2004B16 Explain the physiological processes involved in the development of interstitial oedema.

· Intro

· Starling hypothesis

· Normal starling numbers

· Factors increasing rate of transudation

· Factors decreasing rate of lymphatic drainage

Definition

· Excess fluid in the interstitium i.e. space outside vessel and cells

· Can occur in pulmonary and/or systemic circulation

Aetiology

· Interstitial fluid volume ∝ rate of transudation / rate of lymph removal

· Hence either ↑rate of transudation or ↓rate of lymph removal

· Interstitial hydrostatic pressure increases until a new equilibrium is reached

Traditional hypothesis

· Filtration rate governed by balance of hydrostatic and oncotic pressures either side of the membrane

· Normally 4L/day

· Filtration rate ∝ = K_f[(P_C – P_i) – σ(π_C – π_i)]

Objections to traditional hypothesis

· Net filtration occurs across the whole length of the capillary

· The glycocalyx limits protein extravasation as well as the endothelium

(note falsely predicts net filtration only at the arterial end)

↑Filtration coefficient (K_f)	· ∝ surface area x hydraulic conductivity · e.g. SIRS, sepsis, anaphylaxis -> leaky capillary
↓Staverman reflection coefficient (σ)	· Corrects actual oncotic pressure difference to observed effect · ∝ 1 / (permeability to protein) · e.g. damage to capillary -> ↓ σ -> ↑transudate
↑Capillary hydrostatic pressure (P_C)	· ↑P_Cat arterial end (minor effect) o Severe hypertension o Relaxation of pre-capillary sphincter · ↑P_c at venous end (major effect) o Right heart failure, IPPV o Constriction of post-capillary sphincter
↓Capillary oncotic pressure (π_C)	· ↓ π_p -> ↑transudate · Hypoproteinaemia: e.g. nephrotic syndrome, cirrhosis, malnutrition
Interstitial hydrostatic pressure (Pi)	· N/A
Interstial oncotic pressure (π_i)	· ↑π_i (normally zero) -> ↑transudate

Lymphatic obstruction

· Tumour

Lymphatic damage

· Idiopathic lymphoedema

· Radiation

· Lymphadenectomy