Narrow complex tachycardias, for example, supraventricular tachycardia (SVT, as shown in Figure 8.9), are triggered by an ectopic focus situated above the ventricles, which results in impulses being conducted rapidly along the specialised cells in the intraventricular septum. Because SVT originates above the AV node, the QRS complex will be narrow (<0.12 sec). In extreme cases narrow complex tachycardias can also have a profound effect on cardiac output, resulting in the loss of cardiac output and impaired consciousness. Edhouse & Morris (2002b) also suggest that SVT is more likely to occur in those under 35 years of age.
• Rheumatic heart disease
• Ischaemic heart disease
• Pulmonary embolism
• Patients may be clammy and hypotensive and experience chest pain and palpitations.
• Rhythm may be precipitated by stress or alcohol.
• The rhythm may have an abrupt onset and termination.
Characteristic features of the ECG
Narrow complex tachycardia, presumed to be SVT.
• P waves are not always detectable.
• PR interval is not measurable.
• The relationship between P waves and QRS complex is not discernible.
• There is a QRS rate of 150-250 beats per minute.
• QRS complex tends to be narrow (<0.12sec), unlike VT.
• V leads are discordant (some are positive and others negative) on examination of the 12-lead ECG.
This is largely going to depend on the presenting rhythm and symptoms (ERC 2001).
• Administer oxygen and establish venous access.
• If the rhythm is indicative of AF, then see page 143.
• If the patient is pulseless, follow the left side of the algorithm.
• Vagal manoeuvres such as carotid sinus massage or Valsalva manoeuvre may terminate the narrow complex tachycardias and must only be attempted in the absence of contraindications.
• In carotid sinus massage, the pulse has to be located and then massaged. If there is evidence of a carotid bruit this technique should be avoided due to possible complications; inexperienced personnel should not perform carotid sinus massage. Bradycardia is another side-effect of this technique.
• The Valsalva manoeuvre involves asking the patient to cough or to try to blow into a 20 ml syringe, inducing a strain effect which forces expiration against a closed glottis.
• If vagal stimulants fail, intravenous adenosine 6mg may be administered rapidly in a bolus and flushed with saline. The effect of adenosine is to inhibit or slow conduction of impulses across the AV node. The patient should be warned that they might feel unpleasant sensations in the chest.
• Should the patient be symptomatic and unresponsive to vagal manoeuvres or adenosine and adverse signs and symptoms persist, synchronised cardioversion is recommended. In the presence of adverse signs, the patient must be sedated and synchronised cardioversion with 100 J, 200 J or 360 J should be delivered.
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