Sun21November0452PM 19

One litre of 0.9% N. saline is intravenously infused into a normally fit and well 54-year-old male over a period of 10 minutes. Urine output is measured before and after the infusion using a catheter. The patient weighs 70 kg.

The following urine output data are obtained:

Prior to infusion 50 ml/hour

1 hour following infusion 200 ml/hour

2 hours following infusion 90 ml/hour

3 hours following infusion 60 ml/hour

Which one of the following physiological responses is most likely to explain the dramatic rise in urine output following the fluid bolus?

(Please select 1 option)

Stimulation of arginine vasopressin (AVP) secretion

Increased glomerular filtration rate This is the correct answer

Osmotic diuresis Incorrect answer selected

Stimulation of carotid sinus baroreceptors

Inhibition of renin secretion

Explanation

Basic assumptions are:

Total body water (TBW) is one-third extracellular fluid (ECF) and two-thirds intracellular fluid (ICF).

ECF is one-quarter plasma and three-quarters interstitial fluid (ISF).

The threshold of the volume receptors is 7-10% blood volume change. The osmoreceptors are sensitive to a 1-2% change in osmolality.

Plasma osmolality is normal prior to the transfusion (that is, 287-290 mOsm/kg).

0.9% N. saline has [Na+] of 154 mmol/L, similar to that of extracellular fluid. This limits its distribution within the extracellular space when given intravenously it distributes to a plasma compartment:ISF volume ratio of 1:3.

One litre of 0.9% N. saline will therefore in this time-frame increase plasma volume by about 250 mL, potentially the threshold for activation of the volume receptors in the atria which would release atrial natriuretic peptide (ANP).

0.9% N. saline is isosmotic so plasma osmolality will not change after a 1 L infusion. The hypothalamic osmoreceptors will not detect any changes that will affect antidiuretic hormone secretion.

Normal saline contains no protein so the oncotic pressure in the blood is slightly lowered following the saline infusion. As a result, movement of fluid into the ISF is favoured (Starling's hypothesis) and the lowered oncotic pressure immediately leads to an increase in the glomerular filtration rate (GFR) and a smaller reabsorption of water in the proximal tubule.

Urine flow increases. This is a strictly local effect without any hormonal intermediary. The urine flow increases immediately. Fluid then moves back into the intravascular compartment and the urine flow continues until all the transfused fluid is excreted.

The high-pressure baroreceptors in the carotid sinus are unlikely to be affected by blood pressure changes associated with the infusion of 1 L of fluid.

Specialised cells (macula densa) of distal tubules lie adjacent to the juxta-glomerular cells of the afferent arteriole. The macula densa senses the amount of sodium and chloride ion in the tubular fluid. When NaCl is elevated in the tubular fluid, renin release is inhibited. The hormonal changes are slower in onset than the physical changes governing glomerulotubular balance.

0.9% N saline is not an osmotic diuretic, hypertonic saline is.

Answer Statistics

1

10%

2

43%

3

5%

4

26%

5

18%

Times answered: 240