Comparison of systemic oxygen delivery and uptake with NIR spectroscopy of brain during normovolemic hemodilution in the rabbit.

Incremental hyperoxic normovolemic hemodilution was utilized to progressively decrease oxygen delivery (DO2) in anesthetized rabbits. At decreasing DO2, we compared systemic responses related to the adequacy of DO2, i.e. mixed venous oxygen saturation (SvO2), oxygen consumption (VO2), and arterial lactate concentrations, to near infrared spectroscopy (NIRS) of the brain, a regional measure of intracellular oxygen availability. We sought concomitantly to define critical SvO2 and DO2, beyond which whole body VO2 begins to decline and arterial lactate concentrations increase. NIR Spectroscopy provided the means to test the hypothesis that systemic indicators of inadequate DO2 would not accurately reflect the oxygenation of a critical organ such as the brain. In thirteen rabbits anesthetized with fentanyl, paralyzed and artificially ventilated at an FIO2 of 0.60, hemodilution produced an early decrease in mixed venous oxygen saturation. When mixed venous oxygen saturation decreased below approximately 50%, arterial lactate concentrations began to increase significantly. Further decreases in oxygen delivery precipitated a decline in systemic VO2. Finally, NIRS revealed an increase in the reduction level of brain cytochrome a,a3 after systemic parameters of oxygen delivery had been altered. Analysis of the data indicated that falling SvO2 predicted inadequate DO2 to tissue during early hemodilution under narcotic/relaxant anesthesia and that the brain showed evidence of intracellular hypoxia only after systemic parameters such as SvO2 were affected markedly.     

液体复苏对初进高海拔地区重度失血性休克犬氧合功能的影响

目的 探讨液体复苏对初进高海拔地区重度失血性休克犬氧合功能的影响.方法 13只成年杂种犬由海拔1 510 m地区用1 d时间被运至3 780 m的高海拔地区,随机分为3组.每只犬麻醉后经颈静脉放置漂浮导管进行氧合功能监测,经股动脉放血使平均动脉压(MAP)维持在(35±5)mm Hg(1 mm Hg=0.133 kPa).建立重度失血性休克模型.对照组:制模后不进行液体复苏;乳酸林格液(LR)组:制模1 h输1.5倍失血茸的LR;羟乙基淀粉(HES)组:制模后1 h输入等失血量的6%HES 贺斯);然后B组模型犬均输以5 ml·kg-1·h1LR作为维持量,在不同时间点观察氧合功能的变化.结果 对照组犬在制模后2 h全部死亡.休克1 h,两个液体复苏组氧消耗(VO2)、氧输送(DO2)、氧摄取率(O2ER)、动脉血氧饱和度(SaO2)均较放血前明显降低,而静脉血氧饱和度(SvO2)、肺泡一动脉血氧分压差(A-aDO2)较休克前明显升高(p均<0.05).复苏2 h,LR组VO2、DO2、O2ER、SaO2均较休克1 h显著升高,而A-aDO2则显著降低(P均<0.05);HES组VO:、DO2、O2ER均较休克1 h显著升高.而SvO2显著降低(P均<0.05).结论 对初进高海拔地区的重度失血性休克犬如果不进行有效的液体复苏,死亡率达100%;输入1.5倍失血量的LR后2 h氧合功能达到了预期的复苏指标;而输入等失血量的6%HES后2 h氧合功能没有达到预期的复苏指标.     

