Fluid administration is the first line of treatment in patients with acute circulatory failure.  It is recommended to use dynamic indices to predict fluid responsiveness. The current SARS CoV2 pandemic has posted a real challenge with diverse presentation, with two lung phenotypes, (L-phenotype and H phenotypes) and most patients who required intensive care unit admission had ARDS.1 The circulatory failure in such patients is more complex due to varied prevalence of myocardial injury, distributive shock, cytokine storm and right ventricular dysfunction (proportion of ARDS patients). Recent surviving sepsis campaign suggested restricted fluid strategy in patients with ARDS due to SARS CoV2 and to use appropriate dynamic parameters to decide fluid therapy.2 Various dynamic indices which depend on cardiopulmonary interactions like stroke volume variation (SVV) and pulse pressure variation (PPV), aortic root velocity variation, inferior vena cave distensibility are superior to static indices and shown to accurately predict fluid responsiveness.3 Among all available parameters, the parameters derived using ultrasonography are less invasive, repeatable, easy to do and available at bedside. One such parameter is Inferior vena cava diameter variability (IVC-V), which is routinely measured in all patients with shock, during screening using echocardiography which is calculated as IVC distensibility index (IVC-DI) during controlled ventilation and has been shown to reliably predict fluid responsiveness in mechanically ventilated patients.4,5,6 The IVC-V depends on the variations in intrathoracic pressure, abdominal pressure, and compliance of the vessel. The major limitation for using IVC-DI is first, it is not tested in ARDS patients with low tidal ventilation (≤6mL/kg PBW) and poor lung compliance Crs is <30 mL/cmH2O. Most of the studies that tested IVC-DI used tidal volume > 8 mL/kg PBW and not in low tidal volume ventilation 6,7. When Vt less than 8 mL/kg is used, there may be smaller variations in intrathoracic pressure and in turn intrathrocic blood volume leading to smaller variations in IVC dimensions, thus leading to false negative results irrespective of the volume status.8 Second, it is not feasible to use IVC-DI in patients with prone ventilation. Both these limitations prevent the use of this test in the COVID pandemic where most of the patients are either ventilated with low tidal volume or in prone position or both.  

To overcome the limitation of low TV, a newly developed test “tidal volume challenge,” which involves transiently increasing the TV from 6mL/kg PBW to 8mL/kg PBW for one minute has shown reliability in predicting fluid responsiveness using PPV9. Recent study showed improved reliability of IVC-DI after “tidal volume challenge” with moderate discrimination.10 So â€œtidal volume challenge”, is a potential alternative for using IVC-DI in patient with low tidal ventilation. But all patients were in supine position. IVC-DI cannot be used in prone position due to inaccessibility of echocardiography window in prone position.  An alternative approach to IVC, through the liver, i.e. lateral view or transhepatic view is a potential option for patients in prone position. Few previous studies have shown the feasibility of using this approach and its ability to predict FR.11,12 Also, previous studies have shown moderate correlation of IVC diameter and good agreement in IVC variability between both views in critically ill patients.13,14,15  Also, studies have shown that it is easy to get good view of IVC through lateral approach than subcostal approach15. This opens an opportunity to use the IVC-DI through lateral approach in prone ventilation. Thus, using the lateral approach combining with tidal volume challenge it is possible to use IVC-DI in patients with low tidal volume ventilation and in prone position.  This is easy, repeatable, and feasible in all prone ventilated patients and we hope it would add value to the tests of fluid responsiveness in prone ventilation and it will be of immense help during this CVOID pandemic.  

So, we would like to determine the reliability of IVC-DI at low Vt and whether the “tidal volume challenge” can improve the reliability of IVC-DI in predicting fluid responsiveness in patients receiving low Vt ventilation.

Methodology