, heart sounds, and the changes in ventricular pressure and volume during the heartbeat.