Effect of hyperventilation on oxygenation of the brain cortex of newborn piglets

Portable, high speed blood flow measurements enabled by long wavelength, interferometric diffuse correlation spectroscopy (LW-iDCS)

Diffuse correlation spectroscopy (DCS) is an optical technique that can be used to characterize blood flow in tissue. The measurement of cerebral hemodynamics has arisen as a promising use case for DCS, though traditional implementations of DCS exhibit suboptimal signal-to-noise ratio (SNR) and cerebral sensitivity to make robust measurements of cerebral blood flow in adults. In this work, we present long wavelength, interferometric DCS (LW-iDCS), which combines the use of a longer illumination wavelength (1064 nm), multi-speckle, and interferometric detection, to improve both cerebral sensitivity and SNR. Through direct comparison with long wavelength DCS based on superconducting nanowire single photon detectors, we demonstrate an approximate 5× improvement in SNR over a single channel of LW-DCS in the measured blood flow signals in human subjects. We show equivalence of extracted blood flow between LW-DCS and LW-iDCS, and demonstrate the feasibility of LW-iDCS measured at 100 Hz at a source-detector separation of 3.5 cm. This improvement in performance has the potential to enable robust measurement of cerebral hemodynamics and unlock novel use cases for diffuse correlation spectroscopy.

Open-source FlexNIRS: A low-cost, wireless and wearable cerebral health tracker

Currently, there is great interest in making neuroimaging widely accessible and thus expanding the sampling population for better understanding and preventing diseases. The use of wearable health devices has skyrocketed in recent years, allowing continuous assessment of physiological parameters in patients and research cohorts. While most health wearables monitor the heart, lungs and skeletal muscles, devices targeting the brain are currently lacking. To promote brain health in the general population, we developed a novel, low-cost wireless cerebral oximeter called FlexNIRS. The device has 4 LEDs and 3 photodiode detectors arranged in a symmetric geometry, which allows for a self-calibrated multi-distance method to recover cerebral hemoglobin oxygenation (SO₂) at a rate of 100 Hz. The device is powered by a rechargeable battery and uses Bluetooth Low Energy (BLE) for wireless communication. We developed an Android application for portable data collection and real-time analysis and display. Characterization tests in phantoms and human participants show very low noise (noise-equivalent power <70 fW/√Hz) and robustness of SO₂ quantification in vivo. The estimated cost is on the order of $50/unit for 1000 units, and our goal is to share the device with the research community following an open-source model. The low cost, ease-of-use, smart-phone readiness, accurate SO₂ quantification, real time data quality feedback, and long battery life make prolonged monitoring feasible in low resource settings, including typically medically underserved communities, and enable new community and telehealth applications.

Cerebrovascular Blood Flow Design and Regulation; Vulnerability in Aging Brain

Nutrient delivery to the brain presents a unique challenge because the tissue functions as a computer system with in the order of 200,000 neurons/mm³. Penetrating arterioles bud from surface arteries of the brain and penetrate downward through the cortex. Capillary networks spread from penetrating arterioles through the surrounding tissue. Each penetrating arteriole forms a vascular unit, with little sharing of flow among vascular units (collateral flow). Unlike cells in other tissues, neurons have to be operationally isolated, interacting with other neurons through specific electrical connections. Neuronal activation typically involves only a few of the cells within a vascular unit, but the local increase in nutrient consumption is substantial. The metabolic response to activation is transmitted to the feeding arteriole through the endothelium of neighboring capillaries and alters calcium permeability of smooth muscle in the wall resulting in modulation of flow through the entire vascular unit. Many age and trauma related brain pathologies can be traced to vascular malfunction. This includes: 1. Physical damage such as in traumatic injury with imposed shear stress as soft tissue moves relative to the skull. Lack of collateral flow among vascular units results in death of the cells in that vascular unit and loss of brain tissue. 2. Age dependent changes lead to progressive increase in vascular resistance and decrease in tissue levels of oxygen and glucose. Chronic hypoxia/hypoglycemia compromises tissue energy metabolism and related regulatory processes. This alters stem cell proliferation and differentiation, undermines vascular integrity, and suppresses critical repair mechanisms such as oligodendrocyte generation and maturation. Reduced structural integrity results in local regions of acute hypoxia and microbleeds, while failure of oligodendrocytes to fully mature leads to poor axonal myelination and defective neuronal function. Understanding and treating age related pathologies, particularly in brain, requires better knowledge of why and how vasculature changes with age. That knowledge will, hopefully, make possible drugs/methods for protecting vascular function, substantially alleviating the negative health and cognitive deficits associated with growing old.

Detection of flap tissue ischemia in a rat model: Real-time monitoring of changes in oxygenation and perfusion through injectable biosensors

BACKGROUND

The success of surgical flaps is improved by timely correction of vascular compromise. Current monitoring methods are labor or cost intensive or have limited clinical benefit. We hypothesize that injectable oxygen sensors can identify acute vascular compromise. The purpose of this study was to use a long-term, real-time method of tissue oxygenation detection in a rat flap model with vascular manipulation.

METHODS

Sensors incorporated benzo-porphyrin dye into a microporous hydrogel and were injected intradermally 1 day before flap elevation. Inspired oxygen was modulated between 100% and 12% to confirm sensor O₂ sensitivity. Eight random flaps (4 cm wide, 8 cm long) were elevated. Sensor and clinical observation to temporary clamping of the flap vascular pedicle was recorded. Sodium fluorescein in saline was injected intraperitoneally on postoperative days 0, 3, and 7 with subsequent perfusion area analysis.

RESULTS

Tissue oxygen tension measurements reflected the changes in inspired oxygen levels. Clinical observation of the flaps did not show any significant change in color or temperature with pedicle clamping. However, clamping of the pedicle resulted in a significant decrease in sensor tissue oxygen tension within 70 seconds.

CONCLUSION

Oxygen monitoring of myocutaneous flaps is sensitive and can detect acute vascular occlusion. This technique is faster than current methods and offers a cost-effective and accurate means of monitoring surgical tissues.

In vivo imaging and analysis of cerebrovascular hemodynamic responses and tissue oxygenation in the mouse brain

Cerebrovascular dysfunction has an important role in the pathogenesis of multiple brain disorders. Measurement of hemodynamic responses in vivo can be challenging, particularly as techniques are often not described in sufficient detail and vary between laboratories. We present a set of standardized in vivo protocols that describe high-resolution two-photon microscopy and intrinsic optical signal (IOS) imaging to evaluate capillary and arteriolar responses to a stimulus, regional hemodynamic responses, and oxygen delivery to the brain. The protocol also describes how to measure intrinsic NADH fluorescence to understand how blood O₂ supply meets the metabolic demands of activated brain tissue, and to perform resting-state absolute oxygen partial pressure (pO₂) measurements of brain tissue. These methods can detect cerebrovascular changes at far higher resolution than MRI techniques, although the optical nature of these techniques limits their achievable imaging depths. Each individual procedure requires 1-2 h to complete, with two to three procedures typically performed per animal at a time. These protocols are broadly applicable in studies of cerebrovascular function in healthy and diseased brain in any of the existing mouse models of neurological and vascular disorders. All these procedures can be accomplished by a competent graduate student or experienced technician, except the two-photon measurement of absolute pO₂ level, which is better suited to a more experienced, postdoctoral-level researcher.

