Local PO2 Research Articles

Tissue oxygenation in sepsis; new insights from in vivo EPR.

Nitric oxide (NO) is a key mediator in the maldistribution of oxygen by tissue and organ dysfunction observed in sepsis. Despite this, few techniques are capable of measuring these parameters directly in vivo. We describe here several techniques that have been developed by our group to address this directly by in vivo EPR in animal models of sepsis. Oxygen-sensitive materials can be implanted or administered and report on local tissue pO2. Spin trapping of NO can simultaneously report on tissue NO content. Repeat measures of these parameters can be made directly from a defined tissue site, allowing development of new models and experiments to study the defects in tissue and organ function seen in sepsis.

Interstitial pO2 in Ischemic Penumbra and Core are Differentially Affected following Transient Focal Cerebral Ischemia in Rats

Stroke causes heterogeneous changes in tissue oxygenation, with a region of decreased blood flow, the penumbra, surrounding a severely damaged ischemic core. Treatment of acute ischemic stroke aims to save this penumbra before its irreversible damage by continued ischemia. However, effective treatment remains elusive due to incomplete understanding of processes leading to penumbral death. While oxygenation is central in ischemic neuronal death, it is unclear exactly what actual changes occur in interstitial oxygen tension (pO2) in ischemic regions during stroke, particularly the penumbra. Using the unique capability of in vivo electron paramagnetic resonance (EPR) oximetry to measure localized interstitial pO2, we measured both absolute values, and temporal changes of pO2 in ischemic penumbra and core during ischemia and reperfusion in a rat model. Ischemia rapidly decreased interstitial pO2 to 32% +/- 7.6% and 4% +/- 0.6% of pre-ischemic values in penumbra and core, respectively 1 hour after ischemia. Importantly, whilst reperfusion restored core pO2 close to its pre-ischemic value, penumbral pO2 only partially recovered. Hyperoxic treatment significantly increased penumbral pO2 during ischemia, but not in the core, and also increased penumbral pO2 during reperfusion. These divergent, important changes in pO2 in penumbra and core were explained by combined differences in cellular oxygen consumption rates and microcirculation conditions. We therefore demonstrate that interstitial pO2 in penumbra and core is differentially affected during ischemia and reperfusion, providing new insights to the pathophysiology of stroke. The results support normobaric hyperoxia as a potential early intervention to save penumbral tissue in acute ischemic stroke.

606 純音刺激に対する一次聴覚野の神経活動と局所脳血流量および局所酸素分圧の関係

606 純音刺激に対する一次聴覚野の神経活動と局所脳血流量および局所酸素分圧の関係

Biphasic changes in tissue partial pressure of oxygen closely related to localized neural activity in guinea pig auditory cortex.

An understanding of the local changes in cerebral oxygen content accompanying functional brain activation is critical for making a valid signal interpretation of hemodynamic-based functional brain imaging. However, spatiotemporal relations between changes in tissue partial pressure of oxygen (Po2) and induced neural activity remain incompletely understood. To characterize the local Po2 response to the given neural activity, the authors simultaneously measured tissue Po2 and neural activity in the identical region of guinea pig auditory cortex with an oxygen microelectrode (tip < 10 microm) and optical recording with voltage-sensitive dye (RH 795). In addition, a laser displacement gauge and a laser-Doppler flowmeter were used to monitor the spatial displacement and regional cerebral blood flow, respectively, in the Po2 measurement region. In the activated region, tissue Po2 initially decreased during the approximately 3-seconds after the onset of acoustic stimuli, and then increased during the next approximately 5 seconds. Such biphasic changes are consistently found in cortical layers I to IV. In addition, amplitude of the biphasic change was closely related to detected peak height of the optical signal changes. The results suggest that the initial decrease in tissue Po2 is coupled to the induced neural activity and depends on response time of local increase in cerebral blood flow.

Soluble flt-1 and the angiopoietins in the development and regulation of placental vasculature.

