Ultrasound evaluation of the respiratory changes of the inferior vena cava and axillary vein diameter at rest and during positive pressure ventilation in spontaneously breathing healthy volunteers

Introduction

Ultrasound assessment of the inferior vena cava (IVC) has gained favour in aiding fluid management decisions for controlled, mechanically ventilated patients as well as in non-mechanically ventilated, spontaneously breathing patients. Its utility in spontaneously breathing patients during positive pressure non-invasive ventilation has not yet been determined. The use of the axillary vein, as an alternative option to the IVC due to its ease of accessibility and independence from intra-abdominal pressure, has also not been evaluated. The aim of this study was to assess respiratory variation in IVC and axillary vein diameters in spontaneously breathing participants (Collapsibility Index) and with the application of increasing positive end-expiratory pressure (PEEP) via positive pressure non-invasive ventilation (Distensibility Index).

Methods

The IVC and axillary vein diameters of 28 healthy adult volunteers were measured, using ultrasound, at baseline and with increasing PEEP via non-invasive ventilation. The Collapsibility Index and Distensibility Index of these vessels were calculated and compared for each vessel. The association between increasing PEEP levels and the indices was evaluated.

Results

Positive pressure delivered via non-invasive ventilation produced a similar degree of diameter change in the IVC and the axillary vein, that is, the Distensibility Index was similar whether measured in the IVC or the axillary vein (P=0.21, 0.47 and 0.17 at baseline, 5 and 10 cmH₂O PEEP, respectively). Individual study participants' IVC and axillary veins, however, had variable responses to PEEP; that is, there appeared to be no consistent relationship between PEEP and the diameter changes.

Conclusion

While the axillary vein could potentially be used as an alternative vessel to the IVC to assess for volume responsiveness in controlled, mechanically ventilated patients as well as in non-mechanically ventilated, spontaneously breathing patients, neither vein should be used to guide fluid management decisions in spontaneously breathing patients during positive pressure non-invasive ventilation.

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