The application of a brief SIGH of 4 seconds at 35 cmH20 has shown to reliably predict fluid responsiveness in critically ill patients undergoing pressure support ventilation.

The end-expiratory occlusion test (EEOT) has been also used in the same type of patients, with the same purpose, but in a limited amount of studies.

The aim of this study is to compare the reliability of the the two test in assessing fluid responsiveness.

Assessing preload dependence in critically ill patients is a challenge for the Intensive Care Unit (ICU) physicians. During controlled mechanical ventilation, dynamic indexes can be applied in non-arrhythmic patients with sufficiently high tidal volume (VT), i.e., >8 ml/kg and non-severely impaired lung compliance. The interplay between mechanical ventilation and hemodynamic is more complex in patients with a spontaneous breathing activity, whose respiratory efforts affect intrathoracic pressure and venous return to the right ventricle (RV).

To overcome these limitations, functional hemodynamic assessment, consisting in maneuvers determining a sudden change in cardiac preload, such as Passive leg raising (PLR) or end-expiratory occlusion test (EEO), represents a valuable means for assessment of fluid responsiveness.

Both PLR and EEO have been successfully utilized for assessing fluid responsiveness, regardless of ventilatory assistance and mode of ventilation. Unfortunately, however, some drawbacks limit the extensive use of these maneuvers in clinical practice. One the one hand, PLR cannot be applied in some clinical situations, such as traumatisms of the hip, legs or lumbar spine, deep venous thrombosis, intracranial or abdominal hypertension. Indeed, a recent large observational study showed PLR to be the most common form of assessment of fluid responsiveness, being used, nonetheless, in only 10.7% of the patients necessitating the assessment of fluid responsiveness. On the other hand, rates of EEO failure as high as 22.5 % have been reported, consequent to visible patient's effort against the occluded airway.

It has been recently successfully tested a new approach for assessing fluid responsiveness in patients undergoing partial ventilatory assistance. It has been proposed that the changes from baseline of systolic arterial pressure (SAP), pulse pressure (PP) and stroke volume index (SVI) in relationship to the airway pressure (Paw) generated during a "sigh" maneuver can predict fluid responsiveness in ICU patients undergoing pressure support ventilation (PSV) In the first "proof of concept" study (performed in 40 hemodynamically unstable ICU patients) three sighs has been tested, in a computer-generated random order, at either 15 (SIGH15), 25 (SIGH25) and 35 (SIGH35) cmH2O of total inspiratory Paw (PEEP + PSV). The results showed that only the variation of the pulse pressure recorded after SIGH35 application successfully predicted fluid responsiveness. [AUC of PP after SIGH35 = 0.91 (0.82 - 0.99); sensitivity 75% (47.6%-92.7%) and specificity 91.6 (73.0%- 98.9%); best threshold value of the ROC curve was -35% from baseline].

Hemodynamic Measurements. All patients will be equipped with a continuous hemodynamic monitoring of arterial waveform.

The primary end point will be assessed by means comparison of the areas (AUC) under receiver operating characteristic (ROC) curves: considering the data reported in the literature, we assumed that the AUC of the SIGH35 should be of at least 0.85 to be clinically relevant. This value was compared to the null hypothesis (AUC = 0.65; ratio of samples sizes in negative and positive groups of 1:1): the calculated sample size is 50 patients. Considering the possibility of the occurrence extrasystoles during the beat-to-beat evaluation of the EEOT and the SIGH, the sample size is inflated by the 20% to account for the rate of loss of patients during the post-hoc data analysis. The final sample size is 60 patients.

Fluid responsiveness is defined as a CI increase ≥ 10% after FC infusion.

• Adult (>18 yo) ICU patients undergoing PSV ventilation

• Inspiratory support level (PS) between 8 and 15 cmH2O and positive end-expiratory pressure (PEEP) between 5 and 12 cmH2O,

• Presence of acute circulatory failure defined as:

  • Systolic blood pressure (SBP) ≤90 mmHg or mean arterial pressure (MAP) ≤70 mmHg or requiring vasopressors to maintain SBP >90 mmHg or MAP >70 mmHg or decline of SBP > 50 mm Hg in known hypertensive patients, along with one or more of the following:
  • urinary flow ≤0.5 mL/kg/min for ≥2 hours
  • heart rate ≥100 beats per minute
  • presence of skin mottling and 4) blood lactate concentration ≥4 mmol/L.

Exclusion criteria

• left ventricular ejection fraction <30% or severe valvular dysfunction
• atrial fibrillation
• severe acute respiratory distress syndrome (ARDS)
• abdominal compartment syndrome
• air leakage through chest drains
• artifacts in arterial waveform
• pathological respiratory patterns due to neurological diseases
• signs of fatigue or respiratory distress

The datasets used and/or analyzed during this study are available from the corresponding author on reasonable request.