Anaesthesia - principles

Question 1

Types of anaesthesia

Answer 1

general - producing insensibility in WHOLE BODY, usually causing UNCONSCIOUSNESS - CENTRALLY ACTING DRUGS (hypnotics/analgesics)

regional - producing insensibility in AREA/REGION of body - LA APPLIED to NN. SUPPLYING RELEVANT AREA

local - producing insensibility in ONLY RELEVANT PART of BODY - LA APPLIED DIRECTLY to TISSUES

Question 2

Balanced anaesthesia

Answer 2

• DIFFERENT DRUGS do DIFFERENT JOBS
• TITRATE DOSES SEPARATELY + therefore, MORE ACCURATELY to REQ.
• AVOID OVERDOSAGE
• ENORMOUS FLEXIBILITY

Question 3

Problems

Answer 3

• POLYPHARMACY - increased risk of drug reactions, allergies
• MUSCLE RELAXANTS - NOT SEDATIVE, NOT ANALGESIC - req. for artificial ventilation for airway control
• SEPARATION of RELAXATION & HYPNOSIS - awareness (pt. paralysed but awake)

Question 4

GA: how do they work

Answer 4

• INTERFERE W/ NEURONAL ION CHANNELS in DOSE DEPENDENT FASHION - HYPERPOLARISE NEURONES (opens chloride channels/suppresses excitatory synaptic activity)

* INHALATIONAL = DISSOLVE IN MEMBRANES (direct physical effect)
* IV = ALLOSTERIC BINDING (GABA receptors - open chloride channels)

• IF PROCESS REQUIRES MORE ION CHANNELS = MORE LIKELY TO LOSE IT EARLY (tend to be COMPLEX PROCESSES e.g. LOC 1ST, HEARING LATER, PRIMITIVE FUNCTIONS EVEN LATER, REFLEXES RELATIVELY SPARED - primitive + small no. of synapses - cerebral function lost top down)

Question 5

GA: effects on physiology

Answer 5

CVS:

* CENTRAL = DEPRESS CV CENTRE (reduces symp. outflow + -ve inotropic/chronotropic effect on heart + reduced vasoconstrictor tone - vasodilation)
* DIRECT = -VELY INOTROPIC, VASODILATION due to decreased peripheral resistance, VENODILATION due to decreased VR and decreased CO

RESPIRATORY:

* ALL ANAESTHETIC AGENTS (except ketamine) = RESPIRATORY DEPRESSANTS = REDUCE HYPOXIC + HYPERCARBIC DRIVE, DECREASED TIDAL VOL. + INCREASED RATE
* PARALYSE CILIA
* DECREASE FRC (functional residual capacity) = LOWER LUNG VOL., VQ MISMATCH

Question 6

GA: in practice

Answer 6

• ABC - LONG DRAWN OUT RESUSCITATION
• MANDATES AIRWAY MANAGEMENT - IMPAIRMENT of RESPIRATORY FUNCTION & BREATHING CONTROL
• CV IMPACT
• CARING for UNCONSCIOUS PT.
• IV = RAPID ONSET of UNCONSCIOUSNESS + RAPID RECOVERY
  
  • Rapid onset of consciousness - 1 arm-brain circulation; highly fat soluble drugs so cross membranes v. quickly + leave circulation v. quickly is given as one-off dose as moves into other body cpts.
  • Rapid recovery - disappearance of blood from circulation due to redistribution, metabolism has little impact
  • HEPATIC METABOLISM + RENAL EXCRETION
• INHALATIONAL = UPTAKE & EXCRETION via LUNGS; SLOW INDUCTION, ANAESTHESIA MAINTENANCE, AWAKENING
  
