Microvascular bed volume evaluated by near infrared spectroscopy in healthy and critically ill subjects

Introduction: The microvascular tone regulation can be very useful to be assessed to titrate therapy in critical conditions. In the present study, we use near infrared (NIR) spectroscopy in the frequency domain on critically ill patients and healthy subjects, to obtain the measurement of microcirculatory compliance and the regulation of vascular tone. Methods: Two groups of subjects have been studied: the first group was composed by 17 healthy subjects (H) at rest, in the second group five critically ill patients were treated with noradrenalin infusion (N). In the N group the measurements have been performed at different noradrenalin doses: the first dose (d1) was established by the head physician, then the dose was doubled (d2) and finally reduced at the half of the first dose (d3). The probe of the spectrometer used in this study (ISS incorporated, Urbana, IL, USA), have been applied on the forearm skin, in a zone corresponding to brachioradial muscle. A serial of a pneumatic cuff compressions have been performed at the arm. Measurements of the total hemoglobin [Hbt] in tissue and blood volume, derived from [Hbt] have been obtained. The linear coefficient correlating blood volume to cuff compressions was considered as a measurement of venous and capillary compliance. Results: The [Hbt] and Hb in the blood of the two groups are shown in Table 1, the Hb content in the blood was lower in N than in H group. There was no difference about [Hbt] between the groups. In the H group in no subject [Hbt] change, at cuff pressure of 10 mmHg, was evidenced while only five subjects out of 17 showed a [Hbt] increase at 20 mmHg. In the N group, three subjects out of five (60%) showed a [Hbt] increase at 20 mmHg pressure. The capillary and venular bed compliance is shown inTable 1. Conclusions: From our results is shown clearly that in critically ill subjects the microvascular bed volume rose, in fact even if the Hb in blood was low the [Hbt] in tissue did not change. References: 1. De Blasi RA, Ferrari M, Natali A, et al.: A non invasive measurament of forearm blood flow and oxygen consumption by near–infrared spectroscopy. J Appl Physiolol 1994, 76(3):1388-1393. 2. Fantini S, Franceschini MA, Maier JS, et al.: Frequency domain multichannel optical detector for non-invasive tissue spectroscopy and oxymetry. Opt Eng 1995, 34:32-42. 3. Shrier I, Magder S: Pressure–flow relationship in a vitro model of compartment syndrome. J Appl Physiolol 1995, 79:214-221.