BACKGROUND:During cardiopulmonary resuscitation (CPR) the ventilation/perfusion distribution (VA /Q) within the lung is difficult to assess. This experimental study examines the capability of multiple inert gas elimination (MIGET) to determine VA /Q under CPR conditions in a pig model.METHODS:Twenty-one anaesthetised pigs were randomised to three fractions of inspired oxygen (1.0, 0.7 or 0.21). VA/ Q by micropore membrane inlet mass spectrometry-derived MIGET was determined at baseline and during CPR following induction of ventricular fibrillation. Haemodynamics, blood gases, ventilation distribution by electrical impedance tomography and return of spontaneous circulation were assessed. Intergroup differences were analysed by non-parametric testing.RESULTS:MIGET measurements were feasible in all animals with an excellent correlation of measured and predicted arterial oxygen partial pressure (R(2)  = 0.96, n = 21 for baseline; R(2)  = 0.82, n = 21 for CPR). CPR induces a significant shift from normal VA /Q ratios to the high VA /Q range. Electrical impedance tomography indicates a dorsal to ventral shift of the ventilation distribution. Diverging pulmonary shunt fractions induced by the three inspired oxygen levels considerably increased during CPR and were traceable by MIGET, while 100% oxygen most negatively influenced the VA /Q. Return of spontaneous circulation were achieved in 52% of the animals.CONCLUSIONS:VA /Q assessment by MIGET is feasible during CPR and provides a novel tool for experimental purposes. Changes in VA /Q caused by different oxygen fractions are traceable during CPR. Beyond pulmonary perfusion deficits, these data imply an influence of the inspired oxygen level on VA /Q. Higher oxygen levels significantly increase shunt fractions and impair the normal VA /Q ratio.

INTRODUCTION Partial pressure of oxygen (PO2) is an important physiologic parameter, which has obvious implications in determining the cerebral metabolic rate of oxygen (CMRO2) and the efficacy of radiotherapy, amongst several other functionally and clinically relevant examples. Particularly for determination of CMRO2 via the hyperoxia approach suggested recently (1), a global brain arterial PO2 is inferred from the end-tidal O2 partial pressure since there is currently no method available for mapping PO2 in the brain. In this study, we explore the sensitivity of T1ρ-weighted imaging for changes in PO2 in a controlled in vitro setting and compare it to the known effect of dissolved oxygen on T1 and T2 relaxation times. We also explore T1ρ-weighted MRI detection of PO2 changes in the brain by administration of increased FiO2 to rats and human subjects. METHODS Five phantoms were prepared by adding 15 mL of physiologic saline to 20 mL syringes and balancing gas via a 5 mL syringe filled with appropriate mixture of oxygen and air to yield PO2 of 279, 399, 520, 640, and 760 Torr with an accuracy of ±1 Torr. These phantoms were positioned at an angle of 45° within a clinical extremity coil to allow for a slice through just the liquid portion of the mixtures and then imaged on a 1.5 T clinical MRI scanner. The T2, T1, and T1ρ relaxation times were determined for the phantoms by using a spin echo sequence and varying the echo time, by a saturation recovery approach, and by varying the time of spin-locking length (at a spin-locking frequency of 250 Hz) via a prepared fast spin echo sequence, as previously described in detail (2), respectively. Ten female Sprague-Dawley rats (200-300g) and 2 human subjects were studied on the 1.5 T clinical scanner in accordance with institutionally approved protocols and using the same T1ρ sequence used for the phantoms, with TR = 1s, TE = 17 ms, TSL = 120 ms, ETL = 16, and spin-locking frequency of 250 Hz. The rats were anesthetized with Nembutol® (50 mg/kg) and studied with a head coil specially designed for the application. For gas delivery to the rats, an inhalation mask and a calibrated custom-made ventilation system were utilized to vary FiO2 from 20% to 100% with 20% increments, making up the remainder of the volume with nitrogen. The human subjects were studied with a clinical head coil. 40% FiO2 was administered to the human subjects in the magnet via a clinical face mask and a patient tube extended to a wall-mounted hospital oxygen dispenser providing medical grade O2. RESULTS AND DISCUSSION Figure 1 provides the measured T2, T1ρ, and T1 relaxation times of each phantom with the given PO2. All exponential curvefits for determining these relaxation times resulted in correlation coefficients of 0.943 to 0.999. All three of these relaxation times demonstrate a linear relationship to PO2, with correlation coefficients equal to 0.997, 0.952, and 0.994, for T2, T1ρ, and T1, respectively. The relaxation time difference between the highest and lowest PO2 is 1.3 fold for T2, 4.3 fold for T1ρ, and 2.8 fold for T1, suggesting that T1ρ-weighting is most sensitive to PO2 changes. Figure 2 provides representative change in the normalized T1ρ-weighted MRI signal, measured in a region of interest corresponding to the bilateral frontoparietal cortex of the rat, as a result of stepwise increase in FiO2. As expected from the phantom data, a highly linear relationship (correlation coefficient of 0.998) is seen between the T1ρ-weighted signal and FiO2, which is expected to be linearly proportional to PO2. A representative T1ρ-weighted signal change is also seen in a region of interest consisting of the bi-frontal human cerebral cortex due to increase in FiO2 to 40% from room air. Collectively, these data suggest that T1ρ-weighted MRI is linearly sensitive to PO2, to an extent greater than that seen with both T1 and T2 relaxation. While it remains a focus of our present endeavors to tease out the optimal spin-locking frequency for PO2 detection in the brain and other organs and to pursue a localized PO2 map, the potential utility of T1ρ-weighted imaging for monitoring inspired oxygen induced changes, specifically as they relate to the effect of dissolved oxygen, is readily apparent. A B C

