Assessing Skeletal Muscle Variations in Microvascular Pressure and Unstressed Blood Volume at the Bedside

Abstract

The new NIRS variables we report could be a practical bench-to-bedside tool to assess venous driving pressure for systemic perfusion and measure changes in Vu within the microvascular bed.

Bedside Assessment of the Microvascular Venous Compartment in Cardiac Surgery Patients With Valvular Diseases Undergoing Cardiopulmonary Bypass

“…[26][27][28] Using NIRS, changes in the MBV were measured during cumulative 45-second-to-1-minute steps of Pcuff, increasing from 5 mmHg to 10, 15, 20, 30, 40, and 50 mmHg. 17 Previous studies have demonstrated a close correlation between Pcuff and pressure in veins. 29 The authors of the study presented here limited Pcuff to a value 10 mmHg lower than the individual patient's diastolic pressure, thus affecting veins rather than arteries.…”

“…When examining the microvascular bed, the authors focused mainly on the total Hb tissue concentration contained in venules, small veins, and, to a lesser extent, capillaries, because the Hb contained in the arterial vessels accounts for only 3% of the whole Hb measured with NIRS. 23 The MBV (in milliliters per 100 mL of tissue) was derived from the Hb concentration (g/dL) 17 in blood samples drawn from each patient, taking into account that Hb molecular weight equals 64,500 Da. Because tissue Mb at rest is almost fully oxygenated, 24 it is widely accepted that light absorption measured using NIRS is derived from [HHb] and from oxygenated Hb and Mb.…”

“…Vu (mL/100 mL of tissue) was calculated by subtracting to the MBV, measured before venous occlusions, the Vs and the [MbO 2 ], considered as 40% of the NIRS signal due to [HbO 2 / MbO 2 ], as previously described. 17 To calculate pressure in the MBV (PV), before cuff occlusions, the authors considered Pcuff at the first MBV increase, the sum of this blood volume increase, and baseline MBV, thus deriving pressure by proportion. Pit, pressure in the Vu at which intravascular pressure equals extravascular pressure (transmural pressure ¼ 0), was measured by extrapolating linear regression of the first 2 MBV increases after the steps of Pcuff increase.…”

“…Because the microvascular bed enlarges differently depending on the intravascular pressure values (see Fig 1), the term "capacitance" was used to refer to the ratio between blood volume changes at distending pressures lower than 30 mmHg (reflecting mainly the vascular bed recruitment) 17,30 ; the term microvascular "elastic compliance" (mvec) was used to refer to blood volume changes at distending pressures greater than 30 mmHg (reflecting the elastic properties of the vascular wall). 31 The following new NIRS variables were introduced: the capacitance of Vu or Vs, calculated by the ratios between these volumes and their respective pressures (Vu/Pit, Vs/Ps), and the time constant (τ), calculated by dividing the Vs by the tBF, as reported by Magder.…”

“…Recently, the authors used near-infrared spectroscopy (NIRS), a technology widely used for the in vivo evaluation of tissue oxygenation and blood flow, to measure microvascular bed volume (MBV) partitioning in Vu and Vs, and pressures therein, in humans. 17 This noninvasive, observational study of cardiac surgery patients with valvular diseases and cardiac dysfunction was designed to extend the knowledge of microvascular changes affecting the blood volume reserve for venous return and cardiac load 18 during off-pump surgery and to evaluate the effects of CPB on microvascular bed volume partitioning and tissue blood flow (tBF).…”