You are assessing a patient on the first post-operative day following an aortic valve replacement. There are atrial and ventricular epicardial wires in place and the patient is being paced at 90/min in a DDD mode.
a) Define sensitivity with respect to cardiac pacing. (20% marks)
b) Describe the steps that you would perform to check and set sensitivity. (30% marks)
c) In addition to checking sensitivity, what other daily checks would you perform in regards to the temporary pacing system? (20% marks)
d) The patient becomes completely pacemaker dependent and the pacemaker suddenly completely fails to pace.
List the actions you would perform to troubleshoot the pacemaker.
The ‘sensitivity’ (as numerically represented on the pacing generator) is the minimum current that the pacemaker is able to sense. A lower number thus corresponds to a greater sensitivity.
This is only checked when the patient has an intrinsic rhythm which affords some cardiovascular stability.
- Set the pacemaker rate 10 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 then 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
OR also acceptable-
- Gradually decrease sensitivity (with output set to low value and pacemaker set to 10 below intrinsic rate) and watch for sensing flash on box to disappear. Final setting is usually half this value.
- Underlying heart rate
- Capture threshold
- Appropriate pacing mode for the patient
- Impulse generator- battery
- (Maximum tracking rate, AV interval, post ventricular atrial refractory period (PVARP) should be noted, but do not require daily checks once set)
- Increase output to maximum (20mA)
- Select VOO (fixed ventricular pacing) to overcome inappropriate over sensing
- Check all leads/connections/batteries
- Replace impulse generator if faulty
- Consider reversing the polarity on the v wires
- Consider percutaneous lead through skin
Sensitivity of a pacemaker electrode is the minimum myocardial voltage required to be detected as a P wave or R wave, measured in mV. The lower the number, the higher the sensitivity, which leads to some confusion (when one asks to increase the sensitivity, one decreases the sensitivity mV value).
How to check and set sensitivity:
- Put the pacemaker in a VVI, AAI or DDD mode (i.e. endogenous cardiac activity should inhibit the pacemaker.)
- Set the output as low as possible; you don't want to have any R on T phenomena - you only need to see the pacing spikes.
- Change the rate to one which is much lower than the patients native rate
- Increase the sensitivity value until no cardiac activity is sensed
- Now, keep decreasing the sensitivity until the pacemaker senses every p-wave or QRS interval.
- This minimal sensitivity value is the sensitivity threshold.
- most of the time, you tend to leave the sensitivity turned down to half of the sensitivity threshold to ensure that the cardiac electrical activity will be sensed even if the electrode tip overgrows with filth.
- If you turn the sensitivity value down any more than that, you risk oversensing. Oversensing is described in greater detail elsewhere; briefly, it is an inappropriate inhibition of pacing in response to some sort of trivial non-cardiac signals, like the friendly hum of the nearby microwave.
Daily maintenance checks of the temporary pacemaker:
Care for the epicardial wires:
- Pacing wires must be dressed at least every 72 hours
- When handling epicardial pacing wires, gloves should always be worn to prevent microelectrocution
- Wires not in use should be rolled up in sterile gauze
- Wires in use should be securely taped to the patient's abdomen
Minimum daily box and wire assessment:
- Wound site assesment
- Pacing wire connection check
- Measure and document the wire length every nursing shift
- Note position of wires on daily CXR
- Check the impulse generator battery
Minimum daily paced patient assessment:
- 12-lead ECG
- Underlying rhythm and rate check (turn off the pacemaker for a few seconds)
- Sensitivity check
- Pacing threshold check
- Pacing mode review (is it appropriately selected?)
Occasional (non-daily) checks:
- Maximum tracking rate:the maximum atrial rate at which a pacemaker will deliver a ventricular pacing stimulus following each sensed atrial beat; i.e. if the atria are going at 130bpm and the pacemaker's MTR is set at 120, it will not pace any more frequently than 120.
- AV interval: the interval following a paced or sensed atrial beat allowed before a ventricular pacing impulse is delivered. I.e. how long the pacemaker waits until it decides that the beat was not conducted through the AV node.
- Post ventricular atrial refractory period (PVARP) is a pacemarker refractory period, intended primarily to prevent sensing of retrograde P waves; it prevents the pacemaker from sending another impulse too close to the last QRS, so as not to produce an R-on-T phenomenon.
Why is the pacemaker not pacing?
First, check the patient. Are they now in asystole, or otherwise haemodynamically compromised? If not, the pacemaker can be checked.
Start with the box.
- Is it even on?
- Is the battery dying?
- Are the wires detached from the pulse generator?
- Are the leads connected?
- Was the temporary pacing wire pulled out in course of a recent pressure area care?
- Are the epicardial electrodes displaced? Is the transvenous electrode tip wiggling uselessly in the venticle?
- Is there any weird twitching in the chest wall muscles of the patient? Is the ventilator demonstrating some bizarre sawtooth pattern, suggesting that the diaphragm is being paced?
Ok, so the hardware is intact. if there is output failure, its not because of the leads or the battery. Move on to the software.
First check the sensor threshold.
- Put the pacemaker in a VVI, AAI or DDD mode.
- Change the rate to one which is much lower than the patients native rate.
- Observe the sense indicator.
- Keep increasing the sensitivity.
- Find the sensitivity maximum - where the pacemaker is picking up NONE of the endogenous electrical activity.
- Now keep decreasing the sensitivity.
- Find the sensor threshold - where the sensor picks up EVERY endogenous electical event (i.e. no pacing spikes are visible)
Crank the sensitivity setting up to double the sensor threshold.
This should take care of oversensing as a cause of pacing failure.
Alternatively, as the college recommend, you can simply set the pacemaker in VOO mode.
No sensing - no oversensing.
Now, check the output threshold.
- Set the pacemaker well above the native rate.
- Start reducing the output.
- Find the capture threshold - where a QRS complex no longer follows each pacing spike.
Crank the output to double the capture threshold.
Still not working?
- Roll the patient to one side, and then another. Sometimes this influences the position of the transvenous pacing wire tip just enough to get you some capture.
- Reverse the leads. Sometimes this works, but logically - it shouldnt.
- Convert to unipolar pacing. Attach the negative lead to the positive electrode, and the negative lead to the subcutaneous tissue of the chest.
- Give up. Time to pace externally while waiting for another wire to be floated, or the epicardial leads to be resited.
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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.
Kirk, Malcolm. "Basic principles of pacing." All You Wanted to Know (2008): 1.
Hayes, David L., and Paul A. Levine. "Pacemaker timing cycles." Cardiac pacing and ICDs. Blackwell Publishing Malden (MA), 2002. 265-321.