Venous haemodynamics Part One

A recent tweet about ATLS 10e Shock Classification  @misraasit gave me pause for thought. The message of this now-classic table is that arterial-side blood pressure may be so well defended in haemorrage that it is a poor indicator of  proximity to irreversible shock/ death. Heart rate may be raised by pain and anxiety, so that too is pretty non-specific. Conscious level may be obtunded by trauma, exhaustion or pain killers. Waiting to measure urine output is literally too little too late. It is now 40 years since ATLS was born, and it has to be disappointing that we haven’t identified a simple way to monitor the venous-side consequences of blood loss; an early warning system rather than a moribund identifier.

Of course, hospital-based clinicians will point to the emerging art of HoCUS – POCUS (hospital consultant using science – point of care ultrasound.) If we can see a fat central vein there is probably a decent central venous pressure. But we sadly acknowledge that CVP is probably only informative when it is either very low or very high. Like arterial-side measurements, CVP is protected during haemorrhage and only falls in the later stages of exsanguination. As Guyton showed, CVP is both an inhibitor of venous flow and a booster of atrial filling.

Visitors to fluidphysiology.org know that I urge prescribers to consider capillary pressure, because it is a major determinant of Jv. The patient who gets 30 ml/kg resus bolus may suffer transient capillary hypertension and so hyperfiltration (very raised Jv leading to oedema). Unfortunately the direct intravascular measurement of capillary pressure requires great skill and patience and is not practicable on the road or at the bedside. Wouldn’t it be fantastic if there was a quick and simple non-invasive way to quantify capillary filling pressure and venous filling pressure as well as CVP? A way to assess stressed and unstressed venous volumes? Then we could confidently give the required crystalloid fluid resuscitation in good time, in smaller volumes at slower rate and consequently with less harm and better outcomes. That would be progress. Fantastic!

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