Pacing may be required in the absence (primary prophylaxis) or presence (secondary prophylaxis) of bradycardia.
Primary prophylaxis is warranted when the occurrence of a symptomatic bradycardia is likely, i.e. under the following circumstances.
1. Sick sinus syndrome, particularly if antitachycardic medications are to be used.
2. Conduction is only occurring through one diseased fascicle. ECG shows first-degree heart block with left bundle branch block or first-degree heart block with right bundle branch block and left axis deviation (signifying left anterior hemiblock). Progression to complete heart block may be precipitated by other factors in the sick ICU patient as described above.
3. First-degree heart block complicating endocarditis (suggesting aortic root abscess) demands immediate pacing. Progression to complete heart block is often associated with sudden asystolic cardiac arrest.
Secondary prophylactic pacing is employed under the following circumstances.
1. Further slowing may prove detrimental.
2. There is a risk of escape tachyarrhythmias such as escape ventricular tachycardia.
Any slow rhythm causing cardiovascular compromise should be treated. A poorly functioning ventricle may have a relatively fixed stroke volume; only an increase in heart rate might raise cardiac output without reducing blood pressure.
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