A LED-based phosphorimeter for measurement of microcirculatory oxygen pressure

Quantitative measurements of microcirculatory and tissue oxygenation are of prime importance in experimental research. The noninvasive phosphorescence quenching method has given further insight into the fundamental mechanisms of oxygen transport to healthy tissues and in models of disease. Phosphorimeters are devices dedicated to the study of phosphorescence quenching. The experimental applications of phosphorimeters range from measuring a specific oxygen partial pressure (Po2) in cellular organelles such as mitochondria, finding values of Po2 distributed over an organ or capillaries, to measuring microcirculatory Po2 changes simultaneously in several organ systems. Most of the current phosphorimeters use flash lamps as a light excitation source. However, a major drawback of flash lamps is their inherent plasma glow that persists for tens of microseconds after the primary discharge. This complex distributed excitation pattern generated by the flash lamp can lead to inaccurate Po2 readings unless a deconvolution analysis is performed. Using light-emitting diode (LED), a rectangular shaped light pulse can be generated that provides a more uniformly distributed excitation signal. This study presents the design and calibration process of an LED-based phosphorimeter (LED-P). The in vitro calibration of the LED-P using palladium(II)-meso-tetra(4-carboxyphenyl)-porphyrin (Pd-TCCP) as a phosphorescent dye is presented. The pH and temperature were altered to assess whether the decay times of the Pd-TCCP measured by the LED-P were significantly influenced. An in vivo validation experiment was undertaken to measure renal cortical Po2 in a rat subjected to hypoxic ventilation conditions and ischemia/reperfusion. The benefits of using LEDs as a light excitation source are presented.

Barometric calibration of a luminescent oxygen probe

The invention of the phosphorescence quenching method for the measurement of oxygen concentration in blood and tissue revolutionized physiological studies of oxygen transport in living organisms. Since the pioneering publication by Vanderkooi and Wilson in 1987, many researchers have contributed to the measurement of oxygen in the microcirculation, to oxygen imaging in tissues and microvessels, and to the development of new extracellular and intracellular phosphorescent probes. However, there is a problem of congruency in data from different laboratories, because of interlaboratory variability of the calibration coefficients in the Stern-Volmer equation. Published calibrations for a common oxygen probe, Pd-porphyrin + bovine serum albumin (BSA), vary because of differences in the techniques used. These methods are used for the formation of oxygen standards: chemical titration, calibrated gas mixtures, and an oxygen electrode. Each method in turn also needs calibration. We have designed a barometric method for the calibration of oxygen probes by using a regulated vacuum to set multiple PO2 standards. The method is fast and accurate and can be applied to biological fluids obtained during or after an experiment. Calibration over the full physiological PO2 range (1-120 mmHg) takes ∼15 min and requires 1-2 mg of probe.

Recent advances in ophthalmic molecular imaging

The aim of molecular imaging techniques is the visualization of molecular processes and functional changes in living animals and human patients before morphological changes occur at the cellular and tissue level. Ophthalmic molecular imaging is still in its infancy and has mainly been used in small animals for pre-clinical research. The goal of most of these pre-clinical studies is their translation into ophthalmic molecular imaging techniques in clinical care. We discuss various molecular imaging techniques and their applications in ophthalmology.

A novel technique for monitoring of fast variations in brain oxygen tension using an uncoated fluorescence quenching probe (Foxy AL-300)

BACKGROUND

A novel uncoated fluorescence quenching probe allows fast measurement of oxygen tension in vessels and tissue. The present study reports the first use of the technology for dual measurements of arterial (paO(2)) and brain tissue oxygen tension (ptiO(2)) during hypoxic challenge in a pig model.

METHODS

Eight pigs were anesthetized using fentanyl and propofol. Fluorescence quenching pO(2) probes (Foxy AL-300, Ocean Optics, Dunedin, FL) were placed in the ascending aorta (Foxy-paO(2)) and subcortically at 14 mm in brain tissue (Foxy-ptiO(2)). As reference, a clark-type electrode probe (Licox-ptiO(2)) was placed into brain tissue close to the Foxy probe (Licox, Integra Neurosciences, Plainsboro, NJ). Measurements were taken at baseline (FiO(2) 1.0), during episodes of apnea, and during recovery (FiO(2) 1.0).

STATISTICS

descriptive results.

RESULTS

Individual Foxy-paO(2), Foxy-ptiO(2), and Licox-ptiO(2) courses were related to episodes of apnea. The response time of the Foxy measurements was 10 Hz. Baseline values at FiO(2) 1.0 were Foxy-paO(2) 520±120 mm Hg, Foxy-ptiO(2) 62±24 mm Hg, and Licox-ptiO(2) 55±29 mm Hg; apnea values were Foxy-paO(2) 64±10 mm Hg, Foxy-ptiO(2) 37±12 mm Hg, and Licox-ptiO(2) 31±16 mm Hg; recovery values at FiO(2) 1.0 were Foxy-paO(2) 478±98 mm Hg, Foxy-ptiO(2) 78±26 mm Hg, and Licox-ptiO(2) 62±32 mm Hg.

CONCLUSIONS

The present study demonstrates the feasibility of pO(2) measurements in macrocirculation and cerebral microcirculation using a novel uncoated fluorescence quenching probe. The technology allows for real-time investigation of pO(2) changes at a temporal resolution of 0.05 to 10 Hz.

Physiological oxygen level is critical for modeling neuronal metabolism in vitro

In vitro models are important tools for studying the mechanisms that govern neuronal responses to injury. Most neuronal culture methods employ nonphysiological conditions with regard to metabolic parameters. Standard neuronal cell culture is performed at ambient (21%) oxygen levels, whereas actual tissue oxygen levels in the mammalian brain range from 1% to 5%. In this study, we examined the consequences of oxygen level on the viability and metabolism of primary cultures of cortical neurons. Our results indicate that physiological oxygen level (5% O(2)) has a beneficial effect on cortical neuronal survival and mitochondrial function in vitro. Moreover, oxygen level affects metabolic fluxes: glucose uptake and glycolysis was enhanced at physiological oxygen level, whereas glucose oxidation and fatty acid oxidation were reduced. Adenosine monophosphate-activated protein kinase (AMPK) was more activated in 5% O(2) and appears to play a role in these metabolic effects. Inhibiting AMPK activity with compound C decreased glucose uptake, intracellular ATP level, and viability in neurons cultured in 5% O(2). These data indicate that oxygen level is an important parameter to consider when modeling neuronal responses to stress in vitro.

Hypocarbia and adverse outcome in neonatal hypoxic-ischemic encephalopathy

OBJECTIVE

To evaluate the association between early hypocarbia and 18- to 22-month outcome among neonates with hypoxic-ischemic encephalopathy.

STUDY DESIGN

Data from the National Institute of Child Health and Human Development Neonatal Research Network randomized, controlled trial of whole-body hypothermia for neonatal hypoxic-ischemic encephalopathy were used for this secondary observational study. Infants (n = 204) had multiple blood gases recorded from birth to 12 hours of study intervention (hypothermia versus intensive care alone). The relationship between hypocarbia and outcome (death/disability at 18 to 22 months) was evaluated by unadjusted and adjusted analyses examining minimum PCO(2) and cumulative exposure to PCO(2) <35 mm Hg. The relationship between cumulative PCO(2) <35 mm Hg (calculated as the difference between 35 mm Hg and the sampled PCO(2) multiplied by the duration of time spent <35 mm Hg) and outcome was evaluated by level of exposure (none-high) using a multiple logistic regression analysis with adjustments for pH, level of encephalopathy, treatment group (± hypothermia), and time to spontaneous respiration and ventilator days; results were expressed as odds ratios and 95% confidence intervals. Alternative models of CO(2) concentration were explored to account for fluctuations in CO(2).

