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

·   GFR = 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 ->

↑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 -> ↑GFR

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

Interstitial hydrostatic pressure (Pi)

·   N/A

Interstial oncotic pressure (πBC)

·   ↑πi -> ↑GFR (normally zero)

 

Factors decreasing rate of lymphatic drainage:

Lymphatic obstruction

·   Tumour

Lymphatic damage

·   Idiopathic lymphoedema

·   Radiation

·   Lymphadenectomy

 

 

 

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