The 17O nucleus has been used recently by several groups for magnetic resonance (MR) imaging of cerebral metabolism. Inhalational delivery of 17O2 in very brief pulses could, in theory, have significant advantages for determination of the cerebral metabolic rate for oxygen (CMRO2) with MR imaging. Mechanical ventilators, however, are not typically capable of creating step changes in gas concentration at the airway. We designed a ventilator for large animal and human studies that provides mechanical ventilation to a subject inside an MR scanner through 25 feet of small-bore connecting tubing, and tested its capabilities using helium as a surrogate for 17O2. After switching the source gas from oxygen to helium, the 0-90% response time for helium concentration changes at the airway was 2.4 seconds. The capability for creating rapid step changes in gas concentration at the airway in large animal and human studies should facilitate the experimental testing of the delivery 17O2 in brief pulses, and its potential use in imaging CMRO2.

The multiple inert gas elimination technique (MIGET) represents the gold standard for analysis of ventilation and perfusion distributions in the lung. Modification of this technique allows a much simpler sample processing and hence permits routine clinical application of this technique. MIGET using micropore membrane inlet mass spectrometry (MMIMS) might, therefore, facilitate early diagnosis of lung diseases and monitoring of therapeutic interventions in the future.

Die Multiple-Inertgas-Eliminationstechnik (MIGET) stellt den Goldstandard zur Bestimmung der Ventilations- und Perfusionsverteilungen in der Lunge dar. Eine Modifikation in deren Durchführung erlaubt eine wesentlich einfachere und schnellere Handhabung, sodass diese aufwendige Methode erstmals für den klinischen Routineeinsatz geeignet erscheint. Die MIGET mithilfe der „micropore membrane inlet mass spectrometry“ (MMIMS) stellt damit eine frühere Detektion von Lungenerkrankungen und ein sensitiveres Therapiemonitoring in Aussicht.

INTRODUCTION: Hypoxia influences nitric oxide (NO) production at several levels: gene expression, enzyme activity and substrate availability. Many disease states result in dynamic fluctuations in tissue pO2 e.g. cyclical recruitment in ARDS and nocturnal desaturations in obstructive sleep apnea. Until recently, in vitro studies have been unable to reproduce moderate to high frequency intermittent hypoxia (IH). We hypothesized that IH would induce NOS protein expression and NO production. METHODS: RAW 264.7 macrophages were cultured in a specially designed forced convection culture system. Cultures were randomly assigned to sustained hypoxia (8 Torr O2), IH (cycles of 40 Torr O2 for 90 sec and 8 Torr for 30 sec), or sustained normoxia (40 Torr O2) for 4, 6 or 18 hours. Cell lysates were assayed for inducible nitric oxide synthase (iNOS) and normalized to a constitutive protein (Raf). In the system effluent, collected during the last hour in a limited number of 6 hour experiments, nitrate and nitrite (NOX) were measured by chemiluminescence. RESULTS: Compared to normoxia, both sustained hypoxia and IH resulted in a significant accumulation of iNOS protein at 6 hours which remained increased at 18 hours. Compared to hypoxia, there was a trend for greater iNOS protein in IH samples at 6 hours. NO production was measured at 0.316, 0.877, 1.877 nM NOX/ng cell lysates/hr in normoxia, hypoxia and IH respectively. CONCLUSIONS: Exposure of murine macrophages to IH and hypoxia leads to increased iNOS. The apparent increase in NO production during IH could be due to an increase in active iNOS or in the availability of oxygen. The dynamic nature of oxygenregulated NO production has potential importance in several diseases including ARDS and obstructive sleep apnea. Supported by NIH GM64486

