Oxygen consumption (V˙o₂) is a measurement used to determine the metabolic rate, and is affected by environmental temperature, body temperature, physical activity, blood flow, and nutrition. Measurements of V˙o₂ have been used to study energy balance in newborn infants and to determine the optimal thermal environment for nursing preterm babies.¹ More recently it has been suggested that measurements of peripheral V˙o₂ may provide an indication of the need for circulatory support during critical care.

Methods used to assess V˙o₂ are either based on the Fick’s principle or a gas exchange technique. The standard units used to express V˙o₂ are ml O₂/kg/min. Neonatal cerebral V˙o₂ values using near infrared spectroscopy (NIRS) and jugular venous occlusion have been reported² and have been expressed in ml/100 g/min. NIRS has been used to measure V˙o₂ in limbs,^3–6 and the results are generally expressed in μmol O₂/100 g/min. This non-standard unit makes it impossible to make a direct comparison between global, cerebral, and peripheral V˙o₂ values. We describe a method of expressing NIRS derived peripheral V˙o₂ units in ml O₂/kg/min.

Methods

The basic units for expressing peripheral V˙o₂ by NIRS using the arterial occlusion method are mM HbO₂.cm/min (mmol HbO₂.cm/litre/min). This can be converted into μmol O₂/kg/min^5,6 using 4 × 10/(1.04 × 3.59 × L), on the basis that the molecular ratio of Hb to O₂ is 1:4, and the density of skeletal tissue is 1.04 g/ml.⁴ The distance between the light transmitting and receiving probe is L cm, and the path length correction factor is taken as 3.59.⁷ This is required to correct for scattering of light within the tissues. This equation reduces to 10.7/L.

Conversion of μmol O₂ into ml can be achieved using the molecular mass of oxygen (MO₂) which is 16 and the density (dO₂) which is 1.429 g/l. Consequently 1 μmol O₂ is converted into ml using:

Therefore conversion from mM HbO₂.cm/min into ml O₂/kg/min requires a multiplication factor of 1.1 × 10.7/L. In studies^5,6 where L is 3 cm the conversion factor is simply 3.92.

Results

We used data from previous studies^5,6 to examine the feasibility. Peripheral V˙o₂ was measured by NIRS using arterial occlusion and the oxyhaemoglobin (HbO₂) decremental slope. Global V˙o₂ values were obtained by open circuit calorimetry. Table 1 gives the converted values of peripheral V˙o₂ for comparison with global V˙o₂ values.

Discussion

Conversion of standard NIRS units into those normally recognised for V˙o₂ has been achieved. This allows comparison between global, cerebral, and peripheral V˙o₂ values and comparison between studies. The value of using a range of methods to measure tissue oxygenation is enhanced if the results can be compared through the use of standard units. For example, important relations between global and peripheral V˙o₂ have been described.^5,6

In making the conversion, two key physical variables are used which have so far only been measured in adults. The skeletal tissue density value of 1.04 g/ml has been used for adult muscle studies.⁴ The differential path length factor (DPF) value of 3.59 (0.32) has been reported for adult forearm⁷ for inter-optrode distances over the range 1–6 cm. It has also been shown that the DPF is “almost constant” beyond 2.5 cm. In the studies^5,6 illustrated in table 1, the inter-optrode distance is 3 cm for all infants, hence variation in the calculated peripheral V˙o₂ resulting from changes in DPF is minimal.

Clearly if tissue density and DPF values become available for newborn forearm, then the calculations can be refined. In the meantime, this conversion still provides a valuable method for comparing the relation between cerebral and peripheral V˙o₂. No previous NIRS research using peripheral V˙o₂ methods has been reported using the proposed units. The units commonly used are confusing and difficult to understand. It is recommended that in future peripheral V˙o₂ measurements are reported in ml O₂/kg/min.