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The role of anatomy and physiology in interpreting ECGs

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VOL: 98, ISSUE: 20, PAGE NO: 34

Amanda Roberts, RN, DipHE, is staff nurse, Intensive Care Unit, Royal Cornwall Hospital, Truro

Coronary heart disease is responsible for more than 135,000 deaths a year, according to the National Service Framework for Coronary Heart Disease (Department of Health, 2000). The framework has also established coronary care as a government priority, which means that it is important for nurses to develop a range of skills in keeping with this initiative.

Coronary heart disease is responsible for more than 135,000 deaths a year, according to the National Service Framework for Coronary Heart Disease (Department of Health, 2000). The framework has also established coronary care as a government priority, which means that it is important for nurses to develop a range of skills in keeping with this initiative.

An electrocardiograph measures the electrical activity of the heart. It is a helpful, non-invasive diagnostic tool used in conjunction with other tests, such as coronary angiography (see Box 1). If nurses are able to interpret electrocardiograms (ECGs), not only will their knowledge-base increase but the detection and treatment of chronic or acute cardiac problems will also improve.

Benner (1984) states that patients benefit when nurses are able to draw on a range of skills, both physical and intellectual, to provide therapeutic care. The Scope of Professional Practice (UKCC, 1992) encourages nurses to extend their roles through adequate and relevant postregistration education and training.

Nurses who are trained to read ECGs can improve patient care through the early identification of actual or potential complications and the prompt referral of patients. These nurses are also equipped to explain conditions and treatments to patients when they are asked to do so. It should be remembered that the ECG, like other investigations, is only an aid to diagnosis and is no substitute for the holistic assessment of patients.

Cardiac anatomy
For the purposes of this two-part series of articles, it is recommended that the reader has a basic knowledge of the anatomy and physiology of the heart. Because of the complexity of the subject, these articles cannot cover the subject exhaustively and further study is recommended.

The heart is a four-chamber, muscular pump with its own blood supply and strategically placed valves that prevent the backflow of blood (Fig 1). The two upper chambers, or atria, are smaller and have thinner walls than the two lower chambers, or ventricles, which have the heaviest workload. The left ventricle pumps blood to most of the body, except the lungs, so its walls are thickest.

The ventricles and atria are separated by connective tissue, which also forms the valves inside the heart. Blood pressure is determined by the coordinated contraction and relaxation of the atria and ventricles in turn. The heart also has its own internal conduction system (see Fig 1), which is responsible for the relay of the electrical signals that keep the heart pumping regularly. Good health depends on the synchronised functioning of all components of the heart to maintain cardiac output and blood pressure.

Cardiac physiology
The heart's conduction system is made of specialised neurogenic tissue which is capable of producing and distributing electrical impulses that prompt the heart to beat at a set rate. This ability to independently set the heart rate is known as autorhythmicity. Any disturbances in conduction or damage to other structures of the heart can adversely affect the blood and oxygen supply to the rest of the body.

The sodium-potassium pump and conduction
The conduction system of the heart relies on a metabolic pump (the sodium-potassium pump) that regulates the movement of positively charged sodium (Na+) and potassium (K+) ions (known as electrolytes) in and out of the cells. As the ions are moved in or out through 'gates' in the membranes of neurogenic cells an 'action potential' is created.

The action potential occurs sequentially across all the neurogenic cells in tiny fractions of a second. It is this repetitive wave of impulses sweeping across the conduction system that causes the heart to contract and relax rhythmically. In doing so it pumps blood to the rest of the body and itself.

Disturbances in the electrolyte balance in the body can have a detrimental effect on the conduction system of the heart.

