重型脑损伤局部脑氧饱和度变化及影响因素

目的探讨重型脑损伤后局部脑氧饱和度（ｒＳｃＯ２）变化以及影响因素。方法重型脑损伤５０例为样本，采用近红外光谱血氧仪（ＮＩＲＳ）持续床旁监测ｒＳｃＯ２，颈静脉氧饱和度（ＳｊＯ２）、动脉氧饱和度（ＳａＯ２）、颅内压（ＩＣＰ）、脉搏氧饱和度（ＳｐＯ２）、平均动脉压（ＭＡＢＰ）和脑灌注压（ＣＰＰ）。结果重型脑损伤后ｒＳｃＯ２明显降低（０．４０７５±０．１３９４）。影响局部脑氧饱和度的主要因素为ＩＣＰ、ＣＰＰ、伤后病理及病情转归。结论本研究结果表明，ＮＩＲＳ持续、无创监测ｒＳｃＯ２有重要意义，它能反映重型脑伤后脑氧代谢的信息。局部脑氧饱和度显著降低提示预后不良。     

Assessing oxygenation in the transport environment

The physical signs of hypoxemia often are not seen until profound hypoxemia occurs; therefore a variety of devices commonly are used to assess patient oxygenation. Despite the high reliance on oxygen monitoring equipment when caring for the critically ill, medical personnel often fail to appreciate the advantages and limitations of these devices. In fact, profound hypoxemia can exist in the absence of reliable physical signs.Arterial blood gas measurements, the traditional gold standard for assessing a patient's oxygen and acid base status in the hospital setting, only recently have been simplified for transport use. But their inability to perform real-time continuous measurements limits their acceptance into the transport environment. Although used to assess neonatal hypoxemia, the real value of transcutaneous oxygen monitoring may be in avoiding retinal damage as a result of excessive tissue oxygenation. Finally, despite the simplicity, reliability, and popularity of pulse oximeters, few EMS medical personnel really understand how perfusion states, motion, hemoglobinopathies, intravenous dyes, nail polish, and skin color may lead to interpretation errors.This article discusses the limitations of the physical examination, arterial blood gas measurements, transcutaneous monitoring, and pulse oximetry in determining a patient's oxygenation status.     

Use of transcranial cerebral oximetry to monitor regional cerebral oxygen saturation during neuroendovascular procedures.

Using transcranial cerebral oximetry, we monitored 30 patients who underwent cerebral angiography by the femoral route. Transcranial cerebral oximetry is a noninvasive technique of regional cerebral oxygen saturation measurement that uses near-infrared spectroscopy to differentiate oxyhemoglobin from reduced hemoglobin. Needle puncture, catheterization, and contrast media injection produced no significant peak changes in saturation from baseline. Acute and persistent decreases in oxygen saturation were associated with vascular complications and were detected before development of clinical symptoms. Greater changes in saturation were observed during several neuroendovascular procedures, indicating the development of complications, signaling a need to stop further endovascular manipulation.     

Changes in arterial oxygen saturation during isotope perfusion scans using human macroaggregates of albumin.

Sixty-three patients undergoing isotope ventilation/perfusion scintigraphy for suspected pulmonary embolism were monitored using pulse oximetry. Xenon inhalation had no adverse effect on arterial oxygen saturation. Fifty-seven per cent of patients demonstrated a drop in oxygen saturation of 2-10% within 5 min of injection of macroaggregates. Small changes in arterial oxygen saturation reflect larger changes in the arteriole partial pressure of oxygen. In 10 patients, oxygen saturation dropped to 91% and below, corresponding to an arterial pO2 of less than 60 mm Hg. The effect lasted up to 30 min and is thought unlikely to be simply due to arteriolar blockade. Falls in arterial oxygen saturation cannot be correlated with any specific pulmonary pathology and appear unlikely to be of any clinical significance in most patients.     