Near-infrared and nuclear magnetic resonance spectroscopic assessment of tissue energetics in an isolated, perfused canine hind limb model of dysoxia

This controlled laboratory study examined the efficacy of near-infrared spectroscopy (NIRS) and 31P-nuclear magnetic resonance (NMR) spectroscopy in measuring regional tissue oxygenation in a isolated, perfused hind limb model of tissue dysoxia. Isolated hind limb perfusion was carried out in 20 mongrel dogs and oxygen delivery was varied by manipulating either hemoglobin concentration, oxygen saturation, or flow. Hind limbs from anesthetized mongrel dogs (n = 20) were separated and isolated perfusion performed. NIRS probes for recording relative O2 saturation of tissue hemoglobin (HbO2) and cytochrome a,a3 and NMR probes for measuring 31P-high energy phosphates were placed over the limb. Measurements of physiologic parameters, blood gases, lactate, NIRS values for HbO2 and cytochrome a,a3 redox state, and 31P-phosphate levels were recorded at set intervals throughout the experiment. Measures of tissue oxygen consumption (VO2) correlated with tissue oxygenation as measured by HbO2 and cytochrome a,a3 redox state (NIRS), as well as by 31P-high energy phosphate levels (NMR) throughout the experiment. Delivery-dependent tissue oxygenation was detected at a higher DO2 by NIRS than by VO2 or NMR. Tissue oxygenation as measured by NIRS and NMR shows excellent correlation with oxygen delivery in an isolated, perfused model of shock. NIRS may allow early detection of tissue dysoxia using rapid non-invasive techniques.     

Measurement of the liver tissue oxygenation by near-infrared spectroscopy

Objective To study the relation between the liver tissue oxygenation index (TOI), transcutaneously measured with spatially resolved spectroscopy (a new method of near-infrared spectroscopy or NIRS), the mixed venous oxygen saturation and the blood flow in the different parts of the splanchnic circulation in newborn piglets.Design Tissue oxygenation index of the liver was measured in six newborn piglets at 33°C, 35°C, 37°C and after a decrease in arterial carbon dioxide pressure (PaCO₂).Measurements Mixed venous oxygen saturation, blood gas analysis and peripheral oxygen saturation were measured at each step. Gastric, proximal jejunal, midgut, distal ileal, splenic and hepatic arterial blood flow were measured by injection of coloured microspheres into the left atrium. NIRS optodes were attached to the skin over the liver and TOI was calculated.Results No significant changes of TOI of the liver were seen during the increase in temperature or change in PaCO₂. TOI correlated well with mixed venous oxygen saturation (r=0.85), the mid-ileal blood flow (r=0.57) and the distal ileal blood flow (r=0.72).Conclusions Measurement of the TOI of the liver might be a non-invasive way to measure the distal ileal blood flow.     

Near-infrared spectroscopy measurement of regional tissue oxyhemoglobin saturation during hemorrhagic shock.

Adequate resuscitation of patients from shock states depends on restoration of oxygen delivery (DO2) to tissues. Direct measurement of systemic DO2 during shock states requires invasive techniques such as pulmonary artery catheterization. These experiments were performed to examine the ability of near-infrared spectroscopy (NIRS), to measure regional tissue oxygenation in a large-animal model of hemorrhagic shock, and to compare these measures to global measures of oxygen delivery. Splenectomized female pigs (n = 11) were anesthetized, instrumented, and monitored. NIRS probes were placed on the leg, in the stomach via nasogastric tube, and on the liver during laparotomy. Hemorrhagic shock was induced by phlebotomy of 28% of blood volume. After 1 hour, resuscitation was with shed blood and crystalloid until cardiac output plateaued. Measurements of physiologic parameters, blood gases, lactate, intramucosal pH, and NIRS values for regional tissue hemoglobin oxygen saturation (StO2), and cytochrome a,a3 redox state were recorded at intervals throughout the experiment. Tissue oxygenation as measured by oxyhemoglobin saturation and cytochrome a,a3 redox (NIRS) correlated with measures of systemic DO2 throughout the experiment. The liver probe demonstrated blunted changes in tissue oxygenation suggesting relatively protected circulation. Intramucosal pH did not correlate well with DO2. Regional tissue oxygenation as measured by NIRS shows excellent correlation with global oxygen delivery. NIRS may allow estimation of systemic oxygen delivery using rapid non-invasive techniques.     

