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 BasicsRepresents 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 ECGRepresent the

heart as a single moving dipole

ECG measures projection of the dipole vector

Coarse simplification

ECGVariations in electrical potential at

skin surfaceDepolarizatin of cells near heartExcitation and relaxation of heart

muscleMechanical, electrical, acoustic

MeasurementsHeart 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

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 RateIn 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 CircuitPreAmpLow Pass FilterHigh 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.

fL fH

Pass only

fL to fH attenuate the others.

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

نرجو للجميع الفائده الرجاء زيارتنا في الموقعWWW.AR-BME.COM

24