Fluctuations In pO Research Articles

Brain tissue oxygen dynamics while mimicking the functional deficiency of interneurons.

The dynamic interaction between excitatory and inhibitory activity in the brain is known as excitatory-inhibitory balance (EIB). A significant shift in EIB toward excitation has been observed in numerous pathological states and diseases, such as autism or epilepsy, where interneurons may be dysfunctional. The consequences of this on neurovascular interactions remains to be elucidated. Specifically, it is not known if there is an elevated metabolic consumption of oxygen due to increased excitatory activity. To investigate this, we administered microinjections of picrotoxin, a gamma aminobutyric acid (GABA) antagonist, to the rabbit cortex in the awake state to mimic the functional deficiency of GABAergic interneurons. This caused an observable shift in EIB toward excitation without the induction of seizures. We used chronically implanted electrodes to measure both neuronal activity and brain tissue oxygen concentrations (PO2) simultaneously and in the same location. Using a high-frequency recording rate for PO2, we were able to detect two important phenomena, (1) the shift in EIB led to a change in the power spectra of PO2 fluctuations, such that higher frequencies (8-15 cycles per minute) were suppressed and (2) there were brief periods (dips with a duration of less than 100 ms associated with neuronal bursts) when PO2 dropped below 10 mmHg, which we defined as the threshold for hypoxia. The dips were followed by an overshoot, which indicates either a rapid vascular response or decrease in oxygen consumption. Our results point to the essential role of interneurons in brain tissue oxygen regulation in the resting state.

Impact of Temporal Heterogeneity of Acute Hypoxia on the Radiation Response of Experimental Tumors.

Tumor hypoxia is a major factor inducing resistance to radiotherapy. Spatial limitation in oxygen (O2) diffusion usually leads to chronic hypoxia, whereas temporary shut-down of perfusion or fluctuations in red blood cell flux can cause acute hypoxia. Since the role of temporal heterogeneity of pO2 in acute hypoxia during radiotherapy remains unclear, this study focuses on analyzing the influence of temporal heterogeneity of tumor hypoxia upon radiotherapy by modeling the temporal variance of acute hypoxia. The computational simulation was conducted on digital 2D tumor phantoms. The O2 diffusion and consumption within the tumor tissues were calculated using the reaction-diffusion equation. A total of nine experimental tumor lines (FaDu, GL, C3H, RIF, SCCVII, KHT, MEF, MTG, HT29) were modeled according to known pO2 distributions. Each tumor line was first simulated 36 times with various temporal heterogeneities (dynamic hypoxia) and once again without temporal heterogeneity (static hypoxia). Temporal pO2 fluctuations were modeled according to known red blood cell (RBC) fluxes. All tumor phantoms were irradiated with 30 fractions of 2 Gy. Cell survival was calculated as a function of pO2 and radiation dose via linear quadratic model. The simulation results indicate that the temporal heterogeneity varies with different tumor types, and tumor line HT29 shows the most significant impact of temporal heterogeneity upon the treatment effect. The ratio between the surviving fractions without and with temporal variance ranges from 1.44 to 6.28. Given the same mean pO2, the fraction of killed tumor cells in dynamic hypoxia is higher than in static hypoxia. A temporal heterogeneity index (THI) denoting normalized average pO2 temporal variance is proposed. The results show that for similar mean tumor pO2, a strong inverse correlation between THI and the surviving fraction is observed for each tumor line. THI is highly proportional to the fraction of acute hypoxia and to the RBC flux. The proposed THI corresponds well to the fraction of acute hypoxia.

OP21 - Urine partial oxygen pressure as early marker of acute kidney injury after paediatric cardiac surgery