Venous oxygen measurements in the inferior vena cava in neonates with respiratory failure

BACKGROUND: The present study was undertaken to examine the feasibility of venous oxygen measurements in the inferior vena cava (IVC) via a catheter through the umbilical vein. This may serve as a proxy for mixed venous oxygenation and the complications of right atrial cannulation can be avoided at the same time. It has the added advantage of not being affected by atrial right-left shunting. RESULTS: The study included 22 neonates requiring mechanical ventilation for respiratory insufficiency. The success rate of catheterization of the IVC via the umbilical vein was 81% and there was no catheter-related complications. Fifty paired blood samples were obtained and analyzed while the patients were hemodynamically stable. Linear regression analysis showed a poor correlation between arterial oxygen tension (PaO2) and the arterial-venous oxygen content difference [C(a-v)O2], r = -0.005, and between PaO2 and the fractional oxygen extraction (FOE), r = -0.114. There was also a poor correlation between arterial oxygen saturation (SaO2) and C(a-v)O2, r = -0.057, and between SaO2 and FOE, r =-0.139. The correlations between venous oxygen tension (PvO2) and C(a-v)O2 and between PvO2 and FOE were r = -0.528 and r = 0.592, respectively. There were good correlations between various oxygen saturation (SvO2) and C(a-v)O2, r = -0.634, and between SvO2 FOE, r = -0.712. CONCLUSION: Venous oxygen measurement in the IVC via an umbilical vein catheter is a simple and safe procedure and provides information about the tissue oxygenation status of critically ill neonates.     

Liver tissue oxygenation as measured by near-infrared spectroscopy in the critically ill child in correlation with central venous oxygen saturation

OBJECTIVE: To evaluate the clinical usefulness of near-infrared spatially resolved spectroscopic quantitative assessment of liver tissue oxygenation for simple, non-invasive estimation of global tissue oxygenation in critically ill neonates and children. DESIGN: Prospective observational clinical study. SETTING: A tertiary multidisciplinary neonatal and paediatric intensive care unit (23 beds). PATIENTS: One hundred neonates and children consecutively admitted to the paediatric intensive care unit. MEASUREMENTS AND RESULTS: Near-infrared spectroscopic single-point assessment of liver tissue oxygenation index (TOI(Liver)) was compared with global tissue oxygenation as measured by central venous oxygen saturation (SvO(2)) and derived haemodynamic parameters. Data were compared using linear and multiple regression analysis. Overall correlation between TOI(Liver)and SvO(2) was good ( r=0.72, p<0.0001). Multivariable regression revealed that SvO(2) alone explained 51% of the observed variance of TOI(Liver). However, our data demonstrated large inter-individual differences between SvO(2) and TOI(Liver) values. CONCLUSION: Near-infrared spatially resolved spectroscopic quantitative measurement of liver tissue oxygenation correlates well with SvO(2) in critically ill neonates and children. Large inter-individual SvO(2) to TOI(Liver) differences may prevent its use for non-invasive single-point estimation of global tissue oxygenation. Further clinical studies are required to validate the method with other regional and global haemodynamic parameters and to evaluate its clinical use for continuous non-invasive haemodynamic monitoring.     

Oxygen consumption monitoring by oxygen saturation measurements in mechanically ventilated premature neonates

PURPOSE: To assess oxygen consumption VO2) with arterial and venous oxygen saturation SaO2 and SvO2) from blood measurements for continuous monitoring of mechanically ventilated preterm neonates. METHOD: Twelve preterm neonates with gestational ages ranging from 27 to 34 weeks at birth, who were mechanically ventilated and had umbilical arterial and venous lines in place, were investigated. Analyses were performed with blood samples and continuous monitoring of VO2 from SaO2 and SvO2 measurements. RESULTS: Mean VO2 from blood samples was 3.3 mL/(kg min) (+/-2.49), and that from monitor readings was 8.8 (+/-4.49). Using curve fit analysis to predict the stability of monitor reading, the period from 8 to 28 hours following blood validation was most stable for SvO2 (cubic curve, R=0.5, P<.001). There is a considerable mix of arterial blood in the venous blood, which increased SvO2 and decreased VO2 in the blood samples during first 8 hours of monitoring following blood draws. A multivariate linear mixed model was established for VO2 measurements including related parameters. Ventilatory weaning was associated with decreased VO2. CONCLUSIONS: These findings warrant caution against interpreting VO2 from blood SvO2 and subsequent monitoring readings during mechanical ventilation for preterm neonates.     

