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Analgesia
Emma Archer RVN Dip AVN Surgical VTS Anesthesia
Anaesthesia Technician
Animal Health Trust
Concepts in pain management
Analgesia should be considered with almost all patients nurses are involved with in the veterinary
practice. Because of the flexibility (known as plasticity) of the central nervous system (CNS)
leading to sensitisation, it is more effective to try and prevent pain rather than to treat pain once it
has happened. This is the idea behind pre-emptive analgesia. Pre-emptive analgesia is easy for
patients undergoing elective surgery but may not be possible in all patients, as some will have
arrived already experiencing pain. Even so, giving analgesics before surgery or potentially painful
diagnostic procedures is beneficial.
Multimodal analgesia is the use of a combination of different classes of analgesic drugs which
work at different points of the pain pathway. This is useful because it can be difficult to predict
which analgesics will be effective for a particular patient and its pain. Also, all analgesics have side
effects which can usually be reduced by using lower doses, therefore multimodal analgesia is more
likely to be effective, and less likely to cause adverse effects than the use of a higher dose of a
single drug.
Analgesic drugs
There are several different pharmacological classes of analgesic drugs which exert their effects on
different parts of the pain pathway. The commonly used analgesic drugs will be discussed below.
Opioids
Opioids produce their effect by binding to opioid receptors. There are three receptor types, mu (μ),
kappa (κ), and delta (δ) which are located mainly in the CNS but they also develop at sites of
inflammation in peripheral tissue. The analgesic effects are mainly mediated at μ receptors in the
spinal cord which cause hyperpolarisation of the neuron and prevent neurotransmitter release,
preventing propagation of the nociceptive signal. Action at μ receptors in the brain also contributes
to analgesia.
Types of opioids
Opioids are classified according to magnitude of the effect they exert. Full (pure) agonists eg.
morphine or hydromorphone can exert a maximal effect at the receptor (when an appropriate dose
is given), giving profound analgesia. Partial agonists eg. buprenorphine cannot exert a maximal
effect even at high doses and are more suitable for treating mild to moderate pain. Agonist /
antagonist drugs eg. butorphanol can be an agonist at one receptor type but be an antagonist at
another receptor type. Opioid antagonists such as naloxone, antagonize or reverse the effects of
full opioids.
This can be useful if unwanted side effects such as profound bradycardia or
respiratory depression are seen and also in case of an accidental overdose.
Side effects
CNS effects
Sedation and dysphoria are commonly seen side effects in small animals and are mediated by
action on all three receptor types in the brain. Sedation can be an advantage or a disadvantage
depending on the circumstances.
Cardiovascular effects
The cardiovascular effects of opioids are not generally serious at the doses used clinically.
Opioids increase vagal tone causing a reduction in heart rate or bradycardia. Cardiac output is
generally maintained as the stroke volume normally increases to compensate. Anticholinergics are
occasionally required when opioids are administered in conjunction with other cardiovascular
depressive drugs such as inhalational agents.
Some opioids cause histamine release with
subsequent hypotension and tachycardia. For this reason meperidine (pethidine) should not be
administered by IV injection and if morphine is used IV it should be administered slowly.
Respiratory effects
It is well reported that opioids can potentially cause respiratory depression. This is a significant
problem in humans, however it is much less of a problem in animals. In dogs and cats without
respiratory disease the respiratory depression is not normally significant at doses used clinically.
Respiratory depression may be seen in dogs and cats if opioids are used in conjunction with other
respiratory depressive drugs such as inhalational agents or if highly potent opioids such as
fentanyl or alfentanil are used. The technician should monitor for apnea or hypoventilation and
perform intermittant positive pressure ventilation (IPPV) if required. Generally, it is very unlikely
that a dyspnoeic patient will decompensate when given opioids especially if it is in pain, in fact
post-operative thoracotomy patients or patients with rib fractures may ventilate better once they
are pain free. Animals with intracranial disease may be more prone to respiratory depression
following opioid administration. These animals may already be suffering from an abnormal
respiratory pattern if the brainstem is affected by disease and opioids may exacerbate this.
Respiratory depression is particularly dangerous in these patients as the consequential increase in
arterial carbon dioxide levels can cause cerebral vasodilatation and a further increase in intracranial pressure. Therefore opioids should be used at a lower dose and with caution in patients
with intra-cranial disease and once anesthetized should undergo IPPV.
Some opioids like
butorphanol, codeine and fentanyl are effective antitussives (depress the cough reflex).
Gastrointestinal effects
Full μ agonists (most commonly morphine or hydromorphone) cause vomiting, however this is
rarely seen if the animal is in pain. The risk of vomiting may mean morphine or hydromorphone
should be avoided in certain animals, eg those with raised intracranial pressure, fragile eyes or
megaesophagus. Concurrent administration of acepromazine may reduce the risk of vomiting
when opioids are administered as a premed.
Opioids also cause decreased gut motility and
constipation.
