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<!--nav template-->{{nav|class="toccolours" style="float:right;width:100%; margin: 0 0 1em 1em; background-color: #f2f2f2;"|-|previouspagealign="left" |[[Introduction|previousname='''&laquo; Introduction''']] |nextpagealign="right" | [[Rate|nextname='''Step 1: Heart Rate&raquo;''']] |-|}}<div style="clear:both"></div><!--nav template--> 
{{authors|
|mainauthor= [[user:Vdbilt|I.A.C. van der Bilt, MD]]
==The ECG represents the sum of the action potentials of millions of cardiomyocytes==
The individual [[action potentialImage:Ion_currents_en.png|thumb|right|300px|action potentials]] During the depolarization sodium-ions stream inwards the cell. Subsequently the calcium-ions stream into the cell. These calcium-ions give the actual muscular contraction. Finally the potassium-ions stream out of the individual cardiomyocytes are averagedcell. During the repolarisation the ion concentration is corrected. The final result which On the ECG, an action potential wave coming towards the electrode is shown on as a positive (upwards) signal. Here the ECG electrode is actually represented as an eye.]][[Image:Hart_cells_en.png|thumb|left|280px|The heart consists of approximately 300 trillion cells]][[Image:cells_in_rest_en.png|thumb|left|280px|In rest the average of trillions of microscopic electronical signalsheart cells are negatively charged. Through the depolarization by surrounding cells they become positively charged and they contract.]]{| class="wikitable" font-size="70%" align="rightcenter" width=385px font-size"400px" style="70%margin-top:10px"
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<gallerybr /><br />Image:Hart_cells_en.pngThe individual [[action potential|The heart consists action potentials]] of approximately 300 trillion cellsImage:cells_in_rest_en.png|In rest the heart cells individual cardiomyocytes are negatively chargedaveraged. Through the depolarization by surrounding cells they become positively charged and they contract.Image:Ion_currents_en.png|During the depolarization sodium-ions stream inwards the cell. Subsequently the calcium-ions stream into the cell. These calcium-ions give the actual muscular contraction. Finally the potassium-ions stream out of the cell. During the repolarisation the ion concentration The final result which is corrected. On shown on the ECG, an action potential wave coming towards the electrode is shown as a positive (upwards) signal. Here actually the ECG electrode is represented as an eyeaverage of trillions of microscopic electronical signals.</gallery>
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==The electric discharge of the heart==
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'''The sinal node (SA node) contains pacemakercells which determine the heart frequency.'''
==The different ECG waves==
[[Image:PQRS_origin_en.png|thumb|left|200px| The origin of the different waves on the ECG]]
[[Image:Epi_endo_en.png|thumb| The QRS complex is formed by the sum of the electric avtivity of the inner (endocardial) and the outer (epicardial) cardiomyocytes]]
[[Image:Qrs-shapes.png|thumb| Example of the different QRS configuations]]
Using these 10 electrodes, 12 leads can be derived. There are 6 extremity leads and 6 precordial leads.
===The Extremity Leads===
[[Image:ECGafleidingen.jpg|thumb|left|210px]]
The extremity leads are:
This is done with the use of the height or depth, independent of the wave (QRS, P of T).
Example: if in lead I, the QrS complex is 3 mm in height and in lead III 9mm, the height of the QRS-complex in lead II is 12mm.
 
Other extremity leads are:
#Leads V7-V8-V9 can be used to diagnose a posteriorinfarct. It is means that after V6, leads are placed towards the back. See the chapter[[Ischemia]] for other ways of diagnosing posterior infarction.
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 {{authors||mainauthor= [[user:Technical ProblemsDrj|J.S.S.G. de Jong]]|moderator= [[user:Drj|J.S.S.G. de Jong]]|supervisor= }}<div style="clear:both"></div>==Lead reversals==Sometimes an ECG is not made properly. Mistakes do happen and leads can be switched. Always remain careful to check this or you might come to the wrong conclusions. One of the most common mistakes is to switch the right and left arm. This will result in negative complexes in I, indicating a right axis deviation!<div style="float:right"><gallery>Image:cableReversal1.png|Right and left arm lead reversal can be distinguished from the (much rarer) dextrocardia by looking at the precordial R wave progression.Image:cableReversal2.png|Right arm and left leg lead reversal. Lead II now measures the signal between the left and right leg, which is remote from the heart.Image:dextrocardia.jpg|A patient with dextrocardia (and previous inferior myocardial infarction)</gallery></div>Common mistakes are reversal of:*right leg and right arm:**Hardly any signal in lead II.*right and left arm electrodes; **reversal of leads II and III**reversal of leads aVR and aVL*left arm and left leg:**reversal of leads I and II**reversal of leads aVR and aVF**inversion of lead III*right arm and left leg:**inversion of leads I, II and III**reversal of leads aVR and aVF  It is possible to distinguish lead reversal and [[w:Dextrocardia|dextrocardia]] by watching the precordial leads. Dextrocardia will not show any R wave progression in leads V1-V6, whereas lead reversal will.{{clr}} ==Artifacts==<div align="center"><gallery perRow="6">Image:Noise_move.png|Movement artifactsImage:ECG_Parkinson.png|Increasing movement artifacts in a Parkinson patient. The patient was in sinus rhythm! (which doesn't show on this short recording)Image:BaselineDrift.png|Baseline drift. The amplifier in the ECG machine has to re-find the 'mean'. This often occurs right after lead connection and after electric cardioversion.Image:cardioversion_from_afib.jpg|Cardioversion from atrial fibrillation to sinus rhythm, with clear baseline drift.Image:electric_noise_ecg.png|Electrical interference from a nearby electrical appliance. A typical example is a 100 Hz background distortion from fluorescent lights. Not to be confused with [[Atrial_Fibrillation|atrial fibrillation]].Image:electric_noise_ecg2.jpg|Another example of an artefact caused by an electrical appliance. The patients rhythm is regular. This strip shows 10 QRS complexes.</gallery> <div style="border:1px solid #ccc;padding:5px;width:93%;">Artifacts (disturbances) can have many causes. Common causes are: Movement  Electrical interference</div></div>{{clr}}
==References==
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