Acute kidney injury (AKI) is the common and serious complication which occurs in open-heart surgery (HS) using cardiopulmonary bypass (CPB). The renal medulla is one the first affected tissues from decreased perfusion during the procedure of cardiopulmonary bypass. Partial pressure of the urine taken from the renal pelvis is related to the perfusion of renal medulla was shown. Neutrophil gelatinase-associated lipocalin (NGAL) is accepted as an early biomarker and a predictor of AKI in a variety of clinical settings. We sought to evaluate the relationship between urine pO2 and kidney function-injury biomarkers and contribution of glycocalyx derangement after paediatric cardiac surgery. Twenty paediatric patients with normal preoperative renal function as assessed by the creatinine level, undergoing HS with CPB were included in this prospective clinical observational study. Besides routine hemodynamics and cerebral tissue oxygen saturation monitoring, patients were assessed for the partial pressure of the bladder urine (PuO2) and renal tissue oxygen saturation monitoring by NIRS intraoperatively (after induction, before CPB, after CPB) and postoperative (admission of ICU, 3rd, 6th, 9th, 12th in ICU) periodically. Additionally, syndecan-1, hyaluronan, cystatin-c, and NGAL levels were measured in plasma and urine samples. PuO2 was found wavy throughout the operation (p>0.05) and increased at the 6th (p<0.05), 9th (p<0.001) and 12th (p<0.001) hours of ICU admission. Renal tissue oxygen saturation (rSO2) monitored by NIRS was found stable throughout the operation and postoperative 12 hours. Plasma NGAL and hyaluronan levels were significantly found to rise at the 12th hour of ICU admission compared to the anaesthesia induction (p<0.001); meanwhile syndecan and cystatin-c were remained constant (p>0.05). A significant correlation between PuO2 and other measurements as renal tissue oxygen saturation (rSO2), arterial blood lactate, NGAL and hyaluronan levels was not determined at any time points. Coexistence of urine pO2 fluctuations and increment of plasma NGAL-hyaluronan levels suggest that urine pO2 could be early marker of kidney injury in paediatric patients undergoing HS. Further investigation in the role of urine pO2 plays in ischemic organ injury and its potential diagnostic implications is needed. 1. Clin Exp Pharmacol Physiol. 2008 Sep;35(9):1109-12

Spontaneous Fluctuations of PO2 in the Rabbit Somatosensory Cortex.

In many tissues, PO2 fluctuates spontaneously with amplitudes of a few mmHg. Here we further characterized these oscillations. PO2 recordings were made from the whisker barrel cortex of six rabbits with acutely or chronically placed polarographic electrodes. Measurements were made while rabbits were awake and while anesthetized with isoflurane, during air breathing, and during 100% oxygen inspiration. In awake rabbits, 90% of the power was between 0 and 20 cycles per minute (cpm), not uniformly distributed over this range, but with a peak frequently near 10 cpm. This was much slower than heart or respiratory rhythms and is similar to the frequency content observed in other tissues. During hyperoxia, total power was higher than during air-breathing, and the dominant frequencies tended to shift toward lower values (0-10 cpm). These observations suggest that at least the lower frequency fluctuations represent efforts by the circulation to regulate local PO2. There were no consistent changes in total power during 0.5 or 1.5% isoflurane anesthesia, but the power shifted to lower frequencies. Thus, both hyperoxia and anesthesia cause characteristic, but distinct, changes in spontaneous fluctuations. These PO2 fluctuations may be caused by vasomotion, but other factors cannot be ruled out.

A dynamic model of oxygen transport from capillaries to tissue with moving red blood cells.

Most oxygen required to support the energy needs of vertebrate tissues is delivered by diffusion from microvessels. The presence of red blood cells (RBCs) makes blood flow in the microcirculation highly heterogeneous. Additionally, flow regulation mechanisms dynamically respond to changes in tissue energy demand. These spatiotemporal variations directly affect the supply of oxygen to parenchymal cells. Due to various limiting assumptions, current models of oxygen transport cannot fully capture the consequences of complex hemodynamic effects on tissue oxygenation and are often not suitable for studying unsteady phenomena. With our new approach based on moving RBCs, the impact of blood flow heterogeneity on oxygen partial pressure (Po2) in the tissue can be quantified. Oxygen transport was simulated using parachute-shaped solid RBCs flowing through a capillary. With the use of a conical tissue domain with radii 19 and 13 μm, respectively, our computations indicate that Po2 at the RBC membrane exceeds Po2 between RBCs by 30 mmHg on average and that the mean plasma Po2 decreases by 9 mmHg over 50 μm. These results reproduce well recent intravascular Po2 measurements in the rodent brain. We also demonstrate that instantaneous variations of capillary hematocrit cause associated fluctuations of tissue Po2. Furthermore, our results suggest that homogeneous tissue oxygenation requires capillary networks to be denser on venular side than on arteriolar side. Our new model for oxygen transport will make it possible to quantify in detail the effects of blood flow heterogeneity on tissue oxygenation in realistic capillary networks.

Cxs and Panx- hemichannels in peripheral and central chemosensing in mammals.