Image‐based calculation of perfusion and oxyhemoglobin saturation in skeletal muscle during submaximal isometric contractions

The relative oxygen saturation of hemoglobin and the rate of perfusion are important physiological quantities, particularly in organs such as skeletal muscle, in which oxygen delivery and use are tightly coupled. The purpose of this study was to demonstrate the image‐based calculation of the relative oxygen saturation of hemoglobin and quantification of perfusion in skeletal muscle during isometric contractions. This was accomplished by establishing an empirical relationship between the rate of radiofrequency‐reversible dephasing and near‐infrared spectroscopy–observed oxyhemoglobin saturation (relative oxygen saturation of hemoglobin) under conditions of arterial occlusion and constant blood volume. A calibration curve was generated and used to calculate the relative oxygen saturation of hemoglobin from radiofrequency‐reversible dephasing changes measured during contraction. Twelve young healthy subjects underwent 300 s of arterial occlusion and performed isometric contractions of the dorsiflexors at 30% of maximal contraction for 120 s. Muscle perfusion was quantified during contraction by arterial spin labeling and measures of muscle T₁. Comparisons between the relative oxygen saturation of hemoglobin values predicted from radiofrequency‐reversible dephasing and that measured by near‐infrared spectroscopy revealed no differences between methods (P = 0.760). Muscle perfusion reached a value of 34.7 mL 100 g^?1 min^?1 during contraction. These measurements hold future promise in measuring muscle oxygen consumption in healthy and diseased skeletal muscle. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Validation of in Vivo MR Measurement of Oxygen Saturation after Resuscitation with a Hemoglobin-Based Oxygen Carrier in a Rabbit Model

RATIONALE AND OBJECTIVES: The authors tested whether noninvasive magnetic resonance (MR) oximetry is accurate in the in vivo measurement of oxygen saturation in a stroma-free, hemoglobin-based oxygen carrier (HBOC). MATERIALS AND METHODS: A central venous catheter was placed in the inferior vena cava (IVC) of 10 New Zealand white rabbits (weight range, 2.5-3.2 kg). Each rabbit underwent removal of 20% of blood volume followed by resuscitation with 10 mL/kg of bovine HBOC-200. Oxygen saturation of the blood mixture was measured in vivo at the IVC with MR oximetry, with separate in vitro calibration for each animal. Blood drawn from the IVC was measured with ex vivo oximetry, which was used as the standard of reference. The in vivo and ex vivo measurements were compared. RESULTS: There was no significant difference (P > .1) between measurements obtained with MR oximetry and ex vivo oximetry. The results with in vivo MR oximetry demonstrated excellent correlation with those from ex vivo oximetry (r = 0.99) over a wide range of physiologic oxygen saturation values (16.7%-74.9%) in venous blood. CONCLUSION: Noninvasive in vivo MR measurement of oxygen saturation is valid for whole blood mixed with stroma-free hemoglobin. Therefore, MR oximetry may be clinically useful for assessing the oxygenation status in patients resuscitated with a HBOC.     

Pulse oximetry: basic principles and applications in aerospace medicine

INTRODUCTION: Pulse oximeters are reliable, objective, and noninvasive monitors that have broad application in aerospace medicine. New technology enables pulse oximeters to perform well in adverse environments and measure additional parameters. Small, battery-powered devices can be used to monitor oxyhemoglobin saturation while in flight. THEORY OF OPERATION: Pulse oximeters use spectrophotometry to measure the ratio of oxyhemoglobin (Hbo2) to reduced hemoglobin (Hb) in arterial blood. This value is displayed as oxyhemoglobin saturation (Spo2). A plethysmographic waveform that resembles arterial waveform is also frequently displayed and may indicate relative changes in perfusion and blood volume. Loss and subsequent reappearance of this waveform during occlusion with a cuff has been used to measure systolic blood pressure during helicopter flight. APPLICATIONS: Accurate determination of oxygen saturation requires a high quality arterial signal and is limited by errors resulting from calibration, motion and vibration, and dyshemoglobinemias. Vasoconstriction may result in decreased pulse amplitude and also impair accurate measurement. Conventional fingertip probes may interfere with the performance of required duties, while helmets and other restrictive clothing can impede the use of sensors on the forehead or ear. Recently introduced devices answer some of these limitations and enable measurement of additional parameters. For example, new probe designs permit more freedom of movement and include a contactless camera and a sensor that fits around a finger like an ordinary ring. This article explains the theory of operation and limitations of pulse oximetry, offers an update on new technology, and discusses applications of this technology in aerospace medicine.     

