Electrical Activity of the Heart By: Andrea, Tory, Jackie, Madison, and Aaron

Conducting System Conducting system: a network of specialized cardiac muscle cells that initiates and distributes electrical impulses http://www.nlm.nih.gov/medlineplus/ency/imagepages/18052.htm

Conducting System Made up of two types of cardiac muscle cells that do not contract: Nodal cells: responsible for establishing the rate of cardiac contraction and are located at the sinoatrial (SA) and atrioventricular (AV) Conducting cells: distribute the contractive stimulus to the general myocardium Major sites of the conducting cells are AV bundle, Bundle Branches, and Purkinje fibers

Conducting System Sinoatrial node (SA node): a tissue mass embedded in the posterior wall of the right atrium near the entrance of the superior vena cava Pacemaker cells: cells of the SA node that set the pace of cardiac contraction Atrioventricular node (AV node): transmits signal from SA node to the ventricles

Conducting System AV Bundle: bundle of conducting cells extends along the interventricular septum before dividing into left and right bundle branches Left and Right bundle branches: radiate across the inner surfaces of the left and right ventricles Purkinje fibers: convey the impulses to the contractile cells of the ventricular myocardium

Conducting System The conducting system of the heart The stimulus for contraction moves through the heart in a predictable sequence of events Step 1: SA node activity and atrial activation begin Step 2: Stimulus spreads across the atrial surfaces and reaches the AV nodes Step 3: there is a 100 msec delay at the AV node. Arial contraction begins. Elapsed time = 150 msec Step 4: The impulse travels along the interventricular septum within the AV bundle and the bundle branches to the Purkinje fibers Step 5: The impulse is distributed by Purkinje fibers and relayed throughout the ventricular myocardium. Atrial contraction is completed, and ventricular contraction begins

Conducting System A number of clinical problems result from deviations from normal pacemaker functions Bradycardia: a condition in which the heart rate is slower then normal (less than 60 bpm) Tachycardia: A faster then normal heart rate (100 bpm or more) Ectopic pacemaker: abnormal conducting cell or ventricle muscle cell may begin generating action potentials so rapidly they override those of the SA or AV node

The Electrocardiogram Electrocardiogram (ECG, EKG): a recording of the electrical events occurring in the heart http://www.wpclipart.com/medical/doctor_equipment/electrocardiogram.png.html

QRS Complex: Impulse spreads to ventricles triggering ventricle contraction T Wave: Ventricles return to resting state P Wave: Electrical impulse spreads across atria and triggers atrial contractions and the atria begins contracting 100 msec after the start of the P wave

Electrocardiogram (ECG/EKG) Analyzing an Electrocardiogram(ECG/EKG) involves measuring the size of voltage changes and determining the temporal relationships of the various components Ex. Smaller than normal electrical signal could mean the mass of heart muscle has decreased Cardiac arrthymias: abnormal patterns of cardiac activity 5% of the normal population experiences a few abnormal heartbeats each day Clinical problems appear when arrhythmias reduces hearts pumping efficiency