Thermoregulatory effects
Opioids frequently cause panting in dogs as the hypothalmic thermoregulatory centre is reset to a
lower temperature. Some μ agonists (hydromorphone & oxymorphone) cause hyperthermia in
cats so body temperature should be monitored.
Urinary effects
Opioids can cause water retention and a decrease in urine output by increasing antidiuretic
hormone production.
Ocular effects
Mu agonists produce miosis in dogs and mydriasis in cats.
Full opioid agonists
Morphine
Morphine is a full μ agonist. It also has mild effects at κ, and δ receptors. It can be given by IV and
IM injection and by IV infusions. A preservative free formulation is available which can
be
administered epidurally and given this way its analgesic effects can last up to 24 hours. It can
cause histamine release when given by IV injection so should be administered slowly. A single
dose lasts 2 to 4 hours in dogs. Morphine often causes vomiting although this is observed less if
the patient is in pain.
Methadone
Methadone is a full μ agonist with a similar potency and duration of action to morphine. Less is
known about its pharmacokinetics than morphine. It can be given by SC, IM or IV injection. It has a
faster onset of action than morphine. It very rarely causes vomiting so is useful in cases where
vomiting is undesirable such as increased intracranial pressure.
Merperidine (pethidine)
Pethidine is a full μ agonist which is suitable for treating moderate to severe pain. Pethidine can
not be administered intravenously due to histamine release causing significant side effects. It is
short acting (1 - 2 hours) and stings on injection. Pethidine is unlikely to cause vomiting and
causes light sedation, it may cause less bradycardia than other opioids. It has a similar chemical
structure to atropine so may actually increase the heart rate.
Fentanyl
Fentanyl is a highly potent and very short acting full μ agonist with a rapid onset of action (one two minutes). The duration of action of fentanyl is dose dependent but a low dose lasts about 1520 minutes when given IV, this may be useful for short but painful procedure such as dressing
changes or intra-operatively during times of intense surgical stimulation. It is also useful as an IV
premed or co-induction agent. Fentanyl is often used as a constant rate infusion, intra-operatively
or post-op and causes a dose dependent drop in heart rate and respiratory depression is often
observed under anesthesia. Transdermal fentanyl patches can be useful adjuncts to analgesia.
Sufentanil/alfentanil/remifentanil
Sufentanil, alfentanil and remifentanil are all ultra-short acting derivatives of fentanyl. They are
used as continuous IV infusions under anesthesia to provide analgesia for painful procedures such
as thoracotmies or orthopedic surgery. The main advantage of remifentanil is that unlike fentanyl
and its other derivatives it is not liver metabolized. Instead it is rapidly hydrolysed by tissue and
plasma esterases, meaning accumulation does not occur and it is useful in patients with liver
disease or those undergoing surgery for portosystemic shunt ligation.
Partial opioid agonists
Buprenorphine
Buprenorphine is suitable for treating mild to moderate pain in cats and dogs, although it is likely to
be more effective in cats (as effective as morphine in one study). It can be given IM or IV, and has
recently been shown that in cats, the transmucosal route (oral mucosa) is as effective. It has a
long onset of action (around 45 minutes) and a long duration of action, giving analgesia for 6 to 8
hours, although the duration of action may be very variable in cats, highlighting the importance of
technicians performing regular pain assessments rather than assumming a set duration of
analgesic action. Buprenorphine has a high μ receptor affinity and binds tightly to the receptors.
There is therefore some belief that if buprenorphine has been administered to an animal and
additional analgesia is required, then a higher dose than usual of the full opioid agonist will be
required. This is to displace the buprenorphine from receptors in order to exert the full agonist
effect, however this is questionable.
Opioid agonist/antagonists
Butorphanol
Butorphanol is an opioid agonist- antagonist. It is a κ receptor agonist and a μ receptor antagonist.
It can be administered by IM or IV injection and is a good sedative but poor analgesic, providing
analgesia for mild pain. It has a duration of action of 1-2 hours. Butorphanol can be used to
reverse the effects of a pure μ agonist. The aim is to reverse the undesirable effects of the pure μ
agonist whilst maintaining the analgesia at the kappa receptor.
Opioid antagonists
Naloxone
Naloxone is a full opioid antagonist which can be used to reverse the effects of full μ agonists and
mixed agonists antagonists, however it does not effectively antagonize the effects of the partial
opioid agonist buprenorphine because buprenorphine has a greater affinity for the μ receptors than
naloxone. Naloxone can be administered by IV or IM injection, although when reversing unwanted
side effects such as respiratory depression, IV use is favoured due to the more rapid onset (1-2
minutes). The fact that it can be titrated to effect with the aim being to reverse the negative side
effects while maintaining some analgesic effects. Naloxone has a short duration of action (30-60
minutes), therefore when it is being used to reverse unwanted side effects of the longer acting
opioids such as morphine or methadone the technician should monitor the patient closely for
returning side effects and be aware that a repeated dose may be required.
