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Neurophysiology for Primary FRCA

Basics of Neurophysiology for Primary FRCA exam
by

Ravi Alagar

on 22 May 2011

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Transcript of Neurophysiology for Primary FRCA

Neurophysiology Neurophysiogy for Primary FRCA Ravi Alagar CSF Clear, colourless fluid that bathes brain and spinal cord Total volume 100-150ml 1/3 spinal secreted by choroid plexus (2/3)
& ependyma cells of ventricle walls (1/3)
rate of 0.3ml/min
500-600ml/day Lateral ventricles Third ventricle Fourth ventricle Spinal cord Cerebral hemispheres Foramina of Monro Aqeduct of sylvius Magendie foramen
Lushka foramen Reabsorption
arachnoid villus 90% into dural venules
cerebral venules 10% Protects brain and spinal cord by providing buoyancy
Buffers any increase in ICP
Central chemoreceptor resp. control
Tight control of chemical environment around brain Composition
(differences relative to plasma) Higher pCO2
Lower pH 7.33
Protein <0.3g/L
Higher level of Cl and Na
Lower level of K (40%)
Glucose 2-5mmol/L BBB physiological barrier between CNS and bloodstream Regulates or prevents transmission of ions, toxins, drugs, glucose and neurotransmitters from plasma to brain consists of ultrafiltration barrier chemical barrier in choroid plexus and tight junctions between capillary endothelial cells Substance movement Freely diffusible
Water
CO2
O2
lipid soluble Active transport
sodium
glucose Deficient at hypothalamus and CTZ
Reduced in neonates and meningitis Specific gravity Density of a substance divided by that of water (0.997) Depends on amount of type of solute particles Affected by
acetazolomide (50% reduction)
High dose diuretics (reduction)
Corticosteroids (increase) Plasma - 1.010
CSF - 1.004-1.007
0.5% bupivacaine - 1.004
0.5% bupivacaine with 4% glucose - 1.026 Substance
Protein (g/L)
HCO3 (mmol/L)
Glucose (mmol/L)
Na (mmol/L)
K (mmol/L)
Cl (mmol/L)
pH
Osmolality (mOsm/kgH2O)
pCO2 CSF
0.3
23
4.8
147
2.9
112
7.32
290
6.6 Plasma
70
25
8
150
4.6
100
7.4
290
5.3 Blood Blood supply Arterial
4 major vessels 2 internal carotid arteries
supply 2/3 2 vertebral arteries
supply 1/3 some say 80:20 Blood flow Normal CBF around 50ml/100g/min Grey matter
80ml/100g/min
White matter
20ml/100g/min Totals 750ml/min 15% of cardiac output Regulation Flow = pressure/resistance CBF proportional to CPP inversely proportional to cerebral vascular resistance CPP = MAP - (ICP + CVP) Resistance affected by:
Autoregulation (metabolic, pressure)
PO2 and PCO2
Neural control Neural control of vessels minimal
sympathetic supply from superior cervical ganglia
parasympathetic supply from facial nerve CO2 has vasodilatory effect
4 to 11 kPa
linear relationship with CBF O2 has little effect until <6.7kPa
below this CBF increases
relationship due to shape of O2 dissociation curve Measurement
Kety-Schmidt (Fick priciple using nitrous oxide)
PET (radioactive xenon)
Doppler probes for regional flow Autoregulation Between MAP of 50 and 150mmHg Impaired by:
hypoxia
hypercapnia
trauma
volatiles Increased CBF
Increased CPP (outside autoregulation limits)
Increased PCO2
Decreased PO2
Decreased pH
Increased levels adenosine and K
Volatiles
Ketamine Decreased CBF
Decreased CMRO2
IV induction agents
Hypothermia
Increased plasma viscosity (haematocrit >50%)
Increased venous pressure
Cerebral steal ICP pressure in lateral ventricles of brain relative to atmospheric pressure 10-15 cmH2O (7-11 mmHg) when supine 3 parts
blood (50-75ml)
CSF (75-150ml)
brain (1.4kg) Monro-Kelly doctrine as contents are contained in rigid cranial vault, a change in volume of one compartment is linked to an opposite change in another small increases in volume in one compartment can be compensated for by decreases in volume of another comartment exponential increase in pressure results once volume increases above certain point increased blood volume
increased CBF
reduced venous drainage increased brain volume
space-occupying lesion
cerebral oedema increased vol. of CSF
congenital
acquired (surgery, meningitis, head injury, tumour) CMRO2
cerebral metabolic rate for oxygen
3.5ml/100g/min
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