نشرة المشترك من: Ar-bme.com ECG

Main points What is an ECG? How does it measure? What does it measure? What do the parts of the resulting curve mean? Why use an ECG? What is the circuit to build one?

ECG History and Basics Represents electrical activity (not contraction) Marey, 1867, first electrical measurement from the heart. Waller, 1887, first human ECG published. Einthoven, 1895, names waves, 1912 invents triangle, 1924, wins Nobel Prize. Goldberger, 1924, adds precordial leads (specific chest placement)

Heart Dipole and the ECG Represent the heart as a single moving dipole ECG measures projection of the dipole vector Coarse simplification

ECG Variations in electrical potential at skin surface Depolarizatin of cells near heart Excitation and relaxation of heart muscle Mechanical, electrical, acoustic

Measurements Heart rate (state atrial and ventricular, if different) PR interval (from beginning of P to beginning of QRS) QRS duration (width of most representative QRS) QT interval (from beginning of QRS to end of T)

Parts of the Heart The sinoatrial (SA) node is a section of nodal tissue that is located in the upper wall of the right atrium. The SA node is also referred to as the pacemaker of the heart. The atrioventricular (AV) node is a section of nodal tissue that lies on the right side of the partition that divides the atria, near the bottom of the right atrium. Purkinje fibers are fiber branches that extend from the atrioventricular bundle. They relay cardiac impulses to the ventricular cells causing the ventricles to contract. HIS bundle is part of the specialized conduction system. The HIS bundle rapidly conducts electrical impulses from the AV node to the ventricles. Disease in the HIS bundle can produce a form of bradycardia (too slow heart rate) called heart block.

ECG Waves and Intervals: What Do They mean? P wave: the sequential activation (depolarization) of the right and left atria QRS complex: right and left ventricular depolarization (normally the ventricles are activated simultaneously) ST-T wave: ventricular repolarization U wave: origin for this wave is not clear - but probably represents "afterdepolarizations" in the ventricles PR interval: time interval from onset of atrial depolarization (P wave) to onset of ventricular depolarization (QRS complex) QRS duration: duration of ventricular muscle depolarization QT interval: duration of ventricular depolarization and repolarization RR interval: duration of ventricular cardiac cycle (an indicator of ventricular rate) PP interval: duration of atrial cycle (an indicator or atrial rate)

Rhythm Analysis Really reading an ECG takes LOTS of practice State basic rhythm (e.g., "normal sinus rhythm", "atrial fibrillation", etc.) Identify additional rhythm events if present (e.g., "PVC's", "PAC's", etc) Consider all rhythm events from atria, AV junction, and ventricles Really reading an ECG takes LOTS of practice

Why use an ECG? Patients with unexplained unconsciousness (syncope), near syncope or episodic dizziness in whom the cause is not obvious Patients with unexplained recurrent palpitations With pacemakers Evaluation of frequent symptoms of palpitation, syncope, or near syncope to assess device function, so as to exclude myopotential inhibition and pacemaker mediated tachycardia and to assist in the programming of enhanced features. Evaluation of suspected component failure or malfunction when device interrogation is not definitive in establishing a diagnosis To assess the response to adjunctive pharmacological therapy in patients receiving Implantable Cardioverter defibrillator (ICD) therapy-slows down fast heart rate (pacemakers do opposite)

Heart Rate In normal sinus rhythm, a resting heart rate of below 60 bpm is called bradycardia and a rate of above 90 bpm is called tachycardia. Bradycardia Diseases of sinoatrial node (heart’s pacemaker) Weak, sluggish, dizzy, short of breath Tachycardia Normal response to increased activity/oxygen demands Premature contractions Atrial (PAC, PAB), Ventricular (PVC, PVB) Caffeine, alcohol, decongestants Hyperthyroidism, anemia, hypertension, stress http://www.sjm.com/resources/learnmoreabout.aspx?section=NormalRhythmAndArrhythmias

Now…The Circuit PreAmp Low Pass Filter High Pass Filter

(frequency passes if higher than given frequency) (frequency passes if lower than given frequency)

Noise Several sources 60Hz power lines – shielding, filtering Other biopotentials – filtering Motion artifacts – relaxed subject Electrode noise – high quality electrodes, good contacts Circuit noise – good design, good components

When measuring biopotentials (say ECG), EVERYTHING else creates noise – power line interference – even other biopotentials (like EEG, EMG, EOG) are noise sources. These have characteristic frequencies. So use Band Pass Filters. Pass only fL to fH attenuate the others. fL fH

Frequencies of Biopotentials Signal Frequency range (Hz) Amplitude range(mV) ECG 0.01 – 300 0.05 – 3 EEG 0.1 – 100 0.001 – 1 EOG 0.1 – 10 0.001 – 0.3 EMG 50 – 3000 0.001 – 100

Our frequency range will be 0.05 to 200 Hz

Overall Gain

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