Maintenance in Anaesthesia and avoiding awareness Flashcards
What are the fundamental features of volatile anaesthetic agents that enable us to provide anaesthesia safely and reliably
- Predictable ELIMINATION (delivered by the lungs and removed by the lungs)
- Predictable PHARMACOKINETICS (FiVA of 2 % = ± 2 kPA in blood at equilibrium
- MEASURABLE in real time in a gas mixture: providing confirmation of delivery and depth of anaesthesia
- Negative feedback if breathing spontaneously
What are the physical properties of an ideal volatile agent.
Name the agents that fall short of these ideal physical properties
Chemically stable in storage and use Volatile liquids at room temperature Please odour and non-irritant Non-flammable and non-explosive Low B:G --> rapid induction and recovery
Desflurane is close to its boiling point at room temperature. Therefore, it must be vaporised before adding to the inhaled gases (special vaporizer)
Desflurane and isoflurance are pungent and irritant
What are the administration properties of an ideal volatile agent.
Name the agents that fall short of these ideal administration properties
Easy administration via standard vaporizer
Rapid induction and recovery
Quick uptake/elimination from lungs/body
Desflurane requires a specific vaporiser
Volatiles with higher B:G take longer to achieve maintenance doses and emergence
What are the efficiency properties of an ideal volatile agent.
Sufficient lipid solubility (O:G) to allow high concentrations of O2 to be administered concurrently
What are the metabolism properties of an ideal volatile agent
Which agents are excreted unchanged
Entirely eliminated by the lungs with none metabolised by the body
Desflurane
Xenon
Financial properties of an ideal agent
Cost-effective
Highly fluorinated agents (Sevoflurane and desflurane) are expensive when used with high gas flows.
List the B:G solubility coefficients for the volatile agents
N2O: 0.47
Desflurane: 0.45 Sevoflurane: 0.65 Enflurane: 1.8 Isoflurane: 1.4 Halothane: 2.24
List the factors the influence speed of onset /offset
B:G (solubility) Va (Alveolar ventilation) CO (Cardiac Output) N2O (Second gas effect) FiVA (Concentration effect)
What % of isoflurane is metabolized?
0.2%
What % of Sevoflurane is metabolized?
3%
What % of Desflurane is metabolized?
0%
What % of Halothane is metabolized?
20% (immune hepatitis - rare)
How are the effects of volatile agents different in the elderly?
Elderly patients are more sensitive to volatile agents –> delayed emergence, drowsiness and confusion.
What is the Meyer-Overton hypothesis?
The Meyer-Overton hypothesis states that the potency of a volatile anaesthetic agent is proportional to its lipid solubility. This is illustrated when the Log of the MAC (%) is plotted against the log of the Oil:Gas partition coefficient and demonstrates that as O:G increases, the MAC decreases (increased potency).
What is MAC awake?
This is the MAC of anaesthetic producing unconsciousness in 50% of subjects.
How does MAC awake compare to 1 MAC
It varies between agents but approximates 0.5 MAC.
This concept implies that no matter how little the surgical stimulus (from opioids/RA), patients must receive enough volatile agent (0.5 MAC) to prevent awareness.
What factors increase MAC
Hyperthyroidism
Chronic alcohol use
Acute recreational sympathomimetics
Hyperthermia
What factors reduce MAC
Hypothyroidism
Acute alcohol intoxication
Chronic recreational SNSmimetics
Hypothermia
Hypoxia
CNS depressants
Opioids
Agent mixtures
Above what end tidal MAC of volatile agent has been associated with minimal awareness
0.7 MAC
How does MAC change with age
MAC declines 6% per decade after the 1st year of life - so elderly patients require a lower anaesthetic concentration to achieve 1 MAC
What is an age related iso-MAC chart used for?
Allows for estimation of appropriate end tidal agent concentration over a range of ages and in the presence of 0%, 50% and 67% N2O.
They are used as a guide to anaesthetists on appropriate doses of volatile agents depending on the age of the patient.
