Inhalational Agents Flashcards
Triad of anaesthetics:
- Sleep ( IV or IAA )
- Analgesia
- Muscle relaxation
Concept of inhalation / vapour anaesthetic:
- Liquid > into vaporizer > into lungs > into vasculature > effect
Surrogate measure = alveolar partial pressure (PP)
Dalton’s law:
Total pressure = sum of the partial pressures of the various constituents
Henry’s law:
At T, amount of gas dissolved in liquid is proportional to the partial pressure of the gas
Partition coefficient:
how soluble a gas is in a given liquid (the ratio of the amount of a substance (at equilibrium) present in one phase compared with another, the two phases being of equal volume)
With 1L of blood and 1L of nitrous oxide, there will be 0.47 L of nitrous oxide in the blood at equilibrium that is the partition coefficient. (0.47/1.00 = 0.47 partition coeff)
Ether has a much higher partition coefficient than nitrous oxide. Thus, much more ether will move into the blood at equilibrium than nitrous oxide. Even at the same partial pressure.
**Go find a picture to properly understand this
What is the relation of gas partition pressure and the brain and the alveoli
It is the partial pressure of the inhalational agent in the alveolus that drives the amount of drug into the body. And that partial pressure determines the amount of drug taken up by the brain where it has the effect. We use the alveolar partial pressure as the surrogate measure for the amount of drug that dissolves in the brain – not the actual number of molecules.
incr [IAA] = incr alveolar partial pressure = faster induction &
incr [drug] in brain = incr effect
Phases of anaesthesia:
- Induction putting patient to sleep
- Maintenance keeping patient asleep
- Recovery waking patient up
The three-compartment model
- Alveoli
- Blood
- Brain
factors that influence the distribution of the drug
cardiac output and venous saturation
What is meant by venous saturation affecting distribution of drug in the body
A completely insoluble gas will not put patient to sleep because it will fill the alveoli, but not be able to dissolve into the bloodstream. However, an extremely soluble gas will also not put the patient to sleep as it will be metabolised faster than what it can actually enter the system – you would thus not reach an alveolar partial pressure that can put you to sleep.
Resultantly, the best gases to use for anaesthesia are poorly soluble ones.
Order the inhaled anaesthesia drugs in order of least soluble to most
- Nitrous Dioxide (completely insoluble = will have highest partial pressure in alveoli)
- Desflurane
- Sevoflurane
- Isoflurane
- Halothane
**Go look at the graph of this … Past Paper question
Three phases of inhalation anaesthesia - explain the whole process
- The sleep threshold is the level in the brain at which the patient will fall asleep.
- Turn up the vaporiser. Drug enters the alveoli, the bloodstream, and the brain. Once the sleep threshold is reached, the vaporizer is turned down to maintenance dose to keep patient asleep. Then, at the end of the operation, the vaporiser is turned off to allow recovery.
- When the vaporiser is turned off, the diffusion takes place in reverse. The drug now diffuses from the brain, into the bloodstream, into the alveoli, and then into the environment via expiration.
- The moment the level drops below the sleep threshold, the patient will start waking up.
**Go look at graph explaining this
What is a vapour?
a vapour is a substance that is in gaseous phase at a temperature that is below its critical point
What happens in a vapouriser?
In a vaporiser you have fresh gas flow going in (at least 30% oxygen). It then goes through the splitting valve where a percentage of the fresh gas enters the chamber, mixes with the liquid anaesthetic drug therein, becomes a saturated vapor, and then re-enters the flow to reach the patient. Vaporisers are agent specific!
Minimum alveolar concentration (MAC):
the alveolar concentration of an anaesthetic that prevents movement in 50% of subjects in response to a specific painful / surgical stimulus
- inversely proportional to solubility
- high MAC = low partition coefficient or low solubility
MAC-awake
the MAC of anaesthetic that abolishes response to verbal command in 50% of patients and appears to correspond to the concentration it abolishes learning and memory. This is less than the concentration required to prevent movement in response to surgery.
MAC-intubation
the alveolar concentration required to inhibit movement and coughing response to tracheal intubation.
MAC-BAR
the alveolar concentration adequate to block the autonomic response to skin incision.
Factors affecting MAC:
- Age > MAC is max at 6 months of age
- Pregnancy > MAC decreases
- Synergy
o Nitrous oxide
o Opiates
o Benzodiazepines
o Alcohol - Body temperature > MAC directly proportional to temperature
- Thyroid
- Transmitters
o Adrenaline
o Drugs
o Methyldopa, Reserpine, Prozac
Mechanism of action of inhalational anaesthetic drugs
- Increases / stimulates GABAA receptors
Halothane
Enflurane
Isoflurane
Sevoflurane
Desflurane - Inhibits NMDA receptors
Nitrous oxide
Describe the relationship with the apnea threshold and IAA
As CO2 rises in the body, you develop the need to start breathing faster. However, all inhalation agents cause blunting of your apnoea threshold and the point at which you stop breathing shifts right.
** GO LOOK AT THIS GRAPH
Halothane:
Induction + Maintenance
- Colour code = red
- MAC = 0.75
- Blood-gas partition coefficient = 2.5
- Smells okay
- Cardiovascular function= myocardial suppression, decr HR and CO decr BP
o Can cause ventricular extrasystole in presence of catecholamines and / or carbon dioxide of hypercarbia
- Respiratory function = decr the tidal volume, incr RR decr overall minute volume, leads to bronchodilation (very good for status asthmaticus), and may cause secretions
- Complications = halothane hepatitis
Isoflurane:
Only maintenance
- Colour code = purple
- MAC = 1.15
- Blood-gas partition coefficient = 1.4
- bad smell cannot use for induction
- Cardiovascular function = vasodilation which leads to decrease in BP
o May cause coronary steal … may cause ischaemia in area supplied by collateral
- Respiratory function = suppresses respiration and causes bronchodilation
- better than halothane in patients with traumatic brain injury as it doesn’t cause as much incr ICP as halothane.
- Drug of choice in neuro- and liver surgery
Sevoflurane:
- Induction + Maintenance
- Colour code = yellow
- MAC = 1.85
- Blood-gas partition coefficient = 0.65
o Much less soluble = much faster inductions and much faster recovery - drop in BP is mainly due to decr SVR ± decr contractility
- Smells nice (best drug to use for gas induction) but is quite expensive (great for paediatrics!)
- Hepatic function = causes slight increase in serum fluoride
- Very dry soda lime > may react to create compound A which is nephrotoxic (not in humans)
