Pain management for emergency & critical care patients (Proceedings)

Our patient population has changed fairly dramatically in the last 10 years as our medical skills have progressed and we have become capable of supporting patients with advanced disease and advancing age. Furthermore, our surgical skills have improved and surgery now is often complicated, long and may involve major blood loss and/or major physiological manipulation. Now we must hone our anesthesia and pain management skills in order to support patients that largely don't fit into the 'young, healthy' category and to support patients throughout difficult surgical procedures. It is no longer appropriate to think that safe anesthesia means recovering as many patients as we anesthetize. Instead, we need to focus on what happens to the patient during the anesthetic period - which starts when the patient is admitted for anesthesia and really doesn't end until the patient has returned to 'normal' and free of pain (or at least comfortable). Necessary advances in anesthesia include better preparation of the patient for anesthesia (including the use of pre-anesthetic tranquilizers and analgesic drugs), improved support of the patient during the anesthetic period (including better monitoring and the use of IV fluids) and increased attention to events in the recovery period (including emergence delirium and bouts of pain).

Principles of analgesia:

Also, no matter what anesthetic protocol is chosen, analgesia is imperative. Perioperative analgesia has two monumental advantages: 1) analgesia increases anesthetic safety by decreasing the necessary dosages of anesthetic drugs and 2) analgesia improves our medical success rate because adequate analgesia improves healing and allows a decreased incidence of postoperative stress-related complications. Regardless of which analgesic drugs are chosen, 3 basic tenets of pain management should always be followed: 1) analgesic drugs should be administered preemptively; 2) multimodal analgesia should be used whenever possible; and 3) analgesia should continue as long as pain affects the patient's quality of life.

My favorite drugs for compromised patients:

Reminder: All tranquilizers, induction drugs and inhalant drugs cause some degree of dose-dependent CNS depression and most cause both respiratory and cardiovascular depression. In healthy patients, many of the physiologic effects of anesthetic drugs are well tolerated or can be counteracted by routine measures such as administration of oxygen or intravenous (IV) fluids. In compromised patients, these effects can be exacerbated, further contributing to the demise of the patient. Successful anesthesia in compromised patients is highly dependent on adequate patient stabilization, diligent patient support and monitoring, and the use of appropriate anesthetic agents at appropriate dosages.

Sedatives / tranquilizers

1.Opioids - Butorphanol, buprenorphine, morphine, hydromorphone, oxymorphone, fentanyl

     • Advantages: Provide moderate to profound analgesia, safe, reversible, many are inexpensive, provide sedation, versatile (can be administered PO, IM, IV, SQ, in the epidural space, in the intra-articular space, etc…)

     • Disadvantages: Controlled substances, may provide more sedation than desired (rare), relatively short duration of action

     • NOTE: Opioid-induced respiratory depression is highly overrated. In animals, respiratory depression is almost always related to degree of sedation - in which case, ALL sedatives can be respiratory depressants.

2. Benzodiazepines - Diazepam (Valium®) and Midazolam (Versed®)

     • Advantages: minimal to no cardiovascular or respiratory effects, reversible

     • Disadvantages: won't provide adequate sedation when used alone in young, healthy or excited dogs, no analgesia.

3. Acepromazine – not commonly used in compromised patients; can contribute to hypotension (the exception is patients with upper airway compromise that need long-term sedation and some patients with cardiovascular disease that would benefit from a reduction in afterload).

4. Alpha-2 agonists – not commonly used in compromised patients; will cause an increase in cardiac work (however, they are reversible drugs so they may be appropriate for some critical patients but certainly not for any patient with hemodynamic compromise).

Induction drugs

1. Propofol

     • Advantages: rapid induction and recovery, multiple routes of clearance from the body, good muscle relaxation

     • Disadvantages: somewhat expensive, must be administered IV, causes mild to moderate respiratory and cardiovascular depression

2. Ketamine

     • Advantages: inexpensive, can be administered IM, mild respiratory depression, no cardiovascular depression in heart-healthy patients

     • Disadvantages: can cause cardiovascular depression in patients with cardiovascular compromise, controlled drug, can cause muscle rigidity

3. Etomidate

     • Advantages: no cardiovascular changes

     • Disadvantages: expensive, poor muscle relaxation, vocalization

4. Inhalant induction is not appropriate

     • The dose of the inhalant is entirely too high (side effects of inhalants are dose-dependent), the induction will be stressful and will be prolonged.

Maintenance drugs

Inhalant anesthesia is generally the safest and most effective way to maintain anesthesia that will last 30 minutes or more. However, inhalant anesthetic agents should never be used as the sole anesthetic agent since this group of drugs causes significant hypotension, hypothermia, and hypoventilation. Our goal should always be to keep the vaporizer at the lowest possible setting.

     • Advantages of inhalants: easy to administer, relatively inexpensive, are eliminated with minimal metabolism, require oxygen for delivery, generally require intubation for delivery.

     • Disadvantages of inhalants: dose dependent contribution to hypoventilation, hypotension and hypothermia. monitor, monitor, monitor.

Analgesic drugs

Maintenance of anesthesia is much easier and safer if analgesia is provided prior to the painful stimulus. Most anesthetic drugs, including the anesthetic gases, block the brain's realization that pain has occurred but don't actually block pain. If the pain is severe enough, the brain can still respond and make the animal appear to be inadequately anesthetized. The result is that the vaporizer is turned up and the brain ceases to respond, but the patient is now too deeply anesthetized and can be at a very dangerous physiologic plane. A more appropriate response would be to block the pain and maintain anesthesia at a light, safe depth.