Non-steroidal anti-inflammatory drugs (NSAIDs)
NSAIDs are probably the most commonly used analgesics in veterinary practice. There is an
increased number of licensed NSAID drugs in response to growing clinical demand. They are
effective, long acting analgesics and they have anti-pyretic and anti-endotoxic effects with a wide
range of indications. However, they have a very low threshold for toxicity, and their potential for
causing serious side effects may preclude their use in some ‘sick’ patients under anesthesia.
NSAIDs reduce prostaglandin production by inhibiting cyclo-oxygenase enzymes (COX) which are
responsible for prostaglandin synthesis. There are currently thought to be three COX types: COX
1, COX 2 and (the recently discovered) COX 3. Prostaglandins are mediators of inflammation and
pain, so preventing their production reduces inflammation and pain at the site of injury. Spinal
prostaglandins are involved in central sensitisation, so as well as working peripherally at the site of
inflammation, NSAIDs may also help prevent wind up and the development of chronic pain.
NSAIDs also provide analgesia by inhibiting COX 3 located in the brain (central analgesia) and
reduce fever by a central effect. Prostaglandins, however also have many important roles in the
body, including gastroprotection, maintenance of renal perfusion in situations of reduced renal
blood flow, and maintaining normal coagulation. Reducing prostaglandin production impacts on
these systems and can result in gastro-duodenal ulceration, renal injury and coagulopathy.
Traditionally it was thought that COX 1 was responsible for ‘house-keeping’ roles within the body
while COX 2 was induced during states of inflammation and inhibiting COX 2 would result in
reduced pain and inflammation without affecting housekeeping systems. This theory resulted in a
drive for production of ‘COX 2 selective’ NSAIDs. Unfortunately this is an oversimplification and
both COX 1 and COX 2 are important in gastric and renal protection and in coagulation.
NSAIDs should never be administered concurrently with corticosteroids, and the data sheet dose
should never be exceeded. Several are licensed for use in the peri-operative period and should be
administered as part of the pre-medication to provide pre-emptive analgesia in healthy patients
undergoing surgery. Care should be taken in hypovolaemic and hypotensive patients, especially
under anesthesia when anesthetic drugs may worsen existing hypovolemia and renal perfusion.
Their use should be delayed at least until the patient is fluid rescuscitated or until recovery from
anesthesia. Nurses should be aware which of the many NSAIDs are licensed for peri-operative
use.
α-2 adrenergic receptor agonists
Although traditionally used to provide sedation, α-2 adrenergic receptor agonists also provide
analgesia so can be used as a continuous infusion to provide analgesia as an adjunct to other
methods in the intra and post-operative period, especially when additional sedation is required,
and will contribute to balanced anaesthesia.
As well as causing sedation, they have profound
cardiovascular effects such as decreased cardiac output, decreased heart rate and peripheral
vasoconstriction, which mean their use is reserved for cardiovascularly stable animals.
Ketamine
Ketamine is an NMDA receptor antagonist which helps inhibit central sensitisation. It can be used
as a continuous infusion at subanesthetic doses and used in this way is a good analgesic with
minimal cardiovascular effects. Ketamine infusions reduce inhalational agent requirements (and
therefore their side effects) during anesthesia. It is especially useful for neuropathic pain.
Tramadol
Tramadol is a weak mu receptor agonist that is classified as an atypical opioid. Some American
states have classified it as a controlled substance due to the opioid receptor action. It also inhibits
serotonin and norepinephrine uptake which is thought to contribute to its analgesic effects. It is
normally administered orally by tablet form in patients with chronic cancer pain or those with
arthritis when NSAIDs alone are inadequate or are contraindicated. In the United States, tramadol
is available in tablet form in addition to an extended-release version and in combination with
acetaminophen. An injectable IV preparation is available in some countries. Side effects include
sedation and dysphoria, especially in cats and it also appears to be unpalatable to them making
them salivate. Tramadol use is becomming much more common in veterinary practices over
recent years and although data regarding its use is increasing, there is still work needed into it’s
use in small animals.
Gabapentin
Although gabapentin is an anticonvulsant it has also been shown to also have analgesic effects in
humans with neuropathic pain. In small animals it is sometimes used to control neuropathic pain in
the peri-operative period, especially in patients undergoing surgery for disc extrusions, particularly
if there is nerve root involvement. The exact mechanism is unknown, but it is thought to produce
its effects by increasing production of a neurotransmitter, gamma-aminobutyric acid (GABA). It is
given orally and often causes sedation. Gabapentin is relatively new in small animals, although it is
becoming more commonly used as an adjunct in chronic pain. There is increasing interest in its
use in small animals both for chronic and acute pain. Gabapentin should be used in combination
with other analgesics.
Routes of administration of analgesics
Drug legislation should be considered when administering analgesic drugs. Consideration should
be given as to whether the drug licensed for use in that particular species and by the route the
veterinarian wishes to administer it. The patient’s requirements also should be taken into account.
