Myocardial capture threshold

There are a variety of conditions that can increase the myocardial capture threshold. These include metabolic derangements, medications, and traumatic events such as inadvertent conduction of current down the lead during direct current cardioversion or defibrillation that results in tissue injury at the lead-tissue interface. Metabolic disturbances that increase the myocardial capture threshold include myocardial ischemia and infarction, hyperkalemia, hypoxemia, hypercarbia, acidemia, alkalemia, hyperglycemia, and hypothyroidism (59-62). Hyperkalemia is the most common electrolyte abnormality that can leads to failure to capture (Fig. 16.7), and the threshold typically increases when the serum potassium concentration exceeds 7.0mEq/L (63-65). Increasing the stimulus output is only variably successful and should not be relied on. Inunediate reversal of hyperkalemia should be the first priority. 

Failure to Capture. The absence of myocardial depolarization despite appropriate stimulus delivery from the pulse generator defines failure to capture. Failure to capture is usually diagnosed fiom the ECG by the occurrence of visible pacing artifact without resultant paced P-waves or QRS complexes (Fig. 16.2). Failure to capture can, however, result in no visible stimulus artifact despite stimulus output from the generator when there is complete lead fracture (see above). Failure to capture results from lead failure, lead-tissue interface problems, and increases in myocardial stimulation threshold (Fig. 16.3). 

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