Volatile Agents Flashcards
Give reasons why the following agents are no longer in use: Cyclopropane Ether Chloroform Methoxyflurane
Cyclopropane - flammable
Ether - flammable
Chloroform - hepatotoxic
Methoxyflurane - Nephrotoxic
Which is the only inhalational agent that is a gas at room temperature?
N2O
Define the term volatile
Volatile: Easily evaporated at room temperature
Define volatile anaesthetic agents
Potent halogenated hydrocarbons existing as volatile liquids at room temperature. All of these drugs have a carbon skeleton with Fl, Br, Cl substituting some of the hydrogen molecules
What is the benefit of the halogens constituting volatile agent’s molecular make up
The halogen atoms render the drugs STABLE AND NON-FLAMMABLE
What are the advantages and disadvantages of ether
Advantage
- Cheap
- Minimal side effects
Disadvantage
- Flammable
- Very slow onset and offset of action
Define critical temperature
The critical temperature of a substance is the temperature at which a substance cannot be liquefied regardless of the pressure applied
Define a vaporizer
An agent specifically calibrated device that converts the liquid volatile agent into gaseous form and delivers it into the fresh gas flow towards the patient
What are the colour codes for each different volatile agent
Isoflurane - purple
Sevoflurane - yellow
Desflurane - blue
Halothane - red
Why does desflurane require a unique vaporizer
Unlike the other volatile agents, desflurane’s boiling point (23.5 deg C) is near to room temperature. This means that at STP desflurane is likely to vaporize completely. A special vaporizer that can control the temperature and pressure of desflurane prevents this and ensures that desflurane is maintained at an appropriate temperature and pressure for gauged delivery into the fresh gas flow.
List the uses of volatile agents
- Maintenance
- Induction
- halothane or sevoflurane
- needle phobic children
- compromised airway (no apnoea and quickly turned off) - Analgaesia - N2O only
- Entonox 50/50 O2/N2O for labour and dental
- N2O as adjuvant potentiating hypnosis and providing analgaesia - N2O may speed up the anaesthetic via the second gas effect but cannot provide adequate hypnosis on its own.
List 10 features of an ideal inhalational anaesthetic agent
Cheap: Agent/Equipment Stable: Storage/Transfer/Soda lime/non-flam No metabolism: min S/E Potent: allow for high FiO2 Min long term tox: Staff/Patient/ENV
Airway: Non-irritant Breathing: No RPS depression Circulation: No CVS depression Disability: - Good hypnosis and analgaesia - neuroprotection - readily reversible - Non-excitatory
List the factors that determine the FiAG = Fraction of inspired anaesthetic gas
- % dialed into vaporizer = %entering breathing circuit
- Volume of breathing circuit
- FGF
- Absorption of VA by breathing circuit
List the factors that determine the FAAG = Fraction of Alveolar Anaesthetic Gas
- FiAG
- Increased FiAG –> increased alveolar concentration AND rate of rise of its concentration and hence the speed of induction - Uptake
A) Solubility
Uptake of AG from alveoli –> slows induction as it reduces the FAAG.
- high solubility (higher B:G) agents (e.g. halothane = 2.3) induce slower
- low solubility agents (lower B:G) agents (e.g. sevoflurane 0.6) induce faster.
B) Cardiac Output
- Increased CO results in increased uptake and slows rate of rise of FAAG and slows induction
C) Alveolar to mixed venous blood partial pressure difference
- Gradient depends on amount of AG taken up by tissues and therefore: tissue solubility (O:G), tissue blood flow, conc. gradient between blood and tissues..
- Alveolar ventilation
- Hyperventilation –> constantly replacing the alveoli with AA taken away by the pulmonary blood flow.
List the factors that affect the FaAG = Fraction of arterial Anaesthetic Gas
We assume this to be the same as FAAG but will not be in the instances of intrapulmonary (atelectasis/endobronchial intubation) and intracardiac shunts (ASD/VSD/TOF). Blood within a shunt does not come into contact with a ventilated alveoli and hence does not take up AG. This will then dilute the AG taken up by those capillaries that encounter ventilated alveoli.
Describe the metabolism and elimination of volatile agents
Overall approximation:
98% - eliminated unchanged by lungs
2 % - P450 enzymes
Recovery faster with lower uptake (Solubility | CO | Alveolar-venous gradient) –> anaesthetic depth can also be more rapidly controlled with the less soluble agents.
Define MAC and state its purpose
MAC = Minimum Alveolar Concentration.
The steady-state minimum alveolar concentration at sea level (101.3 kPa) that prevents movement in response to a standard surgical stimulus (defined as skin incision) in 50% of non-premedicated adults. This is the MAC 50.
MAC 95 (95% of adults will not respond) = 1.3 x MAC50
Potency = Oil: Gas partition coefficient - the higher the potency the lower the MAC
MAC has limited clinical use and is an entity used to compare the potency of volatile agents. It is a dosage guide as each patient is unique.