Connexins (Cxs) and Pannexins (Panx) form hemichannels at the plasma membrane of animals. Despite their low open probability under physiological conditions, these hemichannels release signaling molecules (i.e., ATP, Glutamate, PGE2) to the extracellular space, thus subserving several important physiological processes. Oxygen and CO2 sensing are fundamental to the normal functioning of vertebrate organisms. Fluctuations in blood PO2, PCO2 and pH are sensed at the carotid bifurcations of adult mammals by glomus cells of the carotid bodies. Likewise, changes in pH and/or PCO2 of cerebrospinal fluid are sensed by central chemoreceptors, a group of specialized neurones distributed in the ventrolateral medulla (VLM), raphe nuclei, and some other brainstem areas. After many years of research, the molecular mechanisms involved in chemosensing process are not completely understood. This manuscript will review data regarding relationships between chemosensitive cells and the expression of channels formed by Cxs and Panx, with special emphasis on hemichannels.

TEMPORAL DYNAMICS OF OXYGEN CONCENTRATIONS IN A PEAT-PERLITE SUBSTRATE

Anoxic conditions in soilless substrates have been implemented in disease development, reduced growth rates, and denitrification, but there is little quantitative information on oxygen concentrations in soilless substrates. We measured the partial pressure of oxygen (pO2) in peat-perlite substrate planted with petunia (Petunia × hybrid). There are distinct diurnal fluctuations in substrate pO2, and these can be largely explained by changes in substrate temperature, which increase the amount of water vapor in the air in the substrate, diluting oxygen and other gases. Barometric pressure (pair) and substrate volumetric water content (Θ) also affected substrate pO2. Substrate pO2 decreased with decreasing pair and with increasing Θ. Photosynthetic photon flux had a highly significant, but small effect on pO2. Substrate density had no significant effect on pO2. Overall, substrate pO2 was between 19.1 and 20.6 kPa, even after watering the substrate to container capacity. The high air-filled porosity of the substrate (approximately 0.40 m3•m-3 after irrigation) may have prevented the development of anoxic conditions. Since such high levels of pO2 are unlikely to induce any detrimental anoxic effects on plants, our data do not provide any supporting evidence for the idea that anoxia is an important potential problem in peat-perlite substrates.

Antiangiogenic Agent Sunitinib Transiently Increases Tumor Oxygenation and Suppresses Cycling Hypoxia

Structural and functional abnormalities in tumor blood vessels impact the delivery of oxygen and nutrients to solid tumors, resulting in chronic and cycling hypoxia. Although chronically hypoxic regions exhibit treatment resistance, more recently it has been shown that cycling hypoxic regions acquire prosurvival pathways. Angiogenesis inhibitors have been shown to transiently normalize the tumor vasculatures and enhance tumor response to treatments. However, the effect of antiangiogenic therapy on cycling tumor hypoxia remains unknown. Using electron paramagnetic resonance imaging and MRI in tumor-bearing mice, we have examined the vascular renormalization process by longitudinally mapping tumor partial pressure of oxygen (pO(2)) and microvessel density during treatments with a multi-tyrosine kinase inhibitor sunitinib. Transient improvement in tumor oxygenation was visualized by electron paramagnetic resonance imaging 2 to 4 days following antiangiogenic treatments, accompanied by a 45% decrease in microvessel density. Radiation treatment during this time period of improved oxygenation by antiangiogenic therapy resulted in a synergistic delay in tumor growth. In addition, dynamic oxygen imaging obtained every 3 minutes was conducted to distinguish tumor regions with chronic and cycling hypoxia. Sunitinib treatment suppressed the extent of temporal fluctuations in tumor pO(2) during the vascular normalization window, resulting in the decrease of cycling tumor hypoxia. Overall, the findings suggest that longitudinal and noninvasive monitoring of tumor pO(2) makes it possible to identify a window of vascular renormalization to maximize the effects of combination therapy with antiangiogenic drugs.

