Effects of passive leg raising on microvascular venous compartment in critically ill patients

BACKGROUND: Even though fluid loading is thought to improve organ perfusion, the way in which it does so remains unclear. We assessed how the microvascular bed in skeletal muscle reacts to passive leg raising in patients with and without sepsis or septic shock. METHODS: We studied 40 critically ill patients (group A) and 30 healthy controls (group B). The forearm microvascular bed was assessed using near-infrared spectroscopy before and after passive leg raising. We measured stressed and unstressed volumes, inside pressures, blood flow, microvascular compliance and tone. RESULTS: In group A, passive leg raising induced a microvascular bed increase from 4.9 (3.2-6.5) mL/100 mL tissue to 5.7 (3.9-8.1) mL/100 mL tissue (P=0.005), leaving inside pressures unchanged, whereas in group B neither volumes nor pressures changed. Patients without sepsis showed an increase in the stressed volume from 0.22 (0.10-0.28) mL/100 mL tissue to 0.34 (0.23-0.66) mL/100 mL tissue (P=0.039) and a decrease in compliance (P=0.004), whereas, in septic shock, the unstressed volume increased from 4.20 (3.01-5.82) mL/100 mL tissue to 5.32 (4.01-11.50) mL/100 mL tissue (P=0.036). In critically ill patients near-infrared spectroscopy showed no difference in microvascular variables between responders and non-responders to passive leg raising, but responders showed a cardiovascular response shorter than healthy subjects. CONCLUSIONS: Our study provides evidence that macrocirculatory parameters are unreliable to derive measurements of stressed and unstressed volumes. Our results indicate that in septic shock, the enlargement of the unstressed volume associated with passive leg raising induces loss of fluids to the interstitium, thus leaving organ perfusion unchanged or worse.