Air transportation of patients with acute respiratory failure: theory

Adult respiratory distress syndrome (ARDS) that results from severe trauma often occurs in remote places, making it necessary to transport the patients to tertiary medical facilities by air. Since these severely hypoxic patients are exposed to additional risk of reduced inspired oxygen tension due to decreased barometric pressure, the feasibility of transportation of these patients was investigated by computer analysis. Mathematical models of pulmonary gas exchange in patients with ARDS were developed to calculate arterial and mixed venous blood tensions while breathing room air and oxygen at sea level, 8,000 ft, and 40,000 ft. Under each condition the following parameters were varied: alveolar ventilation (VA), cardiac output (Q), metabolic rate (VO2), hematocrit (Hcrit), and membrane diffusing capacity for oxygen (DmO2). Most of the gas exchange problems at altitude could be overcome by breathing oxygen as long as cardiac output and hematocrit were adequate. Hypoxemia in ARDS patients will not be greatly affected by the reduced inspired oxygen tensions at altitude in much the same way that hypoxemia in ARDS is poorly responsive to increased inspired oxygen tensions at sea level.     

Comparative analysis of arterial oxygen saturations during exercise by pulse oximetry, photometric measurements, and calculation procedures

Pulse oximetry allows non-invasive monitoring of arterial oxygen saturation (SO2). To study the validity of pulse oximetry, comparative measurements were performed. During exhaustion limited exercise SO2-values measured by pulse oximetry (SO2puls), calculated SO2-values (algorithms of Kelman, Severinghaus, and Siggaard-Andersen--SO2calc), and as "golden standard" photometric measured SO2-values (SO2meas) were compared. Fourteen triathletes performed a stepwise cycling exercise test in the supine position. SO2calc was determined on the basis of capillary actual blood gas values. SO2puls was measured continuously with a finger probe attached to the second finger. The SO2puls- and SO2calc-values differed from the SO2meas-values (p less than 0.05); however, the differences were of no clinical relevance. Performing linear regression analysis, only SO2puls correlated significantly (r = 0.47, p less than 0.001) with SO2meas. Pulse oximetry is able to replace invasive measurements of arterial oxygen saturation in athletes. It is superior to SO2-calculations and permits reliable, valid and non-invasive continuous monitoring of SO2.     

Monitored arterial and end-tidal carbon dioxide during in-flight mechanical ventilation

Introduction: Mechanical ventilation with monitored arterial carbon dioxide tension is necessary for optimum pulmonary support and hemodynamic stability. Ongoing monitoring is necessary to ensure adequate ventilation parameters. The prospective study purpose was to (1) compare mechanical ventilation to historic manual ventilation, (2) evaluate the effectiveness of institutional tidal volume parameters, (3) determine the effect of institutional tidal volume manipulation on end-tidal carbon dioxide tension, and (4) explore the relationship between in-flight end-tidal carbon dioxide tension and arterial carbon dioxide tension.

Methods: Randomized groups were mechanically ventilated (tidal VOLUME = 12 cc/kg, RATE = 14/min) with a target arterial carbon dioxide tension between 30 and 35 torr. Group I was monitored with in-line end-tidal carbon dioxide tension, and group II was monitored with arterial carbon dioxide tension by means of inflight arterial blood gas.