Imaging Cycling Tumor Hypoxia

Cycling hypoxia is now a well-recognized phenomenon in animal and human solid tumors. Cycling hypoxia can exist more than 100-μm distances from a microvessel, and some of these regions have been shown to exist adjacent to normal tissue. Fluctuations in pO(2) of approximately 20 mm Hg can occur with periodicities of minutes to hours and even days. These fluctuations have been attributed to changes in erythrocyte flux, perfusion, and also development of newer vascular networks. Cycling hypoxia has been shown to induce the expression of hypoxia-inducible transcription factor-1α (HIF-1α) and also confer tumor cells and tumor vascular endothelial cells with enhanced prosurvival pathways, making tumors less responsive to radiation and chemotherapy. Imaging of cycling hypoxia in tumors can provide capabilities to help plan appropriate treatment, by taking into account the magnitude and frequency of fluctuations and also their locations adjacent to normal tissue. Electron paramagnetic resonance imaging (EPRI) provides the ability to distinguish chronic and cycling hypoxic regions and has the required spatial and temporal resolutions to provide quantitative maps of tumor pO(2). EPRI can serve as a valuable tool in examining tumor pO(2) longitudinally in response to treatment and in an experimentally chosen time window to spatially map fluctuations in pO(2) noninvasively in animal models of implanted or orthotopic tumors, with a potential for human applications.

Noninvasive mapping of spontaneous fluctuations in tumor oxygenation using MRI

Acute hypoxia (transient cycles of hypoxia-reoxygenation) is known to occur in solid tumors and may be a poorly appreciated therapeutic problem as it can be associated with resistance to radiation therapy, impaired delivery of chemotherapeutic agents, or metastasis development. The objective of the present study was to use MR 19F relaxometry maps to analyze the spontaneous fluctuations of partial pressure of oxygen (pO2) over time in experimental tumors. The pO2 maps were generated after direct intratumoral administration of a fluorine compound (hexafluorobenzene) whose relaxation rate (1/T1) is proportional to the % O2. The authors used a SNAP inversion-recovery sequence at 4.7 T to acquire parametric images of the T1 relaxation time with a high spatial and temporal resolution. Homemade routines were developed to perform regions of interest analysis, as well as pixel by pixel analysis of pO2 over time. The authors were able to quantify and probe the heterogeneity of spontaneous fluctuations in tumor pO2: (i) Spontaneous fluctuations in pO2 occurred regardless of the basal oxygenation state (i.e., both in oxygenated and in hypoxic regions) and (ii) spontaneous fluctuations occurred at a rate of 1 cycle/12-47 min. For validation, the analysis was performed in dead mice for which acute changes did not occur. The authors thereby demonstrated that 19F MRI technique is sensitive to acute change in pO2 in tumors. This is the first approach that allows quantitative minimally invasive measurement of the spontaneous fluctuations of tumor oxygenation using a look-locker approach (e.g., SNAP IR). This approach could be an important tool to characterize the phenomenon of tumor acute hypoxia, to understand its physiopathology, and to improve therapies.

Low-Field Magnetic Resonance Imaging to Visualize Chronic and Cycling Hypoxia in Tumor-Bearing Mice

Tumors exhibit fluctuations in blood flow that influence oxygen concentrations and therapeutic resistance. To assist therapeutic planning and improve prognosis, noninvasive dynamic imaging of spatial and temporal variations in oxygen partial pressure (pO(2)) would be useful. Here, we illustrate the use of pulsed electron paramagnetic resonance imaging (EPRI) as a novel imaging method to directly monitor fluctuations in oxygen concentrations in mouse models. A common resonator platform for both EPRI and magnetic resonance imaging (MRI) provided pO(2) maps with anatomic guidance and microvessel density. Oxygen images acquired every 3 minutes for a total of 30 minutes in two different tumor types revealed that fluctuation patterns in pO(2) are dependent on tumor size and tumor type. The magnitude of fluctuations in pO(2) in SCCVII tumors ranged between 2- to 18-fold, whereas the fluctuations in HT29 xenografts were of lower magnitude. Alternating breathing cycles with air or carbogen (95% O(2) plus 5% CO(2)) distinguished higher and lower sensitivity regions, which responded to carbogen, corresponding to cycling hypoxia and chronic hypoxia, respectively. Immunohistochemical analysis suggests that the fluctuation in pO(2) correlated with pericyte density rather than vascular density in the tumor. This EPRI technique, combined with MRI, may offer a powerful clinical tool to noninvasively detect variable oxygenation in tumors.