  • HALOGENATED HYDROCARBONS
  • UPTAKE + EXCRETION via LUNGS - gas moves down conc. Barrier from lungs - blood - brain, crosses alveolar BM easily, arterial conc. = alveolar partial pressure (equates quite closely)
  • MAC - minimum alveolar conc., measure of potency (low number means high potency)
  • INDUCTION - SLOW
  • MAINTENANCE - PROLONG DURATION, V. FLEXIBLE
  • AWAKENING - STOP INHALATIONAL ADMINISTRATION, WASHOUT - REVERSAL of CONC. GRADIENT

Question 7

Muscle relaxants: how do they work

Answer 7

• INDISCRIMINANTLY PARALYSE SKELETAL MUSCLE incl. those of RESPIRATORY + AIRWAY
• SYSTEMIC MUSCLE RELAXANT - MUST GIVE HYPONOTIC DRUG (paralysed but awake is extremely uncomfortable)

Question 8

Muscle relaxants: in practice

Answer 8

INDICATIONS:

• VENTILATION + INTUBATION
• IMMOBILITY ESSENTIAL - microscopic surgery, neurosurgery
• BODY CAVITY SURGERY (access)

PROBLEMS:

• AWARENESS - separation of unconsciousness from hypnosis, paralysed but awake
• INCOMPLETE REVERSAL - airway obstruction, ventilatory insufficiency in immediate post-op period
• APNOEA - dependence on airway + ventilatory support
• MAINTAIN + PROTECT AIRWAY while in effect - otherwise death

Question 9

LA + regional anaesthesia: how do they work

Answer 9

• BLOCK Na+ CHANNELS - PREVENTS AXONAL AP from PROPAGATING
• DIFFERENTIAL BLOCKADE due to DIFFERENTIAL PENETRAITON into DIFFERENT TISSUE TYPES
  
  • MYELINATED THICK FIBRES - RELATIVELY SPARED e.g. motor fibres
  • PAIN FIBRES BLOCKED EASILY

Question 10

LA + regional anaesthesia: effects on physiology

Answer 10

• RETAIN AWARENESS/CONSCIOUSNESS
• LACK of GLOBAL EFFECTS of GA
• DERANGEMENT of CVS PHYSIOLOGY - proportional to size of anaesthetised area
• RELATIVE SPARING of RESPIRATORY FUNCTION - may be preferred in pt. w/ respiratory co-morbidities, only relative effect - are consequences for respiratory system in regional techniques

CVS: NEURAXIAL BLOCK

• Similar to CVS effects of GA (venodilation + arterial vasodilation) BUT ALL RA EFFECTS DUE TO SYMPATHECTOMY because of LA BLOCKAGE of MIXED SPINAL NN.

	○ Effects limited to region of body covered by block - greater area covered results in greater physiological impact

RESPIRATORY: NEURAXIAL BLOCK - effects via LA block of mixed spinal nn.

* INSPIRATORY FUNCTION RELATIVELY SPARED - unless high block
* EXPIRATORY FUNCTION RELATIVELY IMPAIRED - cough dependent on abdo muscle function
* DECREASE FRC - AIRWAY CLOSURE comparable to GA
* INCREASED V/Q MISMATCH

Question 11

LA + regional anaesthesia: in practice

Answer 11

LA PROBLEMS:

* TOXICITY!! - limits use
* TOXICITY - HIGH PLASMA LVLS e.g. in IV INJECTIONS; ABSORPTION > RATE of METABOLISM - resulting in HIGH PLASMA LVLS - well-perfused areas have higher absorptions - VASOCONSTRICTORS reduce blood flow and therefore, reduce absorption
* TOXICITY depends on: DOSE USED, RATE of ABSORPTION (site-dependent), PT. WGT., DRUG (bupivacaine > lignocaine > prilocaine)

PRESENTATION of LA TOXICITY: from earliest and least toxic

* CIRCUMORAL + LINGUAL NUMBNESS and TINGLING
* LIGHT-HEADEDNESS
* TINNITUS, VISUAL DISTURBANCES
* MUSCULAR TWITCHING
* DROWSINESS
* CV DEPRESSION
* CONVULSIONS
* COMA
* CARDIORESPIRATORY ARREST