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Diagnostic Testing: Difference between revisions
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==Electrocardiography (ECG)== | ==Electrocardiography (ECG)== | ||
[[Image:Nsr.png|300px|thumb|Figure 1. A short ECG registration of the normal heart rhythm (sinus rhythm). | [[Image:Nsr.png|300px|thumb|Figure 1. A short ECG registration of the normal heart rhythm (sinus rhythm). | ||
Source: http://en.ecgpedia.org/wiki/File:Nsr.png]] | Source: http://en.ecgpedia.org/wiki/File:Nsr.png]] | ||
The electrocardiogram asses the electrical activity of the human heart and translates this into a graphic representation. In Figure 1 the body location for the 10 electrodes of a 12-channel ECG are shown. The exact placement of the electrodes is of utmost importance in obtaining an interpretable ECG. The ECG is a graphic representation of the difference in voltage between the patches over time. | |||
An ECG can be used to directly clarify the mechanism of an irregular heart rhythm detected on physical examination or that of an extremely rapid or slow rhythm. In addition the ECG can help in identifying structural heart disease (i.e. cardiac hypertrophy), ischemic heart disease (i.e. myocardial infarction) or other causes of symptoms outside of the heart (i.e. pulmonary embolism). So called Holter monitoring or other continuous-ECG monitoring devices allow assessment of cardiac rate and rhythm on a continuous and ambulatory basis. The most common use of ECG monitoring is the evaluation of symptoms such as syncope, near-syncope, or palpitation for which there is no obvious cause and cardiac rhythm disturbances are suspected. | An ECG can be used to directly clarify the mechanism of an irregular heart rhythm detected on physical examination or that of an extremely rapid or slow rhythm. In addition the ECG can help in identifying structural heart disease (i.e. cardiac hypertrophy), ischemic heart disease (i.e. myocardial infarction) or other causes of symptoms outside of the heart (i.e. pulmonary embolism). So called Holter monitoring or other continuous-ECG monitoring devices allow assessment of cardiac rate and rhythm on a continuous and ambulatory basis. The most common use of ECG monitoring is the evaluation of symptoms such as syncope, near-syncope, or palpitation for which there is no obvious cause and cardiac rhythm disturbances are suspected. | ||
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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 as the heart rate increases. This problem 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 as the heart rate increases. This problem can be solved by correcting the QT time for heart rate using the Bazett formula: | ||
[[Image:Formule_QTc.png| | [[Image:Formule_QTc.png|200px]] | ||
Thus at a heart rate of 60 beats per minute, 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 beats per minute, 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 of making the correct determination of the end of the T wave. Therefore, it is important to check the QT time manually [ http://en.ecgpedia.org/wiki/Conduction Conduction]. 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. | 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 [ http://en.ecgpedia.org/wiki/Conduction| Conduction]. 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. | ||
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 repolarised, 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 repolarised, 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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Unfortunately, it is impossible to obtain high-quality images or Doppler signals in as many a small percent of patients. Underlying conditions such as obesity, emphysema or chest wall deformities can limit the use of transthoracic echocardiography. A technique to partly cope with these limitations is transoesophageal echocardiography (TEE) [figure 9]. | Unfortunately, it is impossible to obtain high-quality images or Doppler signals in as many a small percent of patients. Underlying conditions such as obesity, emphysema or chest wall deformities can limit the use of transthoracic echocardiography. A technique to partly cope with these limitations is transoesophageal echocardiography (TEE) [figure 9]. | ||
[[Image: | [[Image:Transesophageal echocardiography diagram.svg|thumb|300px|Figure 9. Transoesophageal echocardiography. | ||
source: http://commons.wikimedia.org/wiki/File%3ATransesophageal_echocardiography_diagram.svg]] | source: http://commons.wikimedia.org/wiki/File%3ATransesophageal_echocardiography_diagram.svg]] | ||
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*Myocardial resonance imaging | *Myocardial resonance imaging | ||
[[Image: | [[Image:Stress_test.jpg|thumb|300px|Figure 10. Cardiac stress test making use of a walking treadmill | ||
source: http://commons.wikimedia.org/wiki/File%3AStress_test.jpg]] | source: http://commons.wikimedia.org/wiki/File%3AStress_test.jpg]] | ||
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In case there is an abnormal resting ECG or the main focus of the test is in the area of hymodynamic changes or quantification of myocardial dysfunction, other myocardial imaging modalities can be added. In general additional imaging techniques add to the specificity and sensitivity of ischemic testing, although still not perfect. | In case there is an abnormal resting ECG or the main focus of the test is in the area of hymodynamic changes or quantification of myocardial dysfunction, other myocardial imaging modalities can be added. In general additional imaging techniques add to the specificity and sensitivity of ischemic testing, although still not perfect. | ||
Additional myocardial imaging modalities are chosen based on the scope of the researcher (i.e. hemodynamic or ischemic abnormalities), the quality in relation to the costs, and their availability. | Additional myocardial imaging modalities are chosen based on the scope of the researcher (i.e. hemodynamic or ischemic abnormalities), the quality in relation to the costs, and their availability. | ||