Sensing And Pacing Thresholds

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For manual sensing threshold determination, the device should first be re-programmed below the intrinsic rate to assess sensing. In single-chamber devices, ventricular sensing thresholds may be determined by then decreasing sensitivity (i.e., increasing the millivolt values) in the VVI mode to determine at what value

Pacemaker Mediated Tachycardia Icd

Figure 11.10. Stored atrial (top) and ventricular (bottom) electrograms from a pacemaker with automated electrogram capture triggered by a high ventricular rate. The patient experienced syncope at the time the electrograms were recorded. The documentation of AV dissociation is diagnostic of ventricular tachycardia.

Figure 11.10. Stored atrial (top) and ventricular (bottom) electrograms from a pacemaker with automated electrogram capture triggered by a high ventricular rate. The patient experienced syncope at the time the electrograms were recorded. The documentation of AV dissociation is diagnostic of ventricular tachycardia.

Intracardiac Electrograms
Figure 11.11. Surface ECG, atrial (AEGM) and ventricular (VEGM) intracardiac electrograms, and marker channel (bottom) demonstrating "noise" and oversensing on the atrial lead of a Guidant ICD. This resulted from an insulation break from subclavian crush.
Sensitivity Pacing
Figure 11.12. Atrial triggered pacing mode. Top: Maximal sensitivity of 0.5mV atrial sensing is appropriate. Bottom: With reduction of atrial sensitivity to 5.0 mV there is a failure of atrial sensing with atrial spikes that do not coincide with native P waves.

pacer output is no longer inhibited. The same approach may be applied for establishing atrial-sensing thresholds. The triggered modes may also be used in their respective chambers to determine sensing thresholds. Failure to trigger a pacemaker spike at a given sensitivity value indicates undersensing (Fig. 11.12). Alternatively, triggered pacing by signals other than the P wave or QRS, such as T waves or myopotentials, indicate oversensing. In dual-chamber systems, atrial sensing can be confirmed by programming to a P-wave synchronous ventricular triggered mode, shortening the AV interval so as to trigger ventricular pacing, and reducing atrial sensitivity progressively until paced ventricular events no longer result.Ventricular sensing can either be checked in a VVI mode with the rate set lower than the intrinsic rate or alternatively in the DDD mode as long as the AV delay can be set sufficiently long to inhibit ventricular pacing. Newer devices provide other techniques to check sensing thresholds. Some automatically check atrial and ventricular electrogram amplitudes on a regular basis. The results are available for review upon interrogation. Others require running a sensing protocol that automates the process described above. Finally, in certain devices, the intracardiac signal can be printed on calibrated paper that allows direct reading of the size of the sensed signal. With this technique, small variations may exist compared with the signal measured by the device due to differences in filters between the telemetry circuit and the sensing circuit. In patients with atrial electrical standstill or complete heart block without a viable escape mechanism, sensing thresholds will not be obtainable.

For unipolar systems, particularly in pacemaker-dependent patients, the possibility of myopotential inhibition of ventricular output should be investigated. At increasing ventricular sensitivities, the patient is asked to perform isometric exercise using the arm ipsilateral to the generator, such as pushing the hands together in front of the chest (Fig. 11.13). One should look for "noise" on the surface lead and electrogram with inappropriate sensing of R waves on the marker channel and ventricular pacing inhibition. Likewise, the possibility of myopotential triggering of ventricular pacing from atrial oversensing in a dual-chamber system should be evaluated by having the patient perform del-topectoral isometric exercises at increasing atrial sensitivities. Atrial oversensing

Myopotential Inhibering
Figure 11.13. Two-channel Holter (simultaneous V, and modified V5) showing symptomatic inhibition of pacing by myopotentials.

of myopotential or far-field R waves are frequent causes of inappropriate mode switching. In general, the chronic atrial and ventricular sensitivities settings should be set to a twofold to fourfold safety margin unless oversensing occurs (i.e., for an atrial sensing threshold of 2mV, a sensitivity setting of 0.5 to 1.0 mV would be appropriate).

The pacing threshold determination is an important feature of pacer follow-up evaluations because generator longevity may be significantly enhanced if the output can be programmed to the lowest value that will provide an adequate safety margin for pacing. Particular longevity may be obtained if outputs can be programmed to 2.5 V, i.e., less than the lithium-iodine battery voltage of 2.8 V At this output, the energy inefficient voltage doubling circuit can be avoided. Further decreases in output beyond this point provide diminishing energy savings. In practice, doubling the voltage threshold (at a pulse width of 0.4 or 0.5 milliseconds) or tripling the pulse width threshold (as long as less than 0.3 milliseconds) usually will provide an adequate safety margin. Energy consumption is directly proportional to the pulse width but increases with the square of voltage. If tripling the pulse width results in an interval less than 0.9 milliseconds, this is usually more energy efficient than doubling the voltage. Programming larger safety margins is typical immediately after implant to allow for the usual post-implant threshold increases. This process is almost always complete by 2 to 3 months allowing chronic thresholds to be programmed at that point. Programming larger safety margins should also be considered in the ventricular channel of pacemaker-dependent patients given the possibility of late unexpected threshold rises.32

