Download المحاضرة 02 كيفية رسم وقراءة التغيرات الكهربية للقلب 1

‫سلسلة التعليم الطبى اإللكترونى عن بعد‬
‫‪Online Medical Education‬‬
‫دورة مهارات رسم القلب الكهربائى‬
‫‪ECG Teacher‬‬
‫محاضرة رقم ‪2‬‬
‫كيفية رسم وقراءة التغيرات الكهربية للقلب‬
‫أوال‪ :‬الملخص العربى‬
‫لقياس كهربية القلب يتم توصيل أقطاب كهربية ‪ electrodes‬بالجسم فىى عرىرة أمىاكن ‪ 6‬مىههم برىكل عر ىى‬
‫بالصىىدر تسىىميتهم ‪ V1,V2,V3,V4,V5,V6‬و‪ 4‬باليىىدين والقىىدمين ويهىىتش عىىن ترىىسيل لهىىا رسىىم القلىىب ‪21‬‬
‫رسما بيانيا يسمى كل مههم ( ‪) Lead‬‬
‫فى أى رسم بيانى يسمى الخط األفقى متساوى الكهربية ‪ Isoelectric line‬وهو الخىط الى ى يعهىى عىدو ولىود‬
‫نراط كهربى وأى مهحهى يرسم ألعلى ه ا الخط هو مهحهى مولب وأى مهحهى تحت ه ا الخط هو مهحهى سالب‬
‫المعىىروأ أع عتىىالت اينيهىىين صىىسيرة لىىدا انا مىىا قورنىىت بعتىىالت البطيهىىين و ل ى لئ فىىاع التيىىار الكهربىىائ‬
‫المصاحب ينقبىا اينيهىين سىيكوع صىسيرا و يرمىف لىس فى رسىم القلىب برمىف (المولىة ب) (‪،)P wave‬بيهمىا‬
‫انقبىىا البطيهىىين يرمفلىىس بىىالرمف (مجموعىىة كيىىو ار اس ) ( ‪ ،)QRS complex‬و بعىىدها يرمىىف ينبسىىاط‬
‫البطيهىىين برمىىف ( المولىىة ت) ( ‪، )T wave‬حيىىا اع الىىدورة القلبيىىة تبىىدأ بالنقبىىا اينيهىىين ىىم انبسىىاطهما ىىم‬
‫انقبا البطيهين م انبساطهما ‪ ،‬و كل ه ا يسجل كمولات ف ورقة رسم القلب‪...‬‬
‫نقاط هامة‬
‫‪ -2‬فى حالة إ الة اإلستقطاب ( ‪ ) Depolarization‬انا كاع اتجاه التيار الكهربائ ف القلب إلى القطىب‬
‫( ‪ ) Lead‬فيكوع الهاتش انحهاء أو مولة مولبة (ألعلى) عل ورقة رسىم القلىب ‪ ،‬و العكىن اع كىاع‬
‫اتجاه التيار بعيدا عن القطب فيكوع الهاتش انحهاء سالب (ألسىلل) على ورقىة رسىم القلىب ‪ ،‬وفىى حالىة‬
‫إعىىادة اإلسىىتقطاب ( ‪ ) Repolarization‬يحىىدا العكىىن تمامىىا انا كىىاع اتجىىاه التيىىار الكهربىىائ ف ى‬
‫القلب إلى القطب ( ‪ ) Lead‬فيكوع الهاتش انحهاء أو مولة سالبة (ألسىلل) على ورقىة رسىم القلىب ‪،‬‬
‫و العكن اع كاع اتجاه التيار بعيدا عن القطب فيكوع الهىاتش انحهىاء مولىب (ألعلىى) على ورقىة رسىم‬
‫القلب‪.‬‬
‫‪ -1‬إ الة اإلستقطاب ( ‪ ) Depolarization‬المبكىر دالىل الحىالف البطيهىى فىى إتجىاه مىن اليسىار لليمىين‬
‫يهتش عهس مولة ‪ q‬طبيعية صسيرة فى كل رسوو أقطاب الجهة اليسرى ( ‪)Left sided leads‬‬
‫‪ -3‬ألع الحجىىم العتىىلى للبطىىين األيسىىر أكيىىر وأقىىوى فهىىو يسىىيطر علىىى المولىىة الكهربيىىة ‪ QRS‬الخاصىىة‬
‫بإ الة اإلستقطاب ‪ DEPOLARIZATION‬للبطيهين‪.‬‬
‫‪ -4‬فى رسم القلب للسن الصسير يالحظ أع رسم المولات متهاسق‪.‬‬
‫ثانيا‪ :‬شاهد الفيديو‬
‫‪http://www.youtube.com/embed/MJPt8hUdnt0‬‬
‫ النص اإلنجليزى للمحاضرة كامال‬:‫ثالثا‬
ECG READOUT GENERATION (VIDEO 2)
The leads of the ECG machine detect the movement of the cardiac
depolarisation and repolarisation waves as they spread through the atria
and ventricles. Leads capable of detecting electrical signal are placed on
the patient’s body and the different lead positions record the flow of
current through the heart from different perspectives. In this way, the
ECG recording can give us information about disease process affecting
different anatomical regions of the organ. We’ll see how this works later,
but for now we need to understand how the individual ECG leads analyse
and record cardiac current. In any ECG lead, the flat line recorded on the
readout when no net current is flowing is termed the isoelectric line. It is
very important to realise that all of the leads on the ECG machine are
set up in such a way that depolarising current moving towards a lead
produces a deflection on the ECG paper above the isoelectric line, a
positive deflection, while depolarising current moving away from the
lead produces a deflection below the isolectric line, a negative
deflection. In contrast, repolarising current has the opposite polarity to
depolarising current. Therefore, repolarising current moving towards a
lead produces a negative deflection on the paper, while repolarising
current moving away from the lead produces a positive deflection. As
the depolarisation and repolarisation waves spread over the normal heart
in a well defined and relatively constant pattern, these rules mean that if
we know the position of an ECG lead relative to heart we can predict the
form of readout it records. We’ll come back to the nomenclature and
position of all 12 leads later, but for now we will predict the form of
readout recorded in two of the six so called chest leads V1 and V6. Lead
V1 is placed on the anterior surface of the patients chest in the fourth
right intercostal space to the right of sternum and, therefore, to the right
of the bulk of the ventricles. In contrast, lead V6 is placed on the patients
chest in the 5th intercostal space mid axillary line and looks at the heart
from the left of the ventricles. The atria sit at the back of the chest cavity
and during each cardiac cycle, atrial contraction is associated with a wave
of depolarisation spreading over these chambers. Remember the SA node
is situated towards the back of the right atrium so the atrial depolarisation
wave not only spreads downwards and to the left but also outwards