One of the key roles of the placenta is to mediate exchange of oxygen, nutrients and waste products between the maternal and fetal circulations. While some nutrients are actively taken up, oxygen is passively transported (i.e. it moves down a simple concentration gradient). To ensure an adequate supply as gestation progresses, the vascular structures in the human placenta remodel to facilitate this transfer. Under conditions in which oxygen is limited, at high altitude for example, these adaptations become more pronounced. In placentae obtained from high altitude the mean diameter of vessels is increased and the number of perivascular cells (pericytes) is reduced. Pericytes play an important role in modulating endothelial cell function and in their absence the endothelial cells are sensitive to growth factor withdrawal. The vascular remodelling that occurs in normal pregnancy is in part mediated by soluble factors (some produced by pericytes) and the level of these may in turn be regulated by local oxygen tension. We have shown that the mRNAs encoding both angiopoietins are present in the placenta and both are regulated by local PO2. Ang-2 mRNA transcription is increased under reduced oxygen and the stability of Ang-1 mRNA is reduced under similar conditions. Thus the ratio of mRNAs encoding these antagonistic factors changes (in favour of Ang-2), in the placenta in response to a reduction in PO2. The adaptations seen in the placenta and the mechanism by which they are achieved may be generally applicable.

Soluble flt and the angiopoietins in the development and regulation of placental vasculature

One of the key roles of the placenta is to mediate exchange of oxygen, nutrients and waste products between the maternal and fetal circulations. While some nutrients are actively taken up, oxygen is passively transported (i.e. it moves down a simple concentration gradient). To ensure an adequate supply as gestation progresses the vascular structures in the human placenta remodel to facilitate this transfer. Under conditions where oxygen is limited, at high altitude for example, these adaptations become more pronounced. In placentae obtained from high altitude the mean diameter of vessels is increased and the number of perivascular cells (pericytes) is reduced. Pericytes play an important role in modulating endothelial cell function and in their absence the endothelial cells are sensitive to growth factor withdrawal. The vascular remodelling that occurs in normal pregnancy is in part mediated by soluble factors (some produced by pericytes) and the level of these may in turn be regulated by local oxygen tension. We have shown that the mRNAs encoding both angiopoietins are present in the placenta and both are regulated by local pO2. Ang‐2 mRNA transcription is increased under reduced oxygen and the stability of Ang‐1 mRNA is reduced under similar conditions. Thus the ratio of mRNAs encoding these antagonistic factors changes (in favour of Ang‐2) in the placenta in response to a reduction in pO2. The adaptations seen in the placenta and the mechanism by which they are achieved may be generally applicable.

The present status of the mechanical hypothesis for chemoreceptor stimulation.

Reasons are given to show why the transmitter based hypothesis for the stimulation of chemoreceptors needs to be reviewed. On the other hand evidence is presented to show that chemoreceptors can be stimulated by various mechanical stimuli and how the local PO2 can be sensed by the type II cell which by getting mechanically deformed causes this cell to shrink. This shrinkage is transmitted to the generator region of the nerve terminal thereby leading to the production of propagated impulses at the regenerative region thus making the whole process of generation of information about the local PO2, similar to the generation of sensory information by other sensory receptors.

Comparison of local measurement of cerebral metabolism and to cerebral PvO2 during alterations in intracranial pressure, PaCO2 and arterial pressure--an experimental study in goat.

to assess the changes in local brain PO2, PCO2, pH (PO2br, PCO2br, pHbr) measured by a intraparenchymal probe (Neurotrend, Codeman) and compare them to simultaneous recording of cerebral PvO2 and blood flow (CBF). Arterial, venous longitudinal sinus blood samples and CBF were analyzed in 8 adult anesthetized, ventilated goats. Three step increase of intracranial pressure (ICP) (16, 22, 29 mm Hg) were performed by inflation of an epidural balloon. At each ICP level, similar changes in MAP and in PaCO2 were performed. At constant PaCO2 and MAP, balloon inflation induced a fast response: a decrease of PO2br, PCO2br and pHbr (starting 14 +/- 12 sec, 45 +/- 23 sec and 55 +/- 19 sec after the peak ICP, respectively). Since the second inflation level, PO2br decreased (p < 0.05) despite an ICP returning at 22 mm Hg and a cerebral perfusion pressure (CPP) larger than 90 mmHg. During changes in PaCO2, PO2br paralleled CBF and PvO2 before the second balloon inflation but diverged at higher ICP. In the same time pH-pHbr gradient rose. Hypotension did not induce sizeable changes. The direct metabolic monitoring of cerebral tissue locally compressed show fast response. At steady state, it shows alterations which are not detected by the measurement of the oxygen saturation in the longitudinal sinus or that of CBF. It confirms that the threshold for ICP which may require therapy in presence of focal brain compression is around 20 mm Hg even in the presence of a CPP > 90 mm Hg.

