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

 

List:

        Intro

        Starling hypothesis

        Normal starling numbers

        Factors increasing rate of transudation

        Factors decreasing rate of lymphatic drainage

 

Intro: interstitial oedema

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

 

Starling hypothesis:

Traditional hypothesis

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

   Normally 4L/day

   Filtration rate = Kf[(PC Pi) σ(πC πi)]

Objections to traditional hypothesis

   Net filtration occurs across the whole length of the capillary

   The glycocalyx limits protein extravasation as well as the endothelium

 

Normal starling values:

 

Arterial end

Venous end

Pc

35mmHg

15mmHg

Pc

0

0

πc

28

28

πi

3

3

Net

+10mmHg (net filtration)

-10mmHg (net reabsorption)

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

 

Factors increasing rate of transudation:

↑Filtration coefficient (Kf)

   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 (PC)

   ↑PC at arterial end (minor effect)

o Severe hypertension

o Relaxation of pre-capillary sphincter

   ↑Pc at venous end (major effect)

o Right heart failure, IPPV

o Constriction of post-capillary sphincter

↓Capillary oncotic pressure (πC)

   ↓ πp -> ↑transudate

   Hypoproteinaemia: e.g. nephrotic syndrome, cirrhosis, malnutrition

Interstitial hydrostatic pressure (Pi)

   N/A

Interstial oncotic pressure (πi)

   ↑πi (normally zero) -> ↑transudate

 

Factors decreasing rate of lymphatic drainage:

Lymphatic obstruction

   Tumour

Lymphatic damage

   Idiopathic lymphoedema

   Radiation

   Lymphadenectomy

 

 

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