Inhaled and Gaseous Agents Flashcards
Define Saturated Vapor Pressure
The pressure exerted by a vapour when it is completely saturated and in equilibrium with its liquid form, and is a measurement of volatility
Define Solubility
the ability of a liquid or gas solute to dissolve into a liquid or gas solvent
Define Partition Coefficient (blood:gas and oil:gas)
Blood Gas solubility coefficient (e.g. alveolar-blood interface) –> related to the rate of induction and recovery
- High solubility = halothane = slower speed of induction and recovery
- Low solubility = sevoflurane
Oil Gas solubility coefficient (e.g. blood-brain interface) –> is a measure of anesthetic potency i.e. how much is required to cause anesthesia in the brain
- High solubility = halothane = lower MAC value required to achieve anesthesia = more potent the agent is
- Low solubility = desflurane
Define MAC
The Minimum concentration of anesthetic agent in the alveoli at which 50% of patients will not exhibit a gross purposeful reponse (movement) to a particular surgical stimulus (tail clamp/ skin incision)
- This measurement allows for the comparison of potency of inhalational anesthetics
The higher the MAC, the larger the mount of agent is needed to produce anesthesia, thus the lower the potency
- Halothane MAC = 0.8% = very potent
- Desflurane MAC = 7% = less potent
Isoflurane, Sevoflurane and Desflurane are all __________
Halogenated ethers (Vapours)
Nitrous Oxide and Xenon are both _______
Inhaled Gases
Which of the Halogenated ethers can be used in food-producing animals?
Isoflurane
Isoflurane, Sevoflurane and Desflurane are all __________
Halogenated ethers (Vapours)
Which of the Halogenated ethers can be used in domestic animals?
Isoflurane
Sevoflurane
Why is Nitrous Oxide not used in vet medicine anymore?
- Negative environmental effects
Why is Xenon rarely used in vet medicine?
VERY expensive to purchase and use
What factors increase the speed of induction with inhaled agents?
1) Increase inspired anesthetic concentration
2) Increase alveolar ventilation
3) Decrease removal from alveoli
How can you increase the anesthetic concentration being delivered to a patient?
1) turn up the dial on the vaporizer
2) increase the fresh gas flow (in a circle system)
3) increase vaporization of the agent
4) decrease gas volume of the breathing circuit
note: in order for any of these to work, the patient has to be breathing properly
How can you increase the alveolar ventilation of the anesthetic agent?
1) Increase tidal volume, respiratory rate or both
2) Decrease residual capacity/ dead space ventilation
How can you decrease the removal of the anesthetic agent from the alveoli?
1) Use agents with lower blood:gas solubility coefficients
2) Decrease cardiac output
3) Decrease solubility in tissues
What are the effects on the CNS, CVS, RS, GIT and Renal system of using Inhaled Anesthetics?
CNS: depression + increase blood flow to the brain due to vasodilation –> Increase in intracranial pressure despite lower metabolic activity
CVS: Lower blood pressure + tissue perfusion due to vasodilation, lower cardiac contractility and HR
+/- arrhythmias especially with Halothane and Desflurane
RS: depressed ventilation, resulting in respiratory acidosis and an increased blood flow to the brain
Renal + Cerebral perfusion: both decreased
GIT: reduced lower esophageal tone, thus more likely to regurgitate under anesthesia
What are the effects of Isoflurane specifically on the RS, CVS and environment?
RS: Depressant, leading to hypoventilation
CVS: drop in total peripheral resistance due to vasodilation, and a drop in BP
Environment: has the LEAST ozone-depletion potential of all anesthetics
What are the effects of Sevoflurane specifically on the RS, CVS and environment?
RS: less depression than iso
CVS: similar to isoflurane- drop in total peripheral resistance due to vasodilation, and a drop in BP
Environmental: is the LEAST global warming potential
What are the effects of Desflurane specifically on the RS, CVS and environment?
RS: respiratory depression, airway irritation
CVS: doesnt sensitize the myocardium, but can cause sympathetic storms (arrhythmias)
Why is Sevoflurane stored in plastic bottles instead of glass?
Stored in plastic bottles as the aluminium impurities in glass can lead to reactions with the anesthetic + render it useless
Why does Desflurane need a fancy vaporizer?
This is because Desflurane boils at 22.8 C, unlike the other agents
Why was Halothane taken off the market?
e.g. what were its risk factors?
- Had a profound depression on RS
- Lead to a large drop in blood pressure due to reduced contractility and stroke volume of the heart
- Sensitizes the myocardium, leading to arrhythmias
- Hepatotoxic = halothane hepatitis
What are the gaseous agents used in vet medicine?
