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[[Image:QRSwaves.jpg|thumb|300px|right]] | [[Image:QRSwaves.jpg|thumb|300px|right]] | ||
==The PQ interval== | ==The PQ interval== | ||
[[Image:Conduction_ap.svg|thumb|300px|right|The PQ duration depends on the conduction velocity in the atria, AV node, His bundle, bundle branches and Purkinje fibers]] | |||
'''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.''' | ||
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==The QT interval== | ==The QT interval== | ||
[[File: | [[File:E000571.jpg|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:QTeyeballing.png|thumb|400px|The eyeballing method to estimate QT prolongation. If the QT interval ends before the imaginary boundary halfway two QRS complexes, the QTc is probably normal. If the QTc reaches beyond the halfway line, the QTc is probably prolonged. This method is only 'valid' in registrations with normal (60-100/min) heart rates.]] | |||
'''The normal QTc (corrected) interval''' | '''The normal QTc (corrected) interval''' | ||
The QT interval indicates how fast the ventricles are repolarized, becoming ready for a new cycle. | The QT interval indicates how fast the ventricles are repolarized, becoming ready for a new cycle. | ||
The normal value for QTc is: '''below 450ms for men and below 460ms for women'''. <cite>aha2</cite> | The normal value for QTc is: '''below 450ms for men and below 460ms for women''' as agreed upon by the ACC / HRS. <cite>aha2</cite> | ||
In a recent ACC consensus document an expert writing group suggest that in a hospital setting the upper limit be raised to the 99th percentile of normal: 470ms in males and 480 ms in females, as approximately 10% to 20% of the general population have a QTc > 440m s. For both men and women QTc > 500ms is considered highly abnormal.<cite>TdP</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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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. | 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. | ||
Alternatives to the Bazzett correction formula are the Fridericia, Framingham and Hodges formulas. The latter two perform better at high heart rates (>100 /min). <cite>Indik</cite><cite>T</cite> | |||
*Fridericia: QTc = QT{HR/60}1/3 | |||
*Framingham: QTc = QT + 0.154{1 – (60/HR)} | |||
*Hodges: QTc = QT + 1.75 (heart rate - 60). | |||
Although QT prolongation is potentially lethal, measurement of the QT interval by physicians is not standardized, since 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: | Although QT prolongation is potentially lethal, measurement of the QT interval by physicians is not standardized, since 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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#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. | #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) ). | ||
#Calculate QTc according to Bazett: [[Image:Formule_QTc.png]]. You can use the '''[[QTc calculator]]''' for this. | #Calculate QTc according to Bazett: [[Image:Formule_QTc.png]]. You can use the '''[[QTc calculator]]''' for this. | ||
|} | |} | ||
During ventricular pacing this method overestimates the QTc. The Framingham formula performs better during pacing, but still overestimates the QTc in sinus rhythm (in the same patient) by about 37-43 msec.<cite>Chiladakis</cite> | |||
In a pathological 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]]. | In a pathological 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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The QT interval is prolonged in congenital [[long QT syndrome]], but QT prolongation can also occur as a | The QT interval is prolonged in congenital [[long QT syndrome]], but QT prolongation can also occur be acquire as a results of: | ||
* Medication (anti-arrhythmics, tricyclic antidepressants, phenothiazedes, for a complete list | * Medication (anti-arrhythmics, tricyclic antidepressants, phenothiazedes, for a complete list see [http://www.torsades.org Torsades.org] | ||
* Electrolyte imbalances | * Electrolyte imbalances | ||
* Ischemia. | * Ischemia. | ||
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#Moss pmid=8256751 | #Moss pmid=8256751 | ||
#aha2 pmid=19228821 | #aha2 pmid=19228821 | ||
#Viskin pmid=15922261 | |||
#Indik pmid=16945790 | |||
#T pmid=15842424 | |||
#TdP pmid=20185054 | |||
#Chiladakis pmid=23118006 | |||
</biblio> | </biblio> | ||
}} | }} | ||
[[Category:ECG Course]] | [[Category:ECG Course]] | ||