SAQs 2015

Question 1

Outline the physiological changes that may explain why an otherwise well patient may have a reduced urinary output intraoperatively

Answer 1

outline:
- Intro
- Normal values
- Urine output determinants
Urine = filtration + secretion - reabsorption.
- Factors which effect filtration
- Factors which decrease secretion
- Factors which increase reabsorption

Urine = filtration + secretion - reabsorption.

Normal urine output is 0.5-2ml/kg/hr

FILTRATION physiology:

Renal blood flow = MAP - CVP / Resistance
- Autoregulated MAP 50-150.

MAP: MAP - CVP = CO x TPR
Therefore changes to CO, Preload, afterload, rate, contractility can all affect MAP. These can all be deranged by general anaesthesia.

FILTRATION:
GFR = Kf x NFP.

Changes to Kf:
- Increases in SNS activity e.g. pain will decrease Kf by reducing the available surface area for filtration.

NFP is difference in hydrostatic pressures between arterial and bowman’s capsule minus the oncotic pressure of the blood.

NFP = Phydrostat blood - Pbowmans space - Poncotic blood

Phydrostat blood = MAP - CVP, therefore changes to MAP (e.g. decreased CO, myocardial depression), increases to CVP (e.g. from IPPV, raised abdominal pressure) will decrease NFP and decrease glomerular filtration.

REABSORPTION:
- ADH
–> Released due to:
- Stress from surgery
- Increased osmolality, dehydration from fasting per-operatively
- Decreased blood volume (e.g. haemorrhage)
- Decreased MAP
- AT2
Renal effects:
- Insertion of AQ2 into apical membrane of collecting ducts –> Increased resorption of H20
- Insertion of urea transporters –> Increased urea reabsorption –> Increased medullary interstitial osmolarity, increased ability to concentrate urine.

RAAS:

• SNS stimulation of granular cells (e.g. pain)
• Decreased stretch of afferent arteriolar baroreceptors (e.g. hypotension)
• Effects of AT2:
• • Increased Na/H20 absorption from PCT
• • Release of aldosterone –> Increased sodium reabsorption from principal cells of collecting ducts
• • Increased release of ADH

sympathetic stimulation:

• Baroreceptor reflex:
• Renal effects:
• -Afferent > efferent constriction –> Decreased GFR but preserved GFP.