How quickly is the analgesia required, the patient’s temerament and whether the patient already
has an intravenous catheter should be considered.
Intravenous injection
This is the quickest route for drug absorption, with the most predictable plasma levels. It is useful
when analgesia is required rapidly, however as the drug rapidly enters the circulation the drugs
should be administered slowly. Doses may also need to be adjusted accordingly. Some analgesic
drugs are contraindicated by IV administration such as meperidine.
Intramuscular injection
Most of the opioids (except short acting potent ones like fentanyl) are administered effectively by
IM injection. The main disadvantage is that repeated IM injections can become painful, especially
if large volumes are used. Care should be taken with IM injections in patients with coagulopathies
or severe thrombocytopaenia.
Subcutaneous injection
Other than NSAID administration it is not generally recommended to administer analgesics
subcutaneously as little is known about pharmacokinetics of analgesic drugs following SC
administration. Recently it was shown that SC administration of methadone results in much lower
and less predictable plasma levels than IV administration (reference).
Transmucosal administration
This has been shown to be a very effective way of administering buprenorphine to cats (reference).
It is particularly useful in aggressive cats and avoids repeated painful IM injections, however it is
not effective when administered to dogs in this way. The preservative in some multidose vials of
buprenorphine has anecdotally been reported to be unpalatable.
Transdermal
Fentanyl, buprenorphine and lidocaine are available as transdermal patches, although they are
human preparations. Currently only fentanyl is commonly used transdermally in cats and dogs.
Transdermal fentanyl patches provide long lasting background analgesia. Fentanyl patches take
12-24 hours after application to reach therapeutic plasma concentrations. They last around 72
hours after application and additional analgesia is likely to be required. The patches consist of a
reservoir of the drug covered with a membrane which limits the rate of absorption surrounded by
an adhesive dressing which attaches to the patients skin. The patch should be covered to prevent
ingestion and clearly labeled with date and dose. They are available in different strengths (12, 25,
50, 75 & 100 μg/hr) They are controlled drugs and caution should be used when discharging
patients with these patches still in use, children or even adults can become seriously ill or even die
from ingesting these patches, and the patient should be returned to the practice for removal and
disposal. Under anesthesia drug absorption can be affected. Hypothermia causes decreased
drug absorption whereas heating devices such as forced warm air blankets will increase
absorption which may lead to respiratory depression and bradycardia.
Continuous infusions
Providing analgesia by a continuous infusion prevents the peaks and troughs in plasma
concentrations seen with intermittant boluses, providing less variation in plasma levels and
therefore a more consistant level of analgesia.
Infusions are mainly administered by the
intravenous route, however, can be used for administration of local anaesthetics via an epidural
catheter or a wound catheter. A bolus or loading dose is administered first, which aims to achieve
an effective dose in the plasma and brain before a rate is started in mg/kg/hr or μg/kg/min.
Continuous infusions of analgesic drugs are useful for animals who are expected to require high
levels of analgesia in both the intra-operative period, contributing to balanced anesthesia, in
recovery, or those in the ICU with painful medical conditions such as pancreatitis. Dose rates
should be carefully calculated and clearly written down following logical steps.
The use of
infusions requires the use of syringe drivers or infusion pumps for accurate drug delivery. The
syringe and fluid bags should be clearly labelled with a minimum of the drug and the concentration.
Diluting the drug is recommended for potent drugs when the desired volume would be very low.
Sterile saline 0.9% or Hartmann’s solution should be used and the dilution recorded. Regular
servicing and correct maintenance of equipment is important to ensure accuracy and proper
functioning. The accuracy of infusion pumps can be checked by running a bag of fluids through
the pump. For example a 500 mL bag running at 100 mLhr should take 5 hours to finish. There
are a wide range of drugs which can be administered by continuous infusion, these are discussed
in more detail below.
Opioids
Morphine is the pure agonists commonly used used as continuous infusions. These provide potent
analgesia in the peri-operative period and are useful in recovery as an alternative for patients
which are requiring, or are expected to require, very frequent intermittant boluses of opioid
analgesia. Morphine is often combined with lidocaine and ketamine to produce a combination
known as ‘MLK’ for very painful procedures. Potent opioids such as fentanyl or its derivatives such
as alfentanil or remifentanil are also commonly used intra-operatively in addition to an opioid
premed to provide profound analgesia for painful procedures such as spinal or orthopaedic
surgeries or thoracotomies. These potent opioids may cause respiratory depression, which on top
of the respiratory depressive effects of inhalational agents mean intermittant positive pressure
ventilation is required. As with all pure opioid agonists a vagally induced reduction in heart rate is
seen which may cause profound bradycardia. The technician should monitor this and be prepared
to reduce the dose rate and/or administer anticholinergics to increase the heart rate if required.
