Changes

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).

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:

#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.

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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]].

#T pmid=15842424

#T pmid=15842424

#TdP pmid=20185054

#TdP pmid=20185054

</biblio>

</biblio>

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[[Category:ECG Course]]

[[Category:ECG Course]]