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|nextname=Step 4: Learn how to determine the heart axis | |nextname=Step 4: Learn how to determine the heart axis | ||
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[[Image:QRSwaves.jpg|thumb|300px|right]] | [[Image:QRSwaves.jpg|thumb|300px|right]] | ||
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==The PQ interval== | ==The PQ interval== | ||
'''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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===Correcting the QT for the heart rate=== | ===Correcting the QT for the heart rate=== | ||
The QT interval comprises the QRS-complex, the ST-segment, and the T-wave. One difficultly of QT interpretation is that the QT interval gets shorter if the heart rate increases. This can be solved by correcting the QT time for heart rate using the Bazett formula: | The QT interval comprises the QRS-complex, the ST-segment, and the T-wave. One difficultly of QT interpretation is that the QT interval gets shorter if the heart rate increases. This can be solved by correcting the QT time for heart rate using the Bazett formula: | ||
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[[Image:Formule_QTc.png]] | [[Image:Formule_QTc.png]] | ||
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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. | ||
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On modern ECG machines, the QTc is given. However, the machines are not always capable of recognizing the correct QT time. Therefore, it is important to check this manually. | On modern ECG machines, the QTc is given. However, the machines are not always capable of recognizing the correct QT time. Therefore, it is important to check this manually. | ||
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===Correct measurement of the QT interval: how to measure the end of the T wave=== | ===Correct measurement of the QT interval: how to measure the end of the T wave=== | ||
[[Image:Surawicz.jpg| | [[Image:Surawicz.jpg|120px|thumb|left|Dr Surawicz, still active in November 2006, at age 89.]] | ||
Although QT prolongation is potentially lethal, measurement of the QT interval by physicians is poor as 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 poor as 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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===Examples=== | ===Examples=== | ||
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<gallery> | <gallery> | ||
Image:lastigeQT1.png|The T wave is broad, but the tangent crosses the baseline before the T wave joins the baseline. The QT interval would be overestimated when this last definition of the end of the T wave would be used. | Image:lastigeQT1.png|The T wave is broad, but the tangent crosses the baseline before the T wave joins the baseline. The QT interval would be overestimated when this last definition of the end of the T wave would be used. | ||
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Image:lastigeQT3.png|A bifasic T wave. The tangent to the 'hump' with the largest amplitude is chosen. This can change from beat to beat, making it more important to average several measurements. | Image:lastigeQT3.png|A bifasic T wave. The tangent to the 'hump' with the largest amplitude is chosen. This can change from beat to beat, making it more important to average several measurements. | ||
</gallery> | </gallery> | ||
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===Short QT syndrome=== | ===Short QT syndrome=== | ||
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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== References == | == References == | ||
<biblio> | <biblio> | ||
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#aha2 pmid=19228821 | #aha2 pmid=19228821 | ||
</biblio> | </biblio> | ||
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[[Category:ECG Course]] | |||