A solution is a homogeneous mixture composed of two or more substances. In midwifery and general nursing, we deal primarily with aqueous solutions where water serves as the solvent and various electrolytes, minerals, or drugs serve as the solutes. Understanding how these substances mix, dissolve, and interact within the body is fundamental to safe medication administration and intravenous fluid therapy.
Body fluids depend heavily on the tonicity of administered intravenous solutions. Tonicity describes how an extracellular solution can change the volume of a cell by affecting osmosis across the cell membrane.
Solubility refers to the maximum amount of a solute that can dissolve in a given amount of solvent at a specific temperature. When mixing medications, midwives must observe whether a solute fully dissolves or leaves undissolved particles.
Several factors influence how fast and how well a substance dissolves:
Clinicians must accurately communicate how much medication is packed into a specific volume of solution. Concentration can be expressed in several standard formats:
| Expression Method | Description | Clinical Example |
|---|---|---|
| Percentage Concentration (% w/v) | Grams of solute per 100 mL of solution | 5% Dextrose means 5g in 100 mL |
| Ratio Concentration | Parts of solute to parts of solution | 1:1,000 Adrenaline means 1g in 1,000 mL |
| Weight per Volume (mg/mL) | Milligrams of drug in each millilitre | 40 mg/mL Gentamicin injection |
Safe midwifery practice requires routine calculation of drug doses and intravenous drop rates to prevent toxicity or under-treatment. The basic formula for calculating oral or injectable medication is:
$$\text{Desired Dose} \div \text{Stock Dose} \times \text{Volume} = \text{Amount to Administer}$|
For intravenous fluid infusions, the calculation involves the total volume, the time period, and the drop factor of the infusion set (usually measured in drops per millilitre, such as 15 drops/mL or 20 drops/mL for standard sets). Always double-check calculations with a colleague when dealing with high-risk drugs like oxytocin or magnesium sulfate.
Imbalances in body fluids and electrolytes can lead to serious complications during pregnancy, labour, and the puerperium. Midwives must monitor intake and output charts, assess for signs of dehydration or oedema, and understand how IV fluids affect electrolyte shifts.
For example, infusing large volumes of 0.9% saline too quickly can lead to hyperchloremic metabolic acidosis. Similarly, administering hypotonic fluids to a woman with severe pre-eclampsia can worsen cerebral oedema. Careful selection of solutions based on individual clinical assessment protects maternal and fetal well-being.