anesthesia and analgesia
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- stimulation of nocioceptors, leading to transmission of signal to CNS via sensory nerve pathways
- spinal cord neuron conducts impulse to brain, processes, pain is consciously percieved
can patients under general anesthesia feel pain?
no, but you still need to treat it
- "no sensation"
- a state of controlled and reversible loss of consciousness, in adition to loss of sensation over the entire body
- the CONSCIOUS perception of a noxious stimulus
- "an unpleasant sensory and emotional experience associated with actual or potential tissue damage."
can patients under general anesthesia experience nocioception?
yes. This is why we treat pain under general anesthesia.
reflex responses to nocioception
- nocioception can stimulate autonomic nervous system even when not consciously perceived.
- - ventilatory response (increase tidal volume and/or increase respiratory rate).
- - cardiovascular response (increased sympathetic tone, increased cardiac workload)
- - catecholamine release (increase HR and BP, arrhythmogenic potential)
- **may not be seen due to anesthetic agents decreasing sympathetic NS
nocioceptive reflex response: ventilatory response to noxious stimuli
- Increase tidal volume (or decrease by panting), increase respiratory rate (tachypnea).
- May not be seen because anesthetic agents often diminish responsiveness of sympathetic nervous system. May see all or none, due to drugs.
nocioceptive reflex response: reflex cardiovascular responses to noxious stimuli
- increased sympathetic tone, increased cardiac workload
- catecholamine release
- - increase HR and BP
- - arrhythmogenic potential of catecholamines
- May not be seen because anesthetic agents often diminish responsiveness of sympathetic nervous system
responses to noxious stimuli/pain over time
- Hormonal: catecholamines, cortisol, others
- immunological: hyperglycemia, decreased GI blood flow, impaired healing, increased platelet aggregation (thrombi, pulmonary embolism), decreased immunity
benefits of adequate analgesia
- minimizes patient stress and suffering
- improves healing
- decreases post-op stress-related complications
- "no pain"
- relief or absence of pain
- includes diminished sensory perception of pain and alleviation of emotional components that contribue to experience of pain
- administered prior to noioceptive stimulus
- reduces intensity of pain sensation and increases effectiveness of analgesics
- use of a variety of analgesic drugs to take advantage of additive/synergistic effects
- provides better or longer lasting analgesia than one drug alone
- Different classes of analgesic drugs work at different locations in the pain pathway. With multimodal analgesia, a synergistic response can be achieved by targeting these different locations
multimodal analgesia sites of action: conscious perception
inhibited by general anesthetics, opioids and alpha2 agonists
multimodal analgesia sites of action: spinal cord sensitization
inhibited by opioids, NSAIDs, NMDA antagonists, alpha2 agonists and local anesthetics
multimodal analgesia sites of action: transmission along peripheral nerves
inhibited by local anesthetics and alpha2 agonists
multimodal analgesia sites of action: nociceptor
- inhibited by local anesthetics, opioids and NSAIDs.
- local: lidocaine, bupivicaine (block Na channel)
- loss of sensation or feeling
- generally produced by administering a drug or combination of drugs that depresses nervous tissue activity
- a state of controlled and reversible loss of consciousness, in addition to loss of sensation over the entire body
- Produced by administering drug(s) that REVERSIBLY DEPRESS ACTIVITY OF CNS
Phases of CNS depression
- medullary depression
- medullary paralysis
traditional definition of surgical anesthesia
- depth of general anesthesia at which surgery can be safely performed
- - includes loss of consciousness and sufficient absence of pain sensation and enough sekeltal muscle relaxation to allow surgery to be performed without pain to or movement of the patient
updated definition of surgical anesthesia
- drug-induced unconsciousness in which patient neither perceives nor recalls noxious stimuli
- - achieved by using drugs to:
- - produce anesthesia - block sensory afferent impulses from nocioceptors and autonomic, hormonal and immunological responses to nocioception
- - produce muscle relaxation
- - block efferent motor impulses
- produce a state of controlled and reversible unconsciousness
the period immediately preceding induction of anesthesia
preanesthetic period goals
- to achieve safe, efficient, effective anesthesia with minimal stress to patient
- to anticipate potential complications, take steps to improve patient's condition and take steps to prevent potential complications from arising
preanesthetic period food and drink
- depending on age and species, P is fasted and water withheld (no food for 6-12h and no water for 6h)
- pediatric and geriatric, water until premed to prevent dehydration and hypotension.
- No fasting longer than 4 hours for 6-16 weeks/neonates
preanesthetic patient prep
- administer preanesthetic drugs
- pre-emptive analgesia
- secure venous access
- optimize circulating blood volume with IV fluids
- continuous monitoring essential to reduce risk of vascular overload
- lower dose of induction agent, gas anesthetic
- provide relaxed state
- reduce pain before and after procedure
- prevent pain transmission during procedure
pre-emptive analgesia in preanesthetic period
- prevention or minimization of pain prior to patient consciously experiencing noxious stimulus
- - requires administration of analgesic agents prior to patient experiencing pain
- in humans, analgesic agents proven more effective if administored prior to conscious pain perception
- - also decrease size and decrease frequency of dosage
preanesthetic period fluid therapy reasons
- correct normal ongoing losses
- correct fluid deficits
- help prevent/treat hypotension (MOST COMMON perianesthetic complication)
- facilitate drug excretion
- continue in post-op period
- - correct remaining deficits, provide maintenance requirements and replace ongoing losses
- Continuous monitoring essential to reduce risk of vascular overload
fluid treatment during anesthesia
- pre-warm IV fluids to body temp, esp for smaller animals
- (<10ml/kg/hr, 3 for cats, 5 for dogs)
- set up for induction and surgery, be ready so as short as possible
- - have necessary drugs and equipment prepared in advance of anesthetic induciton
- - check all anesthetic equipment
- - ensure adequate supply O2 and liquid anesthetic agent
- - know doses and routes of administration for all drugs and reversal agents
pre-anesthetic patient evaluation
- gather as much info relevant to anesthesia as possible
- patient history
- procedure to be performed
- physical examination
- diagnostic tests
- determination of physical status and anesthetic risks
signalment relevant to anesthesia.