Nitrous Oxide
Xenon
What are the benefits of using Xenon over Nitrous Oxide?
- Fewer environmental effects
- Lower blood:gas coefficient = rapid onset of induction
- Stable CVS effects
- Provides profound analgesia at the NMDA receptors
- Does NOT activate the SNS, thus no increase in cortisol
- Produces the highest regional blood flow to the brain, liver and kidneys
Can Xenon be used in vet medicine?
Yes, just not readily available. Likely only seen in vet colleges
Can Xenon be used in vet medicine?
Yes, just not readily available due to cost. Likely only seen in vet colleges
What is the lifespan of Sevoflurane, Desflurane and Nitrous Oxide in the environment?
Sevoflurane = 1 year
Desflurane = 14 years
Nitrous Oxide = 114 years
What factors are MAC-sparing, and thus reduce the requirement for volatile agent ?
- The use of sedative and opioid drugs and local techniques
- Other anesthetics (PIVA, CRI’s or N2O)
- Age: the very young and old have reduced requirements
- Temperature: for every 1C lower, this reduces requirement by 5-10%
- Pregnancy: due to the effects of progesterone in the pregnant dam
- Species
- Conditions such as hypotension, anemia and hypoxemia
What are the greatest advantages of using Sevoflurane over Isoflurane?
- Faster speed of induction + recovery
- ## Speed of changing depth of anesthesia is quicker due to less solubility
What are the greatest advantages of using Sevoflurane over Isoflurane?
- Faster speed of induction + recovery
- Speed of changing depth of anesthesia is quicker due to less solubility
- Neuroanesthesia is better: ideal for MRI, CSF tap
What are the advantages of Nitrous Oxide?
- It provides some analgesia
- Reduces MAC of other inhalant agents
- Causes minimal cardiopulmonary depression
- Is not metabolized
- Non-irritating
- Non-arrhythmogenic
- Increases the speed of uptake of other inhaled agents especially at the beginning of anesthesia
Why is Nitrous Oxide not popular in vet medicine?
- Cause easily cause hypoxemia as a minimim of 33% oxygen is needed
- It collects in gas-filled cavities e.g. gastro-intestinal tract and phenomothorax
- At the end of anesthesia it can cause diffusion hypoxia by replacing oxygen in the alveoli
- Health hazards to humans
- Additional monitoring equipment is needed to use it safely
- It is expensive
- Is a green house gas
What MAC-sparing techniques do we have now that have largely replaced Nitrous Oxide in vet medicine?
Analgesia:
- Local blocks (e.g. epidural)
- CRI’s: Medetomidine, ketamine, morphine, lidocaine, fentanyl and various combinations (e.g. MLK = Morphine + Lidocaine + Ketamine)
Describe the use, bolus dose and CRI dose of using a (Dex)Medetomidine CRI
Bolus Dose: 0.5 - 2 ug/kg IV
CRI dose: 1-2 ug/kg/hr
Use:
- Often helps make anesthesia more stable
- Used in combination with opioids to reduce the requirements
- Used when NSAIDs cannot be used
Describe the use, bolus dose and CRI dose of using a Ketamine CRI
Bolus Dose: 0.2-0.5 mg/kg IV
CRI dose: 5-10 ug/kg/min
Use:
- Useful for neuropathic pain
- Reduces the requirement for opioids
caution: bolus dose may cause transient (2min) apnea, which can be handled with positive pressure ventilation
Describe the use, bolus dose and CRI dose of using a Lidocaine CRI
Bolus Dose: 1 mg/kg IV
CRI dose: 15-50 ug/kg/min
Use:
- anti-arrhythmic = Tx for ventricular tachycardia
- pro-kinetic = tx for gut stasis
- reduces inhalent requirements
note: above doses are NOT anti-arrhythmic
caution: may cause CVS depression/ hypotension
caution: only use in dogs, not cats!
Describe the use, bolus dose and CRI dose of using a Morphine CRI
Bolus Dose: 0.1 - 0.2 mg/kg IV
CRI dose: 0.1-0.12 mg/kg/hr
Use:
- USeful post-op in combination with very low dose of acepromazine for increased sedation: must monitor mentation + regurgitation
Describe the MLK and FLK Infusion combination
MLK = Morphine + Lidocaine + Ketamine M 60mg L 500mg K 60 mg - In a 500mL 0.9% saline bag - Infused at 1ml/kg/hr (intra or post op)
FLK = Fentanyl + Lidocaine + Ketamine
