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PACING IN CARDIAC SURGERY

     

       Temporary Dual Chamber Sequential Pacing Box                 Temporary Single Chamber Pacing Box

Pace Medical Inc. website: http://www.pacemedicalinc.com/DesktopDefault.aspx

Temporary Epicardial Pacing:

Both single chamber and dual chamber sequential pacing are routinely used in the postoperative period.

Temporary epicardial pacing wires are inserted onto the outer surface of the atria or the ventricles during cardiac surgery. The pacing end of the wire is looped through or closely sutured to the epicardial surface of the atria or ventricle and the other end is pulled through the chest wall, sutured to the skin and attached to an external pulse generator or pacing box. (As a rule wires exiting the skin on the right sub costal area are atrial wires and those exiting on the left are ventricular wires).

Temporary epicardial pacing is used:
1. For conduction defects and for the prevention of atrial arrhythmias especially atrial fibrillation.
2. To replace the hearts natural pacemaker postoperatively to preserve cardiac output when it fails to meet physiologic demands.


Pacemakers are classified with a universal five-letter code. This describes the expected functions of the device according to the site of the pacing electrodes and the mode of pacing.

The first letter describes the chamber that is paced.
The second letter describes the chamber that is sensed.
The third letter describes the pacemaker’s response to sensing of intrinsic electrical activity.
The fourth and fifth letters describe additional functions that are not used in temporary pacemakers and are only used in implantable permanent pacemakers.


Chamber paced          Chamber sensed         Response to Sensing         Programmability         Antiarrhythmia functions

1.Chamber paced:                       O= NONE        A=ATRIUM             V= VENTRICLE           D= DUAL

2. Chamber sensed:                    O= NONE        A= ATRIUM            V= VENTRICLE           D= DUAL

3. Response to Sensing:            O= NONE        I= INHIBITED         T= TRIGGERED         D= DUAL

4. Programmability :                   O= NONE                 P= SIMPLE PROGRAMMABLE                    M= MULTIProgrammable             

                                                         C=COMMUNICATING       R= RATE MODULATION

5. Antiarrhythmia functions:     O= NONE                 P= PACING                  S= SHOCK              D= DUAL


Connecting a patient to a pacing box

1. A senior staff member performs or supervise the procedure.
2. Check all equipment in bed area prior to receiving a patient from theatre
3. Obtain a pacing box either single chamber (figure 2) or dual chamber (figure 3) depending on patients needs.
4. Check box is not damaged and that battery is charged, and that all leads are intact to the box
5. Carefully identify wires on patient as atrial or ventricular and connect to appropriate terminals on pacing box. (R= Atrial, L= Ventricular)
6. Ensure all connections are tight and wires are secured to skin with gauze swabs and tape (see section on disconnection and dressing pacing wires)
7. Switch box on if required immediately and follow guideline of setting up your pacing box.
Setting up a pacing box

1. Ensure box and leads are in working order and connections are tight
2. Switch box on
For a single chamber pacing box this is the “x1” setting
For a dual chamber pacing box this is the “on” position

3. If dual chamber sequential pacing is required then set the mode of pacing
4. Select rate on pacing box
5. Set the output at
10mA for atrial pacing and
5mA for ventricular pacing, then determine the stimulation threshold
6. Set the sensitivity at
The lowest possible number for atrial pacing and
2mV for ventricular pacing.
When the patient requires only back up demand pacing the sensitivity threshold may then be checked. Note the patient must be in his or her own rhythm for this test.
7. With a dual chamber sequential box, set the box to the lock position (switch on left hand side of box) so that accidental key changes are avoided.



Disconnection from a pacing box and dressing pacing wires

Disconnection from a pacing box may only occur if the patient has not required pacing in the last 24 hours.

1. Unscrew pacing wires from connections
2. The exposed metal end of the pacing wire is a direct route for electrical current from the environment to conduct electricity to the heart, so it is very important to insulate the wires to prevent cardiac arrhythmias especially VF.
3. Insulate the exposed metal end of the pacing wires with transpore tape as shown in figure 4
4. Wrap insulated wires around 2 pieces of folded gauze swab and secure to patients chest with transpore tape or a primapore dressing.
5. The exit site should be clean and covered. Wires are easily dislodged so care must be taken when handling them not to pull them out.
6. Leave the pacing box by the patient’s bed for a further 24 hours just in case they require pacing again.
7. When showering, pacing wires must be covered with an adequate waterproof dressing. Tegaderm is the preferred choice. When patients have completed showering this may be taken off and left dressed as above.

Testing the stimulation threshold
The stimulation threshold is defined as the minimum amount of energy required to capture or to depolarise the heart consistently. The stimulation threshold should be checked at least once every day when a patient is requiring pacing. Scar tissue can form at the attachment site on the surface of the heart with continuous pacing, and this tissue will need more energy to conduct an impulse through the heart. This is why we test the threshold and then double the voltage to ensure capture of the heart muscle.

