Nursing Foundations · Fluid, electrolytes & Acid-base balances

Fluid, Electrolytes & Acid-Base

Why assess fluid balance?

Fluid and electrolyte balance is assessed by combining blood pressure, pulse rate and urine output. Fluid deficit shows hypotension, tachycardia and oliguria; fluid excess shows hypertension and a bounding pulse. All three parameters must be assessed together.

Daily weightdefinition

The most reliable indicator of a patient's fluid balance status.

in simple words: A change of 1 kg corresponds to about 1 litre of fluid gain or loss.

e.g. Weight gain of 2 kg in 24 hours = about 2 litres fluid excess.

Signs of dehydration

Early – Thirst and dark-coloured urine are the first signs.
Late – Poor skin turgor is a late sign, indicating significant fluid loss from interstitial spaces.
Mucous membranes – Dry mucous membranes due to reduced saliva and mucus production.
Compensated shock – Urine output below 30 mL/hour with tachycardia and normal blood pressure suggests compensated shock.

Fluid, Electrolytes & Acid-Base · Nursing Foundations

Regulation & Compensatory Mechanisms

How the body compensates

Dehydration increases serum osmolality, stimulating hypothalamic osmoreceptors to cause thirst. Dehydration also triggers antidiuretic hormone (ADH) release, causing the kidneys to reabsorb water and reduce urine output. Tachycardia is a compensatory mechanism during moderate to severe dehydration.

Compensatory response to fluid deficit

Fluid deficitIncreased serum osmolality
OsmoreceptorsHypothalamus stimulates thirst
ADH releaseKidneys reabsorb water
OliguriaReduced urine output

Insensible water loss – About 800 mL per day

Skin (transepidermal diffusion): 300-400 mL/day
Lungs (respiration): 300-400 mL/day
Increased by fever, hyperventilation and burns
Reduced by heated humidifier on ventilator

Fluid, Electrolytes & Acid-Base · Nursing Foundations

IV Fluid Types & Tonicity

Tonicity matters

Isotonic solutions have a solute concentration similar to blood plasma and do not cause significant fluid shifts. Hypotonic solutions shift water from intravascular space into interstitial and intracellular spaces. Hypertonic solutions draw water from interstitial and intracellular spaces into the intravascular space.

Common IV fluids

Fluid	Tonicity	Osmolality	Use / Note
0.9% NaCl	Isotonic	308 mOsm/L	Extracellular volume replacement
Ringer Lactate	Isotonic	275-295 mOsm/L	ECF replacement; no magnesium
D5W	Isotonic in bag	252 mOsm/L	Becomes hypotonic after metabolism
0.45% NaCl	Hypotonic	—	Cellular dehydration
D5NS	Hypertonic	—	Pulls water out of cells
3% NaCl	Hypertonic	1026 mOsm/L	Severe hyponatremia, cerebral edema

Fluid, Electrolytes & Acid-Base · Nursing Foundations

IV Drip Rate Calculations

Drip rate formuladefinition

Drops per minute = (Total volume in mL × Drop factor) ÷ Infusion time in minutes.

in simple words: Convert hours to minutes first (24 hours = 1440 minutes), then round to nearest whole drop.

e.g. 1000 mL over 8 hours with 15 gtt/mL set: (1000 × 15) ÷ 480 = 31.25 → 31 gtt/min.

Drop factors and conversions

Item	Value
Macrodrip set	15 drops/mL
Microdrip set	60 drops/mL
Burette set	60 drops/mL (microdrip)
1 pint	500 mL (Indian nursing)
1 quart	2 pints = 1000 mL
1 litre	1000 mL

Infusion time

Infusion time in hours = Total volume in mL ÷ Hourly flow rate in mL/hour. For example, 1000 mL at 125 mL/hour takes 8 hours. The hourly rate in mL/hr is not the same as the drop rate in gtt/min.

Fluid, Electrolytes & Acid-Base · Nursing Foundations

IV Complications & Monitoring

Local vs systemic

Infiltration is a local complication causing swelling, coolness and pain at the site, not systemic effects. Circulatory overload from IV infusion causes pulmonary oedema and heart failure. A pyrogenic reaction is a febrile response to contaminants in IV fluid or tubing.

Differentiating IV complications

Complication	Key signs
Infiltration	Cool, pale, swollen site; infusion stops
Phlebitis	Warmth, redness, palpable venous cord
Extravasation	Vesicant drug; tissue necrosis risk
Circulatory overload	Dyspnoea, crackles, distended neck veins, raised BP
Pyrogenic reaction	Fever, chills, malaise

Nursing actions

Infiltration – Stop infusion first, then remove cannula and restart at a new site. Never flush an infiltrated cannula.
Extravasation – Stop infusion but leave cannula in place to give antidote before removal.
Overload – Stop infusion and inform doctor if crackles, dyspnoea or edema appear.
High-alert – Hypertonic saline requires ICU monitoring, preferably central line, slow infusion pump.

Fluid, Electrolytes & Acid-Base · Nursing Foundations

Electrolytes & Special Considerations

Key electrolytes

Sodium is the most abundant cation in extracellular fluid and the primary determinant of extracellular fluid osmolality. Potassium is the major intracellular cation and has minimal effect on extracellular fluid concentration. Magnesium is primarily intracellular, with only about 1% in extracellular fluid.

Thiazide & loop diuretics

Can cause hypokalemia

Potassium-sparing diuretics

Can cause hyperkalemia

Diuretic effects on potassium

Monitor serum potassium because both imbalances can trigger cardiac arrhythmias.

Hypodermoclysis

Slow infusion of isotonic fluid into subcutaneous tissue, used for rehydration when venous access is poor. It is inappropriate for severe dehydration with hypotension, which needs rapid IV fluid resuscitation.