α-2 adrenergic receptor agonists
α-2 adrenergic receptor agonists provide analgesia so can be used by continuous infusion as an
analgesic as an adjunct to other methods in the intra and post-operative period. They cause
sedation so the technician should monitor the patient’s body temeprature and maintain
normothermia as they will often be immobile. They have profound cardiovascular effects, meaning
they are only used on patients without cardiovascular disease.
Basic nursing care such as
frequent turning may also be required. It also may be advisable to check the patient can swallow
properly before offering food and water and to offer water regularly rather than leave water bowls
in with heavily sedated animals.
The dose can be adjusted as required to ensure adequate
analgesia without excessive sedation.
Lidocaine
Lidocaine may be administered by intravenous infusion. Used in this way lidocaine has analgesic
effects as well as other advantages such as anti-inflammatory and anti-hyperalgesic effects and
also reduces the amount of inhalational agent required under anesthesia and improved bowel
function. It appears to be an effective analgesic for visceral and neuropathic pain, making it a
useful analgesic alongside opioids for use in dogs undergoing abdominal surgery as well as
trauma and pancreatitis patients. Metabolism of lidocaine is reliant on adequate hepatic blood flow
so care should be taken in patients with reduced hepatic blood flow, such as hypovolaemic
patients because accumulation and therefore toxicity, may occur. The technician should monitor
the patient for signs of CNS toxicity such as twitching or seizures and discontinue the infusion
immediately if these are observed.
Extra care should be taken under anesthesia as these signs
will not be seen and anesthesia reduces hepatic blood flow. Intravenous lidocaine is not normally
used in cats due to concerns about toxicity.
Ketamine
ketamine is an NMDA receptor antagonist which helps inhibits central sensitisation. Ketamine can
be used as a continuous infusion at subanaesthetic doses (5-10 μg/kg/min after a 0.5-1.0 mg/kg
bolus). Used in this way it is a good analgesic with minimal cardiovascular effects. Ketamine
infusions reduce inhalational agent requirements (and therefore their side effects) during
anesthesia. It is useful for neuropathic pain and used to minimise central sensitization and can be
continued into the post-operative period. Using ketamine in this way has been shown to improve
the demeanour and well-being of dogs following limb amputation as assessed at home by their
owners.
A potential side effect (although unusual at these low doses) is dysphoria and
occasionally seizures (particularly if overdosed) so the nurse should monitor the patient and
discontinue the infusion if any side effects observed.
Local anaesthesia and nerve blocks
Local anaesthetic techniques contribute to multimodal analgesia and balanced anaesthesia and
can be used to safely and effectively provide analgesia for a wide variety of surgical procedures.
They are also less commonly used in small animals to provide total anesthesia, allowing surgical
procedures to be performed in conscious or sedated animals.
Mechanism of action
Local anaesthetics work by causing blockade of sodium channels in the cell membrane of neurons,
stopping conduction of nervous impulses along the nerve until the drug is absorbed into the local
circulation. Both sensory and motor neurons are blocked, causing a temporary loss of motor
function as well as analgesia, although sensory fibres tend to be blocked before motor fibres. The
autonomic nervous system is also effected by local anesthetics, causing sympathetic blockade,
which leads to vasodilation. Local anesthetics cause local vasodilation, speeding up systemic
absorption of the drug, and were traditionally combined with vasoconstrictors such as epinephrine
to prolong the duration of action. However they have little effect on the duration of action of long
acting local anesthetics like bupivacaine. This combined with the potential for delayed wound
healing or tissue necrosis due to ischemia means combining local anaesthetics with epinephrine is
rarely necessary.
Drugs
There are many different local anesthetic agents available, however lidocaine, bupivacaine and
more recently, ropivacaine are most commonly used in small animal medicine.
Lidocaine has a quick onset of action (10-15 minutes), but a short duration of action (1-2 hours). It
is therefore useful for providing rapid intra-operative analgesia rather than providing post-operative
analgesia. Cats are more sensitive to the toxic side effects of lidocaine than dogs so care should
be taken to ensure dose rates are accurately calculated. Lidocaine is very versatile and comes in
a wide range of formulations. It can be used topically, for local infiltration, infiltrated perineurally,
for intravenous regional anesthesia as well as (although rarely used) for epidural and intrathecal
blocks.
Bupivacaine has a slow onset (20-30 minutes) and a long duration of action (4-6 hours). It can be
used for local, perineural, epidural and intrathecal administration. The long duration of action and
selective sensory blockade means it is commonly used to provide intra and post-operative
analgesia in small animals, however it does have a much narrower therapeutic margin of safety
than lidocaine and ropivacaine.
Ropivacaine is structurally related to bupivacaine, has a comparable duration of action, comparible
selective sensory blockade and similar indications to bupivacaine, but a more rapid onset of
action (5-10 minutes).
It is less cardiotoxic than bupivacaine. Because of these advantages
ropivacaine is becoming more commonly used in veterinary medicine.
