Bradycardias and blocks

A heart rate that is either abnormally slow in absolute terms or is too slow to support an adequate circulation needs to be addressed. For example, a heart rate of 65 beats/min may be inappropriately slow for a patient who has developed a low output state. The recommendations for bradycardias and blocks depend on whether there is a recognizable appreciable risk of asystole ( Fig 2). Complete third-degree heart block with a narrow QRS is not in itself an indication for treatment because atrioventricular junctional ectopic pacemakers (with a narrow QRS) often provide an adequate and stable heart rate.

Fig. 2 Algorithm for bradycardia: AV, arteriovenous; BP, blood pressure.

Fig. 2 Algorithm for bradycardia: AV, arteriovenous; BP, blood pressure.

If asystole is believed to be a definite risk, the responder may wish to do no more than establish intravenous access and give atropine before seeking help from others with transvenous pacing skills. If a patient's condition is critical, placement of a transvenous ventricular pacing wire or external pacing may be appropriate. Isoproterenol (isoprenaline) 1 pg/min can be administered by infusing 2.5 mg of isoproterenol in 500 ml of a carrier solution with an infusion pump, initially at 0.2 ml/min. The dose may be increased rapidly. However, attention should be paid to the risk of precipitating or worsening ventricular arrhythmias and increasing myocardial oxygen consumption. Isoproterenol for infusion is not available in all European countries, and orciprenaline is an appropriate alternative.

If there is no perceived risk of asystole, adverse clinical signs including severe impairment of myocardial function, very slow heart rate, or the progression of emerging tachyarrhythmias that require suppression should be sought. Without such signs, observation may be all that is required; however if one or more signs are present, 500 pg of atropine should be administered initially by slow intravenous injection. Increments of 500 pg or 1 mg, up to a total dose of 3 mg, can be given at intervals. Higher doses are not beneficial and may produce unwanted effects. If there is a satisfactory response, only observation may be needed; however, failure to respond to atropine in the presence of adverse signs may require transvenous pacing.

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