Determination of pacing thresholds should be made for both chambers where applicable.33 In patients with intact AV conduction, determination of atrial stimulation threshold is easiest measured in the AAI mode with the pacing rate set 10 to 20bpm above the intrinsic rate. Atrial output (or pulse width) is progressively lowered until a QRS complex is "dropped" indicating loss of atrial capture. In patients with AV block, atrial capture must be measured in a DDD mode. The threshold may then be determined by noting the loss of a P wave on a surface tracing or the appearance of atrial sensed events on a marker channel or spontaneous atrial signal on an electrogram tracing. The latter two findings indicate return of spontaneous sinus node function after loss of atrial capture. Ventricular stimulation thresholds are most cleanly performed in VVI mode. DDD mode may also be used as an alternative particularly in patients who feel poorly with the loss of AV synchrony with VVI pacing. The AV delay must be set sufficiently short to "force" ventricular pacing and minimize fusion and pseudo-fusion which can obscure detection of loss of ventricular capture and result in falsely low perceived stimulation threshold. In pacemaker-dependent patients, loss of ventricular capture may result in asystole. Fortunately, most modern programmers permit rapid restoration of preprogrammed outputs with temporary pacing modes or automated threshold algorithms. The operator must remain vigilant though to terminate the test immediately after loss of capture is first noted.

Automaticity is a feature that has been applied to a number of pacemaker parameters to allow for device regulation without the need for continual clinician input. This is particularly the case with ventricular capture determinations. To date, auto-capture algorithms have been limited to the ventricle due to the relative ease of measuring the large evoked response with capture; however, devices capable of automatically measuring atrial capture are available as well. In some devices, the generator may be programmed to automatically determine the pacing threshold continuously or at periodic intervals and reset the output so as to ensure an adequate safety margin and simultaneously optimize device longevity. Certain devices which check capture on a beat-to-beat basis allow programming the output to as little as 0.25 V over the stimulation threshold. This option is being used increasingly to facilitate device follow-up.34

Programmability of polarity has become increasingly available in current pacemakers and, unfortunately, is not infrequently required. Problems with insulation defects in certain polyurethane leads subject to the "subclavian crush" syndrome, resulting in low impedance values, may be temporarily addressed by re-programming from bipolar to unipolar mode. This maneuver generally will increase the lead impedance in these situations and prevent loss of capture and possible undersensing, but will not prevent oversensing from make—break electrical transients arising from contact between the two conductors. Ultimately, lead replacement is required in the pacer-dependent patient. Occasionally, pacing or sensing thresholds will be significantly better in one polarity compared to the other. This may be helpful in the setting of marginal threshold values. General guidelines for chronic programming of common pacemaker parameters are given in Table 11.4.

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Responses

  • mandy
    How to check sensitivity threshold on transcutaneous pacemaker?
    1 year ago
  • WILLIAM
    What is sensing threshold measured in for pacemaker?
    11 months ago
  • denise driver
    How to check threshold pacing?
    11 months ago
  • Sebhat
    How to check pacemaker thresholds?
    11 months ago
  • lena
    Why is pacing threshold important to note?
    10 months ago
  • FRANZISKA
    What is pacing und sensing?
    9 months ago
  • Carambo
    How to set up atrial sensing ventricular pacing?
    9 months ago
  • Kevin
    What is sensing and capturing in pacing?
    8 months ago
  • sofia
    How to do a sensitivity threshold pacer?
    8 months ago
  • seredic
    What is ventricular pacing capture?
    7 months ago
  • Prisco
    What cause high atrial thresholds in pacemakers?
    7 months ago
  • estella
    Can you check atrial thresholds in DDD mode?
    6 months ago
  • linda
    How to determine the optimal pacing threshold for a pacemaker dependent patient?
    5 months ago
  • charley
    Is output and threshhikd thebsame in a oacer?
    5 months ago
  • tyler
    What is sensing in transcutaneous pacing?
    5 months ago
  • benito
    What are high sensitivity thresholds pacing?
    4 months ago
  • massawa
    What is it called when the atrial channel is sensing the ventricle?
    3 months ago
  • christian
    How to test pacing thresholds?
    2 months ago
  • Tanja Rosendahl
    How to check sensitivity on a external pacer?
    1 month ago
  • caramella
    Why does a sensing and v pacing happen?
    1 month ago
  • frederick
    What is optival pacing?
    28 days ago
  • helen
    What is sensing and captureing in heart problems?
    19 days ago
  • Yolanda
    Why are auto thresholds not measuring pacemaker?
    14 days ago
  • jarno
    How to capture threshold for pacing?
    12 days ago
  • klaus
    How to determine if a pacemaker threshold is adequate?
    2 days ago
  • Aleah
    When you are externally pacing a patient are you ventricular pacing or atrial?
    6 hours ago

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