RESULTS

Both minimum PCO(2) and cumulative PCO(2) <35 mm Hg were associated with poor outcome (P < .05). Moreover, death/disability increased with greater cumulative exposure to PCO(2) <35 mm Hg.

CONCLUSIONS

Hypocarbia is associated with poor outcome after hypoxic-ischemic encephalopathy.

The role of systemic hemodynamic disturbances in prematurity-related brain injury

Premature infants who experience cerebrovascular injury frequently have acute and long-term neurologic complications. In this article, we explore the relationship between systemic hemodynamic insults and brain injury in this patient population and the mechanisms that might be at play.

Permissive hypercapnia to decrease lung injury in ventilated preterm neonates

Lung injury in ventilated premature infants occurs primarily through the mechanism of volutrauma, often due to the combination of high tidal volumes in association with a high end-inspiratory volume and occasionally end-expiratory alveolar collapse. Tolerating a higher level of arterial partial pressure of carbon dioxide (PaCO2) is considered as 'permissive hypercapnia' and when combined with the use of low tidal volumes may reduce volutrauma and lead to improved pulmonary outcomes. Permissive hypercapnia may also protect against hypocapnia-induced brain hypoperfusion and subsequent periventricular leukomalacia. However, extreme hypercapnia may be associated with an increased risk of intracranial hemorrhage. It may therefore be important to avoid large fluctuations in PaCO2 values. Recent randomized clinical trials in preterm infants have demonstrated that mild permissive hypercapnia is safe, but clinical benefits are modest. The optimal PaCO2 goal in clinical practice has not been determined, and the available evidence does not currently support a general recommendation for permissive hypercapnia in preterm infants.

Cerebrovascular injury in premature infants: current understanding and challenges for future prevention

Cerebrovascular insults are a leading cause of brain injury in premature infants, contributing to the high prevalence of motor, cognitive, and behavioral deficits. Understanding the complex pathways linking circulatory immaturity to brain injury in premature infants remains incomplete. These mechanisms are significantly different from those causing injury in the mature brain. The gaps in knowledge of normal and disturbed cerebral vasoregulation need to be addressed. This article reviews current understanding of cerebral perfusion, in the sick premature infant in particular, and discusses challenges that lie ahead.

Layer-specific anatomical, physiological and functional MRI of the retina

Most retinal imaging has been performed using optical techniques. This paper reviews alternative retinal imaging methods based on MRI performed with spatial resolution sufficient to resolve multiple well-defined retinal layers. The development of these MRI technologies to study retinal anatomy, physiology (blood flow, blood volume, and oxygenation) and function, and their applications to the study of normal retinas, retinal degeneration and diabetic retinopathy in animal models are discussed. Although the spatiotemporal resolution of MRI is poorer than that of optical imaging techniques, it is unhampered by media opacity and can thus image all retinal and pararetinal structures, and has the potential to provide multiple unique clinically relevant data in a single setting and could thus complement existing retinal imaging techniques. In turn, the highly structured retina with well-defined layers is an excellent model for advancing emerging high-resolution anatomical, physiological and functional MRI technologies.

Experimental and theoretical studies of oxygen gradients in rat pial microvessels

Using modified oxygen needle microelectrodes and intravital videomicroscopy, measurements were made of tissue oxygen tension (PO(2)) profiles near cortical arterioles and transmural PO(2) gradients in the pial arterioles of the rat. Under control conditions, the transmural PO(2) gradient averaged 1.17+/-0.06 mm Hg/microm (mean+/-s.e., n=40). Local arteriolar dilation resulted in a marked decrease in the transmural PO(2) gradient to 0.68+/-0.04 mm Hg/microm (P<0.001, n=38). The major finding of this study is a dependence of the transmural PO(2) gradient on the vascular tone of the pial arterioles. Using a model of oxygen transport in an arteriole and experimental PO(2) profiles, values of radial perivascular and intravascular O(2) fluxes were estimated. Our theoretical estimates show that oxygen flux values at the outer surface of the arteriolar wall are approximately 10(-5) mL O(2)/cm(2) per sec, independent of the values of the arteriolar wall O(2) consumption within a wide range of consumption values. This also means that PO(2) transmural gradients for cerebral arterioles are within the limits of 1 to 2 mm Hg/microm. The data lead to the conclusion that O(2) consumption of the arteriolar wall is within the range for the surrounding tissue and O(2) consumption of the endothelial layer appears to have no substantial impact on the transmural PO(2) gradient.

Heart, kidney, and intestine have different tolerances for anemia

Organ systems do not respond uniformly to changes in systemic oxygen delivery because of global and local redistributive mechanisms. We hypothesized that progressive hemodilution would evoke a different response in the microvascular oxygenation of the heart compared with kidney and gut. To evaluate this hypothesis, we studied the effect of stepwise isovolemic hemodilution on systemic hemodynamic and oxygenation parameters as well as the relation between systemic hematocrit (Ht) and microvascular PO(2) (microPO(2)) in heart, kidney, and intestines in an anesthetized and mechanically ventilated rat model. Baseline conditions were similar in the hemodilution group and in the control group. In the hemodilution group, Ht was diminished from 46.6 +/- 3.8% to 7.0 +/- 1.8% [mean +/- standard deviation (SD)]. This group had no effect on measured hemodynamics; only when Ht fell below 10% did blood pressure start to decrease. The microPO(2) values in heart, kidney, and intestines did not respond uniformly. Renal microPO(2) (56 +/- 10 mm Hg at baseline) started to decrease at a Ht of 38.5 +/- 8.6%, whereas intestinal microPO(2) (59 +/- 6 mm Hg at baseline) did not start to decrease until Ht reached 17.4 +/- 7.1%. Finally, cardiac microPO(2) (40 +/- 6 mm Hg at baseline) decreased only in the ultimate stage of the experiment at Ht of 8.7 +/- 3.5%. Based on these observations, we conclude that the regulation of microvascular oxygenation during progressive anemia is specific for each organ system. The relation between these observations and organ function and damage needs to be determined.

Methods for noninvasive imaging of tissue hypoxia

The purpose of this review is to provide an overview of the methods available for imaging tissue oxygenation. The following imaging methods are reviewed: phosphorescence, near-infrared (NIR), positron emission tomography (PET), magnetic resonance imaging ((19)F MRI and BOLD MRI), and electron paramagnetic resonance (EPR). The methods are based on different principles and differ in their ability to accurately quantify tissue oxygenation, either the absolute value of a particular measure of oxygenation (partial pressure of oxygen, concentration), or a parameter related to it (oxygen saturation). Methods that can provide images of relative changes in oxygenation or visualization of hypoxia in a specific tissue of interest are also considered valuable tools for biomedical research and clinical applications.