What is potency of an inhalational anaesthetic agent
Potency corresponds to the oil:gas partition co-efficient of the agent. The higher the potency of an anaesthetic agent the lower the MAC and hence the lower the partial pressure of the agent required to produce the desirable level of hypnosis during an anaesthetic. Potency IS NOT a measure of speed of induction, it merely quantifies how much of an agent is required to produce an effect.
Are MACs additive
Yes. 0.7 MAC of Halothane + 0.6 MAC N2O = 1.3 MAC worth of anaesthetic effect.
List factors that increase and decrease MAC
Increase MAC | Decrease MAC
Infancy Elderly/Neonates
| Pregnancy
Hyperthermia | Hypothermia
Hyperthyroidism | Hypothyroidism
Hypernatraemia | Hypotension
Catecholamines/SNS | a- 2 agonists
| Sedatives
Chronic opioid | Acute opioid
Chronic alcohol | Acute alcohol
Acute amphetamine | Chronic amphetamine
| Lithium
List factors that do NOT affect MAC
Biological Sex
Duration anaesthesia
Time of day
Hypocarbia
Why is Xenon not used
Too expensive
What is the MAC of N2O
105%
What is the difference between a gas and a vapour at room temperature
Critical temperature is the temperature above which a substance cannot be liquified however much pressure is applied.
A vapour is a substance that is below its critical temperature at room temperature. A gas is a substance above its critical temperature at room temperature
Why can’t a pressure gauge be used to measure the remaining N2O in a cylinder. How can the remaining N2O in the cylinder be measured?
At room temperature, N2O is below its critical temperature (36.5 deg C). Therefore, inside a cylinder the pressure remains constant as liquid is converted into vapour with incremental depletion of the N2O cylinder and this occurs at a constant pressure ± 5000 kPa.
The cylinder can be weighed and this number subtracted from the tare (unladen) weight. The number of moles can then be calculated using n = m/M
Is N2O flammable/explosive
No. But like O2 it supports combustion.
How is N2O made?
By heating ammonium nitrate and removing impurities
List 5 desirable properties and uses of N2O
Potent analgaesic Weak hypnotic carrier gas, Rapid induction (2nd gas effect) Pleasant smelling Entonox (Labour/small ED procedures/dental)
List and explain the side effects of N2O
- CVS depression when used with high dose opioids
- RSP depression with opioids/induction agents
- Pneumothorax/Pneumocephalus/Intraocular air/Pneumomediastinum/Pneumoperitoneum/Middle ear –> expansion (Higher B:G than N2 therefore diffuses in faster than N2 diffuses out leading to rapid expansion of these air filled spaces.
- Diffusion hypoxia
- When N2O is discontinued –> decrease N2O Alveolus –> increased N2O gradient into Alveolus from blood –> rapid movement N2O into alveolus diluting O2 (diffuses more slowly) –> Rx give FiO2 0.4 - 0.9 for 5 - 10 mins at discontinuation of N2O to prevent diffusion hypoxia. - PONV
- Bone marrow suppression (prolonged administration) - megaloblastic picture by destroying Vit B12
- Addictive
Why are volatile agents called ‘volatile’ agents
Because they exist as liquids that evaporate easily at room temperature.
What is the colour code, MAC, % metabolism, B:G of Halothane
Red. 0.75%. 20%. 2.4
Summarise the physical properties of Halothane
- Halogenated hydrocarbon (no ether bond)
- Colourless liquid decomposed by light
- amber bottles with thymol 0.01% stabiliser - Non-irritant - reasonably pleasant smell
- Induction: 1 - 2 %
- Maintenance: 0.5 - 0.75%
Compare the CVS effects of the volatile anaesthetic agents
Halothane
- Myocardial suppression
- Decreased SVR
- Sensitize to Catecholamines –> Dysrhythmia: Brady and nodal rhythm
Isoflurane
- Myocardial suppression (mild)
- Decreased SVR
- Increase HR and “Coronary steal”
Sevoflurane
- Myocardial suppression (mild)
- Decreased SVR (mild)
- Preferred for cardiac patients
Desflurane
- Myocardial suppression (mild)
- Decreased SVR
- Increase Catecholamines + HR/BP with rapid increase in desflurane
Describe the effects on RSP of all the volatile agents
Increase RR Decreased Vt Increased PaCO2 All Bronchodilate No effect on PVR
Isoflurane and Desflurane are airway irritants
Describe the effects of the volatile agents on the CNS
Increase Cerebral Blood flow
Increase ICP
Decrease CMRO2
Decrease seizures
Isoflurane agent of choice for neuro-anaesthesia –> least effect on cerebral blood flow and ICP
Halothane “hangover”
Describe the neuromuscular effects of the volatile agents
Potentiation of non-depolarising muscle relaxation
Trigger for Malignant hyperthermia
Describe the hepatic effects of the volatile agents
All agents reduce hepatic blood flow
Isoflurane reduces it the least by maintaining hepatic artery blood flow.
