Briefly outline the roles of the hypothalamus
The hypothalamus is an extension of the limbic system, and has important roles in:
- Autonomic nervous system
- Endocrine
- Homeostasis
- (temperature, water/osmolarity, food, emotion) - ANS:
- Anterior pituitary - depressor zone, PSNS response
- Posterior pituitary - stimulation, SNS response - Endocrine
- Via anterior pituitary, FLAGTOP
- Via posterior pituitary, ADH, oxytocin - Homeostasis:
- Temperature, water, food, emotion, puberty
Temperature:
- anterior detects temperature, autonomic and behavioural heat loss
- posterior determines set-point, autonomic and behavioural heat gain
Water:
- detects tonicity and volume status, will increase or decrease ADH secretion as required.
- Increases thirst/water seeking behaviour
Food:
- Detects glucose level and satiety
- Induces hunger
Sleep/wake:
- Suprachiasmic nucleus generates circadian rhythm
Emotion:
- Well connected to limbic system
- Mediates autonomic responses to emotional situation
Describe the normal regulation of cerebral blood flow and outline physiological factors which may alter it. DO NOT discuss the effect of medications or pathology.
Plan:
- Intro
- Cerebral blood flow dynamics
- regulation of CVR
- Regulation of arterial pressure
Intro to brain:
- highly metabolically active
- ——> CMRO2 46ml/min = 3.3L/min/100g
- Well perfused
- ——> 15% CO = 750ml/min = 58ml/min/100G
- Minimal anaerobic capacity. IF interrupted, loss of consciousness, ischaemia ensues quickly
Cerebral BF dynamics
CBF = MAP - (ICP or CVP, whichever is greater) / CVR
Acts as a starling resister.
Therefore the factors which will decrease CBF are:
- Decreased MAP,
- Increased ICP or CVP,
- Increased CVR
Factors which will increase CVR:
Resistance = 8nl/pi.r^4
Therefore factors which increase resistance include decreasing radius of vessels, and increasing viscosity of blood.
Autoregulation of blood flow
(graph of CBF vs MAP)
- Increased flow means increased wall stretch means reflex contraction means decreased radius means decreased flow.
- Occurs between MAPs 50-150, shifted right for chronic hypertension
Physiological variables:
- Pa02 < 50mmHg,
- PCO2 20-80mmHg linear. 3% increase with each 1mmHg change in pco2.
- Decreased temperature: Decreased CMRO2, decreased metabolic rate. 7% per 1*C.
Discuss the cerebral effects of prolonged anaesthesia in the steep head down position
Outline:
- Effects of hydrostatic change
- Effects of respiratory change
- Effects of general anaesthesia
Hydrostatic change: - Monroe-Kellie Doctrine - Increased hydrostatic pressure will increase intracranial blood volume by decreasing venous outflow. Effect: - Mild increase in ICP, IOP - Mild decrease in CPP
Respiratory:
- increased respiratory pressures required
- Cephelad movement of diaphragm
- Decreased thoracic compliance
- V/Q mismatch, increased closing capacity
- Decreased FRC
Effect of prolonged general anaesthesia:
- Drug accumulation
- N/V
- Respiratory depression
- Delirium, cognitive dysfunction)
- other effects are drug dependent
Describe the cardiovascular changes that occur with ageing
Ageing is a physiological, time dependent process resulting in decreased function and physiological reserve.
Primary Cardiac changes:
- Decreased myocytes (number of myocytes)
- –> Increased hypertrophy of remaining cells
- Increased connective tissue/fibrous infultrate
- Downregulation of B receptors
Primary vascular changes:
- calcification
- denegration of coronary vessels
- decreased sensitivity of baroreceptors.
These result in: Structure: - Decreased number of myocytes - Hypertrophy of remaining myocytes - increased energy required for same force of contraction
Diastolic function:
- LVH results in poor compliance, increased filling pressure required
- Poor tolerance of hypovolaemia
- More dependance on atrial kick
HR: decreased resting and maximal HR due to infiltration of fibrous tissue into connective system
Rhythm: Fatty and fibrous infiltration of electrics cause increased risk of SSS, AV block. increase likelihood of dysrhythmias, increased reliance on atrial kick
Blood pressure:
- Increased SBP, decreased DBP, increased pulse pressure due to decreased compliance and decreased windkessel effect
- Increased pulse wave velocity (decreaed aortic-radial delay)
Autonomics:
- Decreased B-adrenoreceptors, decreased catecholamine re-uptake
- Increased circulating [NorAd]
- Results in:
- —- Decreased baroreceptor response, tolerance of valsalva and hypovolaemia.
Outline the clinical laboratory effects of renal function. What are the limitations of each test?
GFR = (urine concentration / plasma concentration) x urine output
Two primary tests performed clinically:
- Creatinine, Urea
Creatinine is released at a steady rate from skeletal muscle. It is almost completely freely filtered in the glomerulus, and is only minimally secreted in the proximal tubule.
As it is freely filtered, not produced in the kidney, not metabolised by the kidney and not reabsorbed by the kidney, it is useful to use to estimate GFR. Because it is secreted by the proximal tubule (minimally) it may over-estimate GFR.
Limitations:
- may over-estimate GFR (see above)
- levels may only drop once approx 50% of nephrons are dysfunctional.
- Increases exponentially with decreasing renal function
- Sensitive, but not specific for renal dysfunction.
- Relies on creatinine being released from muscle, therefore if low muscle mass may over-estimate muscle mass
Urea:
- End product of amina acid deaminiation
- Accumulates in renal impairment
- Freely filtered, 50% reabsorbed at proximal tubule
- Same 50% secreted into thin descending loop of henle
- Same 50% reabsorbed at medullary collecting duct (i.e. urea recycling).
Limitations:
- Dehydration or high protein intake –> Increased urea conc. –> underestimation of GFR
Describe how Morbid obesity might effect washout after inhalational anaesthesia
MO - BMI >35.
changes during obesity:
- upper airway (risk of obstruction, slows offset)
- Lower airway (decreased compliance, decreased FRC, slows offset)
- higher WOB
Metabolic:
- Higher CO and oxygen consumption
Higher muscle mass and lipid mass
Higher uptake of anaesthetic by fat, but blood flow is so low that it’s negligible
Overall - minimal effect on wake up time
