Fluid Physiology
8.2 Infusion of Hypertonic Saline

  

Hypertonic saline 3% has an osmolality (about 900 mosm/l) three times that of plasma. The fluid shifts & osmolar changes that occur with its infusion can be predicted.

Water crosses cell membranes easily and distributes passively in response to osmolar gradients. The Na+ content of the fluid limits the distribution of the infused fluid to the ECF. The hypertonic solution will also draw water out of cells decreasing intracellular fluid volume.

As an example, consider a rapid infusion of 1,000 mls of 3% saline into a 70kg subject with a total body water of 42 liters (ICF: 23 litres, ECF: 19 litres).

Just Before the infusion:
Total body solute content = 42 x 290 = 12,180 mOsm.
ECF solute content = 19 x 290 = 5,510 mOsm
ICF solute content = 23 x 290 = 6,670 mOsm.

Immediately After the infusion :
Total body water = 42 + 1 = 43 liters
Total body solute content = 12,180 + 900 = 13,080 mOsm.
ECF solute content = 5,510 + 900 = 6,410 mOsm
ICF solute content = 6,670 mOsm (ie unchanged)

The prediction is:
Final osmolality = 13,080 / 43 = 304 mOsm/l
ECF volume = 6,410 / 304 = 21.1 litres.
ICF volume = 6,670 / 304 = 21.9 litres.

Is the increase in osmolality enough to be sensed by the osmoreceptors?
Yes. The increase in ECF volume is 2.1 litres with about a quarter of this (say 500 mls) intravascularly. Plasma osmolality has increased by 4.8% and this is well above the threshold (1 to 2%) of the hypothalamic osmoreceptors.

Is the increase in blood volume enough to be sensed by the low pressure (volume) baroreceptors?
Yes. The blood volume has increased by about 10%. The volume receptors respond to changes above about 7 to 10%.

The increase in osmolality will be sensed by the osmoreceptors in the hypothalamus and this will be a potent stimulus to the secretion of ADH to retain water in the kidneys. Thirst will also be increased. The increase in blood volume is at about the lower level of sensitivity of the volume receptors. The effect via the volume receptors will be to inhibit ADH secretion to allow water excretion. In general, volume stimuli tend to be less sensitive but more potent than osmotic stimuli.

There will also be effects on Na+ excretion. The volume expansion will stimulate secretion of atrial natriuretic factor (ANF). Secretion of aldosterone will be inhibited because of a decreased renin and angiotensin II production. ANF also inhibits renin secretion.

The final outcome of all these changes is natriuresis and excretion of the excess water. The increased osmolality causes an increased ADH and this will tend to inhibit the rate of excretion of the excess water.

The decrease in ICF volume may have effects on the brain causing confusion and mental obtundation due to cerebral cellular dehydration and hypertonicity. These effects on cerebral function will probably be the predominant clinical effects. The function of other organs or tissues in unlikely to be significantly affected.

The increase in ISF volume is not sufficient to cause oedema or interfere with gas transfer or nutrient and waste transfers between cells and capillaries.

  

  Last updated   - All material Copyright - Kerry Brandis, 2003 - http://www.anaesthesiaMCQ.com