The T wave represents part of ventricular repolarization. A normal T wave has an asymmetric shape; that is, its peak is closer to the end of the wave than to the beginning. When the T wave is positive, it normally rises slowly and then abruptly returns to the baseline. When the T wave is negative, it descends slowly and abruptly rises to the baseline. The asymmetry of the normal T wave contrasts with the symmetry of T waves in certain abnormal conditions, such as myocardial infarction , and high serum potassium (hyperkalemia).
The QT interval is measured from the beginning of the QRS complex to the end of the T wave. The QT interval primarily represents the return of the stimulated ventricles to their resting state (ventricular repolarization). The normal values for the QT interval depend on the heart rate. As the heart rate increases (RR interval shortens), the QT interval normally shortens; as the heart rate decreases (RR interval lengthens), the QT interval lengthens.
You should measure several QT intervals and use the average value. The QT interval is often difficult to measure when it is long because the end of the T wave may merge imperceptibly with the U wave. As a result you may be measuring the QU interval rather than the QT interval.
Because of this problem, another index of the QT has been devised. It is the rate-corrected QT is obtained by dividing the QT that you actually measure by the square root of the RR interval: QR /. Normally the QTc is less than 0.44 sec.
There are a number of factors that can abnormally prolong the QT interval. For example, certain drugs, such as quinidine and procainamide (Pronestyl, procan SR), and electrolyte disturbances, such as a low serum potassium (hypocalcemia), can prolong the QT interval. The QT interval may also be prolonged with myocardial ischemia and infarction and with subarachnoid hemorrhage. QT prolongation may predispose patients to potentially lethal ventricular arrhythmias.
The QT interval may also be shortened, for example, by digitalis in therapeutic doses or by hypercalcemia (high serum calcium concentration). The lower limits of normal for the QT interval have not been well defined.
The U wave is a small rounded deflection sometimes seen after the T wave. As noted previously, the exact significance of the U wave is not known. Functionally U waves represent the last phase of ventricular repolarization. Prominent U waves are characteristic of hypokalemia (low serum potassium). Very prominent U waves may also be seen in other settings, for example, in patients taking drugs such as quinidine or one of the phenothiazine, or sometimes after cerebrovascular accidents. The appearance of very prominent U waves in such settings, with or without actual QT prolongation, may also predispose patients to ventricular arrhythmias.
Normally the direction of the U wave is the same as the direction of the T wave. Negative U waves sometimes appear with positive T waves. This is abnormal and has been noted in left ventricular hypertrophy and myocardial ischemia.
Calculation of the Heart Rate
There are two simple methods for measuring the heart rate (number of heartbeats per minute) from the ECG.
the easier way, when the heart rate is regular, is to count the number of large (0.2 sec) boxes between two successive QRS complexes and divide the constant (300) by this. (the number of large time boxes is divided into 300, because 300 x 0.2 = 60 and we are calculating the heart rate in beats per minute or 60 seconds.)
For example, the heart rate is 100 beats/min, since there are three large time boxes between two successive R waves (300 ÷ 3 =100). Similarly, if there are two large time boxes between two successive R waves, the heart rate is 150 beats/min. if there are five intervening large time boxes, the heart rate is 60 beats/min.
If the heart rate is irregular, the first method will not be accurate since the intervals between QRS complexes will vary from beat to beat. In such cases an average rate can be determined simply by counting the number of cardiac cycles every 6 seconds and multiplying this number by 10. (A cardiac cycles is the interval between two successive R waves.) Counting the number of cardiac cycles every 6 seconds can be easily done because the top of the ECG paper is generally scored with vertical marks every 3 seconds.
By definition, a heart rate exceeding 100 beats/min is termed “tachycardia” (tachys, Greek, swift) while one slower than 60 beats/min is called “bradycardia” (bradys, slow). Thus, during exercise you probably develop a sinus tachycardia but during sleep or relaxation your pulse rate may drop into the 50s, or even lower, indication a sinus bradycardia.
111. ECG Leads
The heart produces electrical currents similar to the familiar dry cell battery. The strength or voltage of these currents and the way they are distributed throughout the body can be measured by a suitable recording instrument, such as an electrocardiograph.
The body acts as a conductor of electricity, therefore, recording electrodes placed at some distance from the heart, such as on the arms, legs, or chest wall, are able to detect the voltages of the cardiac currents conducted to these locations. The usual way of recording these voltages from the heart is with the 12 standard ECG leads. The leads actually show the differences in voltage (potential) between electrodes placed on the surface of the body.
Taking an ECG is like drawing a picture or taking a photograph of a person. If we want to know what a person’s face really looks like, for example, we have to draw it or take photographs from the front, side, and back.. One view is not enough. Similarly, it is necessary to record multiple ECG leads to be able to describe the electrical activity of the heart adequately. Notice that each lead presents a different pattern.
The 12 leads can be subdivided into two groups: the six extremity (limb) leads and the six chest (precordial) leads.
The six extremity leads--I, II, III, aVR, aVL, and aVF—show voltage differences by means of electrodes placed on thd limbs. They can be further divided into two subgroups: the bipolar extremity leads (I, II, and III) and the unipolar extremity leads (aVR, aVL, and aVF).
The six chest leads—V1, V2, V3, V4, V5, and V6—present voltage differences by means of electrodes placed at various positions on the chest wall.
positions on the chest wall.
The 12 ECG leads can also be viewed as 12 “channels.” However, in contrast to television channels (which can be turned to different events), the 12 ECG channels (leads) are all tuned to the same event (the P-QRS-T cycle), with each lead viewing the event from a different angle.