Twelve-Lead Electrocardiography phần 2

Đoạn ST cũng có thể bị trầm cảm bên dưới đường cơ sở trong một loạt các điều kiện, chẳng hạn như thiếu máu cục bộ và phì đại tâm thất. ST-segment thay đổi của tất cả các loại sẽ được thảo luận chi tiết trong chương sau | 10 Chapter 2 Electrocardiographic Waveforms Figure . The three hallmarks of acute myocardial infarction including ST segment elevation T wave inversion and Q wave formation. The ST segment can also be depressed below the base line in a variety of conditions such as ischemia and ventricular hypertrophy. ST-segment shifts of all sorts will be discussed in detail in later chapters. The T Wave The T wave corresponds to repolarization of the ventricles. It is normally inscribed in the same direction as the predominant deflection of the QRS and has less amplitude than the QRS. Abnormalities of the T wave predominantly take the form of inversion being inscribed in the opposite direction of the QRS as we have seen in BBB left ventricular hypertrophy LVH and AMI. They may also take the form of very large or small amplitudes such as in hyperkalemia and hypokalemia Figure . Figure . Extremely tall pointed T waves seen with hyperkalemia. A Word About Nonspecific ST and T Wave Changes 11 Normal QT interval ------Upper limits normal women ------Upper limits normal men ------Normal women ------Normal men children Figure . Graph plotting normal QT interval against heart rate for men women and children. The QT Interval The QT interval is measured from the beginning of the QRS to the end of the T wave and normal intervals vary with heart rate and the person s sex. Therefore when determining whether a QT interval is normal or not it is best to use a chart that plots normal intervals against heart rate Figure and sex. The primary potential abnormality of the QT interval is prolongation reflecting delays in ventricular repolarization. This is commonly the result of the administration of drugs such as procainamide or quinidine or of electrolyte imbalance particularly as in hypocalcemia. When the QT interval is prolonged there is a greater opportunity for R-on-T phenomenon and a higher incidence of ventricular reentry dysrhythmias