Brain tissue oxygen concentration measurements

Brain function depends exquisitely on oxygen for energy metabolism. Measurements of brain tissue oxygen tension, by a variety of quantitative and qualitative techniques, going back for >50 years, have led to a number of significant conclusions. These conclusions have important consequences for understanding brain physiology as it is now being explored by techniques such as blood-oxygen-level-dependent functional magnetic resonance imaging (BOLD fMRI) and near-infrared spectroscopy (NIRS). It has been known for some time that most of the measured oxygen tensions are less than venous pO2 and are distributed in a spatially and temporally heterogeneous manner on a microregional scale. Although certain large-scale methods can provide reproducible average brain pO2 measurements, no useful concept of a characteristic oxygen tension or meaningful average value for brain tissue oxygen can be known on a microregional level. Only an oxygen field exists with large local gradients due to local tissue respiration, and the most useful way to express this is with a pO2 distribution curve or histogram. The neurons of the brain cortex normally exist in a low-oxygen environment and on activation are oxygenated by increases in local capillary blood flow that lead to increases in hemoglobin saturation and tissue oxygen.

Evidence that renal arterial-venous oxygen shunting contributes to dynamic regulation of renal oxygenation

Renal blood flow (RBF) can be reduced in rats and rabbits by up to 40% without significant changes in renal tissue Po2. We determined whether this occurs because renal oxygen consumption changes with RBF or due to some other mechanism. The relationships between RBF and renal cortical and medullary tissue Po2 and renal oxygen metabolism were determined in the denervated kidneys of anesthetized rabbits under hypoxic, normoxic, and hyperoxic conditions. During artificial ventilation with 21% oxygen (normoxia), RBF increased 32 ± 8% during renal arterial infusion of acetylcholine and reduced 31 ± 5% during ANG II infusion. Neither infusion significantly altered arterial pressure, tissue Po2 in the renal cortex or medulla, nor renal oxygen consumption. However, fractional oxygen extraction fell as RBF increased and the ratio of oxygen consumption to sodium reabsorption increased during ANG II infusion. Ventilation with 10% oxygen (hypoxia) significantly reduced both cortical and medullary Po2 (60–70%), whereas ventilation with 50% and 100% oxygen (hyperoxia) increased cortical and medullary Po2 (by 62–298 and 30–56%, respectively). However, responses to altered RBF under hypoxic and hyperoxic conditions were similar to those under normoxic conditions. Thus renal tissue Po2 was relatively independent of RBF within a physiological range (±30%). This was not due to RBF-dependent changes in renal oxygen consumption. The observation that fractional extraction of oxygen fell with increased RBF, yet renal parenchymal Po2 remained unchanged, supports the hypothesis that preglomerular diffusional shunting of oxygen from arteries to veins increases with increasing RBF, and so contributes to dynamic regulation of intrarenal oxygenation.

A new technique for the mapping of oxygen tension on the brain surface

Most measurements of oxygen tension (PO(2)) in the brain have been performed using oxygen microelectrodes. However, the insertion of microelectrodes into the brain per se causes cortical injury and hence could lead to erroneous PO(2) measurements. The recently developed "quenching lifetime method" requires the injection of fluorescent chemicals into the blood circulation. To address this issue, we tested the feasibility of our O(2)-sensitive fluorescent membrane technique in the rat brain, and visualized the spatial distribution of PO(2) on the brain surface as epifluorescent microscopic patterns. An O(2)-quenching fluorescence dye, tris (1,10-phenanthroline) Ru(2+), was immobilized in a highly gas-permeable, thin silicone-rubber film formed on a microscope coverslip. Unlike the original method, which was intended for transparent rat mesenteric tissue, any change in the redox state in the brain tissue will influence the optical measurement of PO(2). Thus, in the present study, the O(2)-sensing membrane was further coated with a thin opaque silicone-rubber to minimize this type of influence. This new method enabled us to visualize the PO(2) gradient on the rat brain without causing cortical injuries. In an ischemia/reperfusion model using Pulsinelli's four-vessel occlusion rats, the changes in the PO(2) were highly heterogeneous during the ischemic period and this heterogeneity, both temporal and spatial, was higher in the off-arteriolar area than in the peri-arteriolar area.

Both extremes of arterial carbon dioxide pressure and the magnitude of fluctuations in arterial carbon dioxide pressure are associated with severe intraventricular hemorrhage in preterm infants

OBJECTIVE

The goal was to test the hypothesis that extremes of PaCO2 during the first 4 days after birth are associated with severe intraventricular hemorrhage (grades 3 and 4).

METHODS

A single-center retrospective review of clinical and blood gas data in the first 4 postnatal days for 849 infants with birth weights of 401 to 1250 g was performed. The univariate and multivariate relationships of severe intraventricular hemorrhage with maximal and minimal PaCO2, PaCO2 averaged over time (time-weighted PaCO2), and measures of PaCO2 fluctuation (SD of PaCO2 and difference in PaCO2 [maximum minus minimum]) were assessed.

RESULTS

Birth weight (mean +/- SD) was 848 +/- 212 g, and the median gestational age was 26 weeks. Infants with severe intraventricular hemorrhage had higher maximal PaCO2 (median: 72 vs 59 mm Hg) and time-weighted PaCO2 (mean: 49 vs 47 mm Hg) values but lower minimal PaCO2 values (32 vs 37 mm Hg). High PaCO2, low PaCO2, SD of PaCO2, and difference in PaCO2 predicted severe intraventricular hemorrhage, but time-weighted average PaCO2 was not as predictive.

CONCLUSIONS

Both extremes and fluctuations of PaCO2 are associated with severe intraventricular hemorrhage. It may be prudent to avoid extreme hypocapnia and hypercapnia during the period of risk for intraventricular hemorrhage.

Effects of severe hypocapnia on expression of bax and bcl-2 proteins, DNA fragmentation, and membrane peroxidation products in cerebral cortical mitochondria of newborn piglets

BACKGROUND

Hypocapnia occurs in the newborn infant inadvertently or as a therapeutic modality and may result in neuronal and mitochondrial alterations in the newborn brain. Since mitochondria regulate apoptosis, these alterations may initiate a cascade of reactions that lead to apoptotic cell death.

OBJECTIVES

This study tests the hypothesis that hypocapnia results in increased expression of the pro-apoptotic protein Bax, fragmentation of DNA and membrane lipid peroxidation in cerebral cortical mitochondria (mt) of newborn piglets.

METHODS

Studies were performed in three groups of anesthetized normoxic newborn piglets: hypocapnic (H, n = 5), ventilated at a PaCO(2) of 11-15 mm Hg; normocapnic (N, n = 5), ventilated at a PaCO(2) of 40 mm Hg; and corrected normocapnic (CN, n = 4), ventilated as H with CO(2) added to maintain normocapnia. Tissue ATP and phosphocreatine levels were determined. Mitochondrial membrane proteins were separated, transblotted and probed with antibodies to Bax and Bcl-2. Bands were detected by enhanced chemiluminescence and analyzed by imaging densitometry. mtDNA was isolated. Cell and mitochondrial membrane lipid peroxidation products were measured spectrofluorometrically.

RESULTS

ATP and PCr concentrations were similar in the 3 groups. The ratio of Bax/Bcl-2 increased significantly in H compared to N and CN. mtDNA fragmentation was also significantly greater in H compared to N or CN. Membrane lipid peroxidation was higher in H than in N or CN; and in CN compared to N.

CONCLUSIONS

The data demonstrate that severe hypocapnia results in increased Bax expression, DNA fragmentation, and membrane lipid peroxidation in mitochondria of cerebral cortical neurons of newborn piglets, and may result in apoptotic cell death.

