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reduction in morphine requirements (Jeffs et al, 2002 Level II). A combination of
dexmedetomidine and morphine resulted in significantly better pain relief, a lower incidence
of nausea but not vomiting, and significant opioid‐sparing compared with morphine alone
(Lin et al, 2009 Level II).
Magnesium added to morphine was opioid‐sparing and led to better pain relief (Unlugenc et al,
2003 Level II); added to tramadol it was opioid‐sparing but only provided better pain relief for
the first 2 hours (Unlugenc et al, 2002 Level II).
7.1.3 Program parameters for intravenous PCA

Bolus dose
While the optimal sized bolus dose should provide good pain relief with minimal side effects,
there are only limited data available concerning the effects of various dose sizes. In patients
prescribed 0.5 mg, 1 mg and 2 mg bolus doses of morphine, most of those who were
prescribed 0.5 mg were unable to achieve adequate analgesia, while a high incidence of
respiratory depression was reported in those who received 2 mg (Owen, Plummer et al, 1989
Level II). It was concluded that the optimal PCA bolus dose for morphine was therefore 1 mg.
Similarly, in patients prescribed 20, 40 or 60 mcg bolus doses of fentanyl, the larger dose was
associated with an increased risk of respiratory depression and a conclusion was made that
the optimal dose of fentanyl for use in PCA was 40 mcg (Camu et al, 1998 Level II). However in
this study, each dose was infused over 10 minutes, which could alter the effect of that dose.
Four different demand doses of fentanyl (10, 20, 30 and 40 mcg) were assessed for the
management of pain during changes of burns dressings. Pain relief was significantly better
with the 30 mcg and 40 mcg doses; no patient became sedated or experienced nausea and
vomiting (Prakash et al, 2004 Level II).
Rigid adherence to an ‘optimal’ dose may not, however, lead to the best pain relief for all
patients. If the prescribed dose is not ‘optimal’ and not too small, the patient will be able
to compensate to some degree by changing their demand rate. However, they will only
compensate to a certain degree. Even if uncomfortable, patients may only average four CHAPTER 7
demands per hour, even though they could press the PCA button more frequently (Owen,
Plummer et al, 1989 Level II).
Initial orders for bolus doses should take into account factors such as a history of prior opioid
use (see Section 11.7) and patient age (Macintyre & Schug, 2007; Macintyre & Coldrey, 2008);
PCA morphine requirements are known to decrease as patient age increases (Macintyre & Jarvis,
1996 Level IV; Gagliese et al, 2008 Level IV). Subsequent bolus doses may require adjustment
according to patient pain reports or the onset of any side effects. Even though the length of
the lockout interval could allow it, patients may not increase their demand rate enough to
compensate for bolus doses that are too small (Owen, Plummer et al, 1989 Level II).
The number of demands a patient makes, including the number of ‘unsuccessful’ demands, is
often used as an indication that the patient is in pain and as a guide to adjusting the size of the
bolus dose. However, there may be a number of reasons for a high demand rate other than
pain. For example, excessive PCA demands may correlate with anxiety, poor perioperative
adaptation to surgery involving avoidance behaviour and intrusive thoughts, as well as high
pain scores (Katz et al, 2008 Level IV). See also Section 1.2.4 for additional information on the
relationship between pain relief and psychological factors in PCA.
Lockout interval
The lockout interval is a safety mechanism that limits the frequency of demands made by the
patient. For maximum safety it should be long enough to allow the patient to feel the full

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