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276 P R I N C I P L E S A N D P R A C T I C E O F C R I T I C A L C A R E

1. Store current values in memory (for devices with memory). If rhythm difficulty is encountered
during testing, these original settings can be immediately re-established by depressing the
stored values mode selector.
2. Test for underlying rhythm. Gradually decrease rate in 10 beat/min steps until evidence of
underlying rhythm (ULR) emerges:
(a) If ULR present, observe whether sensing is now occurring.
(b) If still pacing at 50/min (no emergence of ULR), return to initial settings; do not continue to
test sensitivity or output thresholds.
(c) Document attempt and ULR less than 50/min.
If underlying rhythm is haemodynamically acceptable, continue.
3. Test sensitivity threshold. Having confirmed haemodynamically adequate underlying rhythm:
(a) Turn pacing rate to half the patient’s rate.
(b) Turn output to minimum (not off).
(NB: Sensitivity testing requires that failure to sense is created for a brief period, so steps 3a
and 3b are designed to minimise danger of arrhythmias.)
While observing the sense indicator on the pacemaker:
• decrease sensitivity (increasing the number) until failure to sense (the sense indicator
stops flashing — pacing indicator will now be flashing);
• increase sensitivity (decreasing the number) until sensing resumes;
• note the value at which sensing returns — this is the threshold value for sensing; and
• set sensitivity to half this value minus 1 mV.
4. Test output threshold (continuing from step 3 above the pacing will now be set at a low rate,
and at minimum output):
• Increase the pacing rate to 10 greater than underlying rhythm.
While watching the monitor:
• gradually increase the output until capture is achieved;
• note the value at which capture occurs — this is the threshold value; and
FIGURE 11.46 Routine temporary • set output to double this value plus 1 mA.
pacemaker testing protocol: underly- 5. Store new values in memory and document settings.
ing rhythm, output and sensitivity
threshold test.

use should be established before undertaking battery undertaken and to report any sensations of lightheaded-
replacement. ness, dyspnoea or other discomfort.
Pacemaker Function Testing Pacemaker testing in the unstable
Routine pacemaker performance checks should be under- pacemaker-dependent patient
taken regularly in the patient with a temporary pace- Greater caution must be applied in the testing of pace-
maker. Temporary pacing leads and wires are prone to maker functions if the patient has marked haemody-
movement and therefore to sensing and capture thresh- namic instability or has little or no underlying rhythm.
old variation. Variations may also be marked when there It is common for pacemaker testing to be avoided
is myocardial, biochemical and haemodynamic volatility altogether in such circumstances although this may be
as often seen in the critically ill patient. Pacemaker tests misguided. Routine testing of pacemaker function as
are performed to reveal the return of underlying rhythm described in Figure 11.46 may not be suitable, but testing
which may be being concealed by pacing, and to measure for underlying rhythm, and some level of testing of
thresholds for both capture and sensing, as these values capture threshold so as to be confident of safety margins
typically change with time and in response to changing is beneficial. For the patient with haemodynamic insta-
myocardial responsiveness. 54,58,62,68 Regular checking bility and/or inotrope use, testing for underlying rhythm
allows detection of threshold changes, and setting of becomes of even greater important as pacing may either
sensing and output safety margins, in order to minimise prevent or conceal the return of sinus rhythm capability,
the development of acute failure to capture or failure and cardiac output may be as much as 50% greater with
to sense.
the atrial kick of sinus rhythm than during pacing (see
The practices employed to test temporary pacemakers Figure 11.47). It may take several seconds for the sinus
vary widely across Australia, as do attitudes to whether node to ‘warm up’ and express itself, so decrease the rate
this may or may not be undertaken by nurses. The sample gradually and only to reasonable levels (sinus rates of less
protocol shown in Figure 11.46 provides an organised than 50 are unlikely to be beneficial). Be sure to gain
approach to testing during which safety has been empha- agreement from the multi-disciplinary team before
sised. Because of the varying attitudes to nursing respon- undertaking testing in this context.
sibilities, the use of this approach should be ratified at
individual institutions before use. Threshold testing in the pacemaker-dependent patient is
also contentious as loss of capture during testing may be
Testing pacemaker thresholds is performed daily or on poorly tolerated. If the capture threshold is not measured,
each shift, but not if the patient is unstable, using the however, a rising threshold and loss of safety margins
steps described in Figure 11.46. The test should be carried cannot be identified, and may only become apparent
out promptly, with attention to avoiding undue bradycar- upon development of acute failure to capture, possibly
dia or periods of asynchronous pacing. The patient with outputs already set to maximum and therefore no
should be advised that pacemaker assessment is being scope for recovering capture. An alternative approach to