The present study aims to evaluate, with non-invasive methods, the relationships existing between PEEP and renal perfusion in patients undergoing surgery and requiring ventilatory support and monitoring in intensive care. Renal perfusion will be assessed at baseline (PEEP 0), subsequently ,at the application of progressive increases of PEEP. At the end of the measurements, the implications of the use of PEEP on the renal vascular system will be analyzed.

The present physiological, monocentric, observational clinical study evaluates the effect of progressive PEEP increases on renal perfusion in mechanically ventilated patients who underwent surgery with the need of ventilatory support and monitoring in intensive care.

Precisely, will be evaluated:

• The morphometric parameters (coronal diameter and cortico-medullary thickness) and the renal resistive indices (IR). The inter-lobar arteries will be sampled and the IRs will be measured as the upper and lower level and the mesorenes. Measurements will be performed both in the kidney and in baseline conditions and incremental values of PEEP (0 - 6 - 12).
• Gas exchanges, FC, systolic BP, diastolic and mean, Pmax, P1 and P2 of the airways, total and ohmic resistances and static respiratory compliance indexed for weight body (RRSmaxI, RRSminI, CrsI will be recorded).

The intra-class correlation coefficient will be measured to estimate the intra-observer reproducibility of the conducted measurement.

Inclusion Criteria:

• 18 Years and older
• Receiving mechanical ventilation, invasive blood pressure monitoring and an urinary catheter to monitor diuresis
• Need for postoperative monitoring in intensive care
• Written informed consent;

Exclusion Criteria:

• Acute kidney injury defined according to KDIGO guidelines;
• Pre-existing chronic renal disease;
• Heart failure;
• ARDS;
• Sepsis and Septic Shock;
• Hemodynamic instability with the need for vasoactive support;
• Chronic or occasional therapy with drugs that could modify the renal blood flow and that can not be suspended two days before the study (Vasoactive agents, Ace-inhibitors, Beta-blockers, NSAIDs, Sartans and Diuretics therapy);
• Presumed or established state of pregnancy;
• Refusal of informed consent;

Patients undergoing surgery and with the need for ventilatory support for the intensive care unit