Abstract 461: Identification of COX-2 as a major actor of the transcriptomic adaptation of endothelial and tumor cells to cyclic hypoxia: impact on angiogenesis and metastases

Abstract Purpose: Cyclic hypoxia in tumors originates from heterogeneities in red blood cell flux and influences tumor cells but also endothelial cells lining tumor blood vessels. Whether pO2 fluctuations, in particular transient reoxygenation periods, alter the well-known hypoxia-inducible factor (HIF)-dependent gene program is largely unknown. Experimental design: We compared the transcriptomic profiles of endothelial and tumor cells exposed to cyclic hypoxia vs continuous hypoxia to uncover a possible differential impact on angiogenesis and metastases. Results: Microarray analyses identified early genes that were selectively induced by cyclic hypoxia in endothelial cells. Among them, we focused on PTGS2 since the observed increase in mRNA expression led to a significant increase in the expression and the activity of COX-2 (the protein product of PTGS2). HIF-1α was shown to be stabilized by cyclic hypoxia (despite reoxygenation periods) and to favor COX-2 induction, as validated by the use of echinomycin and HIF-1α targeting siRNA. Using a specific COX-2 inhibitor and a dedicated COX-2-targeting siRNA, we documented that COX-2 accounted for the higher endothelial cell survival and angiogenic potential conferred by cyclic hypoxia. Cyclic hypoxia also led to a preferential COX-2 induction in tumor cells and, contrary to continuous hypoxia, fostered a higher metastatic take of pre-challenged tumor cells. Conclusions: Our study documents that PTGS2/COX-2 is part of a cyclic hypoxia gene signature and largely accounts for the unique phenotype of endothelial and tumor cells exposed to fluctuations in pO2, thereby offering new perspectives for the clustering of tumors expressing COX-2 together with other cyclic hypoxia-responsive genes. Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 461.

Identification of Cyclooxygenase-2 as a Major Actor of the Transcriptomic Adaptation of Endothelial and Tumor Cells to Cyclic Hypoxia: Effect on Angiogenesis and Metastases

Cyclic hypoxia in tumors originates from heterogeneities in RBC flux and influences not only tumor cells but also endothelial cells lining tumor blood vessels. Whether pO(2) fluctuations, particularly transient reoxygenation periods, alter the well-known hypoxia-inducible factor (HIF)-dependent gene program is largely unknown. We compared the transcriptomic profiles of endothelial and tumor cells exposed to cyclic hypoxia versus continuous hypoxia to uncover a possible differential effect on angiogenesis and metastases. Microarray analyses identified early genes that were selectively induced by cyclic hypoxia in endothelial cells. Among them, we focused on PTGS2 because the observed increase in mRNA expression led to a significant increase in the expression and activity of cyclooxygenase-2 (COX-2; the protein product of PTGS2). HIF-1alpha was shown to be stabilized by cyclic hypoxia (despite reoxygenation periods) and to favor COX-2 induction as validated by the use of echinomycin and HIF-1alpha targeting small interfering RNA. Using a specific COX-2 inhibitor and a dedicated COX-2 targeting small interfering RNA, we documented that COX-2 accounted for the higher endothelial cell survival and angiogenic potential conferred by cyclic hypoxia. Cyclic hypoxia also led to a preferential COX-2 induction in tumor cells and, contrary to continuous hypoxia, fostered a higher metastatic take of prechallenged tumor cells. Our study documents that PTGS2/COX-2 is part of a cyclic hypoxia gene signature and largely accounts for the unique phenotype of endothelial and tumor cells exposed to fluctuations in pO(2), thereby offering new perspectives for the clustering of tumors expressing COX-2 together with other cyclic hypoxia-responsive genes.

The Pervasive Presence of Fluctuating Oxygenation in Tumors

Tumor hypoxia is a persistent obstacle for traditional therapies in solid tumors. Strategies for mitigating the effects of hypoxic tumor cells have been developed under the assumption that chronically hypoxic tumor cells were the central cause of treatment resistance. In this study, we show that instabilities in tumor oxygenation are a prevalent characteristic of three tumor lines and previous characterization of tumor hypoxia as being primarily diffusion-limited does not accurately portray the tumor microenvironment. Phosphorescence lifetime imaging was used to measure fluctuations in vascular pO(2) in rat fibrosarcomas, 9L gliomas, and R3230 mammary adenocarcinomas grown in dorsal skin-fold window chambers (n = 6 for each tumor type) and imaged every 2.5 minutes for a duration of 60 to 90 minutes. O(2) delivery to tumors is constantly changing in all tumors, resulting in continuous reoxygenation events throughout the tumor. Vascular pO(2) maps show significant spatial heterogeneity at each time point, as well as between time points. The fluctuations in oxygenation occur with a common periodicity within and between tumors, suggesting a common mechanism, but have tumor type-dependent spatial patterns. The widespread presence of fluctuations in tumor oxygenation has broad ranging implications for tumor progression, stress response, and signal transduction, which are altered by oxygenation/reoxygenation events.

