The Heart: ECG The Heart: ECG SignalSignal

Basic structure of the heart

The simplified circulatory system. The blood is delivered from the right ventricle to the lung. The oxygenated blood from the lung is then returned to the left atrium before being sent throughout the body from the left ventricle. Deoxygenated blood from the body flows back to the right atrium and the cycle repeats.

The ECG initiates the cardiac cycle and cardiac sounds. The ejection occurs when the pressure in the left ventricle exceeds that in the arteries.

Cardiac Cycle

Duration and characteristics of each major event in the cardiac cycle.

EventEvent Characteristics Characteristics Duration at 75 bpm Duration at 75 bpm (0.8 second cycle) (0.8 second cycle)

Atrial systoleAtrial systole

Ventricular diastole Ventricular diastole

AV valves opened.AV valves opened.

Semilunar valves close.Semilunar valves close.

Ventricular filling. Ventricular filling.

0.4 seconds 0.4 seconds

Atrial systoleAtrial systoleVentricular diastole Ventricular diastole

AV valves open.AV valves open.

Semilunar valves closed. Semilunar valves closed. Ventricular filling. Ventricular filling.

0.1 seconds 0.1 seconds

Atrial diastoleAtrial diastoleVentricular systole Ventricular systole

AV valves closed.AV valves closed.

Semilunar valves open.Semilunar valves open.

Blood pumped into aorta Blood pumped into aorta and pulmonary artery. and pulmonary artery.

0.3 seconds 0.3 seconds

A disposable surface electrode. A typical surface electrode used for ECG recording is made of Ag/AgCl.

Block diagram of an electrocardiograph.

VA

VB

VB

A circuit of an ECG amplifier

Lead I is from RA to LA, lead II is from RA to LL, and lead III is from LA to LL.

Einthoven’s triangle

I aVR V1 V4

I aVR V1 V4

(a) The normal ECG. (b) 1st degree AV block in which the delay from the P wave to the Q wave is lengthened. (c) myocardial infarction (lack of blood flow to heart muscle, which causes tissue to die),

in which the S–T segment is depressed.

I aVR V1 V4

(a)

(b)

(c)

(d) right atrial hypertrophy (increase in muscle mass of the atria), in which V4 is large.(e) ventricular tachycardia with clear AV dissociation.(f) Wolff–Parkinson–White syndrome with atrial fibrillation.

(d)

(e)

(f)

I aVR V1 V4

I aVR V1 V4

I aVR V1 V4

ECG SystemsECG Systems

12-lead ECG12-lead ECG ECG MonitoringECG Monitoring Ambulatory (Holter) MonitorAmbulatory (Holter) Monitor ECG Telemetry SystemECG Telemetry System Exercise Stress TestingExercise Stress Testing

The 1st and 2nd heart sounds are most prominent.

Sound Sound Origin Origin

1st sound 1st sound Closure of mitral and tricuspid valves Closure of mitral and tricuspid valves

2nd sound 2nd sound Closure of aortic and pulmonary valves Closure of aortic and pulmonary valves

3rd sound 3rd sound Rapid ventricular filling in early diastole Rapid ventricular filling in early diastole

4th sound 4th sound Ventricular filling due to atrial contraction Ventricular filling due to atrial contraction

The heart sounds

Heart Sound MeasurementHeart Sound Measurement

StethoscopeStethoscope MicrophoneMicrophone

A stethoscope with bell and diaphragm modes.

R

C

vo

Electrode

Microphone