Electrocardiography

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Electrocardiography is one of the most basic noninvasive diagnostic methods in medicine. Graphic record of the electrocardiography is called electrocardiogram (ECG, EKG). It is based on recording of electric potential generated by heart on body surface. This methode is used in diagnostic of heart disease.

Indications

ECG is indicated if there is suspicion on:

Principle

ECG Principle fast.gif

Everything is based on heart depolarization and repolarization. These processes can be presented by vectors of electrical potentials. ECG is recording of summation of electrical potential vectors from multiple myocardial fibers, not only from cardiac conduction system. Every myocardial fiber can generate electrical potential. These electrical potentials are conducted by body fluids (which are very good conductors) to the body surface, where they are recorded.[1]

ECG Leads

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The 12 Conventional ECG Leads

These 12 leads are divided into two groups:

  • six extremity (limb) leads; they are recording electrical potentials transmitted into the frontal plane;
  • six chest (precordial) leads, they are recording electrical potential transmitted into horizontal plane.[2]

There are the leads with their location and polarity[3]:

limb location of the lead
bipolar extremity leads

-Einthoven's leads-

I + left upper limb - right upper limb
Einthoven.png
II left lower limb right upper limb
III left lower limb left upper limb
unipolar extremity leads

-Goldberg's leads-

aVR right upper limb lead
aVL left upper limb lead
aVF left lower limb lead
unipolar chest leads

-Wilson's leads-

V1 fourth intercostal space, just to the right of the sternum
Chest leads.png
V2 fourth intercostal space, just to the left of the sternum
V3 midway between V2 and V4, fifth intercostal space
V4 fifth intercostal space, midclavicular line on the left
V5 fifth intercostal space, anterior axillary line
V6 fifth intercostal space, midaxillary line

Additional ECG Leads

These leads are used in special situations, when conventional 12-leads ECG can not reliably show the myocardial defect.[2]

There are some examples of additional ECG leads[3]:

lead location of the lead
unipolar chest leads V7 posterior axillary line, on the same level as V6, on the left
V3R-V6R.png
V789.png
V8 scapulary line, in the same level as V6, on the left
V9 paravertebral line on the left, on the same level as V6
VE just to the left of processus xiphoideus
V3R - V6R on the right, same location as V3–V6
unipolar chest leads V1´– V6´ about 1 intercostal space above the V1–V6
V1´´– V6´´ about 2 intercostal space above the V1–V6
esophageal leads E/Oe for example 37.5 cm (left atrium)

Normal ECG

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Basic ECG waveforms, intervals and segments

Electrocardiogram is graphic record of electrocardiography. ECG curve contains waves P, Q, R, S, T, and sometimes U. For description of ECG are very important intervals and segments between waves. Every ECG description has to start with description of heart rhythm (regularly or irregularly, sinus or nonsinus rhythm) and frequency.

Description of Waves, Intervals and Segments

P Wave

P wave should be always before QRS complex, separated by PQ interval. P wave is a sign of normal atrial depolarization.

Parameters:

  • duration: 110 ms;
  • amplitude: 0.25 mV;
  • positivity:
    • positive − always in leads I and II;
    • negative − always in aVR lead.

When is P wave missing? Atrial flutter or fibrillation, …

PQ Interval

PQ interval is a period of atrial contraction. The depolarization is delayed in AV node.

Parameters:

  • duration: 120−200 ms
  • positivity: isoelectric

QRS Complex

QRS complex represents ventricular depolarization and contraction. There are two phases of ventricular depolarization:

  1. depolarization of interventricular septum − the vector is oriented from left to right and anteriorly;
  2. depolarization of ventricles − because the left ventricle is more massive than the right ventricle, the vector oriented from right to left and posteriorly[2].

There are three waveforms in QRS complex:

  • Q wave − the first negative wave following P wave, may not always be presented;
  • R wave − the first positive wave following P wave or Q wave;
  • S wave − the first negative wave following R wave.

Parameters:

  • duration of QRS complex: 100 ms or less;
  • amplitude of Q wave;
  • amplitude of R wave;
  • amplitude of S wave.

ST Segment

ST segment is isoelectric line, period of no electrical activity of the heart. Should be in the same level as PQ interval. Every elevation or depression of this line is pathological.

  • Physiological duration is 320 ms.

T Wave

T wave represents repolarization of ventricles. The positivity or negativity should be the same as the major vector of QRS complex.[2]

  • Physiological duration 160 ms.

U Wave

The U wave is ordinarily small and follows T wave and usually has the same polarity as T wave.[2]

Heart Rhythm

Heart rhythm is physiologically generated by SA node. Sign of its healthy function is P wave and PQ interval. Rhythm generated in SA node is called sinus rhythm.

Heart Frequency

Heart frequency or heart rate is based on frequency of ventricular contraction. Can be easy counted from ECG curve. It is necessary to compare two QRS complexes and measure the time interval between their R waves − RR interval (in seconds):

HR=60/RR

Normal heart rate is 55−90/min.

Links

Related Articles

References

  1. KASPER, Dennis L – FAUCI, Anthony S – LONGO, Dan L, et al. Harrison's principles of Internal Medicine. 16th edition. New York : McGraw-Hill Companies, Inc, 2005. 2607 pp. pp. 1311-1312. ISBN 0-07-139140-1.
  2. a b c d e KASPER, Dennis L – FAUCI, Anthony S – LONGO, Dan L, et al. Harrison's principles of Internal Medicine. 16th edition. New York : McGraw-Hill Companies, Inc, 2005. 2607 pp. pp. 1312-1313. ISBN 0-07-139140-1. Cite error: Invalid <ref> tag; name "Harrison" defined multiple times with different content
  3. a b WikiSkripta. Elektrokardiografie [online]. ©2011. The last revision 2011-07-2, [cit. 2011-07-03]. <http://www.wikiskripta.eu/index.php/Elektrokardiografie>.

Bibliography

  • KASPER, Dennis L – FAUCI, Anthony S – LONGO, Dan L, et al. Harrison's principles of Internal Medicine. 16th edition. New York : McGraw-Hill Companies, Inc, 2005. 2607 pp. pp. 1311-1319. ISBN 0-07-139140-1.