41. PAIN MANAGEMENT

Pain control is critical to prevent physiological and psychological problems. Pain causes an increase in the sympathetic response leading to increases in the heart rate, cardiac work and oxygen consumption. Uncontrolled pain can cause cardiac ischaemia. Pain prevents the patient from being active, which may cause slow circulation, deep venous thombosis and pulmonary embolus. Operations, especially on the thorax and upper abdomen will cause poor respiratory ventilation. The patient will be unable to cough, leading to atelectasis and pneumonia. Pain can slow gut movement and may cause gastric ulceration. Untreated pain may also lead to anxiety, agitation, urinary retention, nausea and vomiting and chonic pain syndromes. The anaesthetist must ensure perioperative pain is eliminated or reduced in every patient with a minimum of side-effects.

Measurement

Pain is a subjective sensation. However it should be quantified in order to estimate how effective the pain management is.

Pain should be measured both at rest and during activity (for example taking two large breaths). This is because increasing activity is one of the goals of pain management.

One of the easiest scales to record pain is the Visual Analogue Scale (VAS). This is a 10cm ruler. The patient indicates where their pain lies on the scale. More commonly, a verbal report of a patient’s pain is recorded. Zero is no pain and 10 is the maximum pain ever experienced by that person. The anaesthetist should aim to provide analgesia so that a patient can move freely in the bed with a pain score less than 3.

Visual analogue Scale (VAS)

No pain                                                                                    Worst pain

   0        1        2        3       4        5        6        7        8        9       10

The patient’s pain should be assessed in the recovery room and the patient should not be sent to the ward until the patient’s pain is less than 3. On the ward, the nurses should continue to assess the patient’s pain. Ideally, an anaesthetist should be available to check each patient with moderate to severe pain each day and to educate hospital staff in pain management.

If a patient has unexpected intense pain associated with a change in their vital signs (e.g. hypotension, tachycardia, fever) the patient may have a complication of the surgery and should be reviewed by the surgical team.

There is a paediatric pain assessment tool called the Wong-Baker faces scale.

From Wong D.L., Hockenberry-Eaton M., Wilson D., Winkelstein M.L., Schwartz P.: Wong's Essentials of Pediatric Nursing, ed. 6, St. Louis, 2001, p. 1301. Copyrighted by Mosby, Inc. Reprinted by permission.

Pathophysiology

Following injury, there is increased responsiveness around the injured area. This is known as peripheral sensitisation. Pain receptors (A delta and C nerve fibres) are stimulated causing the sensation of pain. When there is tissue damage or prolonged stimulation, the pain receptors become more responsive and the original stimulus is amplified.

The inflammatory response due to tissue damage is the release of intracellular contents from damaged cells and inflammatory cells such as macrophages, lymphocytes and mast cells. Nerves also produce peptides that enhance the activity of the sympathetic nerves, causing vasodilatation and the leaking of plasma proteins into the tissues.

The combination of these peptides and chemical mediators such as substance P and products of arachidonic acid metabolism, further sensitise the pain receptors.

Some of these chemicals make good targets for pain relief. For example, non-steroidal anti-inflammatory drugs (NSAID) are a useful component of drug treatment for acute pain. They block the effects of arachidonic acid production.

If the initial injury is extensive or prolonged, there is an increase in pain transmission activity at the level of the spinal cord. This is called central sensitisation. Clinically, the patient will describe pain to light touch in the affected area. This is called allodynia. Also, the patient will show hyperalgesia, which is more pain than would be expected from a painful stimulus. The painful area may also be larger than expected for the type of injury.

With time, there are more permanent anatomical and functional changes in the nervous system.

Central sensitisation indicates that the pain treatment should become more complex. Chonic pain is a result of central sensitisation.

Pain transmission can be altered at several points. For example, peripheral (NSAID, local anaesthetics, morphine), spinal cord (ketamine, morphine, local anaesthetics) or central nervous system (morphine, benzodiazepines).

Pain Management

The choice of pain management will depend on the patient, the surgery, the anaesthetist and the available equipment, drugs and staff.

Operations on the thorax and upper abdomen may be more painful than operations on the lower abdomen, which, in turn, are more painful than operations on the limbs. However any operation involving the body, large joint, deep tissues or a large area should be treated as being moderately to severely painful.