Fetal hypercapnia and cerebral tissue oxygenation: studies in near-term sheep

The precise role of CO2 in cerebral oxygenation is not as well defined as O2, especially in the immature brain. In the ovine fetus, we tested the hypotheses that arterial Pco2 (Paco2) plays a critical role not only in the regulation of cerebral blood flow but also in the regulation of cerebral tissue oxygenation. By use of a fluorescent O2 probe with a laser Doppler flowmeter and the placement of sagittal sinus catheter in six near-term fetal sheep, we measured values of cortical tissue O2 tension (tPo2), sagittal sinus oxyhemoglobin saturation ([HbO2]), and laser Doppler cerebral blood flow (LD-CBF) in response to 20 min hypercapnia induced by having the ewe breathe CO2. In response to moderate to severe hypercapnia, LD-CBF increased above baseline in a curvilinear fashion, cortical tPo2 increased linearly (1 torr per 3.2 torr Paco2), and sagittal sinus [HbO2] increased significantly in a curvilinear manner. Hypercapnia favored cerebral tissue oxygenation of the fetal brain; and cortical tPo2 and sagittal sinus [HbO2] complement or support one another as indices of cerebral oxygenation under hypercapnic conditions.

Dual-wavelength phosphorimetry for determination of cortical and subcortical microvascular oxygenation in rat kidney

This study presents a dual-wavelength phosphorimeter developed to measure microvascular PO2 (microPO2) in different depths in tissue and demonstrates its use in rat kidney. The used phosphorescent dye is Oxyphor G2 with excitation bands at 440 and 632 nm. The broad spectral gap between the excitation bands combined with a relatively low light absorption of 632 nm light by tissue results in a marked difference in penetration depths of both excitation wavelengths. In rat kidney, we determine the catchments depth of the 440-nm excitation to be 700 microm, whereas the catchments depth of 632 nm is as much as 4 mm. Therefore, the measurements differentiate between cortex and outer medulla, respectively. In vitro, no difference in PO2 readings between both channels was found. On the rat kidney in vivo, the measured cortical microPO2 was on average 20 Torr higher than the medullary microPO2 over a wide PO2 range induced by variations in inspired oxygen fraction. Examples provided from endotoxemia and resuscitation show differences in responses of mean cortical and medullary PO2 readings as well as in the shape of the PO2 histograms. It can be concluded that oxygen-dependent quenching of phosphorescence of Oxyphor G2 allows quantitative measurement of microPO2 noninvasively in two different depths in vivo. Oxygen levels measured by this technique in the rat renal cortex and outer medulla are consistent with previously published values detected by Clark-type oxygen electrodes. Dual-wavelength phosphorimetry is excellently suited for monitoring microPO2 changes in two different anatomical layers under pathophysiological conditions with the characteristics of providing oxygen histograms from two depths and having a penetration depth of several millimeters.

Risk factors for cerebral palsy in preterm infants

OBJECTIVE

To identify crucial factors that precipitate cerebral palsy by controlling confounding factors in logistic regression analyses.

DESIGN AND PATIENTS

We retrospectively investigated a cohort of all 922 infants with gestational ages of less than 34 weeks (22-33 weeks), who were admitted to our neonatal intensive care unit between 1990 and 1998. Thirty (3.7%) were diagnosed to have cerebral palsy. We analyzed the prenatal and postnatal clinical variables of the cerebral palsy cases and compared them with 150 randomly selected controls.

RESULTS

Risk factors for cerebral palsy identified in univariate analysis were: twin pregnancy, long-term ritodrine tocolysis, respiratory distress syndrome, air leak, surfactant administration, intermittent mandatory ventilation, high frequency oscillation, lowest PaCO2 levels, prolonged hypocarbia during the first 72 h of life, and postnatal steroid therapy. In a conditional multiple logistic model, long-term ritodrine tocolysis, prolonged hypocarbia and postnatal steroid therapy remained associated with an increased risk of cerebral palsy after adjustment for other antenatal and postnatal variables (OR [Odds Ratio] = 8.62, 95% CI [Confidence Interval], 2.18-33.97; OR = 7.81, 95% CI, 1.42-42.92; OR = 21.37, 95% CI, 2.01-227.29, respectively).

CONCLUSIONS

Our results suggest that long-term ritodrine tocolysis underlines the development of cerebral palsy. Further assessments of the effect of ritodrine on fetal circulation and nervous system are required. Moreover, possible alternatives to systemic postnatal steroids are needed, and carbon dioxide levels should be more strictly controlled.

Quantitative determination of localized tissue oxygen concentration in vivo by two-photon excitation phosphorescence lifetime measurements

This study describes the use of two-photon excitation phosphorescence lifetime measurements for quantitative oxygen determination in vivo. Doubling the excitation wavelength of Pd-porphyrin from visible light to the infrared allows for deeper tissue penetration and a more precise and confined selection of the excitation volume due to the nonlinear two-photon effect. By using a focused laser beam from a 1,064-nm Q-switched laser, providing 10-ns pulses of 10 mJ, albumin-bound Pd-porphyrin was effectively excited and oxygen-dependent decay of phosphorescence was observed. In vitro calibration of phosphorescence lifetime vs. oxygen tension was performed. The obtained calibration constants were kq = 356 Torr(-1) x s(-1) (quenching constant) and tau0 = 550 micros (lifetime at zero-oxygen conditions) at 37 degrees C. The phosphorescence intensity showed a squared dependency to the excitation intensity, typical for two-photon excitation. In vivo demonstration of two-photon excitation phosphorescence lifetime measurements is shown by step-wise PO2 measurements through the cortex of rat kidney. It is concluded that quantitative oxygen measurements can be made, both in vitro and in vivo, using two-photon excitation oxygen-dependent quenching of phosphorescence. The use of two-photon excitation has the potential to lead to new applications of the phosphorescence lifetime technique, e.g., noninvasive oxygen scanning in tissue at high spatial resolution. To our knowledge, this is the first report in which two-photon excitation is used in the setting of oxygen-dependent quenching of phosphorescence lifetime measurements.

Red blood cell velocity and oxygen tension measurement in cerebral microvessels by double-wavelength photoexcitation

Because the regulation of microcirculation in the cerebral cortex cannot be analyzed without measuring the blood flow dynamics and oxygen concentration in cerebral microvessels, we developed a fluorescence and phosphorescence system for estimating red blood cell velocity and oxygen tension in cerebral microcirculation noninvasively and continuously with high spatial resolution. Using red blood cells labeled with fluorescent isothiocyanate to visualize red cell distribution and using the oxygen quenching of Pd-meso-tetra-(4-carboxyphenyl)-porphyrin phosphorescence to measure oxygen tension enabled simultaneous measurement of blood velocity and oxygen tension. We examined how the measurement accuracy was affected by the spatial resolution and by the excitation laser light passing through the targeted microvessel and exciting the oxygen probe dye in the tissue beneath it. Focusing the excitation light into the microvessel stabilized the phosphorescence lifetime at each spatial resolution; moreover, it greatly reduced phosphorescence from the brain tissue. Animal experiments involving acute hemorrhagic shock demonstrated the feasibility of our system by showing that the changes in venular velocity and oxygen tension are synchronized to the change in mean arterial pressure. Our system measures the red cell velocity and oxygen concentration in the cerebral microcirculation by using the differences in luminescence and wavelength between fluorescence and phosphorescence, making it possible to easily acquire information about cerebral microcirculatory distribution and oxygen tension simultaneously.

Hemoglobin-based oxygen carrier provides heterogeneous microvascular oxygenation in heart and gut after hemorrhage in pigs

BACKGROUND

In this study, the hypothesis was tested that resuscitation with hemoglobin-based oxygen carriers (HBOCs) affects the oxygenation of the microcirculation differently between and within organs. To this end, we tested the influence of the volume of an HBOC on the microcirculatory oxygenation of the heart and the gut serosa and mucosa in a porcine model of hemorrhage.

