**4.1 What severely embarrasses or kills the endothelium in a venous valve pocket?**

The clue to what injures the endothelial lining of an intact, functioning, vein was suggested by Drinker (1938) and van Ottingen (1941), whose researches unexpectedly established that copious and ubiquitous thrombus-like coagula formed in the veins of the victims of carbon monoxide poisoning and were associated with endothelial alterations. Evidently, either carboxyhaemoglobinaemia or hypoxaemia could cause the endothelial injury associated with DVT. Drinker proposed this hypothesis to O'Neill (1947) who provisionally validated it, and Samuels & Webster (1952) supported the proposal. Malone & Morris (1978) showed that lesions similar to the white parts of autochthonous and experimental thrombi formed in the veins of oxygen-starved animals by the process of margination and sequestration of platelets and leukocytes. Interestingly, Samuels & Webster (1952) showed that heparin does not prevent the development of a nascent thrombus on a hypoxically injured endothelium.

In those mid-20th century papers, it was debated whether oxygen was supplied to the venous endothelium from the luminal blood or the vasa venarum. Presumably, hypoxia sufficiently severe to kill the endothelial cells and induce leukocyte swarming and phagocytosis of the debris required impaired oxygenation from both sources; it is well established that the venous endothelium can survive moderately prolonged hypoxia (Jackson *et al.*, 1988), depending on anaerobic glycolysis to provide ATP (Berna *et al.*, 2001), and surgeons procure the use of a bloodless field under a 2.5 hour tourniquet. However, hypoxia that is not so severe as to kill the cells but is sufficient to alter their phenotype can also lead to leukocyte and platelet recruitment and local coagulation. During the period 1980-2000, studies in a number of laboratories established that significant but non-fatal hypoxia in cultured venous endothelial cells induces the expression of the early growth response-1 (egr-1) gene, and this unleashes a cascade of gene-expression and phenotypic changes that would promote local coagulation and leukocyte accumulation *in vivo* (Pinsky *et al.*, 1995; Yan *et al.*, 1999a,b; Karimova & Pinsky, 2001; Ten & Pinsky, 2002).

Aetiology of Deep Venous Thrombosis - Implications for Prophylaxis 141

1987) helped to resolve the seeming paradox. Essentially, these studies showed that when venous blood flow is *non-pulsatile (streamline)*, as when the patient's 'calf muscle pump' is inactive and there is no intermittent (*vis a tergo*) upward pressure on the soles of the feet, as in walking, a significant part of the blood within 'backwater' valve pockets is not exchanged with the luminal blood. Under such non-pulsatile flow conditions the valve does not execute

Near the mouth of the valve pocket, the blood is likely to circulate in a spiral vortex, driven by the laminar flow in the vein lumen. Deep in the pocket is a secondary vortex, rotating in the opposite sense to the primary vortex and more slowly (see Fig. 1). Because the blood in this secondary vortex is never evacuated from the valve pocket while flow in the vein remains non-pulsatile, it becomes increasingly hypoxaemic. Therefore, the endothelia lining the depths of the valve pockets are progressively at risk of hypoxic injury when the venous blood flow is non-pulsatile, *irrespective of the flow rate*. This was demonstrated experimentally by Hamer *et al.* (1981), as discussed below. We concur with Schina *et al.* 

Valve cusp leaflets are avascular (Franklin, 1937; Saphir & Lev, 1952a,b; Sevitt, 1974) - they have no vasa venarum. The outer (medial) endothelial surface (luminalis) is oxygenated by the blood flowing through the vein lumen, irrespective of pulsatility, but the inner, lateral endothelium (parietalis) lining the valve pocket is not (see Fig. 1). Therefore, the parietalis endothelium is at greatest risk of hypoxic injury, and potentially of necrotic cell death, when

The initial version of the VCH hypothesis was conceived in 1966 and first outlined in 1977 (Malone, 1977). Its premises were tested critically during the late 1970s and early 1980s. The full version of the validated thesis, with detailed historical and experimental support and scientific and clinical implications, was published some 20 years later (Malone & Agutter,

Under normal (pulsatile) blood flow conditions, the venous valve pockets are emptied and refilled regularly and thus do not become hypoxaemic (Fig. 1). However, if there is sustained non-pulsatile ('streamline') venous blood flow, DVT may occur. Such flow leads to suffocating hypoxaemia in the venous valve pockets, resulting in hypoxic injury to the

This hypoxic injury activates the pleiotropic elk-1/egr-1 pathway within the endothelial cells, which in turn activates a number of chemoattractant and procoagulant factors. When normal pulsatile blood flow is restored, even transiently, leukocytes and platelets, attracted by these factors, may swarm to the site of dead endothelial cells and initiate protective

Prolongation of non-pulsatile flow for multiple hours may kill the accumulated blood cells in the unemptied valve pocket. These dead cells may then form the core of a nascent thrombus (Fig. 4). If periods of non-pulsatile and pulsatile flow continue to alternate in that abnormal sequential pattern (very protracted stasis + very brief normal flow), the ensuing

its usual cycle but remains half-open/half-closed indefinitely.

(1993) that non-pulsatile flow, not slow flow or 'stasis', promotes DVT.

oxygen-starved during sustained non-pulsatile blood flow.

2006, 2008). The VCH thesis is summarised in Figs. 1-4.

inner (parietalis) endothelium of the cusp leaflets (Fig. 2).

coagulant action locally (Fig. 3).

**4.2 The VCH thesis of aetiology: a summary** 
