吊鹽水的原理和作用?

吊鹽水或者葡萄糖水是用作補充病者的水份的。

注射液內裡的鹽份（sodium chloride）或糖份（dextran）是用來使注射液的滲透壓（osmolality）相約於血液的滲透壓的。若病人接受的注射液是清水的話，血液的滲透壓便會因稀釋而降低，血細胞便會因細胞內跟細胞外的滲透壓不同而膨脹至破裂死亡的。因此注射液是須加入適量的溶質來調較其滲透壓的。而鹽份或糖份便是最常用的溶質了。

其實這些注射液內裡的鹽份或糖份對身體的鹽或糖的補充只是十分少的。所以「吊鹽水」或「吊葡萄糖水」的分別不大。病人若需要鹽份或糖份補充，醫生是需要用高濃度的鹽或糖注射液，或另加鹽或糖份於注射液內的。

吊鹽水 is correctly termed -- Intra-venous Fluid (IV fluid) administration.

To understand 吊鹽水的原理和作用, we have to explain some basic knowledge of fluid distribution in our body. The body is make up of 60% water (with variation in different age group). There are (generally speaking) 3 compartments that holds fluid in our body, being -- Plasma (blood), Intra-cellular fluid (inside the cell), and Extra-cellular fluid (outside the cell but not in the blood).

There are many uses of IV fluid, of which 2 is most usual.

1. REPLACEMENT OF BODY FLUID
i) If a person requires ICF and ECF replacement, IV fluid is given as Dextrose (sugar) solution, because the osmolarity of the solution will cause the fluid to distribute mainly to these 2 compartments.

ii) if a person is suffering from blood loss, the loss of fluid is from the plasma. Without known the patients blood type. The best replacement is O- blood, but may not be immediately available. IV fluid is given to maintain the blood pressure. IV fluid is given as a crystalloid, usually Sodium Chloride (but sometimes others, ie. Hartmann Solution) or a colloid, usually an albumin (a type of blood protein) solution. The osmolarity with cause the fluid to stay in the plasma longer, maintaining the blood pressure in a blood loss crisis.

Therefore, a wrong choice in IV fluid may be of enormous consequence.

2. MAINTANENCE OF BODY FLUID BALANCE. In patients who are not allow to eat or NPO (Nil Per Oral), the body still requires fluid for basic metabolism. The minimal fluid a day is approximately 3 L (adjusted according to body weight). Whether this fluid is given as Sodium Chloride or Dextrose solution depends on our body sodium balance. We need about 2 mmol/Kg/day of sodium, translating to 140 mmol/day in a 70 kg man. 1L of 0.9% Sodium chloride (called Normal Saline, or Saline) has 150 mmol of sodium. Therefore, 1L of Normal Saline is enough for sodium balance, and the remaining 2 L is usually given as Dextrose solution.

3. OTHER MISCELLANEOUS USE OF IV FLUID -
a) to maintain and open IV channel, used as a flush to prevent blood clotting.
b) used as IV push, after injection of a drug follow by IV fluid infusion.
c) Fluid challenge.
d) As a medium to administer other drugs or electrolytes.

I am certain that there are further uses.

The practice of IV fluid administration is a amalgamation of science and art as in other areas of medical practice.