METHODS

In anesthetized open-chested pigs (n = 24), a controlled hemorrhage (30 mL/kg over 1 hour) was followed by resuscitation with 10, 20, or 30 mL/kg diaspirin-crosslinked hemoglobin (DCLHb) or isovolemic resuscitation with 30 mL/kg of a 6% hydroxyethyl starch solution (HAES). Measurements included systemic and regional hemodynamic and oxygenation parameters. Microvascular oxygen pressures (microPO2) of the epicardium and the serosa and mucosa of the ileum were measured simultaneously by the palladium-porphyrin phosphorescence technique. Measurements were obtained up to 120 minutes after resuscitation.

RESULTS

After hemorrhage, a low volume of DCLHb restored both cardiac and intestinal microPO2. Resuscitation of gut microPO2 with a low volume of DCLHb was as effective as isovolemic resuscitation with HAES. Higher volumes of DCLHb did not restore cardiac microPO2, as did isovolemic resuscitation with HAES, but increased gut microPO2 to hyperoxic values, dose-dependently. Effects were similar for the serosal and mucosal microPo2. In contrast to a sustained hypertensive effect after resuscitation with DCLHb, effects of DCLHb on regional oxygenation and hemodynamics were transient.

CONCLUSION

This study showed that a low volume of DCLHb was effective in resuscitation of the microcirculatory oxygenation of the heart and gut back to control levels. Increasing the volume of DCLHb did not cause an additional increase in heart microPO2, but caused hyperoxic microvascular values in the gut to be attained. It is concluded that microcirculatory monitoring in this way elucidates the regional behavior of oxygen transport to the tissue by HBOCs, whereas systemic variables were ineffective in describing their response.

High-resolution visualization of oxygen distribution in the liver in vivo

Microcirculatory failure after stress events results in mismatch in oxygen supply and demand. Determination of tissue oxygen distribution in vivo may help elucidate mechanisms of injury, but present methods have limited resolution. Male Sprague-Dawley rats were anesthetized, prepared for intravital microscopy, and received intravenously the oxygen-sensitive fluorescent dye Tris(1,10-phenanthroline)ruthenium(II) chloride hydrate [Ru(phen)3(2+)]. An impaired hepatic oxygen distribution was induced by either phenylephrine or hemorrhage. Intensity of Ru(phen)3(2+) fluorescence was compared with NADH autofluorescence indicating changes in the mitochondrial redox potential. Ethanol was injected to affect the NADH-to-NAD+ ratio without altering the P(O2). Infusion of Ru(phen)3(2+) resulted in a heterogeneous fluorescence under baseline conditions reflecting the physiological acinar P(O2) distribution. A decrease in oxygen supply due to phenylephrine or hemorrhage was paralleled by an increase in Ru(phen)3(2+) and NADH fluorescence reflecting an impaired mitochondrial redox state. Ethanol did not alter Ru(phen)3(2+) fluorescence but increased NADH fluorescence indicating independence of P(O2) and redox state imaging. Intravenous administration of Ru(phen)3(2+) for intravital videomicroscopy represents a new method to visualize the hepatic tissue P(O2). Combined with NADH autofluorescence, it provides additional information regarding the tissue redox state.

Brain oxygenation during cardiopulmonary bypass and circulatory arrest

Quantitative measurements of oxygen distribution in the microcirculation of the brain cortex of newborn piglets were made during different modes of cardiopulmonary bypass. Three groups of animals, anesthetized and mechanically ventilated, were studied. The first group of animals were maintained on normothermic cardiopulmonary bypass (CPB) at a flow of 100 ml/kg/min, while the second and third groups underwent low flow hypothermic cardiopulmonary bypass (40 ml/kg/min at 18 degrees C) (LFCPB) and deep hypothermic (18 degrees C) circulatory arrest (DHCA), respectively. After bypass, the piglets were monitored for a two hours post-bypass recovery period. CPB caused a decrease in the cortical oxygen from 62 +/- 3 mm Hg to 32 +/- 7 mm Hg at the beginning of bypass and to 36 +/- 5 mm Hg at the end of bypass. During the recovery period, cortical oxygenation steadily decreased, reaching 29 +/- 8 mm Hg at the end of the experiment. With initiation of LFCPB, cortical oxygen decreased to 22 +/- 7 mm Hg. Upon rewarming cortical oxygen increased to 37 +/- 5 mm Hg and then decreased again to about 30 mm Hg at the end of two hours of post-bypass recovery. Similar changes in cortical oxygenation were observed during DHCA. In DHCA cortical oxygen decreased to 19 +/- 4 mm Hg and during rewarming and recovery increased to 35 +/- 6 mm Hg. In conclusion, it has been shown that in newborn piglets recovering from CPB, LFCPB and DHCA, when the blood pressure remained above 55 mm Hg and therefore total blood flow should be well maintained, oxygen pressure in the microvasculature is significantly lower than for pre-bypass. It is suggested that the decreased oxygenation is due to increased heterogeneity in resistance in the microcirculatory units, resulting in broadened distribution of flow rates and oxygen levels.

Effect of perfusion flow rate on tissue oxygenation in newborn piglets during cardiopulmonary bypass

BACKGROUND

Our knowledge of the best perfusion flow rate to use during cardiopulmonary bypass (CPB) in order to maintain tissue oxygenation remains incomplete. The present study examined the effects of perfusion flow rate and patent ductus arteriosus (PDA) during normothermic CPB on oxygenation in several organ tissues of newborn piglets.

METHODS

The experiments were performed on 12 newborn piglets: 6 with PDA ligation (PDA-L), and 6 without PDA ligation (PDA-NL). CPB was performed through the chest at 37 degrees C. During CPB, the flow rate was changed at 15-minute intervals, ranging from 100 to 250 ml/kg/min. Tissue oxygenation was measured by quenching of phosphorescence.

RESULTS

For the PDA-L group, oxygen in the brain did not change significantly with changes in flow rate. In contrast, for the PDA-NL group, oxygen was dependent upon the flow rate. Statistically significant decreases in cortical oxygen were observed with flow rates below 175 ml/kg/min. Within the myocardium, liver, and intestine, there were no significant differences in the oxygen levels between the PDA-L and PDA-NL groups. In these tissues, the oxygen decreased significantly as the flow rate decreased below 150 ml/kg/min, 125 ml/kg/min, and 175 ml/kg/min, respectively. Oxygen pressure in skeletal muscle was not dependent on either PDA ligation or flow rate.

CONCLUSIONS

In newborn piglets undergoing CPB, the presence of a PDA results in reduced tissue oxygenation to the brain but not to other organs. In general, perfusion flow rates of 175 ml/kg/min or greater are required in order to maintain normal oxygenation of all organs except muscle.

Blood pO2 and blood flow at the optic disc

A fundus camera-based phosphorometer to noninvasively and quasicontinuously measure the blood partial pressure of oxygen (pO(2,blood)) in the microvasculature of the pig optic nerve using the principle of the phosphorescence quenching by O(2) is described. A porphyrin dye is injected into the venous circulation and the decay of its phosphorescence emission is detected locally in the eye, after excitation with a flash of light. Combined with blood flow measurements by means of a laser Doppler flowmeter mounted on the phosphorometer, we demonstrate the capability of the instrument to determine the time course of optic nerve blood flow and pO(2,blood) in response to various physiological stimuli, such as hyperoxia and hypercapnia. This instrument appears to be a useful tool for the investigation of the oxygenation of the optic nerve.