Muscle perfusion and oxygen consumption by near-infrared spectroscopy in septic-shock and non-septic-shock patients

OBJECTIVE: To measure muscle blood flow (Qtis) and oxygen consumption (VO(2)tis) in septic and non-septic critically ill patients by near-infrared spectroscopy (NIRS). SETTING: Surgical intensive care unit of a university hospital. PATIENTS AND PARTICIPANTS: Four patients with septic shock, eight post-surgical critically ill patients and ten healthy volunteers. MEASUREMENTS AND RESULTS: Oxyhaemoglobin (HbO(2)) and deoxyhaemoglobin (HbH) variations after venous occlusion were measured by NIRS in the brachioradialis muscle. We calculated Qtis by the rate of HbO(2) and HbH increase in the first 30 s of venous occlusion divided by haemoglobin blood concentration. VO(2)tis was calculated by subtraction of the arterial HbH from the initial increase of HbH after venous occlusion extrapolated to 1 min. Tissue oxygenation index [TOI = HbO(2)/(HbO(2)+HbH)] was also measured before venous occlusion. Two measurements in patients with septic shock, and one measurement in non-septic-shock patients and healthy subjects, were obtained. Of the measurements, 35% were repeated because of low-quality NIRS signal. VO(2)tis and Qtis were two times larger ( P<0.05) in patients with septic shock than in patients without and in healthy subjects. The TOI was very similar among the three groups. CONCLUSION: In septic-shock patients the increase in VO(2)tis was associated with an equivalent increase in Qtis. Therefore, tissue O(2) supply does not seem to be a limiting factor for muscle O(2) consumption. NIRS combined with venous occlusion allows a rapid, non-invasive and simultaneous assessment of regional perfusion and oxygen consumption. In case of microcirculatory shunt occurrence, the TOI should be cautiously used to assess tissue oxygenation state.     

丙酮酸乙酯对脓毒性休克犬氧代谢及组织灌注指标的影响

目的 探讨丙酮酸乙酯(EP)对脓毒性休克犬组织氧合及灌注指标的影响.方法 健康雄性杂种犬20只,用脂多糖(LPS)静脉注射复制犬脓毒性休克模型,随机分为对照组(n=8)和EP组(n=12).对照组只接受林格液复苏;EP组除给予林格液复苏外加EP,首剂0.05 g/kg,然后以0.05 g·kg-1·h-1的量持续泵入.脓毒性休克模型稳定后记为0 h,此后12 h内每2 h收集组织氧合及灌注指标:氧输送(DO2)、氧消耗(VO2)、血乳酸(Lac)水平、混合静脉血氧饱和度(S vO2)、尿量、胃黏膜pH值(pHi)、胃-动脉二氧化碳分压差(Pg-aCO2).结果 脓毒性休克模型建立后,DO2、VO2、S vO2、尿量明显下降(P均＜0.05),Lac、Pg-aCO2明显上升(P均＜0.05).EP组DO2逐渐回升,8 h后与对照组比较差异仍有统计学意义(P＜0.05);VO2也有增加趋势,但与对照组比较差异无统计学意义(P＞0.05);S vO2逐渐上升,10 h后已显著超过制模前水平,且与对照组比较差异有统计学意义(P＜0.05);Lac水平逐渐下降,8 h后与对照组比较差异有统计学意义(P＜0.05);尿量在8 h后明显增加,10 h后与对照组比较差异有统计学意义(P＜0.05);pHi在6 h后明显上升,且与对照组比较差异有统计学意义(P＜0.05);Pg-aCO2在休克后8 h其升高程度较对照组低(P＜0.05).结论 EP可以改善脓毒性休克犬氧代谢及组织灌注.     

