Once a patient has a temporary pacemaker inserted, checking its threshold parameters and assuring proper functioning is the nurse's responsibility. To do this you need to understand the concepts and principles of cardiac pacing and how to adjust the settings on the pulse generator.
Let's start with cardiac pacing. A temporary pacemaker is typically indicated for patients with symptomatic bradycardias, such as slow escape rhythms, conduction defects, and heart blocks. The tip of the pacing catheter, or lead wire, sits against the inner wall of the right atrium, right ventricle, or both, depending upon the type of pacemaker placedatrial, ventricular, or AV sequential, for example. The proximal end of the catheter is attached to the pulse generator.
The pulse generator houses the pacemaker's controls and energy source. Electricity is delivered to the myocardium via the lead wire at a prescribed rate. Always check to see that the rate prescribed matches the rate set on the pulse generator.
The mode of pacing can be set on either demand or asynchronous. When it is set on demand, the pacemaker senses the heart's impulses, and paces only when the patient needs it. When it's in the asynchronous mode, the pacemaker fires at a fixed rate regardless of the heart's ability to generate impulses.
The ability of the myocardium to contract when stimulated by the pacemaker is called capture. It's determined by the amount of voltage delivered to the muscle measured in milliamps (mA). The stimulation threshold is the lowest amount of mA needed for capture.
Successful capture will show up on an EKG or telemetry as a spikea perfectly vertical linefollowed by a P wave if the atrium is paced; or by an R wave if the ventricle is paced. (See the image below).
Before determining the stimulation threshold, make sure the pacemaker is in the demand mode. Then, set the pacemaker rate 10 beats above the patient's own heart rate, or as high as needed to see 100% capturea pacer spike with every beat.
Next, slowly turn down the mA until 1:1 capture is lost. The EKG or telemetry strip will show spikes without complexes following them. Slowly turn up the mA until capture is regained. This is the stimulation threshold. The mA should be set two to three times higher than the stimulation threshold.
Delivering the beats
Sensing refers to the ability of the pacemaker to detect the heart's own electrical activity and deliver paced beats only when needed.
To determine if the pacemaker is sensing properly, set the rate 10 beats below the patient's inherent rate. (Note: This step is contraindicated if a patient has no intrinsic rate, or has symptomatic bradycardia.)
Reduce the mA to its lowest level (threshold). Then, turn the sensitivity dial counterclockwise toward a higher numerical setting. (The sensing light, which flashes when sensing, will stop.) The pacemaker is now less sensitive to the patient's heartbeat.
Now, turn the sensitivity dial clockwise (down) until the sensing light starts flashing again. This is the sensitivity threshold. Set the sensitivity, measured in millivolts (mV), at half the sensitivity threshold value obtained. Thus, if the sensitivity threshold is 4, you'll set the dial on 2 mV. Restore the mA and rate to their proper settings.
It's important to note that an atrioventricular (AV sequential) pulse generator may have dials for atrial settings on the left, and ventricular settings on the right. Although it operates like any other pacemaker, you won't need to determine the atrial sensitivity threshold, so there is no dial for this function.
How to troubleshoot
Checking the stimulation and sensitivity thresholds should be done at least daily if the pacemaker is in demand mode. It's unnecessary with the asynchronous mode, though, because this mode is usually reserved for those whose underlying rhythm is asystole.
Troubleshooting, however, applies to both the demand mode and the asynchronous mode and includes the following:
Failure to pace is confirmed when there's no pacer spikes shown on your EKG or telemetry strip.
Always check the batteries and replace them if necessary. (A good rule of thumb is to replace them every two to three days.) Then, check to make sure all connections are tight.
If you're still not seeing pacer spikes it may mean the pacemaker is oversensing. A pacemaker will sense, or pick up and interpret, muscle movement and artifact as heart beats if the sensitivity threshold is set too high, or there are items in the vicinity of the pacemaker that interfere with its signals, or the lead wire is fractured.
If oversensing is the likely cause of the failure to pace, lower the mV on the sensitivity dial, as it raises the sensitivity of the pacemaker, and remove items that could cause electromechanical interference such as electric razors, radios, or cautery devices. You'll need a chest X-ray to rule out a fractured lead wire.
However, not seeing pacer spikes can be normal, such as when the patient's heart is beating faster than the rate set on the pacemaker.
Failure to capture is confirmed when pacer spikes are seen on EKG and telemetry, but the spikes aren't followed by a ventricular or atrial complex. Again, always check the batteries and make sure all connections are tight. Then, try repositioning the patient onto his left side to regain capture, as the lead wire may be dislodged.
If you still don't see capture, the electrical output may be too low. In this case, increase the mA until you regain capture.
Another problem can occur when the pacemaker fails to sense the patient's intrinsic beats. Undersensing can be dangerous: Pacer spikes that fire indiscriminately throughout the cardiac cycle can trigger a lethal dysrhythmia if a spike falls on a T wave. In this case, lower the pacemaker's sensitivity to the heart by turning up the dial to a higher mV.
1. Diepenbrock, N. H. (2004). Quick reference to critical care (2nd ed.), (pp. 119 125). Philadelphia: Lippincott Williams & Wilkins.
2. Breuninger, C., Follin, S., et al. (Eds.). (2001). Handbook of nursing procedures (pp. 603 604). Springhouse, PA: Springhouse Corporation.
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