1. Opioids

     • Advantages: provide moderate to profound analgesia, cause minimal cardiovascular or respiratory effects, are reversible

     • Disadvantages: may cause minor changes in respiratory or cardiovascular function, however, these will be more than offset by the subsequent decrease in the dose of the inhalant anesthetic drug (ie, turn down your vaporizer after you give the opioid!).

2. Local anesthetic drugs

     • Advantages: Inexpensive, easy to administer, very effective

     • Disadvantages: Relatively short duration of action

     • Note: This class of drugs is extremely underutilized yet they are easy to use, inexpensive and highly effective. We should be using more local anesthetics!

3. Constant rate infusion (CRIs)

     • Advantages: EASY, inexpensive, effective, many drug choices (opioids, lidocaine, ketamine, alpha-2 agonists and combinations of all of these drugs)

     • Disadvantages: Almost none because of low dose delivered but side effects from any drug can occur

Sample protocol for compromised dogs

1. Premedication

     • 0.25-0.5 mg/kg morphine OR 0.1 mg/kg hydromorphone IM

     • Can substitute 0.02 mg/kg IM buprenorphine for opioid but probably won't achieve adequate sedation or pain relief for most patients

     • No other sedative required if opioid alone is adequate; if inadequate, add 0.1-0.2 mg/kg midazolam IM (not a good sedative in excited patients)

     • MAY use low-dose ace or medetomidine in some patients - not routine

2. Induction

     • 0.2 mg/kg diazepam followed by 2-4 mg/kg propofol IV

     • ½ normal dose of diazepam/ketamine combination (1 ml/20 kg)

     • etomidate if necessary

     • do not mask

3. Maintenance

     • LOW-DOSE sevoflurane or isoflurane

     • Use local anesthetics, etc… to keep inhalant dosage low

     • ALWAYS use IV fluids, even if the patient is normovolemic since anesthetic-drug induced cardiovascular depression can generally be overcome by counteracting the decrease in cardiac output with an increase in circulating volume. Thus, the routine use of IV fluids should be considered as a beneficial countermeasure to the detrimental cardiovascular effects of the anesthetic drugs. Fluid therapy may not mean crystalloids for all patients, consider use of colloids, whole blood, etc…

     • Monitoring during maintenance and well into the recovery period is absolutely crucial. Monitoring during maintenance should include assessment of heart rate and rhythm, frequency and depth of ventilation, pulse quality and strength, color of mucous membranes, blood pressure, pulse oximetry and end-tidal CO2 (if you don't have one of these yet, put it on your wish list!).

4. Post-op / Discharge

     • Depends on disease but DO NOT withhold treatment - pain is a bigger stressor in compromised patients than in healthy patients

     • Generally, opioids ± NSAIDs in respiratory and cardiovascular disease; opioids ± alternative drugs for other diseases

Sample protocol for compromised cats

1. Premedication

     • Unfortunately, full opioids used alone in cats can cause excitement. Hydromorphone at low dose (0.05-0.1 mg/kg) generally won't cause excitement but concurrent sedation may be required. Buprenorphine at 0.02 mg/kg definitely won't cause excitement but neither will it provide noticeable sedation.

     • If more sedation is required, consider butorphanol (0.2 mg/kg) instead of the other opioids but remember that duration of analgesia is short and supplement with longer lasting drugs

     • No other sedative required if opioid alone is adequate; if inadequate, add 0.1-0.2 mg/kg midazolam IM (not a good sedative in excited patients).

     • MAY use low-dose ace or medetomidine in SOME PATIENTS - not routine

2. Induction

     • 0.2 mg/kg diazepam followed by1-2 mg/kg propofol IV

     • ½ normal dose of diazepam/ketamine combination (1 ml/20kg)

     • etomidate if necessary

     • DO NOT MASK

3. Maintenance

     • LOW-DOSE sevoflurane or isoflurane

     • Use local anesthetics, etc… to keep inhalant dosage low.

     • ALWAYS use IV fluids, even if the patient is normovolemic, anesthetic-drug induced cardiovascular depression can generally be overcome by counteracting the decrease in cardiac output with an increase in circulating volume. Thus, the routine use of IV fluids should be considered as a beneficial countermeasure to the detrimental cardiovascular effects of the anesthetic drugs. Fluid therapy may not mean crystalloids for all patients, consider use of colloids, whole blood, etc…

     • Monitoring during maintenance and well into the recovery period is absolutely crucial. Monitoring during maintenance should include assessment of heart rate and rhythm, frequency and depth of ventilation, pulse quality and strength, color of mucous membranes, blood pressure, pulse oximetry and end-tidal CO2 (if you don't have one of these yet, put it on your wish list!).

4. Post-op / Discharge

     • Depends on disease but DO NOT withhold treatment - pain is a bigger stressor in compromised patients than in healthy patients

     • Generally, opioids and perhaps NSAIDs in respiratory and cardiovascular disease; opioids alternative drugs for other diseases

Anesthetic techniques

Although most of the discussion of anesthesia is centered around drugs, this is actually not totally appropriate since safe anesthetic management in some cases is more about anesthetic technique than about actual drug selection. For example, patients with upper respiratory disease can be anesthetized safely with almost any anesthetic drugs (provided that the dose is appropriate) but appropriate management of the airway is necessary for a successful outcome. Specific techniques are discussed in the diseases presented below.