Intravenous induction sequence Flashcards
What are the main advantages of IV induction of anaesthesia compared to inhalational induction?
The main advantages are that it enables anaesthesia to be achieved far more rapidly and smoothly.
A smooth, rapid and stable loss of consciousness is wanted in all cases of non-RSI IV induction. But why might the required end-point vary between cases?
FACE MASK
IV induction –> relatively light to enable spontaneous ventilation to continue for the introduction of inhalational agents.
LMA
Upper airway reflexes must be suppressed –> requires relatively deep level of anaesthesia (may well produce a short period of apnoea)
ETT
Muscle relaxation required to pass ETT. Short period of intense analgaesia required to suppress SNS response to laryngoscopy.
What is co-induction?
Although it is possible to induce anaesthesia using a single IV agent, a combination of drugs frequently produces a smoother induction. This is called co-induction.
Why is midazolam given? What is an appropriate preoperative dose of midazolam and in which patients is this most beneficial and in which patients are benzodiazepines best avoided
As part of co-induction, benzodiazepines have beneficial characteristics: Amnesia, airway reflexes, akinesia, anxiolytic.
1 - 3 mg IV
Benefit: Young and anxious
Avoid: elderly (postoperative confusion)
Describe the role of opioids in co-induction
- Short lived profound analgaesia (blunt laryngoscopy/intubation SNS response)
- Reduce airway reflexes (for LMA/ETT insertion)
Dose dependent respiratory depression
Minor anxiolytic
How should fentanyl dosing vary according to type of anaesthesia: FM/LMA/ETT
FM - low dose to maintain spontaneous ventilation for the uptake of inhalational agents
LMA - Higher dose to suppress airway reflexes for LMA insertion
ETT - Even higher dose for suppression of airway reflexes and suppression of SNS response to laryngoscopy
Are muscle relaxants considered co-induction agents?
No.
Give the sequence, agents and doses for drugs administered in a non-RSI intubation
- Oxygen (preoxygenation)
- Benzo:
- Midazolam 1-3 mg IV - Opioid
- Fentanyl (SV) 0.5 - 1.0 ug/kg
- Fentanyl (IPPV) 2 - 5 ug/kg
If ETT/IPPV
- Vecuronium 0.1 mg/kg
- Atracurium 0.5 mg/kg
- Rocuronium 0.6 mg/kg
What is the end point for induction of anaesthesia?
Patient has lost consciousness and is unrousable to gentle verbal stimulation: “can you open your eyes”.
What is the usual cause for laryngospasm, coughing during induction and intubation
Under-dosing and stimulation of the patient whilst only lightly anaesthetized
List four situations when inhalational induction is preferred to IV induction
- Paediatric (co-operation/small veins)
- IV drug user (sclerotic veins
- Needle-phobic
- Stridor
Why is stridor an indication for inhalational induction?
In the presence of stridor, the airway is more than 50% narrowed and has the potential to fully obstruct if managed inappropriately.
Inhalation induction –> maintains spontaneous ventilation and anaesthesia can be reversed if the airway is compromised during induction
Why is co-induction an important concept
Co-induction is an important concept as it enables the anaesthetist to combine the beneficial effects from a range of drugs whilst minimizing the side-effects
What are the stages of stages of anaesthesia using a slow inhalational induction using ether. Compare this to inhalational induction with sevoflurane
ETHER (obselete)
Stage 1: Induction to loss of consciousness
Stage 2: LOC –> Excitatory phenomena –> regular breathing.
Stage 3: Surgical anaesthesia
Stage 4: Depression of the medulla oblongata –> death
SEVOFLURANE
Works much faster than ether so no all of these stages can be clinically observed
Are the stages of unconsciousness usually seen after an adequate dose of an intravenous anaesthetic agent?
No. Provided an adequate dose of intravenous anaesthesia has been administered, the patient should pass quickly to the surgical anaesthesia plane.
Are the physiological signs of Stage 2 ever seen after intravenous induction?
They can be. These are signs that the patient is starting to recover from the induction dose and adequate inhalational anaesthesia has not yet been achieved to replace it.
Which drugs can affect the signs defining the stages of anaesthesia during inhalational induction?
Anticholinergics affect pupils, heart rate and reduce secretions.
Opioids may dampen laryngeal reflexes, affect pupils, respiratory rate and response to stimulation.
Benzodiazepines may alter the level of consciousness.
Why is knowledge of the stages of anaesthesia relevant
Knowledge of these stages allows the clinical assessment of depth of anaesthesia, for example whether the patient is ‘too light’ or ‘too deep’, rather than just relying on monitors alone. A balanced anaesthetic can then be titrated against the patient’s physiological signs and the degree of surgical stimulation.
Define and describe Stage 1 anaesthesia (Geudel classification)
From induction to LOC
Awake –> reducing response to pain –> hearing intact
Define and describe Stage 2 anaesthesia (Geudel classification)
From LOC –> excitatory phenomena –> regular breathing
Excitation Irregular breathing Breath-holding/coughing Dilated pupils Movement/struggling Hypertension Tachycardia
Define and describe Stage 3 anaesthesia (Geudel classification)
From regular breathing divided into four phases
- Regular breathing
- Eyes become static
- Progressive loss of muscle tone
- Gradual intercostal paralysis
Surgical anaesthesia
Define and describe Stage 4 anaesthesia (Geudel classification)
Overdose of agent
Hypotension/bradycardia
Apnoea
Cardiac arrest
Death
What clinical indicators exist to inform that the patient is ready for intubation (latter stage 3) without a muscle relaxant
- Regular diaphragmatic breathing
- Slow regular pulse
- Loss of muscle tone
- Dilated pupils
Describe the different types of awareness and their approximate frequency
Explicit awareness with pain (1:3000)
- Memory of events with pain or paralysis
Explicit awareness without pain
- Memory of events but comfortable
Implicit awareness
- Not consciously recalled but affect behaviour subsequently
What important counselling should occur if a regional technique with sedation is used
Patients may be aware of noise in the operating room during procedure –> communicate this beforehand
What are the signs of awareness? Name an anaesthetic-related factor or drug that can mask each symptom of awareness
SNS activation
- tachycardia (Atenolol)
- hypertension (Epidural)
- sweating (glycopyrrolate)
- lacrimation (glycopyrrolate)
- pupils dilated + reactive (Morphine)
- Movement/tachypnoea (Atracurium)
Name 3 causes for awareness
Human error
- Failure to check
- Failure to turn on vaporizer
- Inaudible alarms - noisy theatre
Equipment failure
- poor connection (push and twist)
- Blocked gas sampling line
- Intravenous disconnection (TIVA)
Anaesthesia factors
- TIVA infusion integrity
- Neuraxial anaesthesia masks HPT
Patient factors
- Unstable patients: lower GA agent
- Cardiac surgery: large dose opioid, minimal dose induction agent
- Tolerant CNS (Chronic ETOH/hyperthyroidism)
When is true explicit awareness a significant risk
When muscle relaxants are used
How can ET Volatile agent be used to detect awareness
If ET VA is > 0.8 MAC then awareness is unlikely.
What is the isolated forearm technique
An inflated cuff prevents NMB from reaching the arm - the patient can move their arm or obey commands if aware.
How does a bispectral index monitor (BIS) assist with the detection of awareness?
The BIS gives a dimensionless number between 0 - 100. Adequate anaesthesia is between 40 - 60.
Sedation occurs between 60 -80
What is an audio evoked potential (AEP)
Ear phones - click noises - electrodes detect EEG changes in the auditory cortex –> characteristic waveforms can be processed by a monitor to produce a depth of anaesthesia