Topical application
Lidocaine can be applied topically (available either as a spray or a gel) to desensitize oral, nasal
and laryngeal mucous membranes to facilitate placement of naso- osophageal feeding tubes or
oxygen catheters, urinary catheters or for endotracheal intubation in cats. Systemic toxicity should
be considered for small animals < 5kg and care should be taken to not use excessive amounts.
Most local anesthetics do not penetrate intact skin however a local anesthetic cream (EMLA)
containing a mix of lidocaine and prilocaine provides analgesia of the skin and can be used in
small animals prior to placement of venous or arterial catheters. The cream must be covered with
an occlusive dressing for 30-60 minutes. Proxymetacaine drops can be used on the eye. They
are used to desensitize the cornea to aid ocular procedures such as corneal foreign body removal
or ocular ultrasound.
‘Splash blocks’
Local anesthetics can be directly applied to surgery sites by splashing or dripping onto open
surgical sites before closure. This may provide post-operative analgesia although local infiltration
prior to surgical stimulus is often preferred.
Local infiltration
Local anesthetics can be injected intradermally or subcutaneously to desensitive the skin and
subcutaneous tissues for minor surgery. After the skin is clipped and aseptically prepared over the
surgical site, the local anesthetic is injected subcutaneously to desensitise a small area. This can
be repeated in a ring around a limb, in a line, in a rectangular or triangular pattern in order to
desensitise larger areas. As with all nerve blocks always aspirate before injecting to ensure you
are not injecting into a blood vessel.
Intrapleural blocks
Local anesthetics can be applied into the pleural cavity via a chest drain to provide analgesia postoperatively in patients who have undergone a thoractomy via either a lateral thoracotomy or
median sternotomy. Bupivacaine is most comonly used, the tube should be flushed with sterile
saline afterwards and the patient placed incision down for 10 minutes after administration. The
bupivacaine diffuses across the the pleura and blocks intercostal nerves near the site, onset of
action is rapid and lasts for 4-6 hours. This technique is easy to perform once a chest drain is in
place.
Intravenous regional analgesia (IVRA)
IVRA can be used to produce analgesia of distal limbs in dogs and is useful for toe amputations.
Lidocaine is injected intravenously distally to a tourniquet (An Esmarch bandage is normally
applied to the limb). Onset of analgesia is rapid (around 10 minutes) and persists for the length of
time the tourniquet is in place.
The Esmarch bandage can be left in place for a maximum of 90
minutes. A large dose (2-4 mg/kg) is normally required and the risk of systemic toxicity is high,
therefore its use is not normally recommended in cats.
Intra-articular blocks
Local anesthetics can be used intra-articularly to provide analgesia of up to 24 hours in duration.
Bupivacaine or ropivacaine is commonly used in dogs after surgery to repair ruptured cruciate
ligaments in the stifle. Morphine has also been used this intrarticularly as opioid receptors have
been found in the synovial fluid of inflammed joints.
Perineural infiltration
Perineural infiltration describes the technique of injecting local anesthetics directly around the
nerve. There are many different nerves that can be blocked in this way to provide peri-operative
analgesia in dogs and cats such as intercostal nerve or brachial plexus nerve blocks. Commonly
performed ones will be discussed below.
Specific nerve blocks
Dental nerve blocks
There are four nerve blocks commonly performed to provide excellent analgesia in dogs and cats
undergoing tooth extractions and dental work: infraorbital, maxillary, mental, and mandibular (also
referred to as the inferior alveolar). They are also used for surgery such as maxillectomy and
mandibulectomies. As with all nerve blocks, it is essential to aspirate before injecting the agent to
avoid accidental intravenous injection of the local anesthetic.
Infraorbital
Blocking the infraorbital nerve provides analgesia to the rostral portion of the maxilla on that side;
the canine, and premolar teeth rostral to the fourth premolar and the associated soft tissues. The
infraorbital foramen can easily palpated in most dogs on the maxilla, rostral to the orbit. The
foramen cannot be plapated in cats, although a ridge can be felt. In cats and brachycephalic dogs,
the infraorbital canal is short and care must be taken to avoid inserting the needle beyond the
medial canthus.
Maxillary
The maxillary nerve and its branches provide sensory innervation to the upper dental arcade, soft
and hard palates and the muzzle. A variety of different techniques have been described but the
author favours blocking the maxillary nerve before it enters the maxillary foramen by inserting the
needle ventral to the zygomatic arch at its most dorsal part and directing the needle towards the
maxillary foramen.
Canine skull showing site of needle insertion for infraorbital & maxillary nerve blocks
Mental
The mandibular nerve can be blocked rostrally at the mental foramen which is ventral to the lower
first or second premolar and is useful for lower canine and incisor tooth extractions.
The mental
foramen is very hard to palpate in cats and some dogs, making the mandibular nerve block a
better option in these animals.
