SAQs 2019

Question 1

Briefly outline the roles of the hypothalamus

Answer 1

The hypothalamus is an extension of the limbic system, and has important roles in:

1. Autonomic nervous system
2. Endocrine
3. Homeostasis
   - (temperature, water/osmolarity, food, emotion)
4. ANS:
   - Anterior pituitary - depressor zone, PSNS response
   - Posterior pituitary - stimulation, SNS response
5. Endocrine
   - Via anterior pituitary, FLAGTOP
   - Via posterior pituitary, ADH, oxytocin
6. 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

Question 2

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.

Answer 2

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.

Question 3

Discuss the cerebral effects of prolonged anaesthesia in the steep head down position

Answer 3

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

Question 4

Describe the cardiovascular changes that occur with ageing

Answer 4

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.

Question 5

Outline the clinical laboratory effects of renal function. What are the limitations of each test?

Answer 5

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

Question 6

Describe how Morbid obesity might effect washout after inhalational anaesthesia

Answer 6

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