Dynamics of the development of microvascular reactions in the projection zones of the somatosensory cortex of the brain in rats.

The aim of the present work was to study the nature of local vascular reactions in the whisker projection zones of the somatosensory cortex of the rat brain in conditions of changes in sensory stimulation parameters. Studies were performed on Wistar rats anesthetized with ketamine. Local blood flow, neuron activity, and PO2 were recorded simultaneously in the cortical projection zones during mechanical stimulation of the whiskers. Local development of functional hyperemia was demonstrated in neuronal modules in conditions of threshold stimulation of the corresponding whisker. Increases in the intensity and frequency of stimulation of the whiskers were accompanied by decreases in the latent period and increases in the amplitude of regulatory microvascular reactions and a transformation in the configuration of functional hyperemia from a one-component pattern to a two-component pattern. The first component in the mechanism of development appeared to be associated with the action of factors of extravasal (including neuroglial) origin, while the second component appeared to be endothelium-dependent, as it did not occur in conditions of chemical deendothelialization.

Structural adaptation of vascular networks: role of the pressure response.

Structural reductions in vessel luminal diameters in response to elevated pressure may play a role in the elevation of peripheral resistance generally observed in hypertension. In the present study, a theoretical model is used to simulate the effect of increased driving pressure on flow resistance in microvascular networks. The angioarchitecture (lengths and diameters of all segments, topology) of microvascular networks (n=6) in the rat mesentery was recorded by intravital microscopy. The model simulation of vascular adaptation in response to local wall shear stress, transmural pressure, and tissue PO(2) was used to predict changes in network pressure drop and flow resistance for a given change of driving pressure (DeltaP). For DeltaP increasing from 15% to 190% of the normotensive value, a 3.3-fold increase in flow resistance was observed (structural autoregulation). If vascular reactivity to pressure was suppressed, the resistance increase was abolished. Suppressing pressure sensitivity also led to a rise in mean capillary pressure at normal driving pressure from 23.8+/-7.3 mm Hg to 34+/-6.9 mm Hg. These results indicate that low capillary pressure levels as well as structural autoregulation depend on vascular responses to circumferential wall stress (corresponding to pressure). This tendency of peripheral vascular beds to increase flow resistance for a given increase of bulk flow or driving pressure may amplify and stabilize blood pressure elevation in the development of hypertension.

Adenosine linking reduced O2 to arteriolar NO release in intestine is not formed from extracellular ATP

We have previously reported that adenosine formed in response to reduced arteriolar and/or tissue PO(2) preserves endothelial nitric oxide (NO) synthesis during sympathetic vasoconstriction in the rat intestine. To more precisely identify the site and mechanism of adenosine formation under these conditions, we tested the hypothesis that ATP released in response to reduced O(2) levels serves as a source of adenosine. Direct application of ATP to the wall of first-order arterioles elicited dose-dependent dilations of 15-33% above resting diameter that were reduced by 71-80% by the 5'-ectonucleotidase inhibitor alpha,beta-methyleneadenosine 5'-diphosphate (AOPCP, 4.5 x 10(-5) M) and completely abolished by N(G)-monomethyl-L-arginine (L-NMMA, 10(-4) M). Under control conditions, sympathetic nerve stimulation at 3 and 8 Hz induced arteriolar constrictions of 11 +/- 1 and 19 +/- 1 microm, respectively. These responses were enhanced by 58-69% in the presence of L-NMMA or when local PO(2) was maintained at resting levels. However, in the presence of AOPCP, the enhancing effects of L-NMMA and the high O(2) superfusate on sympathetic constriction were preserved. These results suggest that, although exogenously applied ATP can stimulate arteriolar NO release in the intestine largely through its sequential extracellular hydrolysis to adenosine, this process does not contribute to adenosine formation and sustained NO release during sympathetic constriction in this vascular bed.

