Question 29

College Answer

a)

• This is the minimum amount of current (in mA) required to initiate depolarization of the paced chamber.

• Set rate paced rate 10 above HR (to ensure patient’s rhythm is over ridden minimizing risk of R on T) and set to full demand mode (i.e. high sensitivity- low mV) and output current of 5mA.

• Decrease output until 1:1 capture is lost.

• Slowly increase output till 1:1 capture is regained. This is the pacing (stimulation) threshold.
• The final setting is usually double the pacing threshold.

b)

• This is only checked when the patient has an intrinsic rhythm which affords some cardiovascular stability.
• Set the pacemaker rate10 below patients intrinsic rate
• Set the output to a very low value e.g., 0.1mA

• Setting the pacemaker to asynchronous mode by turning the sensitivity to its lowest value (highest mV setting) AFTER setting the output current to its lowest value (e.g. 0.1mA) so as to not capture but trigger the pacing indicator.

• The sensitivity is the gradually increased (lower mV) until the pacemaker senses the patients intrinsic HR and the pacing indicator no longer illuminates but the sensing indicator does.

• This is the sensing threshold.
• The final setting is usually half this determined value.

c)

• This tests the ability of the pacemaker to sense the patient’s intrinsic cardiac activity when one is present so that the pacemaker does not deliver an inappropriate stimulus in competition with the patient intrinsic rate when it is functioning in demand mode.

• Prevents R on T phenomena

d)

• Poor wire placement, movement of the wire
• Acid Base abnormalities,
• Hypo or Hyperkalaemia,
• Hyperglycaemia,
• Drugs e.g. B blockers, calcium antagonists.

Discussion

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.

• Wire malposition
• Electrolyte derangement
• Hypoglycaemia
• Antiarrhytmics

"Reversible" is the key. No marks would be given to the man who mentions fibrin deposition.

Reade, M. C. "Temporary epicardial pacing after cardiac surgery: a practical review." Anaesthesia 62.3 (2007): 264-271.

Reade, M. C. "Temporary epicardial pacing after cardiac surgery: a practical review: Part 2: Selection of epicardial pacing modes and troubleshooting."ANAESTHESIA-LONDON- 62.4 (2007): 364.

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.