Page 111 Acute Pain Management
P. 111
Opioid‐related adverse effects in surgical patients were associated with increased length of
stay in hospital and total hospital costs; the use of opioid‐sparing techniques can be cost‐
effective (Philip et al, 2002; Oderda et al, 2007 Level IV).
Respiratory depression
Respiratory depression (decreased central CO 2 responsiveness resulting in hypoventilation and
elevated PaCO 2 levels), the most feared side effect of opioids, can usually be avoided by
careful titration of the dose against effect. However, a variety of clinical indicators have been
used to indicate respiratory depression, and not all may be appropriate or accurate.
A number of studies investigating hypoxia in the postoperative period, in patients receiving
opioids for pain relief, have found that measurement of respiratory rate as an indicator of
respiratory depression may be of little value and that hypoxaemic episodes often occur in the
absence of a low respiratory rate (Catley et al, 1985 Level IV; Jones et al, 1990; Wheatley et al, 1990
Level IV; Kluger et al, 1992 Level IV). As respiratory depression is almost always preceded by
sedation, the best early clinical indicator is increasing sedation (Ready et al, 1988; Vila et al, 2005;
Macintyre & Schug, 2007).
Introduction of a numerical pain treatment algorithm in a cancer setting was followed by a
review of opioid‐related adverse reactions. Use of this algorithm, in which opioids were given
to patients in order to achieve satisfactory pain scores, resulted in a two‐fold increase in the
risk of respiratory depression (Vila et al, 2005 Level III‐3). Importantly, the authors noted that
respiratory depression was usually not accompanied by a decrease in respiratory rate. Of the
29 patients who developed respiratory depression (either before or after the introduction of CHAPTER 4
the algorithm), only 3 had a respiratory rates of <12 breaths/min but 27 (94%) had a
documented decrease in their level of consciousness (Vila et al, 2005 Level III‐3). This study
highlights the risk of titrating opioids to achieve a desirable pain score without appropriate
patient monitoring.
In a review of PCA, case reports of respiratory depression in patients with obstructive sleep
apnoea were examined (Macintyre & Coldrey, 2008). It would appear that the development of
respiratory depression may have been missed because of an apparent over‐reliance on the use
of respiratory rate as an indicator of respiratory depression; the significance of excessive
sedation was not recognised (see Section 11.5).
In an audit of 700 acute pain patients who received PCA for postoperative pain relief,
respiratory depression was defined as a respiratory rate of <10 breaths/min and/or a sedation
score of 2 (defined as ‘asleep but easily roused’) or more. Of the 13 patients (1.86%) reported
with respiratory depression, 11 had sedation scores of at least 2 and, in contrast to the
statements above, all had respiratory rates of <10 breaths/min (Shapiro et al, 2005 Level IV).
These studies confirm that assessment of sedation is a more reliable way of detecting opioid‐
induced respiratory depression, although monitoring respiratory rate is still important.
Checking a patient’s level of alertness was considered by the American Society of
Anesthesiologists (ASA) Task Force on Neuraxial Opioids to be important in the detection of
respiratory depression in patients given neuraxial opioids, as well as assessments of adequacy
of ventilation and oxygenation (Horlocker et al, 2009). However, it was noted only that ‘in cases
with other concerning signs, it is acceptable to awaken a sleeping patient to assess level of
consciousness’. In this situation it would be possible for increasing sedation and respiratory
depression to be missed if no attempt is made to rouse the patient.
A workshop convened by the Anesthesia Patient Safety Foundation to discuss this issue in
response to concerns about the safety of IV PCA, recommended ‘the use of continuous
monitoring of oxygenation (generally pulse oximetry) and ventilation in nonventilated
Acute pain management: scientific evidence 63