Protective Role of an Endothelin-Converting Enzyme Inhibitor (FR901533) in Hepatic Ischemia/Reperfusion Injury

Introduction. There is evidence that endothelin (ET) contributes to disturbances of the hepatic microcirculation after warm ischemia/reperfusion (I/R) by causing vasoconstriction and by enhancing leukocyte endothelium interactions. The aim of this study was to investigate a possible protective role of the endothelin converting enzyme (ECE) inhibitor FR901533 in this setting.Methods. In an in vivo model (42 Wistar rats), hepatic ischemia was induced for 30 min by Pringle's maneuver. Sham operated (I), untreated ischemic (II), and treatment (III) groups with FR901533 (1 mg/kg bw iv) were investigated. The effect of FR901533 in I/R was assessed by in vivo microscopy (30–90 min after reperfusion), measurement of local tissue pO2 (30 and 60 min after reperfusion), and determination of AST/ALT levels (2 h, 6 h, and 2, 6, and 14 days after reperfusion).Results. In the untreated ischemic group (II) sinusoidal constriction to 76.3 ± 4.2% of basic diameters was observed, leading to significant decreases in perfusion rate (82.3 ± 3.6% of sham group) and in liver tissue pO2 (43.5 ± 3.2% of sham group) (P < 0.05). In addition, we found an increased percentage of stagnant leukocytes in sinusoids (138.3 ± 9.8) and sticking leukocytes in postsinusoidal venules (155.2 ± 3.3% of sham group) (P < 0.05). Hepatocellular damage (AST/ALT increase to 430.6 ± 47.7 U/L/200.2 ± 23.8 U/L, pre: 27.4 ± 2.7 U/L/28.1 ± 2.7 U/L) was detected 6 h after reperfusion (P < 0.05). Administration of the ECE inhibitor before ischemia significantly reduced I/R injury. Sinusoidal diameters were maintained (102.2 ± 1.7%), while perfusion rate (93.1 ± 1.8%) and tissue pO2 (105.3 ± 2.7%) increased significantly (P < 0.05). Hepatocellular damage was decreased (AST/ALT levels after 6 h of reperfusion: 166.6 ± 26.3 U/L/132.4 ± 22.5 U/L, P < 0.05) and leukocyte sticking and rolling were significantly reduced (P < 0.05).Conclusion. Our results provide evidence that the new therapeutic approach with an ECE inhibitor is effective in reducing hepatic I/R injury.

Reduced PO2 and Adenosine Formation Preserve Arteriolar Nitric Oxide Synthesis during Sympathetic Constriction in the Rat Intestine

Previous reports by this laboratory have indicated that a flow-dependent fall in arteriolar wall PO₂may be a stimulus for the sustained release of endothelial nitric oxide (NO) during sympathetic vasoconstriction in the superfused rat intestine. In this study, we tested the hypothesis that locally formed adenosine serves as the link between the fall in local PO₂ and NO synthesis under these conditions. Adenosine applied via pressurized micropipettes directly onto the wall or at a distance of 25 µm from the wall of first-order arterioles (resting diameter = 54 ± 1 µm) elicited dose-dependent dilations of 15–46% that were significantly reduced by the NO synthase inhibitor N^G-monomethyl-L-arginine (L-NMMA, 10^–4M). Arteriolar responses to sympathetic nerve stimulation were enhanced by 57–66% in the presence of L-NMMA or when tissue PO₂ was prevented from falling under a high O₂ superfusate. Adenosine deaminase (2.0 U/ml) or the selective A₁ receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (4 × 10^–4M) completely blocked the enhancing effect of L-NMMA on sympathetic constriction. These results are consistent with the hypothesis that the fall in arteriolar wall and/or tissue PO₂ that accompanies sympathetic arteriolar constriction in the rat intestine can lead to local adenosine production, which in turn preserves endothelial NO release.

Important Role for Endothelins in Acute Hepatic Ischemia/Reperfusion Injury

The aim of this study was to explore the complex role of endothelins (ETs) in hepatic ischemia-reperfusion injury and to minimize this type of injury by nonselective ET receptor blockade. In an in vivo rat model, hepatic ischemia was induced for 30 min. The rats were divided into three groups: (1) sham operated, (2) untreated ischemic, and (3) group treated with the nonselective ET receptor antagonist bosentan (1 mg/kg body weight iv). Blockage of the ET system during ischemia-reperfusion was assessed by: (1) in vivo microscopic analysis, (2) measurement of local tissue pO2

Effects of nicotinamide and carbogen on oxygenation in human tumor xenografts measured with luminescense based fiber-optic probes

