in practice, a rapid estimation of the cardiac axis can be made simply by inspection of the frontal ECG leads. Convenient leads to use are the standard limb leads si, sii, siii and aVF, which lie at 0, 60, 120 and 90° respectively.
initially, some basic facts about the relationship between a vector and its components should be noted. The QRS amplitudes in these leads are components of the cardiac vector. The amplitude of a component depends on the angle between the component and the vector it is derived from. As this angle increases the amplitude of the component decreases. Note that:
• For angles < 90° the component is positive
• When the angle between vector and component is zero, i.e. the component is acting in the direction of the vector, the component is maximum and equal to the vector
• At 60° the amplitude of the component is half of the vector
• At 90° the amplitude of the component becomes zero
• For angles > 90° the component becomes negative
Applying these simple principles will confirm the following estimations. If:
• sII = 0, the cardiac axis is at -30° (left axis deviation)
• sI = sII, then the cardiac axis is at +30° bisecting the angle between them
• sI and sIII are each half of sII, the cardiac axis is at +60°
• sIII > sII the cardiac axis is at >90° (right axis deviation) Monitoring in the Operating Theatre
During routine non cardiac surgery standard II and V5 are usually used as continuous monitoring leads. The P wave is best detected in standard II that facilitates detection of junctional or ventricular arrhythmias. Chest lead V5 is most sensitive to S-T segment changes and can warn of possible ischaemia. During cardiac surgery all six limb leads are connected for intermittent S-T segment examination and this can increase sensitivity for the detection of ischaemia.
A cardiac rhythm is defined by three characteristics:
• The anatomical origin—a description of where the rhythm originates anatomically, e.g. SA node, atria, AV node or ventricles
• The discharge sequence—a description of the pattern of electrical discharge, e.g. sinus rhythm, tachycardia, bradycardia, fibrillation
• The conduction sequence—a description of abnormalities in conduction of the discharge impulses to the myocardium, e.g. 2:1 SA block, complete AV block
Abnormal cardiac rhythms or arrhythmias can arise as a primary or secondary disorders. Acute arrhythmias occurring during the peri-operative period can seriously compromise perfusion. An arrhythmia must be correctly diagnosed and the precipitating causes should be removed before treatment is considered. Acute arrhythmias are more likely to be reversible. Chronic arrhythmias are usually disease related and relatively stable.
Some departures from a perfectly regular cardiac rhythm occur as a result, of normal physiological responses, as opposed to having an underlying pathological cause. These are outlined below.
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