Brain Tissue and Sagittal Sinus pO2 Measurements Using the Lifetimes of Oxygen-Quenched Luminescence of a Ruthenium Compound

The study was done to assess the performance of a system that measures the partial pressures of oxygen (pO2) from the lifetimes of oxygen-quenched luminescence of ruthenium compounds immobilized at the tip of fiber-optic optodes (Oxylite system). The system was used to measure the pO2 in brain tissue (thalamus and hypothalamus) and in the sagittal sinus of isoflurane-anesthetized rats at different FiO2's. The pO2 recorded in the hypothalamus (HPtO2) was consistently higher than the pO2 in the thalamus (TPtO2) at all FiO2. HPtO2 was closely related to PvO2 during normoxia but not during hypoxia. The equilibrium time of Oxylite system was found to be rapid compared to in vivo tissue response to changes in FiO2.

Permissive hypercapnia

Although lifesaving, mechanical ventilation can result in lung injury and contribute to the development of bronchopulmonary dysplasia. The most critical determinants of lung injury are tidal volume and end-inspiratory lung volume. Permissive hypercapnia offers to maintain gas exchange with lower tidal volumes and thus decrease lung injury. Further physiologic benefits include improved oxygen delivery and neuroprotection, the latter through both avoidance of accidental hypocapnia, which is associated with a poor neurologic outcome, and direct cellular effects. Clinical trials in adults with acute respiratory failure indicated improved survival and reduced incidence of organ failure in subjects managed with low tidal volumes and permissive hypercapnia. Retrospective studies in low birth weight infants found an association of bronchopulmonary dysplasia with low PaCO(2). Randomized clinical trials of low birth weight infants did not achieve sufficient statistical power to demonstrate a reduction of BPD by permissive hypercapnia, but strong trends indicated the possibility of important benefits without increased adverse events. Herein, we review the mechanisms leading to lung injury, the physiologic effects of hypercapnia, the dangers of hypocapnia, and the available clinical data.

The effect of the transfusion of stored RBCs on intestinal microvascular oxygenation in the rat

BACKGROUND

Although it is known that the transfusion of stored RBCs does not always improve tissue O(2) consumption under conditions of limited tissue oxygenation, the efficiency of O(2) delivery to the microcirculation by stored RBCs has never been determined.

STUDY DESIGN AND METHODS

In a rat hemorrhagic shock model, the effects of resuscitation with fresh or 28-day-old RBCs stored in CPD plasma, saline-adenine-glucose-mannitol, and CPDA-1 plasma were investigated. Systemic hemodynamic and intestinal oxygenation measures were monitored. Intestinal microvascular PO(2) was determined with the O(2)-dependent quenching of palladium-porphyrin phosphorescence, and the RBC deformability was measured with a Laser-assisted optic rotational cell analyzer.

RESULTS

Hemodynamic and oxygenation measures were significantly decreased during hemorrhagic shock. Intestinal oxygen consumption and mesenteric venous pO(2) were restored with the transfusion of both fresh and stored RBCs, except for CPD-stored RBCs. The intestinal microvascular pO(2) improved only with the transfusion of fresh RBCs. Deformability of the stored RBCs was significantly decreased.

CONCLUSION

In contrast to that of fresh RBCs, the transfusion of stored RBCs did not restore the microcirculatory oxygenation, possibly because of impaired O(2) unloading, but, except for CPD-stored RBCs, the storage-induced changes were not enough to impair intestinal VO(2) and mesenteric venous pO(2).

Tissue oxygen tension and brain sensitivity to hypoxia

Mammalian brain is a highly oxidative organ and although it constitutes only a small fraction of total body weight it accounts for a disproportionately large percentage of bodily oxygen consumption (in humans about 2 and 20%, respectively). Yet, the partial pressure and concentration of oxygen in the brain are low and non-uniform. There is a large number of enzymes that use O(2) as a substrate, the most important of which is cytochrome c oxidase, the key to mitochondrial ATP production. The affinity of cytochrome c oxidase for oxygen is very high, which under normal conditions ensures undiminished activity of oxidative phosphorylation down to very low P(O(2)). By contrast, many other relevant enzymes have K(m) values for oxygen within, or above, the ambient cerebral gas tension, thus making their operations very dependent on oxygen level in the physiological range. Among its multiple, versatile functions, oxygen partial pressure and concentration control production of reactive oxygen species, expression of genes and functions of ion channels. Limitation of oxygen supply to the brain below a 'critical' level reduces, and eventually blocks oxidative phosphorylation, drastically decreases cellular (ATP) and leads to a collapse of ion gradients. Neuronal activity ceases and if oxygen is not re-introduced quickly, cells die. The object of this review is to discuss briefly the central oxygen-dependent processes in mammalian brain and the short-term consequences of O(2) deprivation, but not the mechanisms of long-term adaptation to chronic hypoxia. Particular emphasis is placed on issues which have been the focus of recent attention and/or controversy.

Hypocapnia and hypercapnia in respiratory management of newborn infants

Recent experimental and clinical data demonstrate that both hypocapnia and hypercapnia during the neonatal period may result in beneficial or adverse consequences. Multiple retrospective studies report a strong association between PaCO2 levels less than 25 to 30 mm Hg and an increased incidence of cystic PVL and CP in preterm infants. Prolonged exposure to PaCO2 values less than 25 to 30 mm Hg is also associated with hearing loss in term and near-term infants. A low tidal volume strategy combined with permissive hypercapnia is potentially a strategy that could prevent lung injury. Clearly, more randomized, controlled trials are needed before this latter strategy or that of permissive hypercapnia can be recommended routinely for preterm, near-term, or term gestation infants with respiratory disorders.

Microvascular and interstitial PO(2) measurements in rat skeletal muscle by phosphorescence quenching

To clarify the transport of O(2) across the microvessels in skeletal muscle, we designed an intravital laser microscope that utilizes a phosphorescence quenching technique to determine both the microvascular and tissue PO(2). After we injected the phosphorescent probe into systemic blood, phosphorescence excited by a N(2)-dye pulse laser was detected with a photomultiplier over a 10 microm in diameter area. In vitro and in vivo calibrations confirmed that the present method is accurate for PO(2) measurements in the range of 7-90 Torr (r = 0.958) and has a rapid response time. This method was then used to measure the PO(2) of microvessels with different diameters (40-130 microm) and of interstitial spaces in rat cremaster muscle. These measurements showed a significant drop in PO(2) in the arterioles after branching (from 74.6 to 46.6 Torr) and the presence of a large PO(2) gradient at the blood-tissue interface of arterioles (15-20 Torr). These findings suggest that capillaries are not the sole source of oxygen supply to surrounding tissue.

Hypocarbia in preterm infants with periventricular leukomalacia: the relation between hypocarbia and mechanical ventilation

OBJECTIVE

The aim of this study was to elucidate the relationship between mechanical ventilation and hypocarbia in infants with periventricular leukomalacia (PVL).

