Maintenance in Anaesthesia and avoiding awareness Flashcards by Charles Pryce

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

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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

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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

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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

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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

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Financial properties of an ideal agent

Cost-effective

Highly fluorinated agents (Sevoflurane and desflurane) are expensive when used with high gas flows.

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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

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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)

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What % of isoflurane is metabolized?

0.2%

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What % of Sevoflurane is metabolized?

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What % of Desflurane is metabolized?

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What % of Halothane is metabolized?

20% (immune hepatitis - rare)

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How are the effects of volatile agents different in the elderly?

Elderly patients are more sensitive to volatile agents –> delayed emergence, drowsiness and confusion.

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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).

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What is MAC awake?

This is the MAC of anaesthetic producing unconsciousness in 50% of subjects.

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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

1. Use Iso-MAC table for Age/N2O guided by the desired depth relevant to the surgery 2. Clinical vigilance (HR, BP, lacrimation, sweating) 3. Judicious use of NMB (guided by neuromuscular monitoring) - respond rapidly 4. IV secure and visible (TIVA) 5. Use of depth of anaesthesia monitors (bispectral index monitor)

Describe specific aspects of emergence that reduce the chance of awareness

1. Ensure NMB has worn off or been reversed prior to waking (remember Scoline apnoea which may be previously undiagnosed = plasma pseudocholinesterase disorders) 2. Early discontinuation of Volatile/Infusion pumps (due to pressure for fast turnaround between patients) 3. 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?

1. 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. 2. Spinal reflexes are not suppressed in the same way as with volatile anaesthetics --> therefore the move/non-move endpoint is less applicable. 3. 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)