Background and purpose: In head and neck cancer, addition of both carbogen breathing and nicotinamide to accelerated fractionated radiotherapy showed increased loco-regional control rates. An assay based on the measurement of changes in tumor pO 2 in response to oxygenation modification could be helpful for selecting patients for these new treatment approaches. Materials and methods: The fiber-optic oxygen-sensing device, OxyLite™, was used to measure changes in pO 2, at a single position in tumors, after treatment with nicotinamide and carbogen in three human xenograft tumor lines with different vascular architecture and hypoxic patterns. Pimonidazole was used as a marker of hypoxia and was analyzed with a digital image processing system. Results: At the position of pO 2 measurement, half of the tumors showed a local increase in pO 2 after nicotinamide administration. Steep increases in pO 2 were measured in most tumors during carbogen breathing although the increase was less pronounced in tumor areas with a low pre-treatment pO 2. A trend towards a faster local response to carbogen breathing for nicotinamide pre-treated tumors was found in all three lines. There were significant differences in hypoxic fractions, based on pimonidazole binding, between the three tumor lines. There was no correlation between hypoxic marker binding and the response to carbogen breathing. Conclusion: Temporal changes in local pO 2 can be measured with the OxyLite™. This system was used to quantitate the effects of oxygen modifying treatments. Rapid increases in pO 2 during carbogen breathing were observed in most tumor areas. The locally measured response to nicotinamide was smaller and more variable. Bio-reductive hypoxic cell marker binding in combination with OxyLite™ pO 2 determination gives spatial information about the distribution patterns of tumor hypoxia at the microscopic level together with the possibility to continuously measure changes in pO 2 in specific tumor areas.

Endothelin/nitric oxide balance influences hepatic ischemia-reperfusion injury.

During ischemia-reperfusion an imbalance between endothelin (ET) and nitric oxide (NO) can be responsible for microcirculatory disturbances. The aim of this study was to restore the ET/NO balance to reduce the ischemia-reperfusion injury. Hepatic ischemia was induced for 30 min in 56 Wistar rats. Sham operation, ischemia and treatment groups with the ET receptor antagonist (ERA) bosentan (1 mg/kg body weight i.v.) and the NO donor L-arginine (400 mg/kg body weight i.v.) were performed. For evaluation of hepatic microcirculation in vivo microscopy was carried out 30-90 min after reperfusion. Local hepatic tissue PO2, laser Doppler flow and aspartate aminotransferase/alanine aminotransferase (AST/ALT) levels were measured. Increased ET caused sinusoidal constriction after reperfusion to 76% of the sham group (p < 0.05), leading to significant decrease in perfusion rate (82%), liver tissue PO2 (6.9 mmHg) and erythrocyte flux (45.2% of sham group). Hepatocellular damage could be detected 6 h after reperfusion by AST/ALT increase (p < 0.05). Sinusoidal diameters were maintained at baseline in the ERA (98%) and NO (102%) groups (p < 0.05). Increased percentage of leukocytes sticking in sinusoids (144%) and venules (435%) was reduced by therapy to 110/253% (ERA) and 111/324% (NO), respectively (p < 0.05). Perfusion rate was increased to 93 and 94% (p < 0.05 vs ischemia). Local hepatic tissue PO2 was improved 30 min after reperfusion in the ERA (11.0 mmHg) as well as in the NO group (11.5 mmHg; p < 0.05 vs ischemia). Measurement with a laser Doppler flow meter revealed significant improved erythrocyte flux in both therapy groups (p < 0.05 vs ischemia). Also, the post-ischemic AST/ALT increase was reduced by therapy. In conclusion, ET evokes strong constriction of post-ischemic sinusoids, leading to microcirculatory disturbances. The maintenance of the ET/NO balance by blocking ET receptors, or providing an NO donor, protects liver microcirculation and reduces hepatic ischemia-reperfusion injury.

