Norepinephrine causes a pressure-dependent plasma volume decrease in clinical vasodilatory shock

Area of Science:

• Critical Care Medicine
• Cardiovascular Physiology
• Pharmacology

Background:

• Norepinephrine increases blood pressure, potentially reducing plasma volume, especially with increased microvascular permeability.
• Vasodilatory shock, often seen in sepsis or post-cardiotomy, requires interventions like norepinephrine to maintain mean arterial pressure (MAP).

Purpose of the Study:

• To investigate the impact of norepinephrine-induced mean arterial pressure (MAP) variations on plasma volume and systemic hemodynamics.
• To determine the relationship between MAP levels and plasma volume changes in patients with vasodilatory shock.

Main Methods:

• Twenty-one mechanically ventilated patients with vasodilatory shock received norepinephrine titrated to target MAPs of 60, 75, and 90 mmHg.
• Systemic hemodynamics, hematocrit, oxygen content, and urine flow were measured at each target MAP.
• Plasma volume changes were calculated using pre- and post-intervention hematocrit values.

Main Results:

• Increasing MAP from 60 to 90 mmHg with norepinephrine led to significant decreases in plasma volume (6.5% to 9.4%).
• Mean arterial pressure (MAP) was identified as an independent predictor of norepinephrine-induced plasma volume reduction.
• Cardiac index, central venous pressure, and norepinephrine dose did not independently predict plasma volume changes.

Conclusions:

• Norepinephrine administration results in a pressure-dependent reduction in plasma volume in patients experiencing vasodilatory shock.
• Transcapillary fluid extravasation is the likely mechanism driving plasma volume loss during norepinephrine infusion at higher MAP targets.