This study prospectively observes changes in critical closing pressure (Pcc) and derived hemodynamic parameters in mechanically ventilated distributive shock patients when clinically indicated norepinephrine dose reduction is performed by the attending physician. Pcc is measured before and 30 minutes after dose reduction using a stepwise inspiratory hold maneuver via PiCCO monitoring.

In distributive shock patients receiving norepinephrine with MAP maintained in the 75-85 mmHg range, attending physicians may reduce norepinephrine dose based on clinical judgment. This study observes hemodynamic changes associated with such reductions without any investigator-initiated intervention. Critical closing pressure (Pcc) and derived parameters including tissue perfusion pressure (TPP), mean systemic filling pressure analogue (Pmsf), and vascular waterfall (VW) are measured at baseline and 30 minutes after dose reduction, provided MAP variability is less than 5 mmHg. Pcc is derived from linear regression of simultaneously measured cardiac output (CO) and mean arterial pressure (MAP) during stepwise inspiratory hold maneuvers. Steady-state CO and MAP values are recorded during the final 3 seconds of 12-second inspiratory holds at plateau pressures of 5, 15, 25, and 35 cmH2O. The x-intercept of the CO-MAP regression line is taken as Pcc.

Inclusion Criteria:

1. Adult distributive shock patients aged ≥18 years;
2. Receiving norepinephrine infusion with MAP in the 75-85 mmHg range;
3. PiCCO catheter in situ;
4. Under mechanical ventilation;
5. Clinically indicated norepinephrine dose reduction per attending physician judgment.

Exclusion Criteria:

1. Significant cardiac arrhythmia affecting CO measurement reliability;
2. Contraindication to airway plateau pressure maneuver;
3. Active titration of other vasoactive agents within 30 minutes prior to measurement;
4. Hemodynamic instability making it impossible to reduce norepinephrine dose.

Adult distributive shock patients receiving invasive mechanical ventilation and norepinephrine treatment