Results: Arterial carbon dioxide tension varied less with monitored mechanical than with manual ventilation (p = 0.001). The gradient between arterial and end-tidal carbon dioxide tension was 5.3 ± 4.4 (mean ± standard deviation [SD]). End-tidal and arterial carbon dioxide tension positively correlated (r = 0.76, P = 0.001), yet end-tidal carbon dioxide tension accounted for only 58% variation of arterial carbon dioxide tension (r² = 0.58).

Conclusion: Mechanical ventilation is more precise but inconsistent in achieving a target arterial carbon dioxide tension with current ventilation parameters. End-tidal carbon dioxide tension is a reasonable estimate of, but cannot exclusively replace, arterial carbon dioxide tension in critically ill patients.     

A near-infrared spectrophotometric method for studying brain O2 sufficiency in man during +Gz acceleration

A technique for the noninvasive monitoring of cerebral oxygen status was evaluated on volunteer subjects on the USAF School of Aerospace Medicine centrifuge. By using multiwavelength near-infrared spectrophotometry, the instrumentation measured changes in the quantities of reduced and oxygenated hemoglobin (and their sum, an indicator of cerebral blood volume), and the quantity of oxidized cytochrome c oxidase within the forebrain. Tests used acceleration of up to 9 G with onset rates from 0.1 to 5.0 G.s-1, anti-G suits and straining maneuvers, and hyperoxic and hypoxic breathing mixtures. In general, +Gz acceleration produced a fall in blood volume within the cerebral microcirculation with a relative increase in the content of reduced hemoglobin and a tendency towards reduction of cytochrome c oxidase. These findings are discussed in relation to accepted changes in arterial blood pressure, cerebral blood flow, and arterial oxygen saturation caused by acceleration exposure.     

Pulse oximetry at high altitude

Pulse oximetry is a valuable, noninvasive, diagnostic tool for the evaluation of ill individuals at high altitude and is also being increasingly used to monitor the well-being of individuals traveling on high altitude expeditions. Although the devices are simple to use, data output may be inaccurate or hard to interpret in certain situations, which could lead to inappropriate clinical decisions. The purpose of this review is to consider such issues in greater detail. After examining the operating principles of pulse oximetry, we describe the available devices and the potential uses of oximetry at high altitude. We then consider the pitfalls of pulse oximetry in this environment and provide recommendations about how to deal with these issues. Device users should recognize that oxygen saturation changes rapidly in response to small changes in oxygen tensions at high altitude and that device accuracy declines with arterial oxygen saturations of less than 80%. The normal oxygen saturation at a given elevation may not be known with certainty and should be viewed as a range of values, rather than a specific number. For these reasons, clinical decisions should not be based on small differences in saturation over time or among individuals. Effort should also be made to minimize factors that cause measurement errors, including cold extremities, excess ambient light, and ill-fitting oximeter probes. Attention to these and other issues will help the users of these devices to apply them in appropriate situations and to minimize erroneous clinical decisions.     

Correlation between oxygen status measures during neonatal air transport

Introduction: The purpose of this study was to determine the strength of the relationship between arterial oxygen tension (Pao₂) and transcutaneous oxygen (Ptco₂), and arterial carbon dioxide tension (Paco₂) and transcutaneous carbon dioxide (Ptcco₂) of premature neonates during fixed-wing air transport.Method: The study admitted 37 randomly selected intubated or nonintubated neonates who received oxygen by mechanical ventilation or face mask and required at least a 1-hour fixed-wing flight. Arterial blood gas samples were taken from the umbilical artery catheter at the end of three 15-minute intervals and stored in an ice slurry for analysis (Radiometer ABL300) on arrival at the receiving hospital. Transcutaneous monitor readings (Radiometer Tina 3) were recorded with each arterial sample.Results: A correlation of 0.88 (p <.001) between Pao₂ and Ptco₂ and 0.86 (p <.001) between Paco₂ and Ptcco₂ existed.Conclusion: Despite strong correlation, the range of difference between the individual transcutaneous values and actual blood oxygen values were from 0 to 31. This difference was not due to change in cabin pressure. Considering the flight environment (cabin temperature, pressure changes, and weather), these values are remarkably stable.     

