Blood flow and oxygen consumption with vascular occlusions

The tissue concentrations of oxy-hemoglobin ([HbO₂]) and deoxy-hemoglobin ([Hb]) provide measures of tissue perfusion and metabolism. Specifically, the total hemoglobin concentration ([HbT] = [HbO₂] + [Hb]) is a measure of blood volume, whereas the oxygen saturation of hemoglobin (StO₂ = [HbO₂]/[HbT]) represents a balance of oxygen supply and delivery. Oxygen supply is determined by the arterial saturation and by blood flow (BF) to the tissue, defined as the amount of blood flowing through tissue per unit time per unit volume (units: ml_blood∕(100 ml_tissue)/min). Oxygen delivery is closely linked to the tissue oxygen consumption (OC), defined as the amount of oxygen consumed by tissue per unit time per unit volume (units: μmol_O2∕(100 ml_tissue)/min). 

Blood flow in skeletal muscle can be measured with NIRS using a venous occlusion. The inflation of a pneumatic cuff placed around a subject's arm or leg to a pressure of 40-60 mmHg blocks the venous outflow in distal tissue while not impacting the arterial inflow. Consequently, distal tissues experience an increase in blood volume (as measured by NIRS through [HbT]) at an initial rate that is directly linked to blood flow (BF):

\(BF=\frac{1}{ctHb}\left.\frac{d\left[HbT\right]}{dt}\right|_0,\)                   (1) 

where ctHb is the concentration of hemoglobin in blood, and the subscript 0 indicates the initial rate of increase of [HbT] immediately after the onset of venous occlusion. 

Oxygen consumption in skeletal muscle can be measured with NIRS using an arterial occlusion. The inflation of a pneumatic cuff around a subject's arm or leg to a pressure >200 mmHg blocks both the arterial inflow and the venous outflow in distal tissue. Consequently, blood stops flowing and hemoglobin starts desaturating at a rate that is directly linked to oxygen consumption. In the absence of blood redistribution in the limb, [HbT] stays constant and the rate of increase in [Hb] is the same as the rate of decrease in [HbO₂]. In this case, the rates of change of [Hb] and [HbO₂] can be combined into a measure of oxygen consumption (OC):

\(OC=4\frac{d}{dt}\left(\frac{[Hb]-[HbO_2]}{2}\right)\),                   (2) 

where the factor 4 accounts for the four binding sites of oxygen in the hemoglobin molecule. 

Figure 1 shows time traces of [HbT] during venous occlusion (Fig. 1(a)), and [Hb] during arterial occlusion (Fig. 1(b)) in the human forearm of a 23 year-old male (ATT: 9.5 mm, MTT: 15 mm). In Fig. 1 we report time traces collected with different frequency-domain NIRS (FD-NIRS) data types (single distance (SD) or dual slope (DS) measurements of intensity \((I)\) or phase \((\phi))\), which feature different spatial sensitivities in tissue.

Fig. 1: (a) [HbT] in the forearm during venous occlusion. (b) [Hb] in the forearm during arterial occlusion. SDII, SDϕϕ: single-distance intensity, phase (at either 25 or 37 mm); DSII, DSϕϕ: dual-slope intensity, phase (at 25 & 35 mm).