Cardiac output from carbon dioxide production and arterial and venous oximetry

OBJECTIVE: To determine cardiac output from measurements of CO2 production (VCO2), and arterial (SaO2) and mixed venous (SvO2) oxygen saturations, using a modified Fick equation, in which cardiac output = VCO2/[k (SaO2 - SvO2)], where k represents a constant. DESIGN: A metabolic measurement cart was used to measure VCO2 and oxygen consumption (VO2) at 3-min intervals. SaO2 and SvO2 were measured via a pulse oximeter and a fiberoptic right heart catheter, respectively. The initial value of k for each study was determined from initial simultaneous measurements of thermodilution cardiac output, VCO2, SaO2, and SvO2 via the equation k = VCO2/[cardiac output (SaO2 - SvO2)]. The value of k was assumed to remain constant for the entire study period. Thereafter, cardiac outputs calculated from k and the measurements of VCO2, SaO2, and SvO2 were compared with the simultaneously obtained cardiac outputs determined by thermodilution. Similarly, cardiac outputs calculated from the traditional oxygen Fick equation, where cardiac output = VO2/[13.4 x hemoglobin (SaO2 - SvO2)], were compared with the simultaneously acquired cardiac outputs determined by thermodilution. SETTING: Surgical ICU in a Veterans Affairs Medical Center. PATIENTS: Seven postoperative patients, mechanically ventilated using the intermittent mandatory ventilation mode, were studied over a mean period of 4 hrs. RESULTS: Cardiac output (obtained from VCO2 and oximetry saturations) was closely related to thermodilution cardiac output: with linear regression showing r2 = .96 and standard error of the estimate = 0.59 L/min, n = 21; and, with bias and precision = 0.17 and 0.68 L/min, respectively. The traditional oxygen Fick cardiac output was also closely related to the thermodilution cardiac output (r2 = .81, standard error of the estimate = 1.46 L/min, n = 22; bias and precision = 0.31 and 1.46 L/min, respectively). CONCLUSION: The proposed method for calculating cardiac outputs solely from VCO2 and oximetry saturations yields results that correspond closely to thermodilution determined cardiac outputs. The method is simple and avoids the difficulties in the Fick method associated with accurate VO2 measurement. This approach may be suitable for continuous cardiac output monitoring in critically ill patients.     

重型乙型肝炎患者原位肝移植围手术期全身氧代谢的变化

目的 探讨重型乙型肝炎（乙肝）与其他肝病患者原位肝移植围术期全身氧代谢变化的特点。方法 12例重型乙肝患者为试验组。10例其他肝病患者为对照组。以咪唑安定、异丙酚、芬太尼、维库溴铵诱导全麻，术中吸入异氟醚维持麻醉。维库溴铵维持肌松，行改良背驼式原位肝移植术。左桡动脉穿刺测有创动脉压，右颈内静脉穿刺置入漂浮导管。分别于术前、无肝前10min、无肝期25min、新肝期30min和术毕监测动脉和混合静脉血氧分压（PaO2和Pv^-O2）、动脉和混合静脉血氧含量（CaO2和Pv^-O2）及动-静脉血氧含量差（CA-vO2）、氧供（DO2）、氧供指数（DO2I）、氧消耗（VO2）、氧耗指数（VO2I）、氧摄取指数（O2EI）和氧摄取率（O2ER）。结果 ①试验组：与术前相比，无肝前期Pv^-O2上升，Ca-vO2、O2EI、O2ER下降，DO2和VO2无明显变化；无肝期DO2、DO2I、VO2和VO2I均明显下降，DO2、VO2分别下降43％和21％，O2EI和O2ER均明显上升；新肝期PvO2上升，DO2和DO2I明显上升。VO2和VO2I回升至术前水平；术毕时DO2和DO2I依然高于术前水平。②对照组：无肝前期PvO2上升。DO2和VO2无明显变化，O2EI和O2ER下降；无肝期DO2、DO2I、VO2和VO2I均明显下降，DO2下降25％，VO2则下降12％；新肝期PvO2上升，Ca-vO2下降，DO2、DO2I明显上升，VO2和VO2I回升至术前水平；术毕时DO2和DO2I依然高于术前水平。结论肝移植围术期中，全身DO2变化大于VO2变化；重型乙肝患者的全身DO2和VO2变化较其他肝病患者剧烈。     

