Bidirectional perforators in the lower limb are not physiologically normal: A brief commentary

Most perforators have at least one valve allowing blood to flow from the superficial to the deep veins, representing normal physiology. ¹ Outward flow in a calf perforator is commonly accepted to signify pathological reflux. It has also been suggested though that the existence of bidirectional flow in perforators may be normal and is not necessarily an essential requirement of the genesis of venous hypertension and venous disease.^2–5 The proposal is that bidirectional flow allows quick equilibration of pressure changes between deep and superficial veins during ambulation in both varicose vein patients and healthy individuals. The implication of this is that calf perforators are essentially incompetent and the bidirectional flow within them is a normal, physiological feature. ²

This view regards calf perforators as re-entry sites, where reflux from the superficial veins drains into the deep system, minimising the impact of reflux on the development of venous hypertension. When the reflux volume exceeds the capacity of the perforator even though it has enlarged and become incompetent, it is the increased reflux that is the cause of worsening venous hypertension and perforator outward flow is not complicit. Consequently, incompetent bidirectional perforators could be deemed harmless and necessary. While in agreement with much of this reasoning, there are aspects of this hypothesis that need to be challenged. There are a number of underlying assumptions presented in several reviews^2–4 which may not be warranted. It may be time to settle the ambivalence regarding the direction of normal perforator flow. ⁶

Bidirectional flow in perforators has been implied to be common. Delis et al. suggested that this occurs in as many as 77% of competent perforators in legs affected by chronic venous disease (CVD), ⁷ whereas Sarin and Scurr reported outward flow in 21% of perforators in normal limbs in patients with contralateral venous disease during a distal compression manoeuvre but none in the critical relaxation phase. ⁸ Labropoulos et al. observed inward and outward flow in 20% of competent perforators in those with CVD but this was much less common (9%) in healthy, normal subjects. ¹ In several studies of younger active subjects devoid of any venous disease, no perforators with outward and inward flow have been observed.^9–11 This suggests that bidirectional flow is not common; we would expect that for a general physiological hypothesis of bidirectional flow, more frequent observations of this would be reported in normal limbs.

Even so, there are discrepancies between these studies related to the compression test used to characterise perforator flow and the populations studied. A standardised distal compression manoeuvre below the perforator of interest was used in all these studies. However they varied in how manual compression was applied (a simple squeeze by hand or with cuff inflation), at different distances from the perforator (in the calf or on the foot), variable duration of compression and its release, and different posture (sitting or standing) – all of which affect the haemodynamic response. The test, while diagnostically helpful, is not physiological. Manual compression includes both deep and superficial compartments simultaneously and produces an initial perforator inflow from superficial to deep (Figure 1). On relaxation of the compression outward flow may result and if prolonged, outward flow is consistent with valve incompetence. Calf muscle action is quite different, compression (systole) affects only the deep veins, leads to valve closure and possible brief outflow or no flow, and with subsequent relaxation (diastole) leading to inflow (Figure 2). The flows induced manually also do not correlate well with those from more physiological manoeuvres.^9,12 We suggest that more physiological manoeuvres better demonstrate the events of perforator flow. Hence, caution is required in using manual compression studies to define normal perforator function.OPEN IN VIEWER

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Figure 2.

Perforator velocity changes demonstrating spontaneous flow, followed by the response during isometric plantar flexion for about 1 s duration and then relaxation. Muscular action onset at ‘flex’ with no flow demonstrated, followed by inflow from superficial to deep veins during relaxation (‘release’).

It has been suggested that the failure to see bidirectional perforators is because of their small size and technical limitations in detecting them. ⁴ While calf perforators are more numerous than can be identified by ultrasound because of their small diameter and low spontaneous flow, with improved technology and careful search particularly in the location of greatest interest, the medial calf, a significant proportion of smaller perforators can be identified down to 0.4 mm in vivo. ⁹ Furthermore, when perforator venous flow is increased following exercise or with passive heating, all perforator flow is inward and obvious.^10,11 Although it is impossible to present evidence that all calf perforators in healthy people are indeed fitted with competent valves that allow only inward flow, one would have expected to recognise some with bidirectional flow in healthy young limbs if this was a normal physiological occurrence.

