Noninvasive tissue oxygen saturation measurements identify supply dependency.

BACKGROUND: Hemorrhagic shock can lead to multiple organ failure and death. We have previously shown that noninvasive measurement of tissue oxygen saturation (StO(2)) has predictive value for outcomes in patients suffering hemorrhagic shock. Our study objectives were twofold: (1) to compare invasive and noninvasive measurements of local and systemic tissue hemoglobin oxygenation and (2) to compare the effects of various physiologic conditions seen in patients in hemorrhagic shock on tissue hemoglobin oxygenation. MATERIALS AND METHODS: We studied pigs in controlled conditions mimicking shock induced by one of the following: hypothermia, isovolemic hemodilution, or manipulations of vascular tone. We obtained both invasive and noninvasive measurements in a hind limb of StO(2), tissue hemoglobin index, femoral artery and venous flows, blood pressures, temperature, pH, pO(2), pCO(2), oxygen saturation, lactate, hemoglobin, and base excess. In all cases, we measured baseline values in both experimental and control hind limbs. RESULTS: We found that tissue hemoglobin oxygenation did not vary significantly over relevant physiologic temperatures. Under all physiologic conditions tested, we found supply-dependent oxygen consumption at oxygen levels less than 7 mL O(2)/min/kg. Similarly, we found that local oxygen delivery in animals subjected to varying degrees of isovolemic hemodilution or altered vascular tone was correlated with supply-dependent oxygen consumption, as measured by local noninvasive StO(2). CONCLUSIONS: Noninvasive StO(2) measurements are valid and durable over a wide range of physiologic conditions and correlate with invasively-measured oxygen delivery.

Near-infrared spectroscopy in patients with severe sepsis: correlation with invasive hemodynamic measurements.

BACKGROUND: Clinicians have begun using near-infrared spectroscopy (NIRS) to monitor tissue perfusion in hemorrhagic shock, as the technique allows continuous noninvasive monitoring of tissue hemoglobin oxygen saturation (StO(2)) and the tissue hemoglobin index (THI). We hypothesized that StO(2) measurements in patients with severe sepsis would be associated with the severity of their illness and would correlate with invasive hemodynamic measurements. METHODS: We measured mean arterial pressure (MAP), serum lactate concentration, blood hemoglobin concentration, StO(2), and THI in nine healthy volunteers and ten patients with septic shock in a surgical intensive care unit (ICU). Enrolled patients had a pulmonary artery catheter, and had family able to give informed consent. The average Acute Physiology and Chronic Health Evaluation (APACHE) II score at enrollment for the patients was 19 +/- 5 (standard deviation) points. Volunteers and patients were similar with respect to age and sex. To collect NIRS data, we used the InSpectra Tissue Spectrometer, Model 325 (Hutchinson Technology, Inc., Hutchinson, MN). For three consecutive days, we obtained invasive hemodynamic measurements three times daily, simultaneously with NIRS measurements, and metabolic cart measurements once daily. RESULTS: Patients with severe sepsis had significantly lower thenar muscle StO(2) values (p = 0.031) than healthy volunteers. Near-infrared spectroscopy-derived mixed venous oxygen saturation (NIRSvO(2)) and StO(2) measured from the thenar eminence in patients with severe sepsis correlated with SvO(2) from the pulmonary artery catheter (p < 0.05). In this group of patients, StO(2) did not correlate significantly with lactate concentration, base deficit, or APACHE II score. CONCLUSIONS: Near-infrared spectroscopic measurements of StO(2) correlated with invasive hemodynamic measurements in patients with severe sepsis but did not correlate with severity of illness. These findings suggest that NIRStO(2) may be a clinically useful measurement in monitoring patients with severe sepsis. Further study of this device in early resuscitation of patients with sepsis is necessary.

Dynamic near-infrared spectroscopy measurements in patients with severe sepsis.