Influence of angular velocity on vastus lateralis and rectus femoris oxygenation dynamics during knee extension exercises

The purpose of this study was to investigate whether changes in angular velocity would alter vastus lateralis (VL) and rectus femoris (RF) oxygenation status during maximal isokinetic knee extension exercises. Eleven recreationally active male participants randomly performed ten maximal knee extensions at 30, 60, 120 and 240° s(-1). Tissue oxygenation index (TOI) and total haemoglobin concentration ([tHb]) were acquired from the VL and RF muscles by means of near-infrared spectroscopy (NIRS). Breath-by-breath pulmonary oxygen consumption (VO(2p)) was recorded throughout the tests. Peak torque and VO(2p) significantly decreased as a function of velocity (P<0·05). Interestingly, RF and VL TOI significantly increased as a function of velocity (P<0·05), whereas [tHb] significantly decreased as a function of velocity (P<0·05). A greater number of muscle fibre recruited at slow velocity, where the torque and VO(2p) were the highest, might explain the lower VL and RF TOI observed herein. Furthermore, the increase in local blood flow (suggested by [tHb] changes) during isokinetic knee extension exercises performed at slow angular velocity might have been induced by a higher intramuscular pressure during the contraction phases as well as a greater microcirculatory vasodilatation during relaxation phases. Implementing slow-velocity isokinetic exercises in rehabilitation or other training programmes could delay the short-term anoxia generated by such exercises and result in muscle metabolism enhancement.     

End-tidal CO2 pressure determinants during hemorrhagic shock

OBJECTIVES: To examine the relationship between end-tidal CO2 (PETCO2) and its physiological determinants, pulmonary blood flow (cardiac output, CO) and CO2 production (VCO2), in a model of hemorrhagic shock during fixed minute ventilation. DESIGN AND SETTING: Prospective, observational study in a research laboratory at a university center. SUBJECTS AND INTERVENTIONS: Six anesthetized, intubated, and mechanically ventilated mongrel dogs. Progressive stepwise bleeding. MEASUREMENTS AND RESULTS: We continuously measured PETCO2 with a capnograph, pulmonary artery blood flow with an electromagnetic flow probe, arterial oxygen saturation (SaO2) with a fiberoptic catheter, and oxygen consumption (VO2) and VCO2 by expired gases analysis. Oxygen delivery (DO2) was continuously calculated from pulmonary blood flow and SaO2. We studied the correlation of PETCO2 with CO and VCO2 in each individual experiment. We also calculated the critical point in the relationships PETCO2/ DO2 and VO2/DO2 by the polynomial method. As expected, PETCO2 was correlated with CO. The best fit was logarithmic in all experiments (median r2 = 0.90), showing that PETCO2 decrease is greater in lowest flow states. PETCO2 was correlated with VCO2, but the best fit was linear (median r2 = 0.77). Critical DO2 for PETCO2 and VO2 was 8.0 +/- 3.3 and 6.3 +/- 2.5 ml x min(-1) kg(-1), respectively (NS). CONCLUSIONS: Our data reconfirm the relationship between PETCO2 and CO during hemorrhagic shock. The relatively greater decrease in PETCO2 at lowest CO levels could represent diminished CO2 production during the period of VO2 supply dependency.     

