Ch.18 Balanced Inhalation Anaesthesia Flashcards
Goal of equine anaesthesia
Maintenance of good intraoperative cardiopulmonary function, followed by calm pain-free and co-ordinated anaesthetic recovery
What is believed to be the cause of foal deaths when using inhalant drugs (iso, servo, des) for induction
Rapid uptake of newer, less soluble inhaled agents in young animals with an immature cardiopulmonary system.
What is drug potency represented by?
MAC
Minimum alveolar concentration -
end tidal alveolar concentration of inhalation anaesthetic that prevents movement in 50% of subjects in response to a noxious stimulus.
MAC of Halothane
0.88%
MAC of Isoflurane
1.31%
MAC of Sevoflurane
2.31%
MAC of Desflurane
7.6%
How are Iso, sevo and des metabolised and at what rate
Liver
ISO - 0.2%
SEVO - 3-5%
DES - 0.02%
How do iso, sevo and des decrease blood pressure
Decreased peripheral resistance
How does Desflurane affect CO at 1 MAC
Does not depress CO
What plasma levels of lidocaine are required to reduce the MAC of Halothane by 50-70%
> 5ug/ml
Lidocaine at 1.3mg/kg over 15mins followed by 50ug/kg/min decreased the MAC of Sevo by what %
27%
At what plasma levels are signs of ataxia seen in the standing horse
1.85-4.53ug/ml
Intra op lidocaine protocol dose
0.65-1.3 mg/kg over 15 - 20mins
Followed by CRI 25 - 50 ug/kg/min
Intra op CRI rate of Medetomidine
3.5 ug/kg/hr
Intra op CRI rate of Dexmedetomidine
1.75 ug/kg/hr
Intra op CRI rate of Xylazine
0.3 - 0.6 mg/kg/hr
Intra op CRI rate of Detomidine
10 ug/kg/hr
Intra op CRI rate of Romifidine
18 -40 ug/kg/hr
Intra op CRI rate of Ketamine
0.5 - 1 mg/kg/hr
Intra op CRI rate of S (+) Ketamine
0.5 - 1 mg/kg/hr
What is the maximum length of time a Ketamine CRI is advised for owing to rough recoveries
1.5 hours
Which horses are at risk of toxicosis following lidocaine
Ones with compromised cardiovascular function with resultant reduced liver blood flow and metabolism
How does lidocaine decrease the MAC of inhalant anaesthetics
Dose dependently
How do Alpha 2 adrenoreceptor agonists decrease the MAC of inhalant anaesthetics
Dose dependently
For how long does the bolus of alpha 2 adrenoreceptor agonists negatively impact cardiopulmonary function in the anaesthetised horse
20 -120 mins
Which alpha 2 adrenoreceptor agonists have the shortest half life
Medetomidine and dexmedetomidine
For how long does the bolus of alpha 2 adrenoreceptor agonists negatively impact cardiopulmonary function in the standing sedated horse
10 mins
Which has a better anaesthetic recovery Medetomidine CRI or Dexmedetomidine CRI
Dexmedetomidine
How does recovery following a lidocaine balanced anaesthesia compare to medetomidine balanced
Medetomidine recovery is longer but of better quality
How does management of a lidocaine balanced anaesthesia compare to medetomidine balanced
Medetomidine - fewer drugs administered during the anaesthesia
How does monitoring a horse’s eye differ when balancing anaesthesia with medetomidine
Brisker eye reflexes
Only nystagmus may indicate the horse is light
How does xylazine 1mg/kg/hr CRI in balanced anaesthesia affect the cardiovascular system and recovery
HR Lower
BP Higher
Recoveries longer but better quality
On average Alpha 2 adrenoreceptor agonists reduce MAC by what%
30%
What is the currently licensed form of Ketamine?
Racemic mixture of S-Ketamine and R-Ketamine
What is postoperative wind up
A state of reactivity of the cns that can occur following tissue damage leading to maintenance of pain even after the injury has healed
What duration of ketamine infusion has been associated with undesirable excitatory CNS effects
Over 1 - 2 hours
or
repetitive IV boluses
Describe the emergence reactions associated with ketamine
Muscle tremor and rigidity
Mydriasis
Oculogyric movements
Sweating
Excitation
Ataxia
Schizophrenia like behaviour
What is accountable for emergence reactions when using Ketamine
Plasma conc
Length of drug infusion
Formation of S-norketamine
How to minimise emergence rxns when using ketamine
Lower dose
Gradual decrease to stop 15/20 mins before end of sx
Alpha 2 adrenoreceptor agonists for recovery
or use 1.1mg/kg IV S-Ketamine
and maintain on CRI AT 0.5 mg/kg/hr with iso and oxygen
What is associated with better recovery when combined with guafenesin Racemic Ketamine or S-Ketamine
S-Ketamine
Experimental studies of opiod use during inhalant anaesthesia shows what changes in MAC
No change
Except for high doses of Fentanyl
Changes in MAC from Morphine CRI
None
Effects of 0.02 mg/kg Butorphanol on anaesthesia when combined with Iso
Deeper anaesthesia
Sympathetic stimulation caused by sx is blunted
Former name of Guaifenesin
Glyceryl Guaiacolate
Advantages of using Guaifenesin
Potentiates other sedatives/anaesthetics drug
Good relaxation of laryngeal and pharyngeal muscles
Relaxation of skeletal mm
Does not affect diaphragmatic or respiratory function
No effect on CO or BP
What is the dose range of Guaifenesin
50 - 100 mg/kg
Risks of Guaifenesin use
Irritant to vessels
10% Guaifenesin very high risk (7/7) of thrombus formation and reports of hemolysis
Do benzodiazepines have analgesic properties?
No but they potentiate analgesic properties of co-administered drugs
Risks of using benzodiazepines intra-operatively and how to avoid
Ataxia after standing in recovery
Reverse using benzodiazepine antagonist such as flumazenil
How is lidocaine metabolized
By the liver
True or False
Lidocaine has a long half life?
False
In healthy awake horses, lidocaine toxicity has been observed in plasma levels exceeding..
1.85 - 4.35 ug/ml
True or False
Alpha 2 adrenoreceptor agonists are potent analgesics
True
True or False
Medetomidine has a short half life
True
Does Ketamine depress CO
No
Causes sympathetic stimulation which may help maintain cardiac function when anaesthetised
Does Guafenisin have a wide or narrow window of safety
wide
Where does Guaifenesin elicit its affects
Brain stem - potentiating other sedative and anaesthetic drugs
Does Guaifenesin affect respiratory function
No effect on diaphragm or resp function when used in clinical doses of 50-100mg/kg
What % Guaifenesin has been assoicated with Hemolysis
10%
