Glossary

The Starling principle. That fluid filtration from capillary to interstitium depends on a driving hydrostatic pressure difference and a reverse osmotic pressure gradient. As described by Ernest Starling and still essentially correct.

The revised Starling principle. Recognises the endothelial glycocalyx layer as the filter across which the osmotic pressure difference exists.

The steady state Starling principle.
Emphasises that fluid filtration is kept low and fairly constant because the subglycocalyx osmotic pressure Πg rises by diffusion of large molecules from the interstitial space when fluid filtration falls, and will be close to the plasma osmotic pressure as fluid filtration approaches zero. Therefore filtration does not go into reverse (absorption does not occur). Therefore the age-old concept of filtration at the arterial end, and absorption at the venous end of a capillary is wrong. Therefore biophysical osmotic therapy with a colloid solution will not boost the intravascular volume beyond the volume of the solution infused.

The glycocalyx model. The mechanism, described separately by Michel and Weinbaum, that maintains the steady state of transendothelial fluid filtration.

Michel-Weinbaum model. As above.

The Revised Starling Equation and Glycocalyx Model. A clumsy title I gave to a paradigm for rational fluid prescribing in clinical practice. Sorry.

The Starling Equation. Jv = Lp A (delta P - sigma delta Π) The informed Starlingista knows that Starling gave us a Principle, but Staverman gave us Starling’s Equation almost half a century after Starling’s death.

Jv. The transendothelial volume flow of solvent. You could also call it the transendothelial filtration rate, especially if you use the filtration coefficient Kfc in the equation instead of Lp A.

Lp. The hydraulic conductivity of the endothelial barrier. Conductivity is the reciprocal of resistance (1/R)

Transendothelial resistance.
Rte, I term I propose clinicians use instead of hydraulic conductance because they are more familiar with the concept of resistance in electrical current analogies. The Starling equation then becomes Jv / A = (delta P - sigma delta Π) / Rte. The transendothelial resistance to the extravascular circulation of extracellular fluid is caused by the glycocalyx layer, the tight mesh of collagen that forms the endothelial basement membrane, and the looser intercellular matrix.

A (or SA). Surface area for filtration. Think of the surface area of the junction gaps and any other “pores”. Can be increased by recruitment of capillaries in hyper dynamic states as well as by increase in area of existing junction gaps.

Kfc. Filtration coefficient, an alternative term that includes both Lp and A.

Delta P. The hydrostatic pressure difference under consideration. In this case, the capillary pressure Pcap – the interstitial pressure Pint.

Delta Π. Pi is a Greek letter representing an osmotic pressure; delta Π is the osmotic difference difference across the glycocalyx layer, and the grand idea of the revised Starling principle is that the subglycocalyx osmotic pressure is close to zero, making delta Π much larger and Jv much smaller than they would be if we used the general interstitial osmotic pressure Πint. Πint is no longer to be considered a direct determinant of Jv.

sigma. Staverman’s reflection coefficient. A number that adjusts the actual delta Π for the effective delta Π. You can think of it as a descriptor of the health of the glycocalyx layer.

J-curve and J-point.
J-curve describes the relationship between Jv and Pcap; Jv is low and steady at low capillary pressures, but rises proportionate to delta P once Πg is zero and delta Π is maximal. The inflection point is the J point.

Js. The transendothelial mass flow of solute. Of most interest to clinicians is Js for albumin, but researchers may use alternative marker molecules, typically radioloabelled. The much-cited Glasgow study by Adam Fleck et al reported the Transcapillary Escape Rate for Albumin (TCER Alb) in surgical, septic and cancer patients.

Permeability. The ability of a solute to diffuse through the endothelial barrier. Many molecules, including albumin, are actively transported across the endothelial cell. Many people talk of increased permeability when describing increases in both Jv and Js, though it must be remembered that raised Pcap can increase Jv, and so Js by convection, with no change in endothelial permeability.

Leaky capillary. A loose and indefinable term widely used to explain oedema in the absence of high venous pressure. Not to be encouraged.

Context-sensitive fluid therapy. The response to fluid infusion varies with the context in which it is infused. Healthy volunteer, surgical patient, anaesthesia, critical illness, haemorrhage etc. RSE&GM attributes these findings to capillary pressure and the J-curve.

The colloid paradox. A laboratory finding that the ability of colloids to inhibit filtration was unrelated to their effect on colloid oncotic pressure was described as the COP paradox.

The colloid delusion. Though colloid solutions are very effective for diluting the erythrocyte concentration (i.e. Inducing anaemia), they are much less effective for restoring or preserving haemodynamic stability after haemorrhage.

Clinical Endotheliology. regards the vascular endothelium as an organ whose physiology and pathophysiology need to be understood by critical care practitioners.