Oxygen delivery-dependent oxygen consumption in acute respiratory failure

OBJECTIVE: To investigate whether oxygen consumption (VO2) is dependent on oxygen delivery (DO2) in adult respiratory distress syndrome (ARDS) and non-ARDS acute respiratory failure. DESIGN: Intervention study of a consecutive sample of patients admitted to the ICU with the diagnosis of acute respiratory failure. SETTING: Tertiary care center. PATIENTS: Thirteen consecutive patients with a diagnosis of ARDS and 11 with a diagnosis of respiratory failure not due to ARDS. Patients were monitored with an oximetric pulmonary artery catheter and mechanically ventilated. INTERVENTIONS: DO2 was decreased by the application of positive end-expiratory pressure (PEEP) (20 cm H2O), and subsequently increased by an iv infusion of dobutamine (10 micrograms/kg.min). RESULTS: After the application of PEEP, DO2 decreased significantly in both groups. However, VO2 decreased significantly (p less than .01) only in the ARDS group. When dobutamine was infused, DO2 increased significantly (p less than .01) in both groups, but VO2 increased only in ARDS patients. DO2 correlated significantly with VO2 both in ARDS (r2 = .81, p less than .01) and in non-ARDS (r2 = .38, p less than .05) patients. The correlation coefficient was significantly higher for ARDS than for non-ARDS patients. Comparing the slopes of the regression lines, a stronger dependency of VO2 on DO2 was found in ARDS than in non-ARDS respiratory failure (p less than .001). The oxygen extraction ratio correlated with DO2 in non-ARDS patients (r2 = .49, p less than .05), but not in ARDS patients. CONCLUSIONS: VO2 is dependent on DO2 over a wide range of DO2 values in acute respiratory failure. This dependency phenomenon is much stronger in ARDS than in respiratory failure due to other causes. Due to the abnormal dependency of VO2 on DO2, changes in the oxygenation status may not be reflected by changes in mixed venous oxygen saturation in ARDS.     

Effects of positive end-expiratory pressure on tissue gas tensions and oxygen transport

To determine whether positive end-expiratory pressure (PEEP) impairs peripheral tissue oxygenation, hemodynamic variables including blood and subcutaneous tissue gas tensions were measured at 0, 5, 10, 15, and 20 cm H2O of PEEP, in 9 patients who were being mechanically ventilated for acute pulmonary failure. Increasing the level of PEEP produced parallel decreases in cardiac output and oxygen delivery (DO2 = cardiac output X arterial oxygen content); however, there were no significant changes in mean arterial blood pressure (MAP), oxygen consumption (VO2), mixed-venous oxygen tension (PvO2), pH, or base excess. Subcutaneous tissue oxygen (PtO2) and carbon dioxide (PtCO2) tensions, which were directly measured in the femoral region by a mass spectrometer, also remained at their baseline levels (zero end-expiratory pressure). We concluded that peripheral tissue oxygenation is not impaired up to the level of 20 cm H2O of PEEP, even though DO2 significantly decreases.     

Changes in cerebral oxygenation and haemodynamics during postural blood pressure changes in patients with autonomic failure

Patients with autonomic failure suffer severe postural hypotension that may be associated with symptoms of cerebral hypoperfusion. This study utilized near-infrared spectroscopy (NIRS) to measure changes in cerebral oxygenation and haemodynamics during the head-up tilt table test in 18 patients with autonomic failure and 10 healthy age-matched volunteers. Heart rate, blood pressure (MAP), oxygen saturation, cerebral tissue oxygen index (TOI) and total cerebral haemoglobin concentration [HbT] were measured continuously. In patients with autonomic failure there was a mean (SD) reduction in MAP of 46.7 (26.5) mmHg (p < 0.005) associated with a reduction in TOI of 8.6 (6.2)% (p < 0.005) during the head-up tilt table test. In healthy volunteers mean (SD) MAP rose by 12.3 (8.0) mmHg (p < 0.005) and TOI fell by 2.6 (3.2)% (p < 0.05). There was a mean (SD) reduction in [HbT] of 3.09 (2.82) micromol l(-1) (p < 0.005) in patients, equivalent to a decrease in cerebral blood volume of 0.2 (0.18) ml/100 g. There were no changes in [HbT] in the healthy volunteers. Postural hypotension in patients with autonomic failure is associated with a substantial decrease in absolute cerebral oxygenation measured by NIRS and this might reflect a critical reduction in cerebral oxygen delivery.     