towards the front of chest towards the chest leads. As this depolarising
current is moving towards the leads it produces a positive deflection on
the ECG paper, this is the P wave of atrial depolarisation. After a short
delay, in which no current is flowing, the AV node allows the
depolarisation signal to travel into the ventricles. As we’ve seen, the
midzone of the interventriclular septum is the first piece of
ventricular muscle to depolarise and it does so by signal spreading
across the septum from the left to right bundle branch. This early
depolarisation signal is moving towards V1 and therefore produces a
positive deflection on the ECG paper in the recording from this lead.
However, early septal depolarisation is moving away from lead V6
producing an initial negative deflection in this lead. As the septum
continues to depolarise, the depolarisation wave spreads out over the
muscle mass of the ventricles. To understand what happens next in the
recordings, it is important to realise that the magnitude of the electrical
signal generated by depolarising muscle is directly proportional to the
mass of muscle generating it, what this means is, that the more muscle
present the more electrical signal generated and the more signal the ECG
machine detects. The left ventricle has a much greater muscle mass
than the right and so dominates the electrical signal of ventricular
depolarisation in all leads. Therefore, as the wave of electrical activity
reaches the main muscle mass of the ventricles, the left ventricular signal
overwhelms all other signals and as it is moving away from V1, the
deflection produced on the ECG recording from this lead becomes
negative. In contrast, however, this signal is moving towards lead V6
producing a strong positive deflection. The flow of depolarisation around
the ventricles is recorded as the QRS complex and the morphology of the
QRS complex differs predictably in the ECG leads depending on their
position relative to the heart. We’ll come back to the precise
nomenclature of the QRS complex later. When ventricular depolarisation
is complete there is a brief period when no current is flowing and the
recording returns to the isoelectric line. This period ends with the onset of
ventricular repolarisation. Remember, repolarising current has the
opposite polarity to the depolarisation wave and, therefore, when it is
moving towards a lead it produces a negative deflection on the ECG
paper and a positive deflection when moving away from a lead. We’ve
also learned that repolarisation spreads through the ventricles in the
opposite direction to the depolarisation wave beginning in the epicardium
and spreading from the epicardial to the endocardial surface of the
ventricles. The deflection produced on an ECG by ventricular
repolarisation is again dominated by the signal from the left ventricle. As
this repolarising current is moving towards V1 the deflection produced is
negative in this lead. In contrast, this repolarising signal is moving away
from lead V6 producing a positive deflection. The deflection produced by
ventricular repolarisation is termed a T wave. Cardiac repolarisation
spreads relatively slowly through the muscle mass, outside the conducting
system. Hence, the T wave is considerably longer in duration and,
therefore, broader on the ECG paper than the QRS complex.
The fact that repolarising current moves through the ventricles in the
opposite direction to the depolarisation wave means that in leads with an
overall positive QRS complex that is the positive deflection is larger than
the negative deflection, the T wave also tends to be positive above the
isoelectric line, while in leads with an overall negative QRS complex the
T waves tend also to be negative, inverted below the isoelectric line. To
use the jargon, in non-diseased hearts the QRS complexes and T waves
tend to be concordant. There are important exceptions to this rule which
we’ll deal with shortly. Just to tie up a loose end, atrial repolarisation
produces a relatively weak electrical signal which is buried in the QRS
complex and is generally not detectable on a standard 12 lead ECG.
KEYPOINTS
1. Depolarising current moving towards a lead produces a positive
deflection on the ECG paper, while depolarising current moving
away from a lead produces a negative deflection.
2. Repolarising current moving towards a lead produces a negative
deflection on the ECG paper, while repolarising current moving
away from the lead produces a positive deflection.
3. Early left to right septal depolarisation may produce small
physiological q waves in left sided leads.
4. The left ventricle has a much greater muscle mass than the right
and so dominates the electrical signal of ventricular depolarisation in
all leads.
5. In young, non-diseased, hearts the QRS complexes and T waves
tend to be concordant.
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