Erythrocyte-associated transients in capillary Po2: an isovolemic hemodilution study in the rat spinotrapezius muscle

Mathematical simulations of oxygen delivery to tissue from capillaries that take into account the particulate nature of blood flow predict the existence of oxygen tension (Po(2)) gradients between erythrocytes (RBCs). As RBCs and plasma alternately pass an observation point, these gradients are manifested as rapid fluctuations in Po(2), also known as erythrocyte-associated transients (EATs). The impact of hemodilution on EATs and oxygen delivery at the capillary level of the microcirculation has yet to be elucidated. Therefore, in the present study, phosphorescence quenching microscopy was used to measure EATs and Po(2) in capillaries of the rat spinotrapezius muscle at the following systemic hematocrits (Hct(sys)): normal (39%) and after moderate (HES1; 27%) or severe (HES2; 15%) isovolemic hemodilution using a 6% hetastarch solution. A 532-nm laser, generating 10-micros pulses concentrated onto a 0.9-microm spot, was used to obtain plasma Po(2) values 100 times/s at points along surface capillaries of the muscle. Mean capillary Po(2) (Pc(O(2)); means +/- SE) significantly decreased between conditions (normal: 56 +/- 2 mmHg, n = 45; HES1: 47 +/- 2 mmHg, n = 62; HES2: 27 +/- 2 mmHg, n = 52, where n = capillary number). In addition, the magnitude of Po(2) transients (DeltaPo(2)) significantly decreased with hemodilution (normal: 19 +/- 1 mmHg, n = 45; HES1: 11 +/- 1 mmHg, n = 62; HES2: 6 +/- 1 mmHg, n = 52). Results suggest that the decrease in Pc(O(2)) and DeltaPo(2) with hemodilution is primarily dependent on Hct(sys) and subsequent microvascular compensations.

The Implications of Arterial Po2 Oscillations for Conventional Arterial Blood Gas Analysis

In a surfactant-depletion model of lung injury, tidal recruitment of atelectasis and changes in shunt fraction lead to large Pao2 oscillations. We investigated the effect of these oscillations on conventional arterial blood gas (ABG) results using different sampling techniques in ventilated rabbits. In each rabbit, 5 different ventilator settings were studied, 2 before saline lavage injury and 3 after lavage injury. Ventilator settings were altered according to 5 different goals for the amplitude and mean value of brachiocephalic Pao2 oscillations, as guided by a fast responding intraarterial probe. ABG collection was timed to obtain the sample at the peak or trough of the Pao2 oscillations, or over several respiratory cycles. Before lung injury, oscillations were small and sample timing did not influence Pao2. After saline lavage, when Po2 fluctuations measured by the indwelling arterial Po2 probe confirmed tidal recruitment, Pao2 by ABG was significantly higher at peak (295 +/- 130 mm Hg) compared with trough (74 +/- 15 mm Hg) or mean (125 +/- 75 mm Hg). In early, mild lung injury after saline lavage, Pao2 can vary markedly during the respiratory cycle. When atelectasis is recruited with each breath, interpretation of changes in shunt fraction, based on conventional ABG analysis, should account for potentially large respiratory variations in arterial Po2.

Direct Demonstration of Instabilities in Oxygen Concentrations within the Extravascular Compartment of an Experimental Tumor

To test the hypothesis that temporal variations in microvessel red cell flux cause unstable oxygen levels in tumor interstitium, extravascular oxygenation of R3230Ac mammary tumors grown in skin-fold window chambers was measured using recessed tip polarographic microelectrodes. Red cell fluxes in microvessels surrounding pO2 measurement locations were measured using fluorescently labeled red cells. Temporal pO2 instability was observed in all experiments. Median pO2 was inversely related to radial distance from microvessels. Transient fluctuations above and below 10 mm Hg were consistently seen, except in one experiment near the oxygen diffusion distance limit (140 microm) where pO2 fluctuations were <2 mm Hg and median pO2 was <5 mm Hg. Vascular stasis was not seen in these experiments. These results show that fluctuations in red cell flux, as opposed to vascular stasis, can cause temporal variations in pO2 that extend from perivascular regions to the maximum oxygen diffusion distance.