Cerebral oxygenation monitor during head-up and -down tilt using near-infrared spatially resolved spectroscopy

Reflectance near-infrared spectroscopy (NIRS) has become a suitable and easily manageable method to monitor cerebral oxygenation changes in presyncopal and syncopal symptoms caused by postural changing or standing. A new clinical tissue oxygenation monitor has been recently developed which measures absolute tissue haemoglobin saturation (Tissue Oxygenation Index, TOI) utilizing spatially resolved spectroscopy (SRS). The present study examined the effects of postural changes on cerebral oxygenation as reflected in SRS-NIRS findings. Cerebral oxyhaemoglobin (O2Hb), deoxyhaemoglobin (HHb), and the TOI were recorded from both sides of the forehead in five healthy male subjects (age range, 28-40 years) during 90 degrees head-up tilt (HUT) and -6 degrees head-down tilt (HDT). Three series of measurements were carried out on separate days. O2Hb was decreased during HUT. TOI was significantly lower in HUT than in the supine position (SUP). There was no significant change in TOI during HDT. A significant session effect was observed in the left forehead TOI during SUP, but not in the right. SRS-NIRS measurements confirmed sub-clinical alterations of cortical oxygenation during HUT. NIRS data from the left side of the forehead, which may vary with cognitive or emotional activation, were more variable than those from the right side.     

Venous oxygen saturation and lactate gradient from superior vena cava to pulmonary artery in patients with septic shock

Monitoring of central venous oxygen saturation (ScvO2) is considered comparable with mixed venous oxygen saturation (SvO2) in the initial resuscitation phase of septic shock. Our aim was to assess their agreement in septic shock in the intensive care unit setting and the effect of a potential difference in a computed parameter, namely, oxygen consumption (VO2). In addition, we sought for a central venous to pulmonary artery (PA) lactate gradient. We enrolled 37 patients with septic shock who were receiving noradrenaline infusions, and their attending physicians had placed a PA catheter for fluid management. Blood samples were drawn in succession from the superior vena cava, right atrium (RA), right ventricle, and PA. Hemodynamic and treatment parameters were monitored, and data were compared by correlation and Bland-Altman analysis. Mixed venous oxygen saturation was lower than ScvO2 (70.2% +/- 11.4% vs. 78.6% +/- 10.2%; P < 0.001), with a bias of -8.45% and 95% limits of agreement ranging from -20.23% to 3.33%. This difference correlated significantly to the noradrenaline infusion rate and the oxygen consumption and extraction ratio. These lower SvO2 values resulted in computed VO2v higher than the VO2cv (P < 0.001), with a bias of 104.97 mL min(-1) and 95% limits of agreement from -4.12 to 214.07 mL min(-1). Finally, lactate concentration was higher in the superior vena cava and RA than in the PA (2.42 +/- 3.15 and 2.35 +/- 3.16 vs. 2.17 +/- 3.19 mM; P < 0.01 for both comparisons). Thus, our data suggest that ScvO2 and SvO2 are not equivalent in intensive care unit patients with septic shock. Additionally, the substitution of ScvO2 for SvO2 in the calculation of VO2 produces unacceptably large errors. Finally, the decrease in lactate between RA and PA may support the hypothesis that the mixing of RA and coronary sinus blood is at least partially responsible for the difference between ScvO2 and SvO2.