SINUS AND SUPRAVENTRICULAR TACHYCARDIA

    DEFINITION

    Supraventricular tachycardia (SVT) is a heart rate greater then 100
    beats/minute that originates above the ventricle.  It includes sinus
    tachycardia, paroxysmal SVT (PSVT) and atrial tachycardias.  

    Sinus tachycardia (ST) is a heart rate faster than 100 beats/minute
    (in adults) that originates in the sinus node. The rate does not
    usually exceed 160 beats/minute but can be as high as 180 beats/minute
    in young adults and children.

    Paroxysmal SVT is a regular rhythm with rates from 160 to 220
    beats/minute.

    TOXIC CAUSES

    Many toxic substances cause SVTs, usually sinus tachycardia. 
    Important examples include:

         Albuterol and other beta-2 agonists (reflex tachycardia)
         Amphetamines and related stimulants
         Atropine and other anticholinergic drugs and plants
         Beta-1 adrenergic agents
         Caffeine
         Carbon monoxide
         Cocaine
         Cyanide
         Ephedrine, pseudoephedrine and other related decongestants
         Salicylates
         Theophylline
         Thyroid hormone
         Tricyclic antidepressants

    In addition, supraventricular tachycardias may be observed in many
    intoxications as a secondary response to hypoxaemia, hyperthermia,
    methaemoglobinemia, hypovolaemia or peripheral vasodilation.

    NON-TOXIC CAUSES

    Anxiety 
    Ethanol or other sedative-hypnotic drug withdrawal
    Dehydration
    Exercise
    Fever 
    Hypoxemia
    Intrinsic conduction system disorders (paroxysmal SVT) 
    Pain

    CLINICAL FEATURES

    Sinus and other supraventricular tachycardias are often well-tolerated
    and the only finding is a fast heart rate.  Conscious patients may be
    aware of palpitations.  If the rate is very fast, and the patient has
    pre-existing cardiovascular compromise or is hypovolaemic, impaired
    cardiac filling may lead to hypotension and hypoperfusion.  In such
    cases, the patient may experience dizziness, weakness, altered mental
    status, anxiety, chest pain or syncope.

    DIFFERENTIAL DIAGNOSIS

    Atrial fibrillation
    Ventricular tachycardia

    RELEVANT INVESTIGATIONS 

    Cardiac monitor
    ECG
    Toxicology screening or specific drug levels as indicated

    TREATMENT 

    In most cases, the only treatment is initial cardiac monitoring,
    establishment of intravenous access, supplemental oxygen and simple
    observation.

    Where there is associated hypotension and hypoperfusion, intravenous
    fluids should be administered, initially as boluses of 250 to 500 mL. 
    If the patient does not respond to these simple measures, further
    management should proceed as detailed in the Shock and Hypotension
    Treatment Guide.

    Rarely, specific drugs may be indicated.  Examples include:

          Beta blockers for drugs causing excess adrenergic stimulation
          Benzodiazapines for drugs causing central nervous system
         stimulation

    DC cardioversion may be attempted.

    CLINICAL COURSE AND MONITORING 

    The patient with mild sinus tachycardia who is otherwise asymptomatic
    does not need prolonged monitoring. Patients with symptoms or signs of
    hypoperfusion and those with suspected serious ingestions, should be
    closely observed until toxicity resolves. 

    LONG-TERM COMPLICATIONS

    Complications and sequelae from simple ST or mild SVTs are uncommon.

    AUTHOR(S)/REVIEWERS

    Author:        Dr Kent R. Olson, University of California,
                   San Francisco, USA.

    Reviewers:     London 3/98:  T. Della Puppa, T.J. Meredith, L. Murray,
                   A. Nantel, J Pronczuk
                   Geneva 8/98, D. Jacobsen, L. Murray, J. Pronczuk.