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Conduction: Difference between revisions
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|previousname=Step 2: Rate | |previousname=Step 2: Rate | ||
|nextpage=Heart axis | |nextpage=Heart axis | ||
|nextname=Step 4: Learn how to determine the heart axis | |nextname=Step 4: Learn how to determine the heart's conduction axis | ||
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'''The PQ interval starts at the beginning of the atrial contraction and ends at the beginning of the ventricular contraction.''' | '''The PQ interval starts at the beginning of the atrial contraction and ends at the beginning of the ventricular contraction.''' | ||
The PQ interval (sometimes referred to as the PR interval as a Q wave is not always present) indicates how fast the action potential is transmitted through the AV node (atrioventricular) from the atria to the ventricles. Measurement should start at the beginning of the P wave | The PQ interval (sometimes referred to as the PR interval as a Q wave is not always present) indicates how fast the action potential is transmitted through the AV node (atrioventricular) from the atria to the ventricles. Measurement should start at the beginning of the P wave and end at the beginning of the QRS segment. | ||
'''The normal PQ interval is between 0.12 and 0.20 seconds'''. | '''The normal PQ interval is between 0.12 and 0.20 seconds'''. | ||
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[[File:Tangent QT.png|thumb|400px| The QT interval starts at the onset of the Q wave and ends where the tangent line for the steepest part of the T wave intersects with the baseline of the ECG. ]] | [[File:Tangent QT.png|thumb|400px| The QT interval starts at the onset of the Q wave and ends where the tangent line for the steepest part of the T wave intersects with the baseline of the ECG. ]] | ||
'''The normal QTc interval''' | '''The normal QTc interval''' | ||
The QT interval indicates how fast the ventricles are repolarized | The QT interval indicates how fast the ventricles are repolarized, becoming ready for a new cycle. | ||
The normal value for QTc( | The normal value for QTc(corrected) is: '''below 450ms for men and below 460ms for women'''. <cite>aha2</cite> | ||
If QTc is < 340ms [[Short_QT_Syndrome|short QT syndrome]] can be considered. | If QTc is < 340ms [[Short_QT_Syndrome|short QT syndrome]] can be considered. | ||
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Thus at a heart rate of 60 bpm, the RR interval is 1 second and the QTc equals QT/1. The '''[[QTc calculator]]''' can be used to easily calculate QTc from the QT and the heart rate or RR interval. | Thus at a heart rate of 60 bpm, the RR interval is 1 second and the QTc equals QT/1. The '''[[QTc calculator]]''' can be used to easily calculate QTc from the QT and the heart rate or RR interval. | ||
On modern ECG machines, the QTc is given. However, the machines are not always capable | On modern ECG machines, the QTc is given. However, the machines are not always capable of making the correct determination of the end of the T wave. Therefore, it is important to check the QT time manually. | ||
[[Image:Surawicz.jpg|120px|thumb|right|Dr Surawicz, founder of the tangent method, still active in November 2006, at age 89.]] | [[Image:Surawicz.jpg|120px|thumb|right|Dr Surawicz, founder of the tangent method, still active in November 2006, at age 89.]] | ||
Although QT prolongation is potentially lethal, measurement of the QT interval by physicians is | Although QT prolongation is potentially lethal, measurement of the QT interval by physicians is not standardized, sine different definitions of the end of the T wave exist.<cite>Viskin</cite> Most QT experts define the end of the T wave as the intersection of the steepest tangent line from the end of the T-wave with the base line of the ECG.<cite>Lepeschkin</cite> This leads to the following stepwise approach: | ||
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#Use lead II. Use lead V5 alternatively if lead II cannot be read. | #Use lead II. Use lead V5 alternatively if lead II cannot be read. | ||
#Draw a line through the baseline (preferably the PR segment) | #Draw a line through the baseline (preferably the PR segment) | ||
#Draw a tangent against the steepest part of the end of the T wave. If the T wave has two positive | #Draw a tangent against the steepest part of the end of the T wave. If the T wave has two positive deflections, the taller deflection should be chosen. If the T wave is biphasic, the end of the taller deflection should be chosen. | ||
#The QT interval starts at the beginning of the QRS interval and ends where the tangent and baseline cross. | #The QT interval starts at the beginning of the QRS interval and ends where the tangent and baseline cross. | ||
#If the QRS duration exceeds 120ms the amount surpassing 120ms should be deducted from the QT interval (i.e. QT=QT-(QRS width-120ms) ) | #If the QRS duration exceeds 120ms the amount surpassing 120ms should be deducted from the QT interval (i.e. QT=QT-(QRS width-120ms) ) | ||
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In | In a (serious) prolonged QT time, it takes longer than the normal amount of time for the myocardial cells to be ready for a new cycle. There is a possibility that some cells are not yet repolarized, but that a new cycle is already initiated. These cells are at risk for uncontrolled depolarization, induction of [[Torsade de Pointes]] and subsequent [[Ventricular Fibrillation]]. | ||
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