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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 | 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 rhthm 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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#T pmid=15842424 | #T pmid=15842424 | ||
#TdP pmid=20185054 | #TdP pmid=20185054 | ||
#Chiladakis pmid=23118006 | |||
</biblio> | </biblio> | ||
}} | }} | ||
[[Category:ECG Course]] | [[Category:ECG Course]] | ||