1. Check that the patient is in a paced rhythm
2. Gently decrease the output voltage while watching the patients monitor until capture is lost, and then turn the voltage up until capture is once again seen. This is the stimulation threshold.
3. Now double this number to ensure capture.
Testing the sensitivity threshold
The sensing circuit controls how sensitive the pacemaker is to intrinsic cardiac depolarisations. Intrinsic activity is measured in (mV), and the higher the number the larger the signal. A fence analogy is used to help explain sensitivity. When pacemaker sensitivity needs to be increased to make the pacemaker “see” smaller signals, the sensitivity number must be decreased. So if the sensitivity were a fence standing between the pacemaker and what it wants to see such as the ventricular QRS signals, the fence would need to be lowered if it was too high. For example if the fence was 10 metres high (a 10mV sensitivity) then the pacemaker would not be able to see what the ventricle is doing, so if the fence was lowered to 2 metres (2mV sensitivity) then the pacemaker may be able to “see” the intrinsic depolarisations. Therefore when we increase the sensitivity of the pacemaker we decrease the number of mV (bring down the fence).
1. Check that the patient is in his or her own rhythm and the pacemaker is in sense mode.
2. Slowly decrease the sensitivity by turning the number up.
3. When there are pacing spikes in competition with the intrinsic rhythm or the sensing indicator on the box stops flashing, this is the threshold.
4. Set the sensitivity at half the threshold, e.g. if the threshold was 4mV set it to 2mV.


Changing The Batteries

It is recommended to change the pacing box rather than the batteries on a pacing dependent patient. The PACE dual chamber pacing box is designed with two batteries and will run with just one in situ. You can remove one battery with the box still functioning replace it and then remove the other battery for replacement. Pacing is maintained throughout This allows almost continual pacing of the patient, as you do not have to wait for the pacing box to turn on again and complete it’s self-test. You must already have a box standing by on the patient’s settings.
1. Obtain a new pacing box and ensure battery is charged.
2. Switch new box on and set to the same parameters as the existing box
3. Loosen the connections on the NEW box in readiness for the cables.
4. Unwrap wires and expose connection points.
5. Ensure that the patient is appropriately monitored.
6. Unplug the cable from the existing box and place in the new charged pacing box. The patient should not have dropped more than 2 or 3 complexes and should be pacing again adequately.

Note:
If there are no replacement pacing boxes available to complete this procedure then the batteries may be changed manually very quickly. The dual chamber pacing box should continue to pace on only one battery (providing that there is enough charge) and this can aid the battery changing procedure. You can change one battery at a time but always make sure that both batteries are changed. However in the single chamber pacing boxes there is only one battery to change and so it has to be done quickly if the patient is being paced.

Changing batteries one at atime should be standard practice but to have a second box with the current setting already programmed ready as well as your plan B



Evaluating pacemaker function

Evaluating pacemaker function enables staff members to trouble shoot the pacing box. There are three main problems that can occur in temporary pacing, they are loss of capture, under sensing and over sensing. .


Loss of capture is defined as the presence of pacing spikes that are not followed by paced complexes. To troubleshoot loss of capture:

1. Pacing wire not contacting
Check all wires and connections for tightness
2. The battery may be low
Check the indicator and change the battery
3. Inadequate signal strength
You need to increase the electrical output on the box


Under sensing is when the pacemaker fails to sense intrinsic activity that is present. To troubleshoot under sensing:

1. There is a break in the connections
Again check all connections and change the battery if low
2. The pacemaker may be in fixed rate mode
Change the box to demand mode
3. Intrinsic QRS voltage is to low to sense
Increase the sensitivity by reducing the number


Over sensing is when the pacemaker is set so sensitive that it inappropriately senses internal or external signals as QRS complexes and inhibits pacemaker output. To troubleshoot over sensing:

1. Sensitivity is set to high
Reduce the sensitivity by increasing the number
2. Electromagnetic interference may be caused by equipment near to the pacemaker
3. There are also internal sources of interference such as large P or T waves



Other complications of pacing

R on T phenomenon
The pacemaker discharges during ventricular repolarisation
Fusion beats
When an intrinsic depolarisation and a pacing stimulus occur at the same time
Pacemaker syndrome
Adverse clinical signs due to inappropriate timing of AV contraction
Pacemaker mediated tachycardia
A tachycardia induced by competition between the pacemaker and the intrinsic rhythm, sustained by the continued participation of the pacemaker.
Infection of the pacing wires or of the exit site
The presence of pacing wires in the body is an infection risk
Cardiac tamponade
Can occur in removal of pacing wires
Arrhythmias
Any direct manipulation of cardiac tissue can cause arrhythmias. The most likely of which will be atrial ectopic beats, which can lead to atrial fibrillation or flutter
Bleeding
Can occur on removal of pacing wires
Pain
The patient may experience pain on removal of pacing wires. A pulling sensation in the chest be felt and they may experience a sharp pain in either shoulder tip due to the wires touching the phrenic nerve on the way out of the body



Removing Pacing Wires
Pacing wires are only removed after discussion with the surgical team. They should only be removed during normal operating theatre hours.
Before removal:
• The patient should be in normal sinus rhythm or in a controlled atrial fibrillation.
• A 12 lead ECG is performed before the pacing wires are removed
• If the patient is taking warfarin, ensure their therapeutic INR is less than 2.5
• If the patient is on intravenous heparin for a sub therapeutic INR then it should be stopped 2 hours prior to removal, then started 1-hour post removal.
The pacing wires are removed by the nurse caring for the patient
• Atrial wires are removed first with a slow constant traction and then the ventricular wires in the same fashion.
• If the nurse is unable to remove wires he or she will notify a senior staff nurse or surgeon
• The patient is monitored closely during and after the removal of the wires
• The patient must have bed rest for 1-hour post procedure.
• Record on care pathway that wires have been removed.



Electrical safety

Temporary pacing wires provide a direct route for current to the heart bypassing the protective resistance of the skin. A very small electrical current is enough to cause an arrhythmia such as atrial or ventricular fibrillation. Some considerations for electrical safety are outlined below:

• Wear gloves when handling wires
• Make sure all connections are tight
• Cover exposed metal ends of pacing wires that are not in use with insulating tape, and wrap around gauze
• Keep dressings dry
• Make sure all other electrical equipment in the room is earthed and in good working order