Accuracy of the cylinder approximation for susceptometric measurement of intravascular oxygen saturation

Susceptometry‐based MR oximetry has previously been shown suitable for quantifying hemoglobin oxygen saturation in large vessels for studying vascular reactivity and quantification of global cerebral metabolic rate of oxygen utilization. A key assumption underlying this method is that large vessels can be modeled as long paramagnetic cylinders. However, bifurcations, tapering, noncircular cross‐section, and curvature of these vessels produce substantial deviations from cylindrical geometry, which may lead to errors in hemoglobin oxygen saturation quantification. Here, the accuracy of the “long cylinder” approximation is evaluated via numerical computation of the induced magnetic field from 3D segmented renditions of three veins of interest (superior sagittal sinus, femoral and jugular vein). At a typical venous oxygen saturation of 65%, the absolute error in hemoglobin oxygen saturation estimated via a closed‐form cylinder approximation was 2.6% hemoglobin oxygen saturation averaged over three locations in the three veins studied and did not exceed 5% for vessel tilt angles <30° at any one location. In conclusion, the simulation results provide a significant level of confidence for the validity of the cylinder approximation underlying MR susceptometry‐based oximetry of large vessels. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Maximal lactate steady state does not correspond to a complete physiological steady state

The purpose of this study was to verify whether the maximal lactate steady state (MLSS) corresponds to a physiological steady state. Eight male trained subjects performed a 30-min test on a cycle ergometer at a constant power corresponding to their own MLSS which had been previously determined. No significant variation was observed between the 10th and the 30th min for arterial lactate concentration, redox state, arterial oxygen pressure, arterial oxygen saturation, bicarbonates concentration, base excess, hematocrit, hemoglobin concentration, plasma volume, oxygen uptake, carbon dioxide output, gas exchange ratio, minute ventilation, ventilatory equivalents for oxygen and carbon dioxide, and arterial systolic blood pressure values. However, arterial carbon dioxide pressure and pH values were significantly different between the 10th and the 30th min (p < 0.01). Respiratory rate values and heart rate significantly increased (p < 0.01). These results indicate that MLSS does not correspond to a complete physiological steady state.     

A comprehensive approach using MR imaging to diagnose acute segmental mesenteric ischemia in a porcine model.

OBJECTIVE: Acute mesenteric ischemia is a lethal disease that lacks a noninvasive diagnostic test. We evaluated the abilities of contrast-enhanced MR angiography, MR oximetry, and real-time interactive MR imaging to diagnose segmental mesenteric ischemia in a porcine model. MATERIALS AND METHODS: Segmental mesenteric ischemia was created by subselective Gelfoam embolization of the mesenteric circulation in eight pigs. Conventional digital subtraction angiography (DSA), MR oximetry, and real-time interactive MR imaging of the small bowel were performed before and after embolization. Changes in the perfusion pattern seen on DSA established the regions of true ischemia. Postembolization DSA and MR angiography were compared with this gold standard. RESULTS: Both MR angiography and DSA had high sensitivity (91% and 100%, respectively) for detecting ischemic regions. The difference was not statistically significant (p > .2). MR angiography yielded lower specificity than DSA (80% and 90%, respectively; p < .01). After embolization, the oxygen saturation in the superior mesenteric vein (SMV) dropped significantly (p < .005). After embolization, the SMV also showed oxygen saturation significantly lower than that in the inferior vena cava (p < .005). In two of the animals, segmental hypomotility of the small bowel was observed. CONCLUSION: MR oximetry is capable of detecting oxygen desaturation caused by segmental ischemia. A loss of oxygen saturation in the SMV relative to that in the inferior vena cava provides a convenient marker of mesenteric ischemia. Contrast-enhanced MR angiography has sensitivity and specificity approaching those of DSA. Both MR techniques hold promise for the detection of acute mesenteric ischemia.     