Halothane:
- 20% metabolized in liver (excreted over many days)
- Transient postoperative transaminitis quite common
- Halothane hepatitis
- Rare complication (1 in 35 000)
- Fulminant hepatic necrosis with 50 - 75% mortality
Isoflurane
- 0.2 % metabolized
- agent of choice for patients with liver disease –> preserves hepatic arterial flow
Sevoflurane
- 3 - 5% metabolized in the liver
Desflurane
- 0.02% metabolized
Describe the effects of the volatile agents on the kidneys
Sevoflurane
- 3 - 5% metabolized in liver releases: inorganic fluoride –> not sufficient quantities to cause renal failure
- Compound A (Sevo + Soda lime) is potentially nephrotoxic –> no clinical significance at normal concentrations. Some countries limit fresh gas flows to >2 L/min to minimize accumulation of compound A but no evidence for this.
- Avoid Sevoflurane in patients with established renal failure
Other volatile agents decrease RBF, GFR, UO.
Describe the effects of the volatile agents on the Uterus
All agents cause uterine muscle relaxation increasing the risk of bleeding
What are the effects of the volatile agents on IOP
All Decrease IOP
List the the VAs in order of washout time and state the MACs and B:G of each
Name | B: G | MAC
- Desflurane | 0.45 | 6.6
- Sevoflurane | 0.7 | 2.0
- Isoflurane | 1.4 | 1.2
- Enflurane | 1.8 | 1.7
- Halothane | 2.4 | 0.75
State 3 features of Desflurane which necessitate a special vaporizer design
- SVP = 89.2kPA (other volatile agents ± 25 kPa)
- Higher SVP means the number of molecules above the liquid is much higher –> standard flow over vaporizers will collect more vapour molecules versus the other agents –> this would require a very high FGF to dilute this to clinically useful concentrations (uneconomical) required to be PRESSURIZED - Very high latent heat of vaporization = amount of energy required to convert a liquid state into its vapor state.
Liquid state - tighter bonds less kinetic energy
Vapour state - higher kinetic energy
Where does this extra energy come from –> the molecules draw heat energy from the liquid when they become vapors = latent heat of vaporization —> very fast cooling of the vapor requiring HEATER. (need consistent vapor output and constant temperature) - Boiling Point is near room temperature at 23 deg C –> with standard flow over vaporizers possibility for rapid and wasteful vaporization
Describe the adaptations made to the TEC 6 vaporizer to deliver Desflurane
- Heater
- Diaphragm
- Differential pressure transducer
- Flow control valve (dependent on differential pressure transducer)
- Second flow control valve (controlled by anaesthetist)
- Heater to heat Desflurane to 39 deg C –> this increases the SVP to 173 kPa and is done combined with the other components to achieve constant, predictable and controlled VA output
- Diaphragm that separates the fresh gas flow from the vaporizing chamber –> connected to a differnetial pressure transducer which senses the amount of FGF –> this controls another flow control valve which permits a specific amount of desflurane vapour to be released depending on the FGF sensed by the diaphragm and differential pressure transducer
- A second Flow control valve is controlled by the anaesthetist and allows for micture of the specified amount of desflurane with the fresh gas flow arm of the desflurane vaporizer
Why is O2 stored as a liquid
Oxygen is stored as a liquid in larger hospitals as it is more economical. A larger quantity of Oxygen can be stored as a liquid in a large insulated, refrigerated and pressurised container (as it can only be liquified when stored below its critical temperature. This lasts longer and has fewer labor, transport and personal costs for the hospital than does storing O2 in multiple tanks via a manifold system
How is liquid oxygen stored in large hospitals
- As a liquid
- Below its critical temperature of -119 deg C
- In a large pressurised, refrigerated and insulated tank. The insulation is achieved with a surrounding chamber containing a vacuum to minimise heat transfer between the environment and the liquid oxygen.
Describe how surgical diathermy works
Electrosurgical units (ESUs) = surgical diathermy
ESUs generate an ultra high frequency electrical current that passes from a small active electrode (the cautery tip) through the patient and exits by way of a large plate electrode (the dispersal pad or return electrode)
- The HIGH CURRENT DENSITY at the cautery tip allows for tissue coagulation or cutting (depending on the electrical waveform)
How is ventricular fibrillation prevented by the design of the diathermy
Through use of Ultra-high electrical frequencies:
- Surgical diathermy = 0.1 - 3 MHz
- Line power = 50 - 60 Hz
What part of the design of the dispersal pad (large plate electrode) prevents burns to the patient
The large surface area of the low impedence electrode at the currents point of ‘exit’ provides a LOW CURRENT DENSITY
How does the diathermy cause electrical interference
The high power levels of electrosurgical units (ESUs) can cause INDUCTIVE COUPLING with monitor cables leading to electrical interference
How can malfunction of the dispersal pad occur
- Disconnection from the Electrosurgical Unit (Surgical Diathermy)
- Inadequate patient contact
- Insufficient conductive gel
What are the consequences of dispersal pad (large plate electrode) malfunction
The current will find another place to exit the body.
E.g.
1. ECG pads
2. Metal parts of the operating table
This may result in burns
What precautions can be taken to prevent diathermy burns
- Proper dispersal pad attachment
- Proper dispersal pad site
- Away from: metallic implants, bony prominances,
