Inhalational Agents Flashcards
Halogenated inhalational agents
Halothane Enflurane Isoflurane Sevoflurane Desflurane
Inorganic molecule used as inhalational anaesthetic agent
Nitrous Oxide
Old anaesthetics no longer used
Chloroform - liver toxicity
Ether
Methoxyflurane - kidney toxicity
Cyclopropane
Critical temperature (defn)
Temperature above which a substance cannot be liquefied, regardless of how much pressure applied (ie. It is a gas)
*anaesthetic agents are below critical T at room temperature (vapours)
Inhalational agent with boiling point near room T, requiring special vapouriser
desflurane
Uses of inhalational agents (4)
- Maintenance of anaesthesia (ventilator/spontaneous breathing)
- Sevoflurane/halothane –> Induction
- N2O –> Analgesia (labour, intra-op)
- N2O –> second gas effect, diluting potent agents in maintenance
The ideal inhalational anaesthetic agent (10)
Cheap Stable (non-flammable, easy storage) No metabolism (less toxic effect) Potent (can use hi O2 concentration) No long term effects/resp depression/cardiovascular depression Non-irritant Hypnotic and analgesic Readily reversible Neuroprotective Non excitatory
Second gas effect
N2O’s effect of speeding up an inhalational induction
The agent analyser’s end tidal value reading indicates ___________
Alveolar concentration of anaesthetic agent
Benefits of inhalational induction
Use in Children
Safer if Compromised Airway (inhalational - pt breathes spontaneously, IV - risk of apnea and loss of airway)
Stages of anaesthesia can be observed and used to ensure anaesthetic adequacy and prevent overdose
FiAA
Inspired concentration of anaesthetic agents
FaAA
Alveolar concentration of anaesthetic agent
Factors which INCREASE MAC (1+1+2+3)
Infancy Hyperthermia Hyperthyroidism Hypernatraemia Chronic opioid use Chronic alcohol intake Acute Amphetamine intake
Factors affecting FaAA
- Inspired concentration (Fi)
- Uptake
- Alveolar ventilation
The greater the uptake, the _____________ the rate of induction and recovery/emergence
Slower
?Dye dropping into water: If water is flowing fast, the dye will continually be diluted, and take longer to change color
?People getting onto a train: if the train is in a rush, it stops for a short time and will take longer for the train to fill.
Respiratory depression and Airway obstruction cause ____ventilation and will ____________ induction
Hypoventilation —> deccelerate
Factors affecting anaesthetic uptake
Solubility of agent in blood (BGPC)
Cardiac output (train analogy)
Alveolar to mixed venous partial pressure difference**
V-Q shunt
NO2 uses
Analgesia intra-op and in labour
Second gas effect during induction
Reducing potent volatile agents during maintenance
Blood : gas partition coefficient (BGPC)
Ratio of the amount of volatile present in blood compared with the alveolar gas, when the two phases (gas and blood) are of equal volume and at equilibrium
Indicates agent’s solubility in blood
Hyperventilation will ________ induction
Accelerate
Halothane (BGPC = 2,3) is a __________(more/less) soluble volatile; therefore it is a ___________(fast/slow) induction agent.
Halothane is a highly soluble volatile, and therefore a slow induction agent.
Hi solubility –> large quantities removed from lung by circulation –> lo PaAA –> slow rise in arterial and brain partial pressures
Factors affecting FiAA
- Volume of breathing circuit
- Fresh gas flow rate
- Absorption of inhalational agent by breathing circuit
The substance used in muscle relaxants, initially used by Pygmy hunters to shoot monkeys in trees
Curare
Shunts affecting arterial concentration of anesthetic agent (FaAA)
Intrapulmonary: atelectasis, bronchial intubation
intracardiac: ASD, VSD, Fallot’s tetralogy
Factors which DECREASE MAC (non-drug)
Neonates and elderly Pregnancy Hypotension Hypothermia Hypothyroidism
MAC of Halothane
0.75
Which class of drugs significantly lowers MACbar
Opiates
Sevoflurane is a __________(more/less) soluble volatile; therefore it is a ___________(fast/slow) induction agent.
Sevoflurane has low solubility therefore it is a fats induction agent
Factors which DECREASE MAC (drug)
Acute opioid use Acute alcohol intake Chronic Amphetamine intake Lithium A2 agonists Sedatives
Desflurane, Sevoflurane and NO2 are __________(poorly/highly) soluble agents with ______(slow/rapid) recovery and induction.
These are poorly soluble agents with RAPID induction and recovery.
2 volatiles which are metabolized in the liver as well as via the lungs
HE! Halothane and enflurane!
Minimum alveolar concentration (MAC)
The steady-state minimum alveolar concentration at sealevel that prevents movement to a standard surgical stimulus (skin incision) in 50% of non premedicated adults. (MAC50)
POTENT ANALGESIC POOR ANAESTHETIC
NO2
Desflurane has a LOW potency; therefore it requires a ______ (higher/lower) MAC
HIGHER. The MAC of Desflurane is 6.
Halothane has a HIGH potency due to its HIGH fat solubility therefore it requires a ______(higher/lower) MAC.
halothane requires a LOW MAC I.e. 0.75
In a clinical situation, what is the appropriate amount of agent (in terms of MAC) that will provide surgical anaesthesia in the patient
1.3 x MAC
MACawake
Average of concentrations above and below those permitting VOLUNTARY RESPONSE to COMMAND
MAC of NO2
105
Factors which do not affect MAC
Gender
Duration of anaesthesia
Hypocarbia
Time of day
MACbar
The alveolar concentration that Blocks Autonomic Response to surgical stimulation.
MAC of Desflurane
6