Phases of the action potential
Depolarisation describes the point at which negatively charged cell interiors become packed with positively charged sodium ions and some positively charged potassium ions. This occurs because the positively charged ions are attracted by the negatively charged cell interior via sodium and potassium gates in the cell membrane. These gates respond directly and in thousandths of a second to the voltage state in the cell. When the cell reaches a point called a threshold (when the interior measures about 30 millivolts), the gates close swiftly to keep the positively charged ions inside. This positive state allows an electrical impulse to flow from one positively charged cell to the next along the length of specialised fibres.

After the impulse has been 'fired' the cells repolarise as positive ions slowly diffuse back out of each cell membrane via the sodium and potassium gates and the cells' interiors again become negatively charged. The whole sequence of events then repeats itself.

The sinoatrial node: the master pacemaker
Electrical impulses normally arise from a specialised area of neurogenic tissue, known as the sinoatrial (SA) node, located in the right atrial wall of the heart. The SA node is also known as the master pacemaker because it usually sets the rhythm for the rest of the heart but is subject to 'outside intervention' by the autonomic nervous system. Heart rate is usually set by the SA node at between 60 and 100 beats a minute at rest, although this varies between individuals.

The cells of the SA node generate and conduct an action potential to 'fire' an electrical impulse which spreads out in a wave over both atria and ventricles in turn, via further specialised networks of muscle fibres. This impulse first causes the atria to momentarily contract, reaches the atrioventricular node, travels down the atrioventricular bundle (bundle of His) and down the left and right bundle branches. It then moves over the central surfaces of the ventricles along the Purkinje fibres (which stimulates contraction of the larger ventricles) and on into the conduction fibres of the left and right ventricle interiors (see Fig 1).

The electrocardiogram waveform
An electrocardiogram depicts the depolarisation and repolarisation of the cells of the heart's conduction system in a two-dimensional form. The recording is made on graph paper, which is divided into small squares that are each further divided into 25 smaller squares. The horizontal axis represents time, while the vertical axis represents the intensity of the electrical impulse in millivolts.

An ECG recording is made on graph paper because this makes it much easier to detect any slight deviation from the norm which may indicate a problem or potential problem in the heart. The graph paper makes it possible to accurately assess the ECG. The normal individual waveforms, or QRS complexes, seen on an ECG tracing take about 0.6 seconds to be generated, which puts into perspective the length of time it takes for each specialised cell to contribute to a single QRS complex (see Fig 2).

The P-wave at the beginning of each complex corresponds with atrial depolarisation as the electrical impulse spreads across the atria, causing them to contract. The time taken from the start of the P-wave to the start of the R-wave represents the time taken for the impulse to make its way from the atria, through the conduction system to the ventricles to be 'distributed' by the Purkinje fibres. The ventricles contract, causing ventricular depolarisation. This larger surge of activity is depicted by the large 'spikes' of the QRS complex.

Finally, the T-wave equates to the repolarisation of the ventricles. A waveform to correspond with the repolarisation of the atria cannot be seen on the ECG as it is 'hidden' by the larger QRS complex.

Making an ECG recording
Although they are often referred to as 'leads', in fact only 10 electrodes (six on the chest and one on each limb) interpret the electrical impulses generated by a 12-lead ECG. Put simply, the four limb electrodes produce six viewpoints of the heart along a vertical plane by 'looking' at one another while the six chest electrodes produce another six viewpoints of the heart on a horizontal plane. There are a total of 12 images.

As an electrical current travels towards an electrode it causes a positive or upward deflection on the ECG, whereas an electrical current flowing away from the electrode will produce a downward deflection. An electrical current that flows at right angles to the electrode will produce an isoelectric waveform.

The QRS complex is normally positive or points upward in 'lead' I because the flow of electrical activity is coming towards it. On the other hand, 'lead' C1, located on the right chest over the fourth intercostal space, should have a negative deflection because the flow of electricity is heading away from it. As it heads towards C6 the deflection is positive.

Once you know which electrode is looking at which area of the heart, it is possible to pinpoint areas of damage because damaged tissue does not conduct impulses normally.n

- Part two of this series will appear in the next issue.

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