The shape of the normal ECG waveform has important similarities, whatever the orientation. The first deflection is caused by atrial depolarization, and it is a low-amplitude slow deflection called a P wave. The QRS complex results from ventricular depolarization and is sharper and larger in amplitude than the P wave. The T wave is another slow and low-amplitude deflection that results from ventricular repolarization. The atrial repolarization wave is not seen in a conventional ECG because it is low in voltage and is hidden by the QRS complex. The PR interval is the length of time from the start of the P wave to the start of the Q RS complex. It is the time taken for activation to pass from the sinus node, through the atrium, AV node and the His-Purkinje system to the ventricle.
The QT interval extends from the start of the QRS complex to the end of the T wave. This interval represents the time talcen to depolarize and repolarize the ventricular myocardium.
The ST segment is the period between the end of the QRS complex and the start of the T wave. In the normal heart, all cells are depolarized by this phase of the ECG. The normal values for the electrocardiographic intervals are indicated in Table 11.9. A normal ECG is shown in Fig. 11.18. Leads that face the lateral wall of the left ventricle have predominantly positive deflections, and leads looking into the ventricular cavity are usually negative. Detailed patterns depend on the size, shape and rhythm of the heart and the characteristics of the torso.
At any point in time during depolarization and repolarization, electrical potentials are being propagated in different
directions. Most of these cancel each other out and only the net force is recorded. This net force in the frontal plane is known as the cardiac vector. The mean QRS vector can be calculated from the six
standard leads ; it normally lies between -30° and +90°. Left axis deviation lies between -30° and -90° and right axis deviation between +90° and +150°. Calculation of this vector is useful in the diagnosis of cardiac disorders.
This is a technique used to assess the cardiac response to exercise. The ECG is recorded whilst the patient walks or runs on a motorized treadmill or cycles on a stationary cycle ergometer. Recording the ECG after the exercise is not an adequate form of stress test. Normally there is little change in the T wave or ST segment. Myocardial ischaemia provoked by exertion results in ST segment depression (>I mrn) in leads facing the affected area. Although most abnormalities are detected in leads Vs (anterior and lateral ischaemia) or AVF (inferior ischaemia), it is best to record a full 12-lead ECG. The form of ST segment depression provoked by ischaemia is characteristic: it is either planar or shows down-sloping depression (Fig. 11.20). Up-sloping depression is a non-specific finding. During an exercise test the exercise tolerance, blood pressure and rhythm responses to exercise are also assessed. Exercise causes an increase in heart rate and blood pressure. A sustained fall in blood pressure indicates severe coronary artery disease. The usual indications and contraindications for the test are shown in Table 11.10. Its use in angina is described.
24-Hour ambulatory taped electrocardiography This is a technique for recording transient changes such as a brief paroxysm of tachycardia, an occasional pause in the rhythm, or intermittent ST segment shifts. A conventional 12-lead ECG is recorded in less than a minute and usually samples less than 20 complexes. In a 24-hour period over 100000 complexes are recorded. Such a large amount of data must be analysed by automatic or semiautomatic methods. This technique is called ‘Holter’ electrocardiography after its inventor.
Event recording is another technique that may be used to record rare arrhythmias. The patient is provided with a pocket-sized device that can record and store a short segment of the ECG. The device may be kept for several days or weeks until the arrhythmia is recorded. Most units of this kind will also allow transtelephonic ECG transmission so that the physician can determine the need for treatment or the continued need for monitoring. Heart rate variability (HR V) can be assessed from a 24- hour ECG. HRV is decreased in some patients following myocardial infarction and represents an abnormality of autonomic tone or cardiac responsiveness. Low HRV is a major risk factor for sudden death and ventricular arrhythmias in patients discharged following myocardial infarction.
Ambulatory ECG recordings may also be used to monitor the level of the ST segment and to record transient ST segment depression. Tilt testing Patients with suspected neurocardiogenic syncope should be investigated by upright tilt. The patient is secured to a table which is tilted to +600 to the vertical for 45 min or more. The ECG and blood pressure is monitored throughout. If neither symptoms nor signs develop, isoprenaline may be slowly infused and the tilt repeated. A positive test results in hypotension, sometimes bradycardia (Fig. 11.21) and presyncope/syncope. If symptoms and signs appear, they can be quickly reversed by placing the patient flat. The effect of treatment can be evaluated by repeating the tilt test.
Carotid sinus massage
Carotid sinus massage may lead to asystole (2: 3 s) and/or fall of blood pressure (> 50 mmHg). This hypersensitive response occurs in many of the normal (especially elderly) population, but may also be responsible for loss of consciousness in some patients with carotid sinus syndrome .
The application of a sensitive microphone to the chest wall allows heart sounds and murmurs to be recorded. Usually cardiac, carotid or jugular pulsations are recorded at the same time. The technique is difficult and, except for research purposes, has been largely superseded by echocardiography and other non-invasive techniques.