Fluctuations inpO2in Irradiated Human Melanoma Xenografts

Several studies have demonstrated that untreated tumors may show significant fluctuations in tissue oxygen tension (pO(2)). Radiation treatment may induce changes in the tumor microenvironment that alter the pO(2) fluctuation pattern. The purpose of the present study was to investigate whether pO(2) fluctuations may also occur in irradiated tumors. A-07 human melanoma xenografts were irradiated with single doses of 0, 5 or 10 Gy. Fluctuations in pO(2) were recorded with OxyLite probes prior to irradiation and 24 and 72 h after the radiation exposure. Radiation-induced changes in the tumor microenvironment (i.e. blood perfusion and extracellular volume fraction) were assessed by dynamic contrast-enhanced magnetic resonance imaging. Seventy-two hours after 10 Gy, tumor blood perfusion had decreased to approximately 40% of that prior to irradiation, whereas the extracellular volume fraction had increased by approximately 25%. Fluctuations in pO(2) were seen in most tumors, irrespective of radiation dose and time after irradiation. The mean pO(2), the number of fluctuations around the mean pO(2), the number of fluctuations around threshold pO(2) values of 1, 2, 3, 5, 7 and 10 mmHg, and the amplitude of the fluctuations were determined for each pO(2) trace. No significant differences were detected between irradiated and unirradiated tumors. The results showed that pO(2) fluctuations may occur in irradiated tumors and that the pO(2) fluctuation pattern in A-07 tumors exposed to 5 or 10 Gy is similar to that in untreated tumors. Consequently, these doses did not induce changes in the tumor microenvironment that were sufficient to cause detectable alterations in the pO(2) fluctuation pattern.

Fluctuations in pO 2 in poorly and well-oxygenated spontaneous canine tumors before and during fractionated radiation therapy

Background and purpose The spatial heterogeneity in oxygen tension (pO 2) in tumor tissue has been studied extensively, whereas, the information about the temporal heterogeneity is sparse. The purpose of the present study was to search for pO 2 fluctuations in untreated and irradiated spontaneous canine tumors, and to investigate whether there is a relationship between overall tumor oxygenation status and pO 2 fluctuation pattern. Patients and methods Six dogs scheduled for radiation therapy of head and neck cancer were included in the study. The primary tumors were irradiated with 18 fractions of 3 Gy. Eppendorf polarographic electrodes and OxyLite fluorescence probes were used to measure overall oxygenation status and pO 2 fluctuation pattern, respectively. Tissue pO 2 was recorded at three subsequent days prior to treatment, and immediately before radiation fraction 4, 7, and 10. Results Overall oxygenation status differed substantially among the tumors. Radiation therapy had no consistent effect on overall oxygenation status. Fluctuations in pO 2 were detected in untreated as well as irradiated tumors, and independent of whether the tumors were poorly or well oxygenated. Conclusions Fluctuations in pO 2 can occur in untreated and irradiated spontaneous canine tumors. There is no correlation between pO 2 fluctuation pattern and overall tumor oxygenation status.

Intra- and postoperative monitoring of transplanted flaps. Measurement of the partial pressure of oxygen in tissue

According to Schmelzeisen et al. (1996), the failure rate for microvascular free flaps is 5%. While surface tissue oxygenation can be assessed clinically, if necessary by a puncture, the oxygen supply to deeper areas mostly cannot be checked. We therefore wished to find whether measurement of tissue pO(2) would prove to be an objective and practical technique that could be used for continuous and accurate intra- and postoperative evaluation of flap perfusion. A Clark-type microcatheter was used intra- and postoperatively to monitor tissue pO(2) in 5 pedicled pectoralis major flaps and 32 free revascularized flaps (9 jejunal flaps, 5 latissimus dorsi flaps, 6 radial forearm flaps and 12 scapular flaps). The mean values for tissue pO(2) were significantly lower in pedicle grafts than in free revascularized flaps. Within in each flap group the pO(2) values measured did not vary significantly over an observation period of up to 77.2 h after transplantation. Continuous measurement of tissue pO(2) by means of a Clark-type microcatheter combined with clinical examination constitutes a reliable method of monitoring tissue oxygenation in pedicle grafts and free revascularized flaps during the intra- and postoperative phases. Analysis of small and of wide fluctuations in pO(2) values may help in the diagnosis of early arterial and venous obstructions in flaps and may in the future result in new insights into the tissue oxygenation in surgical flaps allowing some alleviation of the problems currently experienced in clinical monitoring.