Endothelin/Nitric Oxide Balance Influences Hepatic Ischemia-Reperfusion Injury

Summary: During ischemia-reperfusion an imbalance between endothelin (ET) and nitric oxide (NO) can be responsible for microcirculatory disturbances. The aim of this study was to restore the ET/NO balance to reduce the ischemia-reperfusion injury. Hepatic ischemia was induced for 30 min in 56 Wistar rats. Sham operation, ischemia and treatment groups with the ET receptor antagonist (ERA) bosentan (1 mg/kg body weight i.v.) and the NO donor L-arginine (400 mg/kg body weight i.v.) were performed. For evaluation of hepatic microcirculation in vivo microscopy was carried out 30-90 min after reperfusion. Local hepatic tissue PO2, laser Doppler flow and aspartate aminotransferase/alanine aminotransferase (AST/ALT) levels were measured. Increased ET caused sinusoidal constriction after reperfusion to 76% of the sham group (p < 0.05), leading to significant decrease in perfusion rate (82%), liver tissue PO2 (6.9 mmHg) and erythrocyte flux (45.2% of sham group). Hepatocellular damage could be detected 6 h after reperfusion by AST/ALT increase (p < 0.05). Sinusoidal diameters were maintained at baseline in the ERA (98%) and NO (102%) groups (p < 0.05). Increased percentage of leukocytes sticking in sinusoids (144%) and venules (435%) was reduced by therapy to 110/253% (ERA) and 111/324% (NO), respectively (p < 0.05). Perfusion rate was increased to 93 and 94% (p < 0.05 vs ischemia). Local hepatic tissue PO2 was improved 30 min after reperfusion in the ERA (11.0 mmHg) as well as in the NO group (11.5 mmHg; p < 0.05 vs ischemia). Measurement with a laser Doppler flow meter revealed significant improved erythrocyte flux in both therapy groups (p < 0.05 vs ischemia). Also, the post-ischemic AST/ALT increase was reduced by therapy. In conclusion, ET evokes strong constriction of post-ischemic sinusoids, leading to microcirculatory disturbances. The maintenance of the ET/NO balance by blocking ET receptors, or providing an NO donor, protects liver microcirculation and reduces hepatic ischemia-reperfusion injury.

Controlled Vasoregulation of Postischemic Liver Microcirculation - A Therapeutic Approach

One of the changes produced by ischemia and reperfusion is endothelin (ET)-mediated constriction of the hepatic vascular bed. This leads to microcirculatory disturbances and reduced blood flow, thereby causing local hypoxia and liver damage. Our aim was to induce stepwise changes of microvascular vessel diameters so as to define the best protective vessel width that could be produced by drug therapy and thereby to minimize ischemia/reperfusion injury. The mixed ET receptor antagonist bosentan was used in different dosages in a rat liver ischemia/reperfusion model with splenocaval shunt. In vivo microscopy was performed 30-90 min after reperfusion and local hepatic tissue pO2

Oxygenation and perfusion of rabbit tibialis anterior muscle subjected to different patterns of electrical stimulation.

Dual amperometric microelectrodes were used to measure local pO2 and perfusion at multiple sites in the fast-twitch tibialis anterior muscles of anaesthetized rabbits. Six muscles were stimulated continuously at 10, 5, or 2.5 Hz. For all three frequencies, perfusion declined to about 50% of resting levels and recovered after stimulation. These changes corresponded to a rise followed by a fall in extracellular pO2. The highest levels of pO2 were reached during stimulation at 10 Hz. Eight muscles were stimulated tetanically at 100 Hz for 200 ms with duty cycles that were varied between 1.3 and 20.0%. Perfusion rose to 8.7 +/- 2.0 ml s(-1) 100 g(-1) at a duty cycle of 5% and declined with further increases in duty cycle. pO2 was depressed for duty cycles less than 10% but rose above resting levels at higher duty cycles. It is suggested that the paradoxical combination of elevated pO2 and depressed perfusion is attributable to stimulation conditions that exceed the oxygen transport capacity of a fast muscle.

Quantification of Regional Intrapulmonary Oxygen Partial Pressure Evolution during Apnea by 3He MRI

We present a new method to determine in vivo the temporal evolution of intrapulmonary oxygen concentrations by functional lung imaging with hyperpolarized 3Helium (3He→). Single-breath, single-bolus visualization of 3He→ administered to the airspaces is used to analyze nuclear spin relaxation caused by the local oxygen partial pressure pO2(t). We model the dynamics of hyperpolarization in the lung by rate equations. Based hereupon, a double acquisition technique is presented to separate depolarization by RF pulses and oxygen induced relaxation. It permits the determination of pO2 with a high accuracy of up to 3% with simultaneous flip angle calibration using no additional input parameters. The time course of pO2 during short periods of breathholding is found to be linear in a pig as well as in a human volunteer. We also measured the wall relaxation time in the lung and deduced a lower limit of 4.3 min.