Subclavian Vein Diameter Variations to Assess Vascular Filling

Acute circulatory failure is encountered in shock, regardless of its origin, when physiological compensatory mechanisms are overwhelmed. The initial tachycardia and reflex vasoconstriction of shock aims to maintain sufficient arterial pressure and tissue perfusion pressure. Beyond these mechanisms of compensation for relative and/or absolute hypovolaemia, arterial hypotension and tissue hypoperfusion appear, expressed clinically by functional signs (e.g. asthenia) and physical signs (arterial hypotension, consciousness disorders, mottling, oliguria, etc.).

It has been observed that organ failure induced by circulatory insufficiency is associated with a poor prognosis in septic shock. In daily clinical practice, acute circulatory failure is most often characterised by haemodynamic failure as reflected by arterial hypotension and/or tachycardia. Early treatment of acute circulatory failure is a recognised prognostic factor in trauma patients and patients with septic shock (2,3), leading to a reduction in mortality.

The first line of treatment for circulatory failure is usually early vascular filling due to relative and absolute hypovolaemia. The choice of fluids depends on the clinical situation, although for septic shock crystalloids are recommended as first-line therapy. Although early vascular filling is the first line of treatment, overfilling, also known as "induced" vascular filling, is alone recognised as a factor associated with excess mortality in various situations of acute circulatory failure. It is now accepted that early administration of inotropic and/or vasopressor agents, i.e. before the end of vascular filling, is necessary to restore arterial pressure and therefore limit the consequences of tissue hypo perfusion. In order to determine the choice between continued vascular filling and the administration of vasopressor and/or inotropic agents, predictive indicators of vascular filling requirements have been developed. These aim to determine whether or not vascular filling will increase cardiac output or one of its surrogates such as the subaortic time velocity integral (SATI).

Numerous indicators have been described for more than 30 years in intensive care units. Dynamic criteria are those that have the best predictive performance compared to static indicators. Their time-consuming implementation requires an invasive approach for most of these criteria due to their technical nature and the need to respect sometimes restrictive validity criteria.

The technological development of ultrasound scanners has allowed their liberal use and the validation of dynamic ultrasound criteria predictive of the response to vascular filling (respiratory variations of the sub-aortic velocity time integral, S wave, index of distensibility of the inferior vena cava, index of collapsibility of the superior vena cava, central venous pressure). The mechanism underlying superior vena cava collapsibility is complex, involving variations in intra-thoracic pressure as well as the inherent movements of the lung parenchyma. These pathophysiological mechanisms have not yet been described.

A recent study has demonstrated the interchangeability between delta PP and respiratory variations in subclavian vein diameter in intensive care patients and in the operating theatre.

The advantages of the transthoracic ultrasound approach to assessing respiratory variations in subclavian vein diameter are its rapidity of implementation, ease of use, non-invasive nature and reproducibility.