This study evaluated near-infrared spectroscopy (NIRS)-derived measurements in hemodynamically stable patients with severe sepsis, as compared with similar measurements in healthy age-matched volunteers. Prospective, preliminary, observational study in a surgical intensive care unit and clinical research center at a university health center. We enrolled 10 patients with severe sepsis and 9 healthy age-matched volunteers. For patients with severe sepsis, we obtained pulmonary artery catheter and laboratory values three times daily for 3 days and oxygen consumption values via metabolic cart once daily for 3 days. For healthy volunteers, we obtained all noninvasive measurements during a single session. We found lower values in patients with severe sepsis (versus healthy volunteers), in tissue oxygen saturation (StO2), in the StO2 recovery slope, in the tissue hemoglobin index, and in the total tissue hemoglobin increase on venous occlusion. Patients with severe sepsis had longer StO2 recovery times and lower NIRS-derived local oxygen consumption values versus healthy volunteers. In our preliminary study, NIRS provides a noninvasive continuous method to evaluate peripheral tissue oxygen metabolism in hemodynamically stable patients with severe sepsis. Further research is needed to demonstrate whether these values apply to broader populations of patients with systemic inflammatory response syndrome, sepsis, severe sepsis, and septic shock.

Noninvasive method for measuring local hemoglobin oxygen saturation in tissue using wide gap second derivative near-infrared spectroscopy.

A simple continuous wave near-infrared algorithm for estimating local hemoglobin oxygen saturation in tissue (%StO2) is described using single depth attenuation measurements at 680, 720, 760, and 800 nm. Second derivative spectroscopy was used to reduce light scattering effects, chromophores with constant absorption, baseline/instrumentation drift, and movement artifacts. Unlike previous second derivative methods which focused primarily on measuring deoxyhemoglobin concentration; a wide 40 nm wavelength gap used for calculating second derivative attenuation significantly improved sensitivity to oxyhemoglobin absorption. Scaled second derivative attenuation at 720 nm was correlated to in vitro hemoglobin oxygen saturation to generate a %StO2 calibration curve. The calibration curve was insensitive to total hemoglobin, optical path length, and optical scattering. Measurement error due to normal levels of carboxyhemoglobin, methemoglobin, and water absorption were less than 10 %StO2 units. Severe methemoglobinemia or edema combined with low blood volume could cause StO2 errors to exceed 10 StO2 units. Both a broadband and commercial four-wavelength spectrometer (InSpectra) measured %StO2. The InSpectra tissue spectrometer readily detected limb ischemia on 26 human volunteers for hand, forearm, and leg muscles. A strong linear correlation, r2>0.93, between StO2 and microvascular %SO2 was observed for isolated animal hind limb, kidney, and heart.

Use of near-infrared spectroscopy in early determination of irreversible hemorrhagic shock.

BACKGROUND: In field situations, patient triage may require early determination of patients progressing to irreversible shock. We investigated the utility of near-infrared spectroscopy (NIRS) in early detection of irreversible hemorrhagic shock. METHODS: Twenty instrumented pigs were treated with a protocol involving 35% blood volume hemorrhage, 90 minutes of shock, and stepwise resuscitation with lactated Ringer's. Hemodynamics and NIRS measurements of skeletal muscle (leg), stomach, and liver tissue oxyhemoglobin saturation (StO2) were measured at baseline, every 30 minutes during shock, and after each resuscitative step. Measurements were compared between animals that expired during resuscitation (unresuscitatable) and animals that survived all resuscitative steps (resuscitatable). RESULTS: Neither global oxygen delivery, oxygen consumption, nor lactate distinguished resuscitatable from unresuscitatable animals. Invasive measurements of SvO2 did distinguish resuscitatable from unresuscitatable animals. After the first fluid bolus, both stomach and leg StO2 differed significantly between resuscitatable and unresuscitatable animals. Regression analysis revealed skeletal muscle (leg) StO2 obtained after the first resuscitative step was a significant mortality predictor despite resuscitation (r2=0.45) (p = 0.005). CONCLUSIONS: Non-invasive NIRS monitoring of leg and stomach StO2 differentiates resuscitatable from unresuscitatable animals after the initial resuscitative bolus. Use of this non-invasive tool may guide appropriate use of resuscitative fluids and has possible point-of-care applications.