The Human Body

Electrocardiography is a fundamental part of cardiovascular assessment. It is an essential tool for investigating cardiac arrhythmias and is also useful in diagnosing cardiac disorders such as myocardial infarction. Familiarity with the wide range of

patterns seen in the electrocardiograms of normal subjects and an understanding of the effects of non­cardiac disorders on the trace are prerequisites to accurate interpretation.

The contraction and relaxation of cardiac muscle results

from the depolarisation and repolarisation of myocardial cells. These electrical changes are recorded via electrodes placed on the limbs and chest wall and are transcribed on to graph paper to produce an electrocardiogram (commonly known as an

ECG).

The sinoatrial node acts as a natural pacemaker and initiates atrial depolarisation. The impulse is propagated to the ventricles by the atrioventricular node and spreads in a coordinated fashion throughout the ventricles via the specialised conducting tissue of the His­Purkinje system. Thus, after delay in the atrioventricular mode, atrial contraction is followed by rapid and coordinated contraction of the ventricles.

The electrocardiogram is recorded on to standard paper

travelling at a rate of 25 mm/s. The paper is divided into large squares, each measuring 5 mm wide and equivalent to 0.2 s. Each large square is five small squares in width, and each small square is 1 mm wide and equivalent to 0.04 s.

The electrical activity detected by the electrocardiogram

machine is measured in millivolts. Machines are calibrated so that a signal with an amplitude of 1 mV moves the recording stylus vertically 1 cm. Throughout this text, the amplitude of waveforms will be expressed as: 0.1 mV = 1 mm = 1 small square.

The amplitude of the waveform recorded in any lead maybe influenced by the myocardial mass, the net vector of depolarisation, the thickness and properties of the interveningtissues, and the distance between the electrode and the myocardium. Patients with ventricular hypertrophy have a relatively large myocardial mass and are therefore likely to have

high amplitude waveforms. In the presence of pericardial fluid, pulmonary emphysema, or obesity, there is increased resistance to current flow, and thus waveform amplitude is reduced.

The direction of the deflection on the electrocardiogram

depends on whether the electrical impulse is travelling towards

or away from a detecting electrode. By convention, an electrical

impulse travelling directly towards the electrode produces an

upright (“positive”) deflection relative to the isoelectric baseline,

whereas an impulse moving directly away from an electrode

produces a downward (“negative”) deflection relative to the

baseline. When the wave of depolarisation is at right angles to

the lead, an equiphasic deflection is produced.

The six chest leads (V1 to V6) “view” the heart in the

horizontal plane. The information from the limb electrodes is

combined to produce the six limb leads (I, II, III, aVR, aVL, and

aVF), which view the heart in the vertical plane. The

information from these 12 leads is combined to form a

standard electrocardiogram.

The arrangement of the leads produces the following

anatomical relationships: leads II, III, and aVF view the inferior

surface of the heart; leads V1 to V4 view the anterior surface;

leads I, aVL, V5, and V6 view the lateral surface; and leads V1

and aVR look through the right atrium directly into the cavity

of the left ventricle.

Rate

The term tachycardia is used to describe a heart rate greater than 100 beats/min. A bradycardia is defined as a rate less than 60 beats/min (or < 50 beats/min during sleep). One large square of recording paper is equivalent to 0.2 seconds; there are five large squares per second and 300 per minute. Thus when the rhythm is regular and the paper speed is running at the standard rate of 25 mm/s, the heart rate can be calculated by counting the number of large squares between two consecutive R waves, and dividing this number into 300.

Alternatively, the number of small squares between two consecutive R waves may be divided into 1500.

Some countries use a paper speed of 50 mm/s as standard; the heart rate is calculated by dividing the number of large squares between R waves into 600, or the number of small squares into 3000.

“Rate rulers” are sometimes used to calculate heart rate; these are used to measure two or three consecutive R­R intervals, of which the average is expressed as the rate equivalent. When using a rate ruler, take care to use the correct scale according to paper speed (25 or 50 mm/s); count the correct numbers of beats (for example, two or three); and restrict the technique to regular rhythms.

When an irregular rhythm is present, the heart rate may be calculated from the rhythm strip (see next section). It takes one second to record 2.5 cm of trace. The heart rate per minute can be calculated by counting the number of intervals between QRScomplexes in 10 seconds (namely, 25 cm of recording paper)

and multiplying by six.

Rhythm

To assess the cardiac rhythm accurately, a prolonged

recording from one lead is used to provide a rhythm strip. Lead II, which usually gives a good view of the P wave, is most commonly used to record the rhythm strip.

The term “sinus rhythm” is used when the rhythm originates in the sinus node and conducts to the ventricles.

Young, athletic people may display various other rhythms, particularly during sleep. Sinus arrhythmia is the variation in the heart rate that occurs during inspiration and expiration.

There is “beat to beat” variation in the R­R interval, the rate

increasing with inspiration. It is a vagally mediated response to

the increased volume of blood returning to the heart during

inspiration.

Cardiac axis

The cardiac axis refers to the mean direction of the wave of

ventricular depolarisation in the vertical plane, measured from a zero reference point. The zero reference point looks at the heart from the same viewpoint as lead I. An axis lying above this line is given a negative number, and an axis lying below the line is given a positive number. Theoretically, the cardiac axis may lie anywhere between 180 and - 180°. The normal range for the cardiac axis is between - 30° and 90°. An axis lying beyond - 30° is termed left axis deviation, whereas an axis > 90° is termed right axis deviation.

Several methods can be used to calculate the cardiac

axis, though occasionally it can prove extremely difficult to

determine. The simplest method is by inspection of leads I, II,

and III.

A more accurate estimate of the axis can be achieved if all

six limb leads are examined. The hexaxial diagram shows each lead's view of the heart in the vertical plane.

The direction of current flow is towards leads with a positive deflection, away from leads with a negative deflection, and at 90° to a lead with an equiphasic QRS complex. The axis is determined as follows:

1. Choose the limb lead closest to being equiphasic. The axis lies about 90° to the right or left of this lead

2. With reference to the hexaxial diagram, inspect the QRS complexes in the leads adjacent to the equiphasic lead. If the

lead to the left side is positive, then the axis is 90° to the

equiphasic lead towards the left. If the lead to the right side is

positive, then the axis is 90° to the equiphasic lead towards the

right.