27. Electrocardiogram, analysis of ECG

Question 1

ECG

Answer 1

fibers can be measured on the surface of the body (ECG). Einthoven invented a device to detect slight electric changes (mV). Pairs of electrodes are placed around the heart – as dipole – creating the „Einthoven’s Triangle”. Potential differences (U) are measured between the pairs of electrodes. During rest: the sum of the potential differences around the dipole (integral vector) is always zero. The graphical pattern representing the potential changes during rhythmic cardiac activity is called electrocardiogram. This tool has been most important diagnostic possibility in cardiology for decades. Beside its diagnostic importance, its understanding may help to develop a better insight into the physiology of the heart. Ultrasound made it a less important tool, but it is still an important tool in diagnostics.

Question 2

The heart as a dipole

Answer 2

All single working fibers can be looked at as a dipole as the different phases of the action potential travel alongside its membrane. The dipole can be characterized by a three-dimensional vector which has direction, measure and polarity.
Einthoven’s standard bipolar leads are the potential differences measured between two of the three electrodes placed onto the surface of the body around the heart at the points of a uniform triangle.

Question 3

Einthoven’s standardbipolar leads

Answer 3

Einthoven’s standardbipolar leads

• Lead1:referenceelectrode=RA,investigating electrode=LA
• Lead2:referenceelectrode=RA,investigating electrode=LL
• Lead 3: reference electrode = LA, investigating electrode = LL

Question 4

Analysis of ECG curves

Answer 4

Deflections on the ECG curve are called waves. The part of the line that falls between waves is called segment. Larger parts that may contain one or more waves and segments are called intervals or complexes.

• P-wave: The atrial muscle fibers are being depolarized.
• PQ-segment: Total activation of atrial fibers and the period of atrioventricular conduction. QRS-complex: depolarization of ventricular fibers & repolarization of the atria.
• Q-wave: transmission of the excitation from the bundle of His to the ventricular muscles, downward deflection.
• R-wave: ventricles are being depolarized, largest deflection of the ECG curve.
• S-wave: right ventricle depolarizes from the endocardium close to the apex towards the epicardial surfaces of the base of the heart.
• ST-segment: all ventricular muscle fibers are activated, this causes a shift from the isoelectric line T-wave: ventricular repolarization
• TP-segment: represents the resting phase after full repolarization.

Question 5

How can we measure ECG?

Answer 5

By an oscilloscope, the asymmetry of the heart makes measurable the total electric activity of it. ECG can only be measured because the heart is an asymmetrical object of changing wall thickness.

Question 6

Types of ECG

Answer 6

• Unipolar: RA, LA & LL connected to each other via high resistance forming 0 potential reference point. The potential diff. between 0-point and different points on the surface are detected.
• His bundle ECG & Esophageal ECG: fine analyses of SA,AV nodes + the conduction system
• Vector cradiography: provides info about the anatomy of the heart.
• Vector loop: provides info about the functioning of certain territories.
• ECHO cardiography: provides even more detailed picture about the anatomy of the heart.