ECG 3 chapters 1–7:Layout 1 10/09/2014 11:08 Page 1
Chapter 1
Recording a readable electrocardiogram (ECG) An ECG is a graphic tracing of electrical patterns produced by the heart. This test is frequently used for patients who have heart problems and is an important diagnostic procedure. However, the ECG has limitations and so it is important to evaluate it in conjunction with the patient’s clinical status. ECG abnormalities can occur in healthy individuals; and it can also be possible for a person to have a heart attack and yet have a normal ECG. The nature of the abnormality and its effect on the patient influence the clinical importance of the findings, so the ECG should never be used in isolation. Before we start to interpret the ECG, it is important to learn how to obtain a readable recording (see fig 1.1). We will learn in the following chapters that slight changes can have huge implications for the patient. These days it is common for ancillary staff to take on the task of recording ECGs. To the untrained eye a recording may seem readable but it is not until we learn to interpret an ECG recording that we really gain an understanding of the importance of producing a readable tracing. It is possible to misdiagnose patients or miss their diagnosis if the recording is not clear. Before interpreting the ECG, it is therefore essential to ensure that the recording was obtained correctly. Common errors are incorrect paper speed and standardisation, artefact and incorrect lead placement. Any of these problems can make it extremely difficult, and in some cases impossible, to measure the intervals and the segments that we are going to learn about in this book.
Paper speed and standardisation The ECG is made up of a series of horizontal and vertical lines that measure the duration and amplitude of the various deflections. The small boxes on the paper are 1 millimetre (mm) in height and I mm in width. The amplitude of the ECG deflection is measured vertically and the duration of the ECG event horizontally. Recordings are usually made at 25 mm per second (mm/s). It is therefore important to ensure that the ECG machine has not been set, at say, 50mm/s before the ECG is recorded. The paper speed should be printed on the ECG itself when it is recorded (see fig 1.2). A standard deflection (a box that looks like half a rectangle) should be inscribed at the beginning or end of the ECG. The ECG is usually standardised so that the amplitude of a 1 millivolt impulse causes a deflection of 10 mm (see fig 1.3). An increased amplitude (or voltage) usually indicates increased muscle mass of the heart. Note: If the ECG is not set at 25 mm/s, all the usual ECG measurements that we are about to learn will not apply.
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