Mandibular (Inferior alveolar)
Desensitizes the lower dental arcade and the associated soft tissue, including the tongue on the
side of the block. Bilateral mandibular nerve blocks are not usually recommended due to the risk
of completely desensitizing the tongue causing the potential for accidental biting and damage to
the tongue. The foramen is located on the medial side of the mandibular ramus, just rostral to the
angular process and can easily be be palpated in most dogs from the inside of the mouth. The aim
is to block the mandibular nerve before it enters the foramen as the foramen cannot be directly
entered.
canine skull showing site for
mental 7 mandibular nerve
blocks
Intercostal nerve blocks
Intercostal nerve blocks can be used to help manage peri-operative pain from lateral
thoracotomies or to manage pain from fractured ribs. The intercostal nerves run alongside the
caudal border of each rib, along with blood vessels. Two intercostal nerves should be blocked both
cranially and caudally to the affected site. The injection is performed to the proximal aspect of the
rib and as with all local injections care should be taken to ‘draw back’ before injecting the drugs to
ensure you are not injecting into a blood vessel, this is particularly important here as there are
blood vessels in close proximity.
Intercostal nerve blocks may improve ventilation in trauma
patients with fractured ribs and post-thoractomy patients. Pneumothorax is a potential risk so the
technician should observe the ventilation for 20-30 minutes after.
Brachial plexus nerve blocks
The brachial plexus is a collection of five spinal nerves located in the axillary space slightly cranial
to the first rib.
The brachial plexus block causes blockade of the radial, median, ulnar,
musculocutaneous and axillary nerves desensitizing the forelimb from the elbow distally, providing
analgesia for fractures involving the ulnar and radius or other soft tissue surgery involving the
forelimb, below the elbow. The block should be performed using an aseptic technique and sterile
gloves worn. A long needle is required and a spinal needle preferred (the length of the needle
should be about the length of the humerus. The needle is inserted in the axillary space at the
shoulder, directing it towards the first rib There is a risk of entering the pleural cavity and causing a
pneumothorax if the needle is directed too far towards the midline. When the stylet is removed the
technician should listen for any ‘whoosh‘ of air which will be heard if the pleural cavity is entered.
The needle should be withdrawn if this occurs.
Unilateral blockade of the phrenic nerve &
hemiparalysis of the diaphragm has been reported.
Another alternative technique has been
recently described called the paravertebral brachial plexus block.
Radial, ulnar, median, musculoskeletal (RUMM) nerve block
An alternative to the brachial plexus block for providing analgesia to the distal forelimb (below the
elbow) is the RUMM block which blocks the radial, ulnar, median and musculoskeletal nerves. A
nerve stimulator specifically designed for locating peripheral nerves and performing nerve blocks
greatly aids this technique.
Auriculotemporal and greater auricular nerve blocks
These nerve blocks provide analgesia to the external ear canal and the auricular cartilage making
it a useful nerve block for dogs under going total ear canal ablation and bulla osteotomy surgery
whose pain is often difficult to control. These blocks may also be useful to provide a stable plane
of anesthesia during ear flushes in dogs with chronic or extreme pain. The auriculotemporal nerve
is blocked by inserting a needle between the rostral aspect of the vertical ear canal and the caudal
aspect of the zygomatic arch. To block the greater auricular nerve place the needle caudal to the
vertical ear canal and cranially & ventrally to the wing of the atlas, keeping the needle parallel to
the vertical ear canal.
Retrobulbar nerve block
Retrobulbar nerve blocks can be used to provide excellent analgesia and a central eye position for
a wide variety of ophthalmic procedures. Retrobulbar blocks are much more commonly performed
in large animals due to the risk of complications which include retrobulbar haemorrhage,
perforation of the globe and injection of local anaesthetic into the subarachnoid space which may
cause respiratory or cardiac arrest. However in the author’s experience these blocks are very
useful to provide analgesia and reduce the anesthetic agent requirements for dogs and cats
undergoing enucleation or evisceration with intrascleral prosthesis surgeries. This is providing the
person performing the block is properly trained and has a thorough understanding of the anatomy
in the area and an understanding of the risks. Before injecting the local anesthetic drugs close
attention should be paid to the hub of the needle to ensure there is no CSF or blood present.
Specifically designed curved, blunt needles are available to aid injection into the retrobulbar space
which greatly facilitates this procedure.
Wound catheters (continuous wound infiltration)
Excellent post-operative analgesia of long duration can be provided by the use of a wound
infiltration catheter. These can be placed into the surgical site before the wound is closed and are
useful to provide analgesia in the post-operative period for a wide variety of procedures such as
total ear canal ablation (TECA), limb amputations and extensive tumour resections.
Specific
wound infusion catheters are commercially available through a number of companies.
The
catheter should be pre-flushed with local anesthetic before placement. To minimise the risk of
wound infection the catheter should exit the skin at a site separate from the wound as bacteria can
migrate up the surface of the catheter.
Bupivacaine or ropivacaine may be applied intermittantly through the drain (every 4-6 hours)
depending on the patient’s pain, or lidocaine may be used as a continuous infusion using a syringe
driver or a commercial reservoir infusion pump.
