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The formation of oedema. Part 1: pathophysiology, causes and types

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Oedema formation - whether external and visible, or internal and invisible, and whatever the cause - is relatively common, occurring in most diseased states. Prescribed interventions aim to ensure:

Sharon Edwards, MSc, RN, DipN, PGCEA.

Senior Lecturer, Department of Nursing and Midwifery, University of Hertfordshire

- Adequate oxygen supply to meet demand

- Adequate nutrients to meet metabolic needs

- Fluid replacement to sustain a depleted circulating volume.

If a patient has oedema due to an acute illness, this eventually subsides and the pain, dysfunction or immobilisation they are currently experiencing improve. However, in chronic conditions the formation of oedema may be controlled only through medical interventions or prescribed drugs.

Oedema

Oedema is an abnormal collection of fluid in the tissues, which can collect in either the interstitial or intracellular spaces. The causes of both types of oedema are varied (Table 1). Interstitial and intracellular oedema are not mutually exclusive. Interstitial oedema can lead to swelling, which can cut off blood supply, leading to intracellular oedema. Intracellular oedema can lead to cellular damage, which will stimulate the release of mediators and the inflammatory response (IR).

Oedema is a problem of fluid distribution and does not necessarily indicate fluid excess (McCance and Huether, 1997). It is usually associated with weight gain, swelling and puffiness, tight-fitting clothes and shoes, limited movement of an affected area, and symptoms associated with an underlying pathological condition.

Body fluid compartments

Total body water is commonly divided into two volumes:

- The extracellular fluid (ECF) volume

- The intracellular fluid (ICF) volume.

In an average male, intracellular water constitutes about 40% of total body weight and extracellular water about 20%. The extracellular fluid is further subdivided into:

- Plasma

- Interstitial fluid (ISF).

A third fluid compartment, known as transcellular fluid, is a distinct collection of fluids, of small volume, and is generally counted as interstitial fluid (Table 2).

Electrolyte composition of body fluid compartments

The solute compositions of the extracellular fluid and intracellular fluid compartments are electrolytes, that is, products of ionic compound dissociated in solution, and are markedly different in each compartment (Edwards, 2001). Cations carry a positive charge, and anions carry a negative charge.

The main cation of the extracellular fluid is sodium (Na+), whereas the main cation of the intracellular fluid is potassium (K+). The main anions of the extracellular fluid are chloride (CL-) and bicarbonate (HCO3-), and those of intracellular fluid are proteins - which are predominantly negatively charged - and organic phosphates.

Interstitial oedema

There are many different types of interstitial oedema. They are named by the mechanisms that cause it and may be localised or generalised (Table 3).

Interstitial oedema is formed in three ways:

- Changes in capillary dynamics due to increased hydrostatic pressure or decreased plasma oncotic pressure

- Stimulation of the inflammatory immune response

- Lymphatic system obstruction.

Changes in capillary dynamics

The blood in the capillaries is always under pressure. Fluid leaks out of the capillaries all the time into the interstitial space to allow nutrients to enter the cell (Marieb, 2001). However, this leakage does not affect the circulating volume because the movement of fluid in the opposite direction balances it.

Counteracting forces determine the fluid moving from the plasma to the interstitial space and vice versa. The fluid moving out of the capillaries and into the interstitial space is called filtration, and a fluid move into capillaries from the interstitial space is called absorption (Germann and Stanfield, 2002).

Two forces govern the movement of fluid across the wall of a capillary:

- The hydrostatic pressure (HP) gradient

- The osmotic pressure gradient.

The hydrostatic pressure gradient

The hydrostatic pressure gradient is the difference between the hydrostatic pressure of fluid inside the capillary and outside the capillary, determined by the blood pressure. Where hydrostatic pressure is higher, water tends to move from the side with the higher hydrostatic pressure to the lower one, driving water out of the capillaries. The hydrostatic pressure in the capillary varies because the pressure of blood declines continually as blood flows from the arteriolar end of the capillary to the venous end. In contrast, there is no variation in hydrostatic pressure outside the capillary.

The hydrostatic pressure inside the capillary declines from 38mmHg at the arterial end to 16mmHg at the venous end, and the hydrostatic pressure outside the capillary is 1mmHg. Therefore, the hydrostatic pressure drops from 38 - 1 = 37mmHg at the arterial end to 16 - 1 = 15mmHg at the venous end.

The osmotic pressure gradient

The osmotic pressure gradient is the difference between the osmotic pressure of fluid inside the capillary and outside the capillary. When an osmotic pressure gradient exists, water tends to flow from the side where the osmotic pressure is higher. This is determined by the protein concentration between the plasma and the interstitial fluid, because it creates a difference in osmotic pressure between the inside and outside of capillaries. The osmotic pressure exerted by proteins is referred to as oncotic pressure (OP).

Because the concentration of proteins in the plasma is higher than the concentration of proteins in the interstitial fluid, the oncotic pressure gradient is directed inwards and it tends to drive water into the capillaries.

Under normal conditions, the concentration of proteins in the plasma is 6-8 grams per 100ml, which is many times the protein concentration in interstitial fluid. The oncotic pressure of plasma is approximately 25mmHg, whereas that of interstitial fluid is negligible. Therefore, the oncotic pressure gradient across the capillary wall is 25 - 0 = 25mmHg.

Net filtration pressure (NFP)

The direction of water flow across the wall of a capillary is determined by the net filtration pressure, which is the difference between the hydrostatic pressure and the oncotic pressure: NFP = HP - OP

When the sign of the net filtration pressure is positive, the hydrostatic pressure gradient is greater than the oncotic pressure gradient, and fluid flows outward (filtration); when it is negative the oncotic pressure gradient is greater than the hydrostatic pressure gradient, and fluid flows inward (absorption).

Assuming that the hydrostatic pressure gradient of 37mmHg at the arterial end of the capillary and the oncotic pressure gradient is 25mmHg, the net filtration pressure is 37 - 25 = 12mmHg, which favours filtration. Assuming that the hydrostatic pressure falls to 15mmHg at the venous end of the capillary, the net filtration pressure at the end is 15 - 25 = -10mmHg, which favours absorption. Filtration and absorption occur within the same capillary to allow nutrients (glucose) to cross over into the cell.

Most of the fluid filtered out of the extracellular fluid is returned to the circulation, but there is a net deficit of 2mmHg. One might assume that this small amount of fluid remains in the interstitial space, and leads to oedema formation or a reduction in blood volume. But this is not the case because about three litres of filtered fluid is picked up from the interstitial space and returned to the circulation by the lymphatic system.

- Part 2 will examine different causes of oedema

 

 

 

 

 

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