Introduction: Using forced convection cell culture, we previously reported rapid reversible inhibition of nitric oxide (NO) production by cytokine-stimulated macrophages due to acute hypoxia (seconds). The apparent inducible nitric oxide synthase Km for oxygen was 28 Torr, much higher than 5 Torr, the reported Km in cell lysates. We suspect the increased Km is due to multiple mechanisms, one of which could be increased NO consumption during hypoxia. Methods: Using an NO electrode, effluent NO from the forced convection system was directly and continuously measured. Experiments were performed with and without cells at oxygen tensions between 8 and 160 Torr (n?2). Maximum effluent NO was defined as the NO concentration measured without oxygen or cells in the system. Cellular consumption of exogenous NO was measured after completely inhibiting endogenous NO production with 1400 W. Results: The effluent NO concentration was independent of oxygen tension and the presence of cytokinestimulated macrophages. Conclusions: NO consumption did not affect the apparent Km in our model. The lack of consumption by cytokine-stimulated RAW 264.7 cells is consistent with reports that these stimulated cells do not produce significant amounts of superoxide, the predominant intracellular scavenger of NO. We are currently investigating other mechanisms to explain the high apparent Km detected in our model.FigureFunding Source: NIH GM64486

Introduction: Acute lung injury (ALI) is recognized as a heterogeneous condition that progresses over time. Dependent regions of the lung are prone to collapse and/or alveolar flooding; mid-regions are prone to tidal recruitment and intermittent hypoxia; and non-dependent regions of the lung are prone to overdistension and stretch. We hypothesized that neutrophil accumulation would vary with spatial position and with time in a surfactant depletion model of acute lung injury. Methods: Following saline lavage in NZW rabbits, we adjusted mechanical ventilation to provide tidal recruitment in approximately 25% of the lung as determined by a previously described fast intra-arterial fluorescence quenching oxygen probe. Lungs were harvested at prior to or at 0, 1.5, 3, or 6 hours after lavage; then formalin fixed at a constant distending pressure of 30 cm H2O; sectioned into 8-9 slices from nondependent to dependent regions; and cut into histologic sections. After staining with H&E, investigators blinded to the origin of the section obtained 10 digital images from each section. Semi-automated image analysis and manual cell counting was performed for each image. The influence of time and location on the number of alveolar neutrophils was tested by repeated measures 2 way ANOVA. Results: Both time and location were significantly related to alveolar neutrophil accumulation. The dependent regions had significantly higher neutrophil counts than the nondependent regions. Alveolar neutrophil counts increased over time. Conclusions: In this surfactant depletion model of ALI, neutrophil accumulation showed substantial spatial and temporal heterogeneity. Dependent regions of the lung exposed to cyclical recruitment had greater neutrophil accumulation than nondependent regions exposed to excessive stretch. Supported by DFG Ma2398/3-1, NIH GM64486

Objective: An ultrafast responding fluorescent-quenching PO2 probe allows time-resolved, in vivo measurement Of PO2. This study describes several validation experiments of this new device in vitro, and reports its first use during cardiopulmonary resuscitation in an animal model of cardiac arrest.Methods: The influence Of CO2, temperature and motion artefacts on the signal response of the PO2 probe was analysed in vitro by systematic variation of these values. Thereafter, with approval of the Review Board for the care and use of animals, CPR was performed in four pigs. The PaO2 course was recorded continuously at time resolution of < 80ms in the abdominal aorta using an uncoated fluorescence-quenching probe (Foxy AL-300, OceanOptics Inc., USA).Results: In vitro experiments showed that signal intensity is dependent on CO2 concentration (Delta PfaO2 = 4 mmHg/vol.% CO2) and temperature (Delta PfaO2 = 16 mmHg/degrees C), but it is robust with regards to probe motion. In the animal experiments, the uncoated fluorescence-quenching probe was calibrated by repeated simultaneous measurements with the Paratrend 7 (R) sensor to correct the PfaO2 for a potential signal drift, PCO2, and temperature variations. In all animal experiments. the individual PaO2 courses were clearly related to therapeutic interventions and their haemodynamic effects during CPR and allowed recording of ultrafast PO2 changes with a time resolution of 80 ms.Conclusions: The results demonstrate the feasibility of ultrafast PO2 measurement during CPR and low-flow states. They also demonstrate very rapid systemic effects of CPR upon aortic PO2. Among many other useful applications, the information derived from this technique may help to define the optimum conditions for successful defibrillation and restoration of spontaneous circulation. (c) 2004 Elsevier Ireland Ltd. All rights reserved.