A patient in the Intensive Care Unit develops complete heart block with hypotension and has a temporary transvenous pacing wire inserted.
b) Define the pacing threshold and describe how you would test and set it.
c) Describe how you would check the pacing sensitivity.
c) What is the purpose of setting the pacing sensitivity?
The bedside nurse informs you that the output has been increased markedly over the course of his shift to maintain capture.
d) What reversible factors might cause this problem?
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b)
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d)
In general, the details of sensitivity and output settings of the temporary pacemaker are discussed in the section concerned with mechanical haemodynamic support. In order to simplify revision and to reduce the total volume of reading, a brief guide to troubleshooting the pacemaker circuit is also available.
b) Define the pacing threshold and describe how you would test and set it.
The college answer is sufficiently succinct. "This is the minimum amount of current (in mA) required to initiate depolarization of the paced chamber". It is otherwise known as the capture threshold.
Basically, you set the pacing rate to higher than the patient's native rate, and then down-titrate the current of the output until you lose capture. The point at which you barely capture is the pacing threshold. You may want to set the current to somewhere above this (say, double), to ensure capture.
c) Describe how you would check the pacing sensitivity.
Pacing sensitivity is checked by adjusting the sensitivity setting up, and then down. First, you need to set the pacemaker at a heart rate lower than the native rate. Then, increase the sensitivity setting until the pacemaker no longer senses (and starts to pace asynchronously). Then, you adjust it down (until no pacing occurs whatsoever). This lowermost sensitivity is the sensor threshold.
c) What is the purpose of setting the pacing sensitivity?
Well, if the pacemaker is sensing appropriately, it wont compete with the native pacemakers, and you wont get R on T phenomena.
The bedside nurse informs you that the output has been increased markedly over the course of his shift to maintain capture.
d) What reversible factors might cause this problem?
There are numerous reasons why the pacemaker might need higher current to capture.
The college answer is appropriate and brief.
"Reversible" is the key. No marks would be given to the man who mentions fibrin deposition.
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Gammage, Michael D. "Temporary cardiac pacing." Heart 83.6 (2000): 715-720.
Sanders, Richard S. "The Pulse Generator." Cardiac Pacing for the Clinician. Springer US, 2008. 47-71.