2011B15 Outline the physiological changes that might explain why an otherwise well patient
may have a reduced urine output intra-operatively.

List:

· Intro

· Glomerular filtration physiology

· Factors decreasing filtration

· Factors increasing reabsorption

Intro:

Urine output determinants

= filtration + secretion – reabsorption

Normal urine output

0.5mL/kg/h

Peri-op effects

↓filtration: hypotension

↑Reabsorption: stress response, dehydration

Hence ↓urine output

Glomerular filtration physiology:

Starling equation

GFR = [K(P_c – P_B) – σ(π_c – π_B)]

· P_c (capillary hydrostatic pressure) is the major variable peri-operatively

· Dependent on a) renal blood flow b) afferent and efferent arteriolar tone

Renal blood flow

RBF = mAP – CVP / renal vascular resistance (RVR)

· Myogenic autoregulation effective if mAP 70-170mmHg

Mean arterial pressure

mAP – CVP = CO x TPR

· Factors ↑CO: ↑preload, ↓afterload, ↑rate, ↑contractility

Factors decreasing filtration:

↓mAP

↓Cardiac output

· ↓preload (general or neuraxial anaesthesia, IPPV, dehydration)

· ↓inotropy (e.g. propofol)

↓SVR:

· General anaesthesia

· Neuraxial anaesthesia

↑CVP

· IPPV

· Heart failure

· Raised intra-abdominal pressure

↑RVR

Angiotensin 2 (AGTR1) and SNS (α₁) -> efferent > afferent arteriolar constriction

Factors increasing reabsorption:

↑ADH

Source: hypothalamus -> posterior pituitary

Release stimuli:

· Stress response from surgery

· Many drugs

· ↑osmolality (sensitive to ∆2%) – dehydration from pre-op fasting

· ↓blood volume (sensitive to ∆10% but overrides osmolality)

· ↓mAP

· Angiotensin 2

Renal effects:

· Insertion of aquaporin 2 into apical membrane of collecting ducts -> ↑H₂O reabsorption

· Insertion of urea transporters -> ↑ medullary interstitial osmolarity

· ↑Na⁺ reabsorption in thick ascending loop of Henle

↑RAAS

Source: granular cell of juxtaglomerular apparatus

Release stimuli:

· ↑SNS activation of granular cells (β₁) - e.g. pain

· ↓stretch of afferent arteriolar baroreceptors - i.e. hypotension

· ↓NaCl delivery to macula densa -> ↑PGE₂, ↓adenosine release - i.e. ↓GFR

Renal effects of angiotensin 2

· ↑Na⁺/H₂O reabsorption from PT (via Na⁺/H⁺ exchange) (AGTR1)

· ↑release of aldosterone

· ↑release of ADH

Actions of angiotensin-3:

· 40% of pressor activity

· 100% of aldosterone induction

Renal effects of aldosterone

· ↑Na⁺ reabsorption from principal cells of connecting tubule and collecting ducts (MR)

· Indirectly causes H₂O reabsorption via ↑ECF osmolality hence ↑ADH

↑SNS

Stimulus: baroreceptor reflex

Renal effects:

· Constrict efferent > afferent arterioles(α₁)

· Hence ↓GFR but preserved GFP

· ↑ proximal tubular reabsorption of Na⁺/H₂O (α₁)