Patients have different expectations of pain after surgery and pain thesholds. Always ask the patient if they have pain. Some patients may suffer in silence. Patients who are scared or anxious may have more pain. Patients may have had poor pain treatment in the past. During the preoperative assessment the anaesthetist should warn the patient of what pain to expect and tell the patient how the pain can be treated.

The current practice in pain management is to combine more than one analgesic agent. This is known as a multi-modal approach. The aim is to reduce the doses and side-effects of each drug and treat the pain at more than one part of the pain pathway. Treating moderate to severe pain with only one analgesic may not be effective. (There are many reasons why treating pain only with intramuscular morphine or pethidine may be ineffective. For example, the effect of morphine varies between patients, side-effects may limit the dose, doctors and nurses may have fears about respiratory depression, there may be reduced blood supply to the muscle and there may be a delay in giving the dose).

Pain is harmful. The anaesthetist must try to reduce pain with all available drugs. The main problems with acute postoperative pain management are a failure to regularly assess the patient’s pain (VAS), a failure to understand the variability between patients, a failure to use adequate doses of opioids (morphine) and a failure to use a multimodal approach. 

More complex techniques of analgesia, for example epidural opioids and patient controlled analgesia, can give better patient satisfaction but simple analgesic techniques, for example regular opioid, paracetamol and NSAID, does not cause an increase in morbidity or mortality.

A good pain service needs education of patients, nurses and doctors. Guidelines should be written.

Preoperative Pain Management

Patients with pain should be treated preoperatively. Oral analgesics can be given with a small amount of water to patients who are fasting. Alternatively, for more severe pain, the patient may receive an intramuscular or intravenous opioid.

All adults can be given an oral loading dose of paracetamol (at least 1 g). Children may be best given a loading dose (30 mg/kg) intra-operatively in suppository form or as an oral premedication (20 mg/kg).

Intra-operative Management

Multi-modal analgesia is the best approach for moderate to severe pain. Single analgesics are adequate for minor surgery only.

A good combination of drugs for adults is local anaesthetic (infiltration or regional blocks), paracetamol (1 g oral premedication or intra-operative suppository), NSAID (injectable or suppository) and intravenous opioid. Inhalation induction agents (e.g. halothane or sevoflurane) and intravenous induction agents (e.g. thiopentone, propofol) are not analgesics. Antihistamines and benzodiazepines will aid sedation but are not analgesics.

Postoperative Management

In recovery, patients should receive small doses of opioids until pain control is adequate. For example, 2 mg morphine IV every 5 minutes.

The same drugs used intraopertaively for adult pain control may be continued postoperatively. Again, multi-modal management is required until pain is minimal. For example, an open cholecystectomy may require thee analgesics for up to one week after surgery. Pain must be assessed.

Analgesics need to be given by an appropriate route (oral, subcutaneous, intramuscular, intravenous, rectal, sublingual, transdermal or epidural).

Paracetamol can be continued 1 g four times a day (oral or rectal) as well as NSAID (e.g. diclofenac 50 mg oral/rectal thee times a day) for up to a week. An opioid (e.g. morphine) should be given (IV/IMI or patient controlled analgesia PCA) until the pain score is consistently less than 3.

This analgesic regimen is enhanced by the addition of regional blockade.

Morphine (and pethidine) is the most important analgesic in the perioperative period. It is usually required in 90% of operations. The side-effects of nausea and vomiting are easily managed with antiemetics such as antihistamines. The risk of respiratory depression is low.

Analgesic Drugs

Paracetamol (acetaminophen) should be given preoperatively and postoperatively to patients. It enhances the action of NSAID and opioids. Adults may be given 1 gram orally or rectally up to 6 g per day. After 2 days the maximum dose should be reduced to 4 g per day. Children may receive a loading dose of 20 to 30 mg/kg, then maintenance of 15 mg/kg up to a maximum of 90 mg/kg/day. Neonates should not receive more than 60 mg/kg/day.

Paracetamol should be given initially as a regular dose during the first 48 hours after surgery, rather than on demand.

Large doses may cause liver toxicity. Hepatocellular necrosis may occur if more than about 7.5 g are taken. Patients may be asymptomatic for 24 hours. Early symptoms include nausea and vomiting, anorexia and abdominal pain. Liver damage becomes maximal in about 3 to 4 days.