Early anatomical studies with dissections of small veins describe perforators being valveless.^13,14 More recent developments including resin cast studies of the lower leg are capable of showing small veins (<50 μm) in greater detail, and the presence of microvalves is undisputable. ¹⁵ These studies confirm the complexity of perforators and show that all have valves, often multiple, and always directed for inward flow.^16,17

Perhaps the controversy is one of semantics and dependent on the meaning of bidirectional flow, in particular what is meant by outward flow. Outflow has been described as follows: ‘During calf muscle contraction, the pressure in the posterior tibial vein is higher than in the great saphenous vein; it induces the harmless outward flow within calf perforators, which runs further via great saphenous vein in the physiological direction toward the heart. ³ It is well recognised that venous valve closure in this phase may be associated with a brief moment (or ‘flicker’) of reverse flow of short duration and small volume,^18–20 though this is not a general requirement of valve closure. ²¹ Such flow reversal is dependent on the method used to initiate valve closure and to create sufficient pressure differential and flow velocity across the valve. ²² The duration of this reversal of flow for calf perforators in active normal limbs is brief (150 ms, 95%CI 145–155 ms) and involves only small volumes based on distal compression testing^7,23 and does not constitute pathological reflux by common definitions. When tested more physiologically with foot movement valve closure may be more discrete (Figure 3).^5,12 This outward ‘wisp’ in a well-functioning valve is physiological but should not be considered incompetence nor be called bidirectional, as these terms implicate that there is no valve or the valve is dysfunctional. In contrast, when there is valve incompetence the pathophysiological outward flow with cuff relaxation is distinctively more prolonged (generally considered reflux at >350 ms ²³ or >500 ms ⁷ ).OPEN IN VIEWER

A critical postulate has been that bidirectional flow is needed to ‘allow a quick equilibration of pressure changes between deep and superficial veins during ambulation, and that the bidirectional flow allows this by conjoining the deep veins and the saphenous system of the lower leg’. ² This suggests that perhaps discontinuity or disruption of the continuity of blood with valve closure prevents pressure equilibration between the deep and superficial networks. To clarify this, it has been proposed that ‘the competence or incompetence of the whole system can be assessed preferentially by the simultaneous recordings of venous pressure in deep and superficial veins during calf pump activity’. ⁴ Unfortunately, evidence of this is scarce as this technique has not been performed often, especially in more recent times.^24–31 Furthermore, it would be preferable to acquire simultaneous measurement of both pressure and volume flow, as conventionally, definitions of competence or incompetence are based on flow. Even fewer such studies have been done.^28,32–34

Regardless, what these studies have shown is that the concept of needing open conjoined vessels with bidirectional flow for pressure equilibration is unfounded and unnecessary. While standing at rest, the pressures are similar in the deep and superficial systems in the lower leg which is understandable with similar hydrostatic pressures at similar positions. During ambulation, while the pressure profiles appear to be similar, they are not the same (Figure 4). Pressure waves in the deep veins within the muscle compartments are obviously generated by muscle compression within the compartments.^24,28 The superficial veins have no comparable compression mechanism and the pressure pulse wave observed is propagated from the deep veins. Pressure is propagated as a wave with velocity in m/s (compared to flow in cm/s ³⁵ ) and this is almost instantaneous in an antegrade direction up the deep axial vessels and retrograde through the perforators into the superficial vessels. The shape and amplitude of the pressure wave along its path is determined by the vessel characteristics including wall compliance, flow direction, tributaries and any intraluminal projections such as valves. All of these features generally attenuate wave propagation but do not prevent it. In both the superficial and deep veins the wave phasicity is the same and the effects of ambulation to lower pressure are similar but the waveforms are different (Figure 4). Neglen also demonstrated differences in magnitude and rate of pressure changes. ³³ These observations suggest that luminal continuity throughout the pump cycle is not required. However, pressure wave analysis to detect valve competency has not been reported for the perforator. In contrast, venous flow which is stopped by the closed valve provides a clearer indicator of competence.OPEN IN VIEWER

The pressure wave behaviour is complex and is undoubtedly influenced by the degree of valve closure. Complete closure is relatively short lived for flow to be sustained. The propagation of pressure pulse waves across closed valves in the venous system is readily demonstrable as for example by propagation of a cough impulse detected down the length of the great saphenous vein with all its intact valves to the ankle.^26,32,33 Absence of competent valves does allow more ready propagation of the cough impulse amplitude that can be simply detected as in the well-known clinical test by palpation of the great saphenous vein at the ankle. Similarly, the presence of intact venous valves does not prevent propagation of the low amplitude cardiac and respiratory pressure waves in the forearm for clinical venous wave analysis.^36,37

Our observations are that:

• Outflow is physiological when it is brief and minimal, occurring at the time of valve closure at the commencement of systole. It varies with the method used to demonstrate valve closure.

We suggest that ‘bidirectional’ perforator flow as a physiological description should be avoided as it is conventionally used to describe the abnormal pathophysiological flow pattern of valve failure of the incompetent perforator (i.e. reflux) as observed with venous ultrasound. It is unfortunate that in earlier days, venous pressure measurements held most sway in understanding perforator function and their incrimination in venous hypertension. Subsequently venous flow measurements have become more accessible and shed further light on the contributing role of the perforator to superficial reflux.^4,38 Both pressure and flow measurements have important implications for understanding the role of perforators in venous hypertension and resolving the dilemma of what to do clinically when they are truly incompetent with pathological outflow and inadequate compensatory inflow.