Failure to capture

This criterion works well for adequate conceptual schemes. But inadequate conceptual schemes seem to pose a special problem. The inadequacy of a conceptual scheme may consist in its leading us to false sentences. Remember the zebralopes. The deficient scheme makes us assert that certain animals belong to the same species, whereas, as a matter of fact, they belong to different species. Or, for a better example, consider what Aristotle says about motion. Many things he says are false. Yet, his theory of motion shares a domain with mechanics. But false sentences do not describe facts. So how can the criterion be applied when one conceptual scheme does not describe facts (This is a problem for metaphysical realism as well. Metaphysical realism holds that sharing a domain means capturing the same mind-independent structures. But inadequacy is just a failure to capture the real structures. So how can two conceptual schemes capture the same structures, if one of them does not capture these structures )... 

September 8 to November 4, 1998 March 17, 1999 Failure to establish and maintain procedures Failure to capture all trends and incidents for corrective and preventative action Failure to validate a process Failure to establish a quality plan Degree of control over a process not adequate to assure conformance... 

Antidysrhythmic drugs increase the pacing threshold, but failure to capture is a rare consequence. It has, however, been reported twice with flecainide (28,29). 

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). 

The only other situation that mimics lead fracture in both failure to capture and high lead impedance is a faulty connection between the lead connector... 

Fig. 16.2 Failure to capture due to insufficient current delivery in a patient with a pulse generator at end of life. The first and third QRS complexes are paced the second complex represents true fusion (the QRS complex resulting from both the pacing stimulus and spontaneous depolarization) the fourth complex represents pseudofusion (the QRS complex resulting from spontaneous depolarization despite pacing stimulus output) the remaining complexes are spontaneous.

Lead insulation breaks also result from mechanical stresses such as clavicular crush. A break in the outer insulation will result in current leak into the surrounding tissue. A break in the inner insulation that separates the conducting wires in a bipolar lead will result in a short circuit. In either case, insufficient current reaches the myocardium and failure to capture results. 

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. 

Inspect the connector block. Is connector pin (or pins) completely through the coimector block A loose connection could explain intermittent or complete failure to output or intermittent failure to capture. 

Does the lead position appear radiographically acceptable Inadequate lead position may explain failure to capture or sense. Compare current and previous radiographs if possible. Is ventricular lead redundancy, or slack, adequate Is atrial J adequate ... 

Inspect the entire length of the lead for lack of integrity, such as fracture, compression, or crimp. Intermittent or complete failure to capture or sense or output could be secondary to lead conductor coU fracture or loss of insulation integrity. Attempt to follow each lead along its course, assessing the conductor coil. Also, inspect for any crimping of the lead as it passes under the clavicle. 

The purpose of inspecting the connector block is to determine that the connector and pin are firmly in contact. If the pin of the pacing lead is not firmly in the connector block, intermittent or permanent disruption of the circuit occurs. In Figure 18.6, the chest radiograph demonstrates a dual-chamber pacemaker with a lower pin that is only partially advanced. At presentation, the patient had intermittent failure to capture the ventricle and intermittent failure to deliver a ventricular pacing output. 

As with all manmade devices there is the possibility that the device can be faulty, either due to a fault in the manufacturing process or in the programming of the device. Pacemaker malfunctions can often be seen on the 12-lead ECG. The most commonly encountered problems associated with pacemaker systems include failure to sense, failure to capture, failure to pace and over sensing. 

Fig. 8.7 Failure to Capture. Arrows show presence of pacing spikes on the ECG without a subsequently paced QRS complex or P wave...

Integral- or diaphragm-tyTpe abutments are always included in the model of the bridge, as a continuation of the deck model in addition to the body of the abutment, they also include the foundation, which typically consists of piles (often relatively flexible ones). It is essential that the flexibility of the abutment foundation be modeled otherwise the displacements of the integral bridge are seriously underestimated besides problems with earthquake analysis, failure to capture the flexible end supports of the bridge also results in unrealistic stresses from... 

This is a relatively sophisticated statistical approach which is based on historic price and market information. Econometric models seek to identify those independent variables (such as changes in economic activity, metal stocks, production etc) which best explain changes in the dependent variable, in this case, lead prices. The relationship normally takes the form of a linear equation. These models help to focus on key influences and provide an insight into relationships between past prices and other variables. However, their usefulness as forecasting tools are limited by the quality of available historic data, by the need for forecasts of the explanatory variables themselves, and by their failure to capture satisfactorily dynamic structural changes in the industry under examination. 

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