Bolus Dose and Lockout Interval

To a great extent, the success of IV PCA depends on prescribing an adequate bolus dose that can be self-administered frequently enough for patients to manage their pain effectively. Small doses of analgesia (e.g., 1 mg of morphine or equivalent) and short lockout (delay) intervals (e.g., 5 to 10 minutes) are best for opioid-naïve patients so as to prevent excessive sedation at peaks (the highest blood levels of opioid) and to prevent breakthrough pain at troughs (the lowest blood levels of opioid); however, if the dose is too small, patients may have difficulty maintaining analgesia (Macintyre, Coldrey, 2008). Larger bolus doses at lockout intervals of 15 to 30 minutes are commonly required and are usually well tolerated by opioid-tolerant patients with cancer or persistent noncancer pain. The optimal dose should provide consistent, satisfactory analgesia without excessive or dangerous adverse effects (Macintyre, Coldrey, 2008).

There is surprisingly little research to guide selection of an optimal PCA bolus dose for an opioid-naïve patient. An early randomized controlled trial found comparable efficacy, morphine consumption, and adverse effects with PCA doses of 1 mg every 6 minutes, 1.5 mg every 9 minutes, and 2 mg every 12 minutes (Badner, Doyle, Smith, et al., 1996). PCA attempts, successful injections, missed injections, and dose adjustments were greatest in the group receiving 1 mg; however, one patient in the group receiving 1.5 mg and one in the group receiving 2 mg required naloxone for respiratory depression. The researchers concluded that 1 mg every 6 minutes represents appropriate titration but may result in lower patient satisfaction. Another study concluded similarly that 0.5 mg of IV morphine was not large enough to control pain, and 2 mg was associated with adverse effects, but 1 mg was an optimal bolus dose for postoperative pain in opioid-naïve patients (Owen, Plummer, Armstrong, et al., 1989). A randomized controlled study of IV fentanyl use during burn dressing changes showed that PCA bolus doses of 30 to 40 mcg of fentanyl with a lockout of 5 minutes produced better analgesia and demand/delivery ratios than did 10-mcg and 20-mcg bolus doses (Prakash, Fatima, Pawar, 2004). No research could be found on the optimal PCA bolus dose or lockout interval for hydromorphone, the other opioid commonly used for IV PCA; however, 0.1 to 0.2 mg of IV hydromorphone is considered roughly equal to 1 mg of IV morphine in this setting (Knotkova, Fine, Portenoy, 2009).

There is also very little research on the optimal lockout (delay) interval for IV PCA (Macintyre, Coldrey, 2008). The length of the lockout interval should allow for adequate analgesic coverage during times when patients need the most opioid. For example, postoperative patients should be able to activate PCA before and frequently during potentially painful activities, such as ambulation, self-care, and physical therapy or respiratory therapy treatments. The lockout interval also should be long enough for a patient to appreciate the effect of one bolus before self-administering another (Macintyre, Coldrey, 2008). The characteristics of the opioid, such as the onset and peak times, are primary determinants of the length of the lockout interval. The American Pain Society (APS, 2003) recommends a lockout interval of between 5 and 10 minutes for most of the IV opioids used for acute pain. For example, commonly used lockout intervals are 5 to 6 minutes for IV fentanyl and 6 to 8 minutes for IV PCA morphine and hydromorphone for acute pain management.

The customary adult starting dose should be reduced by 25% to 50% for opioid-naïve older patients because research has shown that analgesia requirements decrease with increasing age (Gagliese, Jackson, Ritvo, et al., 2000; Macintyre, Coldrey, 2009). In a review of more than 6000 patients aged 65 and older who had received IV PCA, bolus doses greater than 1 mg/dose and intraabdominal surgery were cited as risk factors for hypoxemia and respiratory depression during IV PCA (Sidebotham, Dijkhuizen, Schug, 1997). Also, the lockout interval is sometimes increased to 10 minutes in this population.

Hour Limit

The hour limit is the maximum amount of opioid a patient has access to in an hour-limit time period. Most PCA pumps can be programmed for a 1-hour or 4-hour limit, and some pumps allow this parameter to be bypassed altogether. If an hour limit has been programmed and the patient consumes the programmed amount before the time period has expired, the pump denies the patient any further PCA bolus doses until the next hour-limit time period begins. The programmed hour limit includes the number of PCA bolus doses and the basal rate (if one is programmed). Some PCA pumps also include clinician-administered bolus doses in the hour-limit amount.

There is no consensus about whether hour limits should be used. Those who choose not to use them do so because they think patients should have access to as much opioid as they need to manage their pain, restricted only by the amount of the dose and the length of the lockout interval. Those who choose to use an hour limit do so because they think it offers an additional safeguard against overdosing; however, this thinking may provide false reassurance; the only built-in safeguard of PCA is patient-only use (see discussion of PCA by proxy later in this chapter). A benefit of programming an hour limit is that it alerts caregivers that increased doses might be necessary. Hour limits seem to be used more commonly in opioid-naïve than in opioid-tolerant patients. If used, a 1-hour limit is preferable to a 4-hour limit for at least two reasons: (1) caregivers are alerted to the need for an increase in the opioid dose sooner (i.e., within 1 hour instead of within 4 hours); and (2) it eliminates a scenario in which an increase in dose is neglected in patients who use the 4-hour amount in less than 4 hours (i.e., a patient who uses the 4-hour amount in 2 hours and is left for 2 hours without analgesia). In an analysis of data submitted to MEDMARX and the United States Pharmacopeia (USP) Medication Errors Reporting Program between 1998 and 2003, the USP identified this scenario as a common PCA-related underdosing error (USP, 2004). It is essential that the hour limit be adjusted up or down as necessary on the basis of patient response. If used, the APS (2003) recommends that the hour limit be set at 3 to 5 times the projected hourly IV requirement for at least the first 24 hours.