Total Body Water (TBW) is approximated by multiplying a fractional factor with the patient’s body weight. TBW is then unequally distributed among two compartments : intracellular and extracellular. The extracellular compartment is then divided into two components—interstitial and intravascular. The following equations demonstrate the approximate partritions of each compartment respectively.
$$ Total\ Water =\ a\ \times\ Body\ Weight\ \begin{cases} a=0.45\ in\ eldery\ women\\ a=0.5\ in\ women\ and\ elderly\ men\\ a=0.6\ in\ men\ and\ children \end{cases} $$
$$ Total\ Water=\ ICF\ +\ ECF\ =\ ICF\ +\ (ISF\ +\ IVF) $$
$$ V=\ \frac{2}{3}V\ +\ \frac{1}{3}V= \ \frac{2}{3}V\ +\ (\frac{3}{12}V\ +\ \frac{1}{12}V) $$
${Volume} = {100× a}\ +50× b\ +20× c = 1500\ +(20× c)$
a+b+c = patient’s bodyweight; where a and b max at 10 kg (i.e. c = remaining BW beyond 20)
Dehydration Assessment
| Grade | Clinical | Portion of Deficit | Signs / Symptoms |
|---|---|---|---|
| Mild | Thirsty | Infants : 50 ml/kg (5%) | |
| Children (3%) — Adult : 30 ml/kg | Pulse rate may increase | ||
| Moderate | Irritable / Lethargic | Infants : 100 ml/kg (10%) | |
| Children (6%) — Adult : 60 ml/kg | Tachycardia with delayed cap refill (1.5-2 secs), cool and pale skin | ||
| Severe | Depressed / Unconscious | Infants : 150 ml/kg (15%) | |
| Children (9%) — Adult : 90 ml/kg | Rapid and weak or absent pulse, hypotension, cap refill > 3 secs, cold mottled skin with poor turgor |
Water deficit in hypernatremia
$$ V \ (L)=\ Total\ Body\ Water \times(\frac{Serum\ [Na⁺] \ -\ 140}{140})\ $$
Fluid Products with their respective components (in mmol/L)
| Fluids | Na⁺ | Cl⁻ | K⁺ | Other | Dextrose (g/L) | Osmolarity | pH |
|---|---|---|---|---|---|---|---|
| NSS | 154 | 154 | 308 | ||||
| Ringer’s Lactate | 130 | 109 | 4 | Ca²⁺ 2.7, Lactate 28 | 273 | ||
| Ringer’s Acetate | 130 | 109 | 4 | Ca²⁺ 2.7, Lactate 28 | 273 | ||
| NSS/2 | 77 | 77 | 154 | ||||
| 5%DNSS | 154 | 154 | 50 | 560 | |||
| 5%DNSS/2 | 77 | 77 | 50 | 406 | |||
| 5%DW | 50 | 252 | |||||
| 3%NaCl | 513 | 513 | 1026 | ||||
| 5%Albumin | 130-160 | 100-130 | <2 | 309 | |||
| 10%Dextran 40 in NSS | 154 | 154 | 310 | ||||
| ORS in 240 ml of water | 90 | 80 | 20 | Citrate 20 | Glucose 111 | 311 | |
| rORS | 60-70 | 60-70 | 20 | Citrate 10 | Glucose 75-90 | 210-260 | |
| SEA ORS | 90 | 80 | 20 | Citrate 10 | Glucose 111 | ||
| Babi-lyte | 45 | 35 | 20 | Citrate 10 | Glucose 138 |
Consists of Effective Osmolarity (Tonicity)* and Ineffective Osmolarity
$$ Total\ Osmolality =\bold{(2 \times [Na⁺]\ +\ \frac{Glucose}{18})}\ +\ \frac{BUN}{2.8} $$
Effective* = able to effect transcellular fluid shift