the part of the veterinary medical record dealing with species, breed, age, sex
signalment relevant to anesthesia: species
- cats have low capacity for hepatic glucuronidation, watch dosages and dosage intervals
- morphine, lidocaine, NSAIDs
signalment relevant to anesthesia: breed
- anatomical differences lead to anesthetic complications
- brachycephalic, chows, shar peis: rapid induction, intubation and recovery, and extra-vigilant monitoring after extubation
- pharmacokinetic differences - mutant multi-drug resistance gene (MDR1)
- size - toy breeds are hypothermic, big dogs with thick hair are hyperthermic
- Von Willebrands
- narrow trachea
- stenotic nares - malformed nostrils that are narrow or collapse inward during inhalation
- everted laryngeal saccules - tissue in airway in front of vocal cords is pulled into trachea and partially obstructs airway
- elongated soft palate - soft palate is too long so tip protrudes into airway
multi-drug resistance gene
- encodes a protein called P-glycoprotein
- - a specific transporter ound in brain capillary endothelial cells needed to pump many drugs and toxins out of brain tissue
- - butorphanol, acepromazine, ivermectin, milbemycin, loperamide, etc.
- Collies, shelties, aussies, old english sheepdogs, g.sheps, long-haired whippets, silken windhounds, etc.
Von Willebrand's Disease and anesthesia
- check von Willebrand's and buccal mucosal bleeding time before surgery
- dobermans, scotties, shelties, german shepherds, etc.
signalment relevant to anesthesia: neonates
- pediatric patients (up to 12 weeks old) are prone to dehydration, hypothermia and hypoglycemia
- immature liver is less able to biotransform drugs
- less able to handle volume overload or blood loss
- renal insufficiency may decrease ability to excrete drugs and predispose to subsequent kidney failure
signalment relevant to anesthesia: geriatric patients
- liver changes may increase half life of some anesthetic drugs
- more susceptible to hypoxia, hypercarbia, hypovolumia and hypothermia
- need particularly close monitoring and support during procedure and recovery
signalment relevant to anesthesia: weight
- determines drug dosages and fluid administration rates
- determines O2 flow rates and type of anesthetic equipment used
- obesity compromises cardiovascular system, restricts diaphramatic movment
- extreme thinness may prolong anesthetic recovery, increase heat loss, require padding to decrease pressure
signalment relevant to anesthesia: sex/sexual status
- intact females may be in estrus/pregnant which can increase chance of bleeding during OHE
- many drugs may effect fetus or cause abortion
physiological changes caused by pregnancy
- increase HR, increased CO
- decreased lung volume, increased respiratory rate (more rapid induction, need lower vaporizer setting), decreased PaCO2, decreased MAC
- Very susceptible to hypoxemia, preoxygenate
signalment relevant to anesthesia: temperament
aggressive animals may be impossible to evaluate prior to anesthesia, leading to greater risks
patient evaluation: patient history relevant to anesthesia
- past or present illness, recent drug therapy
- last eat
- recent weight gain/loss, V/D
- previous problems with anesthesia
- drug allergies
patient evaluation: physical exam
- day of procedure, to discover any acute abnormalities that may affect plan for anesthesia
- essential during preanesthetic period to observe behavior, look for abnormalities, determine baseline parameters for comparison before and after anesthesia
- Emergency procedure, E after classification
- Try to stabilize dehydration, hypovolemia, anemia, hypoproteinemia, cardia dysfunction, respiratory distress, renal dysfunction, hemostatic defects, temperature abnormalities
determination of anesthestic risk
Potential for survival of anesthesia, determined by type of surgery, surgeon's competence and speed, anesthetist's competence, animals physical status
classification of patient physical status: Class I
- normal healthy paitent, no systemic disease, minimal risk.
- Basic spay/neuter on a healthy animal
classification of patient physical status: Class II
- patient with milk systemic disease, no systemic signs. Slight risk.
- neonate or geriatric, controlled diabetes mellitus, no organ damage
classification of patient physical status: Class III
- Patient with severe systemic disease limiting activity but not incapacitating. Moderate risk.
- Fever, anemia, anorexia, moderate dehydration, low grade heart murmur, low grade kidney dz
classification of patient physical status: Class IV
- Patient with incapacitating systemic disease that is a constant threat to life. High risk.
- toxemia, decompensated failure, ruptured bladder, early shock, cachexia, severe dehydration
classification of patient physical status: Class V
- Moribund patient not expected to live 24h without surgery. Grave risk.
- Severe trauma, advanced cardiac, kidney, liver, lung disease, terminal malignancy, late shock.
classification of patient physical status: Class VI
- A declared brain-dead patient whose organs are being removed for donor purposes
- Not used much in animal meds.
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