False indication of arterial oxygen desaturation and methemoglobinemia following injection of methylene blue in urological surgery

The pulse oximeter serves as an indicator of arterial oxygen saturation. We present two cases in which methylene blue injection during urological surgery appeared to cause arterial oxygen desaturation by pulse oximetry and methemoglobinemia by arterial blood gas co-oximetry. Methylene blue interferes with light absorption and gives a false estimate of the percentage of oxyhemoglobin and arterial oxygen saturation. The co-oximeter interprets methylene blue as methemoglobin and gives a false indication of methemoglobinemia. The surgical team must be familiar with conditions and agents that interfere with their ability to safely monitor surgical patients.     

Precise control of end‐tidal carbon dioxide and oxygen improves BOLD and ASL cerebrovascular reactivity measures

In‐depth investigation of cerebrovascular blood flow and MR mechanisms underlying the blood oxygenation level dependent signal requires precise manipulation of the arterial partial pressure of carbon dioxide and oxygen, measured by their noninvasive surrogates, the end‐tidal values. The traditional methodology consists of administering a fixed fractional concentration of inspired CO₂, but this causes a variable ventilatory response across subjects, resulting in different values of end‐tidal partial pressures of CO₂ and O₂. In this study, we investigated whether fine control of these end‐tidal partial pressures would improve stability and predictability of blood oxygenation level dependent and arterial spin labeling signals for studying cerebrovascular reactivity. In 11 healthy volunteers, we compared the MR signals generated by the traditional fixed fractional concentration of inspired CO₂ method to those of an automated feed‐forward system, a simpler, safer, and more compact alternative to dynamic end‐tidal forcing systems, designed to target constant end‐tidal partial pressures of CO₂ and O₂. We found that near square‐wave changes in end‐tidal partial pressure of CO₂ of 5, 7.5, and 10 mm Hg (±1.01 mm Hg within two to three breaths) and constrained changes in the end‐tidal partial pressure of O₂ (<10 mm Hg) induced cerebral vascular reactivity measurements with faster transitions, together with improved stability and gradation, than those achieved with the traditional fixed fractional concentration of inspired CO₂ method. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Improvement in hypoxemia at 4600 meters of simulated altitude with carbohydrate ingestion.

BACKGROUND: Carbohydrate ingestion increases the relative production of carbon dioxide which results in an increase in ventilation in normal individuals. An increase in ventilation at altitude can result in improvement of altitude-induced hypoxemia. HYPOTHESIS: Carbohydrate ingestion will increase the arterial blood oxygen tension and oxyhemoglobin saturation during acute high altitude simulation. METHODS: There were 15 healthy volunteers, aged 18-33 yr, who were given a 4 kcal x kg(-1) oral carbohydrate beverage administered 2.5 h into an exposure to 15,000 ft (4600 m) of simulated altitude (5.5 h after the last meal). Altitude was simulated by having subjects breath a 12% oxygen/balance nitrogen mixture while remaining at sea level. Arterial blood gas samples were drawn at baseline and at regular intervals up to 210 min after carbohydrate ingestion. Subjects were evaluated for AMS by use of the Environmental Symptoms Questionnaire (ESQ) and a weighted average of cerebral symptom score (AMS-C). RESULTS: Baseline PaO2 increased significantly (p < 0.01) from 43.0 +/- 3.0 mmHg at 4600 m before carbohydrate ingestion to 46.8 +/- 6.2 mmHg at 60 min after carbohydrate ingestion. Arterial oxygen saturation rose significantly (p < 0.01) from a baseline of 79.5% +/- 5.1 to 83.8% +/- 6.42 at 60 min. CONCLUSIONS: Carbohydrate consumption significantly increased oxygen tension and oxyhemoglobin saturation in arterial blood of normal subjects during simulated altitude. Effects reached statistical significance across all subjects at 60 min. There was no significant difference in arterial oxygen levels or arterial oxygen saturation in subjects who developed AMS vs. those who did not develop AMS.