Patient Controlled Analgesia (PCA) is most commonly used for intravenous opioid demand dosing although the principle can be applied to other treatment modalities e.g. epidural, oral, iontophoresis and intranasal. This article refers to intravenous Patient Controlled Analgesia.
- Some examples of the indications for the use of intravenous PCA are as follows:
- Post operative pain management
- Sickle Cell Crisis
Essentially any patient who is nil by mouth and is requiring consistent amounts of strong opioid analgesia.
In addition to effecting a minimum effect analgesic concentration (MEAC) the main advantages of PCA are as follows:
- Empowering patients to have some degree of control over their pain will in turn help to alleviate anxiety which will in turn reduce pain experience.
- It is immediate and effective and the patient does not have to wait to have analgesia administered.
- Extremely useful for incident pain e.g. dressing changes, physiotherapy
- The treatment regimen can be tailored to the needs of the individual
- Improved quality of recovery and care
- Decreased bed occupancy time with resultant economic benefit
More positive perception of hospital stay
However, it is not a concept that will suit all patients and the key to successful use is ensuring that the patient understands the principle, is compliant and that compliance is monitored. The following minority groups would be inappropriate for PCA:
- Those who are confused or have learning difficulties
- Those with poor manual dexterity
- The very young (< 5 years) should have Nurse Controlled Analgesia (NCA)
- The critically ill (NCA)
- The terminology relating to the concept of Patient Controlled Analgesia is defined below.
Loading dose- this is the total opioid dose, which is initially required to provide analgesia. It is administered either by pre-setting of the PCA pump and allowing automatic administration, or by nurse administration in the recovery setting. Most commonly it is the latter.
Bolus dose- this is the quantity of analgesia given to the patient at each self-administration demand. It is assumed that patients’ will demand doses of analgesic until pain has been relieved, but the size of the demand dose influences the patient’s perception of how effective the treatment has been. If the dose is too small the patient will fail to achieve adequate analgesia.
If the dose is too large the plasma concentration will gradually increase with repeated doses until it reaches a level causing excessive sedation and possibly respiratory depression. The optimal dose is the minimum dose to produce appreciable analgesia consistently without producing objective or subjective side effects. For morphine this has been quoted as 1 mg, for pethidine 10 mg for adults (, ) and for fentanyl 20 microgrammes .
Lockout interval- this is the time period between patient demands during which the machine will not administer a further dose despite any further demands made by the patient. The lockout interval is determined by the size of the bolus dose, the pharmacokinetics of the drug and the pharmacodynamics of that drug in the patient (in particular the length of time for the drug to reach peak plasma concentrations after intravenous bolus injection). For morphine, peak concentration after intravenous bolus is achieved after about 4 minutes. It would be inappropriate to set a lockout time shorter than this time to peak plasma concentration.
Background infusion- the basis of PCA means that the patient experiences pain before demanding relief or in anticipation of pain e.g. coughing or moving will use the PCA. If a drug with a short half-life is used analgesia is rapidly achieved but a high demand frequency is necessary. If the patient falls asleep they will frequently wake up in pain, which then requires several demands, depending on the length of the lockout interval, to attain analgesia.
These facts lend weight to the potential benefits of a background infusion . However, the addition of a constant rate background infusion of morphine neither improved the effectiveness of analgesia by PCA nor reduced the number of demands (, ) Studies with other opioids confirm this general principle, even when using short acting opioids. The addition of a background infusion can also reduce the inherent safety of PCA. The complete control by the patient has been removed and the patient will start to receive analgesia even if they do not need it. In particular, studies have shown that respiratory depression can occur when a background infusion is used (, ).