Читать книгу Understanding Anatomy and Physiology in Nursing - John Knight - Страница 98

Ideally when learning the components of an ECG, it is suggested that you should continually refer to the cardiac conductive system, and for this reason the two are presented together (Figure 3.6a and b). The electrical activity shown in Figure 3.6b corresponds to a single heartbeat and consists of:

 The P wave: This small wave corresponds to the action potentials generated by the heart’s natural pacemaker, the sinoatrial node (SAN), and their passage across the atria. Therefore, nurses can regard this small initial peak (P) as corresponding exactly to the time when the patient’s atria are undergoing atrial systole. It may be a useful memory aid to think of the P wave as corresponding to activity generated by the pacemaker (p for pacemaker).

 The P-R interval: This short time period corresponds to atrial contraction and the short delay in action potentials that occurs at the AV node to allow ventricular filling.

 The QRS complex: Since the ventricles are so thick and muscular when action potentials spread through these lower chambers, a much larger electrical signal is generated. This is recorded on an ECG as a large spike termed the QRS complex. It is useful to regard this portion of the ECG as corresponding to the time when the ventricles are undergoing ventricular systole.

 The T wave: This final major electrical peak corresponds to the ventricles returning to their resting state (ventricular repolarisation). Nurses can regard the T wave as corresponding to the time when the ventricles are relaxing as they undergo ventricular diastole.

 The S-T segment: The period of electrical activity between ventricular depolarisation and repolarisation. Characteristic changes to this segment are frequently seen in patients with CAD, particularly when the patient has their ECG recorded when on a treadmill (stress ECG).