Explain how an iso-MAC chart is used
The iso-MAC tables give you the appropriate END TIDAL VOLATILE CONCENTRATION for a given age and N2O concentration to achieve a specific MAC (0.8 MAC, 1.0 MAC, 1.2 MAC). The target MAC must reflect the level of surgical stimulation
What is the incidence of all-type awareness during use of muscle relaxant
1:1000
Describe specific aspects of preoperative checks that reduce the chance of awareness
Preoperative checks
- Adequate pre-assessment to ID those at higher risk of awareness
- Equipement/machine checks
- Drug labelling and draw up
Describe specific aspects of induction that reduce the chance of awareness
Induction
- Secure, visible IV access (TIVA)
- Adequate monitoring ETCO2/ETVA
- Benzo as co-induction: amnesia
- Adequate dose IV induction agent
- Ensure LOC before NMB
- Additional doses of induction agent may be required for difficult intubation (time for equilibration of volatile)
What should be done if airway management takes longer than expected in GA
An additional dose of IV induction agent should be administered as the time to equilibration for volatile agent is delayed.
Describe specific aspects of maintenance that reduce the chance of awareness
- Use Iso-MAC table for Age/N2O guided by the desired depth relevant to the surgery
- Clinical vigilance (HR, BP, lacrimation, sweating)
- Judicious use of NMB (guided by neuromuscular monitoring) - respond rapidly
- IV secure and visible (TIVA)
- Use of depth of anaesthesia monitors (bispectral index monitor)
Describe specific aspects of emergence that reduce the chance of awareness
- Ensure NMB has worn off or been reversed prior to waking (remember Scoline apnoea which may be previously undiagnosed = plasma pseudocholinesterase disorders)
- Early discontinuation of Volatile/Infusion pumps (due to pressure for fast turnaround between patients)
- Misinterpretation: patient’s may misinterpret sounds in the OT during emergence as awareness – counsel.
What should be considered with regard to avoiding awareness during TIVA?
- Propofol has more inter-patient PK and PD variability and so has less predictability than the volatile agents. Some infusion devices use pharmacokinetic modelling to estimate the brain concentration, but this is NOT a measured value.
- Spinal reflexes are not suppressed in the same way as with volatile anaesthetics –> therefore the move/non-move endpoint is less applicable.
- RELIABLE DRUG DELIVERY - new IV access, visible and secure.
How are clinical signs of awareness limited in anaesthesia
Drugs and regional anaesthesia may mask SNS response
What is the best method to estimate brain concentration of volatile anaesthetic
ET VA
Why is EEG analysis difficult for monitoring depth of anaesthesia
Raw EEG data is difficult to interpret and is agent dependent.
Bispectral index is used
What is auditory evoked potentials
Auditory evoked potentials are reduced in a dose-dependent fashion with anaesthetic agents. The characteristic waveform of the early cortical EEG in response to the sounds undergoes a characteristic change with anaesthesia
What is the isolated forearm technique and is this used in clinical practice
Torniquet isolates the forearm from muscle relaxation
The torniquet can be let down after a period of time when the relaxants have been distributed and bound.
This allows movement of the arm and patients have responded to command
How does remifentanyl mask signs of awareness
By reducing the SNS response
How do anticholinergic drugs mask awareness
Reducing sweating, lacrimation and pupillary response
What is the PRST score
Pressure (BP)
< 15 mmHg from baseline –> 0
15 - 30 mmHg from baseline –> 1
>30 mmHg from baseline –> 2
Rate (HR)
<15bpm from baseline –> 0
15-30 bpm from baseline –> 1
> 30 bpm from baseline –> 2
Sweating
Nil –> 0
Skin Moist –> 1
Visible beads of sweat –> 2
Tears
None –> 0
Excess tears in open eyes –> 1
Tears overflowing –> 2
PRST score value between 0 - 3 = adequate anaesthetic depth (= BIS 40 - 60)
