Slow Rhythms_The Bradycardias

Slow rhythms <60 beats per minute: The Bradycardias

On a simple level, the bradycardia rhythms can be divided into:

·         Sinus bradycardia

·         Heart blocks

-          1^st degree: prolonged PR interval (>0.2s or 5 small squares on the ECG)

-          2^nd degree type 1 (Mobitz 1, Wenckebach phenomenon) where the PR interval prolongs with subsequent beats until there is a dropped beat (no QRS after P)

-          2^nd degree type 2 (Mobitz 2) where PR intervals are constant until there is a dropped beat (no QRS after P)

-          3^rd degree (Complete Heart Block, CHB) where there is no relationship between P waves and QRS complexes at all.

Sinus Bradycardia: regular rhythm, more than 5 large boxes between r waves, PR intervals <0.2s. There is a P-wave before every qrs complex, and a qrs after every P

1st degree Heart Block: regular rhythm, PR intervals >0.2s. There is a P-wave before 

every qrs complex, and a qrs after every P

2nd degree Heart Block type 1: The Wenckebach phenomenon.  PR intervals increase until there is a dropped beat.  There are therefore more P waves

than qrs complexes.

2nd degree Heart Block type 1: "Mobitz 2"

  PR intervals are constant until there is a dropped beat.  There are therefore more P waves than qrs complexes.

Third degree or Complete Heart Block.  There is no relationship between P waves and qrs complexes.

Management of STABLE AND UNSTABLE BRADycardias:

First of all, as with tachycardias, the questions we need to ask are:

• How is the patient? (stable or unstable)
• Is there an identifiable reason for the bradycardia?

SINUS BRADYCARDIA: treatment is of the cause in the first instance.  Causes of sinus bradycardia include:

• high vagal tone (eg in athlete)
• hypothermia
• hypothyroidism
• action of a therapeutic drug  (eg beta blockers,  calcium channel blockers, digoxin etc)
• Cushing's triad with raised intracranial pressure (bradycardia, high SBP, abnormal RR)
• problems in the heart's conducting system
• acute myocardial infarction
• electrolyte imbalance: eg Mg++, K+

Causes of bradycardia should be sought and rectified.  The reason we treat tachycardia is because the fast heart rate increases oxygen demand; the reason we treat bradycardia is because the slow heart rate makes the patient symptomatic or unstable.  If treatment is necessary, atropine 1mg can be given every 3-5 minutes up to a total atropine dose of 3mg.  If the patient fails to respond to atropine or becomes increasingly unstable, epinephrine (2-10mcg/min) or dopamine (5-20mcg/kg/min) infusions can be considered, or, if available, Transcutaneous Pacing (TCP).  TCP is a bridge to more definitive therapy (transvenous pacing), but is useful because it can be commenced quickly and can be controlled rapidly.  

HEART BLOCKS: Broadly speaking, provided there are no confounding complexes such as ectopic beats, the regular slow rhythms (ie rhythms with regular qrs complexes) are sinus bradycardia, 1st degree heart block, and 3rd degree heart block.  The slow rhythms with irregular qrs complexes are the 2nd degree heart blocks, due to dropped beats.  Note that Mobitz 2 may present as a regular rhythm if beats are dropped regularly with a 2:1 block, making it difficult to differentiate from 3rd degree heart block.  Mobitz 2 may progress to complete heart block, and the treatment considerations in both are similar. 

1ST DEGREE HEART BLOCK AND MOBITZ 1 (WENCKEBACH PHENOMENON): causes include high vagal tone, medications, electrolyte disturbances and myocardial problems.  Treatment considerations are as for sinus bradycardia.  These blocks tend to occur above or at the AV node, and therefore they can be expected to respond to atropine therapy.

MOBITZ 2 AND COMPLETE HEART BLOCK: These blocks are more likely to be associated with structural heart disease, such as myocardial infarction or congenital heart disease.  They may also be associated with electrolyte disturbances such as hyperkalaemia or with drug ingestion.  Atropine may have no effect in infranodal blocks.  Atropine should also be used with extreme caution in a patient who is infarcting, as it increases oxygen demand.  


SUMMARY

• the first line therapy that can be considered for all bradycardias is atropine 1mg +/- repeat every 3-5 minutes up to 3mg max.
• second line therapy should be considered rapidly in patients with higher degrees of block (Mobitz 2 and CHB), in patients who are infarcting, and in patients who fail to respond or deteriorate after 1st line therapy.  Note that many practitioners opt straight for pacing in patients with higher degrees of block or those having acute myocardial infarction.

TRANSCUTANEOUS PACING (TCP)

• Machine needs to read rhythm through the leads and deliver energy as needed through the pads, therefore patients should have pads and leads on from a single machine 
• Having selected an adequate heart rate and "Demand" mode on the pacer, the mAmp should be increased in steps until capture is achieved.  Capture is known to have occurred if all pacing spikes are followed by a qrs complex which differs in morphology from patient's intrinsically generated qrs complexes
• The patient should be checked to ensure that electrical capture is accompanied by mechanical output, by checking pulse (femoral is best), BP and full vital signs.
• The output should then be increased by one step to consolidate capture
• Sedation and/or pain relief will probably be required by the patient
• Transfer to cardiology for definitive care - TCP is a bridging treatment