STUDY DESIGN

Matched pair analysis was conducted for 26 infants with PVL and 26 with normal development, who were born between 27 and 32 weeks' gestational age and required mechanical ventilation. The time-averaged carbon dioxide (CO(2)) index, PaCO(2), and pH were calculated every 24 hours for samples obtained from indwelling arterial catheters within the first 72 hours of life. The time-averaged respiratory rate of the ventilator (RR), peak inspiratory pressure (PIP), mean airway pressure (MAP), and ventilator index (VI) were also determined. The time-averaged total respiratory rate (TRR) was determined by observing the movement of the chest wall. The patients' characteristics, antenatal and neonatal variables, and electroencephalographic findings were also compared.

RESULTS

The time-averaged CO(2) index was larger, the time-averaged CO(2) lower and the time-averaged pH higher in infants with PVL than in those with normal development on the third day of life. There was no significant difference in the time-averaged RR, PIP, MAP, or VI on any day. TRR was larger in the PVL group than in the control group on each day, but there was no significant difference. No significant difference was observed in the clinical characteristics or neonatal variables. Electroencephalographic abnormalities within 48 hours of life were more frequent in infants with PVL than in those with normal development.

CONCLUSION

Hypocarbia was associated with PVL because the time-averaged CO(2) index was larger and the time-averaged PaCO(2) lower in infants with PVL than in those with normal development. However, the ventilator settings were similar among the infants with and without PVL.

Hypocapnia under hypotension induces apoptotic neuronal cell death in the hippocampus of newborn rabbits

We investigated the adverse effect of hypocapnia on the neonatal rabbit brain. Two-week-old Japanese white rabbits were assigned to three groups, hyperventilation (H group), ischemia (I group), or hypocapnia with ischemia (HI group) and then subjected for 1.5 h with simultaneous measurement of the mean arterial blood pressure (MABP) and intracranial Hb concentration changes. Marked reductions of PaCO2 and MABP were induced in the hyperventilation-loaded groups and the ischemia-loaded groups, respectively. The intracranial oxyhemoglobin and total Hb concentrations decreased slightly in the H group and markedly in the I and HI groups after the start of experimental protocols, although there were no statistical differences between the I and HI groups. Animals were killed at 24 h after experiments and then subjected to pathologic examination. Damaged neurons with shrunken cell bodies and nuclear changes were found on light microscopic examination, mainly in the pyramidal cell layer of the subiculum and cornu ammonis 1. The numerical density of damaged neurons was significantly higher in the HI group than those in the H or I groups (p < 0.05). These damaged neurons were positive on DNA nick end labeling. A DNA ladder was detected on electrophoresis with a DNA sample extracted from hippocampal tissue in the HI group, but not in the other two groups. On electron microscopic examination, not only condensation of the nucleus but also disruption of mitochondria and the cell membrane were detected. These results suggested that hypocapnia under hypotension might cause neuronal cell death in the hippocampus of neonatal rabbit. Not only ischemia but also a metabolic change induced by hypocapnia might contribute to this apoptotic neuronal cell damage.

Effect of hyperventilation on brain tissue oxygenation and cerebrovenous PO2 in rats

Previous studies have shown that cortical tissue oxygenation is impaired during hyperventilation. However, it is important to quantify the effect of hyperventilation on brain tissue PO(2) and cerebrovenous PO(2) simultaneously especially since cerebral venous oxygenation is often used to assess brain tissue oxygenation. The present study was designed to measure the sagittal sinus PO(2) (PvO(2)), brain tissue PO(2) in the thalamus (PtO(2)), and brain temperature (Bt) simultaneously during acute hyperventilation. Isoflurane-anesthetized rats were hyperventilated for 10 min during which time the arterial carbon dioxide tension (PaCO(2)) dropped from 40.3+4.9 mmHg to 23.5+2.8 mmHg. PtO(2) declined from 26.0+/-4.2 mmHg to 14.8+/-5.2 mmHg (P=0.004) while brain temperature decreased from 36.5+0.3 degrees C to 36.2+0.3 degrees C (P=0.02). However, PvO(2) and arterial blood pressure (BP) did not change during hyperventilation. The maintenance of PvO(2) when perfusion is thought to decline and PtO(2) decreases suggests that there may be a diffusion limitation, possibly due to selective perfusion. Therefore, cerebrovenous PO(2) may not give a good assessment of brain tissue oxygenation especially in conditions of acute hyperventilation, and deeper brain regions other than the cortex also show impaired tissue oxygenation following hyperventilation.

Preservation of intestinal microvascular Po2 during normovolemic hemodilution in a rat model

The effect of hemodilution on the intestinal microcirculatory oxygenation is not clear. The aim of this study was to determine the effect of moderate normovolemic hemodilution on intestinal microvascular partial oxygen pressure (Po2) and its relation to the mesenteric venous Po2 (Pmvo2). Normovolemic hemodilution was performed in 13 anesthetized male Wistar rats. Systemic hemodynamic and intestinal oxygenation parameters were monitored. Intestinal microvascular Po2 was measured by using the oxygen-dependent quenching of palladium-porphyrin phosphorescence. Hemodilution decreased systemic hematocrit from 45.0% +/- 0.1% (average +/- SEM) to 24.6% +/- 1.6%. The mesenteric blood flow did not change from baseline values, resulting in a linear decrease in intestinal oxygen delivery (from 2.77 +/- 0.15 to 1.42 +/- 0.11 mLxkg(-1)xmin(-1)). The intestinal oxygen extraction ratio increased significantly from 24% +/- 1% to 42% +/- 4%. Pmvo2 decreased significantly (from 57 +/- 2 to 41 +/- 2 mm Hg), but intestinal oxygen consumption and microvascular Po2 remained unaffected. As a result, the difference between microvascular Po2 and Pmvo2 increased significantly during hemodilution. Intestinal microvascular Po2 and oxygen consumption were well preserved during moderate normovolemic hemodilution. These results might be explained by the notion of others that hemodilution induces recruitment of capillaries, resulting in redistribution of the intestinal blood flow in favor of the microcirculation, which allows a more efficient extraction of oxygen. These findings further indicate that the use of venous Po2 values as indicators of microvascular oxygenation may be misleading.

Regional expression of heat shock protein 72 mRNA following mild and severe hypoxia in neonatal piglet brain

The present study examined the effect of hypoxia on expression of 72-kDa heat shock protein (hsp72) mRNA in the newborn brain. The studies were carried out in anesthetized and mechanically ventilated newborn piglets, age 3-5 days. Hypoxic insult was induced by decreasing the fraction of inspired oxygen (FiO2) from 21% to 6% or 10% for 1 h. Oxygen pressure in the microvasculature of the cortex (cortical pO2) was measured by oxygen dependent quenching of the phosphorescence of phosphor dissolved in blood. Following the two hours of normoxic recovery, regional expression of the 72-kDa heat shock protein (hsp72) mRNA was determined using in situ hybridization and autoradiography. Two grades of hypoxia were studied. Mild hypoxia (cortical pO2 = 10-30 mm Hg) induced the expression of hsp72 mRNA predominantly in the subcortical white matter. In individual animals of this group, the extent of expression varied from isolated regions to widespread involvement of the white matter. Severe hypoxia (cortical pO2 = 3-10 mm Hg) induced the expression of hsp72 mRNA in both white and gray matter regions, with strong expression occurring in the cerebral cortex of individual animals. The present results indicate that immature white matter is more sensitive than gray matter to the hypoxia induced expression of hsp72 mRNA. Further, increased expression of hsp72 mRNA may be an indicator of a pathologic degree of hypoxic stress, and the observed increase may indicate that in the newborn brain the immature white matter is particularly sensitive to injury by hypoxia-ischemia and reperfusion.