Non-Steroidal Anti-Inflammatory Drugs (NSAID) act by decreasing inflammatory mediators at the site of tissue injury. They help reduce the amount of opioid required. Side-effects include gastritis and ulceration, decreased renal function (especially if associated with hypovolaemia, nephotoxic antibiotics, elderly and renal impairment) and decreased platelet function. The maximum adult oral dose is indomethacin 200 mg/day, diclofenac 150 mg/day, ibuprofen 1600 mg/day and naproxen 1000 mg/day. The maximum adult intravenous dose is ketorolac 90 mg/day (less than 65 years), 60 mg/day (older than 65 years), parecoxib 40 mg/day (20 mg/day in the elderly). The maximum adult rectal dose is indomethacin 100 mg/day. NSAIDs should be given initially as a regular dose rather than on demand.

Opioids (morphine) are the best analgesia for moderate to severe postoperative pain. They have side-effects that are dose dependent. The dose (and side-effects) can be reduced by multi-modal analgesia with regular doses of paracetamol and/or NSAIDs.

Morphine is the agent of choice in most situations. Pethidine is an alternative for patients with a true allergy or excessive nausea and vomiting. Pethidine is metabolised to nor-pethidine which is capable of causing convulsions. Codeine produces its analgesic effect by being metabolised to morphine. Some patients (5 to 10%) do not metabolise codeine to morphine.

It is important to realise that the opioid dose is more closely related to age than to weight. Also there is an 8 to 10 fold variation in opiate requirements in people of the same age and weight.

Side-effects include nausea and vomiting, pruritus, constipation, urinary retention, sedation, hallucinations, constricted pupils and allergy (very rare). Addiction is not a problem when opioids are used to treat acute pain after surgery. Morphine has little direct effect on the cardiovascular system. Relieving pain may cause a small fall in the blood pressure. Significant hypotension after giving morphine is usually due to other causes such as hypovolaemia. The most dangerous side-effect is respiratory depression. Respiratory depression is unlikely unless the dosage is high or the patient frail. The sedation score is the best indicator of early respiratory depression. A reduced respiratory rate is a late sign. All patients who are given morphine must be observed for sedation and respiratory depression.

Sedation Score        

1.         awake

2.         sedated/asleep, easily aroused

3.         sedated/asleep, hard to rouse

4.         unrousable

Patients with a sedation score of 3 or more or respiratory rate of less than 8 breaths per minute should be given oxygen, naloxone 200 micrograms intravenously and be carefully observed.

Patients with a sedation score of 2 and a respiratory rate greater than 8 breaths per minute should have their morphine dose reduced.

Morphine can be given orally, sublingual, rectally, subcutaneously, intramuscularly, intravenously or injected into the epidural or subarachnoid space.

Regular oral, rectal, sublingual, intramuscular, intravenous or subcutaneous plus “on demand doses” for break though pain is an effective technique for postoperative pain control. Epidural/subarachnoid and patient controlled analgesia dosing of morphine may produce better patient satisfaction but requires more equipment, staffing and experience.

Intravenous morphine has a quick onset. Peak analgesic effect occurs within 15 minutes. Patients need close observation (5 minutely for 30 minutes) so should only be used when individual nursing (1:1 or 1:2) is available e.g. recovery or intensive care. Intravenous morphine is the analgesia of choice for the control of moderate to severe pain in recovery. Patients should have their loading doses of paracetamol and /or NSAID plus intravenous opioids during anaesthesia. They should receive 1 to 2 mg intravenously every 5 minutes in PACU until the pain score is less than 3. Patients need close observation for 30 minutes after the last intravenous dose of morphine.

Intramuscular or subcutaneous morphine is easy to administer, cheap and requires no special training. It has a slower onset of action. Peak effect occurs in 30 minutes. Intramuscular/subcutaneous morphine orders need to be adjusted for each patient. It is best to write a range of doses and time intervals. A 2 hour dosing interval is usually more appropriate than 4 hours. Remember there is a marked variation between patients in morphine requirement, and age is more important than weight.

If the patient’s pain score is checked and documented and the patient is reviewed intramuscular/subcutaneous morphine can be successfully used plus regular paracetamol and/or NSAIDS.

The anaesthetist should aim for

a sedation score of 1,

a respiratory rate above 8 breaths per minute and

a pain score of less than 3 at rest and with coughing.

Acknowledgement

I would like to thank Dr. Charlotte Johnstone for her advice and help with preparation of this chapter.