Care should be taken not to produce local
anesthetic toxicity. For bupivacaine, it has been suggested not to exceed a total daily dose of
4mg/kg in dogs and 2mg/kg in cats. (Vet Clinics ICU Pain update). The low pH of the bupivacaine
may cause discomfort on injection. If this occurs, the injection should be discontinued, wait 10
minutes for the injected amount to desensitize the area, then the injection can often be continued
without discomfort. The catheters should be handled aseptically. The technician should regularly
assess the patient’s pain and the catheter can be left in place while they are still requiring frequent
doses of local anesthetic (normally 1-3 days).
Epidural analgesia & anaesthesia
The terms epidural and extradural are used interchangably to describe the space outside the dura
mater (the tough fibrous outermost layer of the meninges surrounding the spinal cord). Epidural
administration of local anesthetics and opioids are frequently used in small animals in combination
with general anaesthesia as part of a balanced anesthetic technique to provide analgesia, to
reduce anaesthetic agent requirements and to provide muscle relaxation. They provide profound
peri-operative pain relief of surgical procedures involving the hind limbs, pelvis and caudal
abdomen. Epidural anesthesia refers to the complete sensory, motor and autonomic blockade
produced by local anaesthetics injected epidurally, whereas epidural analgesia refers to the
profound analgesia produced by epidural injection of opioids.
Drugs used
Local anaesthetics
Lumbosacral epidural administration of local anaesthetics desensitise the nerves leaving the spinal
cord in the area of the spread of the local anaesthetic. How far the drug spreads cranially (&
therefore how cranially the analgesic effects reach)
depends on the volume administered.
Lidocaine can be used, it has an onset of action of < 10 minutes and provides 1-2 hours of
anaesthesia while epidural administration of bupivacaine or ropivacaine provides 4-6 hours of
anaesthesia (onset of action less than 15 minutes).
Opioids
Morphine (0.1-0.2 mg/kg) is the opioid most commonly used for epidural injection as it produces
longer lasting analgesia with fewer side effects than more lipid soluble opioids such as fentanyl or
hydromorphone. The dose of morphine is usually diluted with sterile 0.9% saline to increase the
volume of injection (1 mL/5 kg) or combined with local anaesthetics such as bupivacaine.
Preservative free preparations should be used to prevent the risk of damage to the spinal cord by
preservatives. A single epidural injection of morphine has an onset of action of 30-60 minutes but
provides 12-24 hours of analgesia. Hydrophilic agents like morphine spread cranially over time,
therefore lumbosacral morphine injections provide good analgesia for upper abdominal and
thoracic procedures as well as for more caudal surgery. Epidural injections of opioids avoid the
risk of motor blockade and hypotension caused by sympathetic blockade seen with local
anaesthetics however they may cause urinary retention and pruritis.
Other drugs
Alpha-2 agonists are occasionally used epidurally.
Systemic absorption occurs so along with
analgesia, medetomidine injected epidurally causes sedation, marked bradycardia and an increase
in blood pressure.
These side effects mean they are not suitable for use in animals with
cardiovascular disease.
Patient selection
To avoid complications the technician should consider suitable patient selection. Contraindications
of epidural injections include coagulopathies, skin infection/infection around the lumbar sacral
junction, or sepsis. Other conditions such as anatomical abnormalities of the pelvis or lumbar
spine (either congential or acquired through trauma) and obesity which may obscure landmarks
can make the procedure difficult and may be best left to experienced anesthetists or avoided
altogether. Due to the potential of epidural injection of local anaesthetics to cause vasodilation
through sympathetic blockade, local anaesthetics should not be given epidurally in hypovolaemic
or cardiovascularly unstable patients.
Side effects
Due to vasodilation caused by sympathetic blockade, epidural injection of local anaesthetics can
cause significant hypotension even in healthy patients (although this is often off set by the reduced
isoflurane requirements) therefore the technician should monitor blood pressure, reduce the
vaporizer setting of the inhalational agent as much as possible and administer appropriate fluid
therapy. Depending on the dose, volume and type of local anaesthetic used, motor nerves as well
as the sensory nerves may be blocked. Post-operatively this may result in hind limb ataxia and
weakness or a patient that is unable to walk until the injection wears off.
Nurses should be
prepared to turn the patient if prolonged recumbency occurs and assist the patient with walking
using a sling under the abdomen if necessary. Urinary retention can occur so all patients should
have their bladder expressed at the end of surgery and the bladder checked periodically in the
post-operative period. Delayed hair growth over the lumbo sacral junction may occur so the owner
should be suitably informed and pruritis has also been reported with epidural injection of opioids.
Adverse effects of local anesthetics
Nurses should be aware of the symptoms of the toxic side effects of local anaesthetics. Central
nervous system (CNS) toxicity is seen at much lower doses than those required to produce
cardiovascular toxicity. Signs of CNS toxicity include muscle twitching and seizures. To prevent
the risk of toxicity, total doses of local anaesthetic should be calculated carefully and based on
lean bodyweight.