College Answer

In marking this question it was realised that the candidates come from all parts of the world, especially Australia, Hong Kong and New Zealand. The choice of fluids reflected that diversity.

In  “comparing  and  contrasting”  it  was  expected  that  the  candidate  would  cover  content, manufacture, fate in the circulation, effects on organ function and idiosyncratic effects, not merely listing the properties but contrasting the properties within that list.

Eg. Normal Saline versus 4% Human Albumin (CSL).

Normal Saline is a sterile solution of 150mmol each of Na and Cl in I litre water whereas 4% albumin is prepared from human-donor, pooled blood by complex fractionation.   The albumin cannot be regarded as sterile, but is heated to 60o C for 10 hours and prepared at low pH. Prion transfer is feasible. It contains 140 mmol/L Na, 128 mmol/L Cl.

Saline would be expected to distribute 25% intravascularly and 75% interstitially whereas albumin, theoretically, is iso-oncotic and expands the vascular compartment by the administered volume. This may not be true in the critically ill with high albumin turnover and capillary leak.

Saline will have effects via expansion of the appropriate compartments and will lead to increased cardiac output proportionately. In large volumes it may lead to oedema formation, hypernatraemia and hyperchloraemic acidosis. On the other hand, colloid,eg albumin, in one meta-analysis has been associated with higher mortality. It also contains pre kalikrein activator (PKA) which, although present in low amounts, may produce hypotension and bradycardia in conjunction with ACEI use.

The half-life of albumin is said to be 20 days. The distribution half-life of saline is short (30mins) and elimination half-life will depend on the hormonal milieu (ADH, ANP, aldosterone levels) due to hypovolaemia and stress.

Cost : Saline- $1-2 per litre

Albumin – free to users in Australia, theoretical cost ~$80 for 500mls.

Discussion

Though this question is unique, it is difficult to come up with an answer which does not duplicate a series of other answers.

Instead, I will present here a copy of the table of colloid solutions (from Question 29, second paper of 2007) and a table of intravenous fluid content from the chapter on the applied physiology on fluid and electrolyte replacement.