Quinone Catalysis Modulates Proton Transfer Reactions in the Membrane Domain of Respiratory Complex I

Complex I is a redox-driven proton pump that drives electron transport chains and powers oxidative phosphorylation across all domains of life. Yet, despite recently resolved structures from multiple organisms, it still remains unclear how the redox reactions in Complex I trigger proton pumping up to 200 Å away from the active site. Here, we show that the proton-coupled electron transfer reactions during quinone reduction drive long-range conformational changes of conserved loops and trans-membrane (TM) helices in the membrane domain of Complex I from Yarrowia lipolytica. We find that the conformational switching triggers a π → α transition in a TM helix (TM3ND6) and establishes a proton pathway between the quinone chamber and the antiporter-like subunits, responsible for proton pumping. Our large-scale (>20 μs) atomistic molecular dynamics (MD) simulations in combination with quantum/classical (QM/MM) free energy calculations show that the helix transition controls the barrier for proton transfer reactions by wetting transitions and electrostatic effects. The conformational switching is enabled by re-arrangements of ion pairs that propagate from the quinone binding site to the membrane domain via an extended network of conserved residues. We find that these redox-driven changes create a conserved coupling network within the Complex I superfamily, with point mutations leading to drastic activity changes and mitochondrial disorders. On a general level, our findings illustrate how catalysis controls large-scale protein conformational changes and enables ion transport across biological membranes.

Molecular Basis of the Electron Bifurcation Mechanism in the [FeFe]-Hydrogenase Complex HydABC

Electron bifurcation is a fundamental energy coupling mechanism widespread in microorganisms that thrive under anoxic conditions. These organisms employ hydrogen to reduce CO₂, but the molecular mechanisms have remained enigmatic. The key enzyme responsible for powering these thermodynamically challenging reactions is the electron-bifurcating [FeFe]-hydrogenase HydABC that reduces low-potential ferredoxins (Fd) by oxidizing hydrogen gas (H₂). By combining single-particle cryo-electron microscopy (cryoEM) under catalytic turnover conditions with site-directed mutagenesis experiments, functional studies, infrared spectroscopy, and molecular simulations, we show that HydABC from the acetogenic bacteria Acetobacterium woodii and Thermoanaerobacter kivui employ a single flavin mononucleotide (FMN) cofactor to establish electron transfer pathways to the NAD(P)⁺ and Fd reduction sites by a mechanism that is fundamentally different from classical flavin-based electron bifurcation enzymes. By modulation of the NAD(P)⁺ binding affinity via reduction of a nearby iron-sulfur cluster, HydABC switches between the exergonic NAD(P)⁺ reduction and endergonic Fd reduction modes. Our combined findings suggest that the conformational dynamics establish a redox-driven kinetic gate that prevents the backflow of the electrons from the Fd reduction branch toward the FMN site, providing a basis for understanding general mechanistic principles of electron-bifurcating hydrogenases.

Redox-controlled reorganization and flavin strain within the ribonucleotide reductase R2b-NrdI complex monitored by serial femtosecond crystallography

Redox reactions are central to biochemistry and are both controlled by and induce protein structural changes. Here, we describe structural rearrangements and crosstalk within the Bacillus cereus ribonucleotide reductase R2b-NrdI complex, a di-metal carboxylate-flavoprotein system, as part of the mechanism generating the essential catalytic free radical of the enzyme. Femtosecond crystallography at an X-ray free electron laser was utilized to obtain structures at room temperature in defined redox states without suffering photoreduction. Together with density functional theory calculations, we show that the flavin is under steric strain in the R2b-NrdI protein complex, likely tuning its redox properties to promote superoxide generation. Moreover, a binding site in close vicinity to the expected flavin O₂ interaction site is observed to be controlled by the redox state of the flavin and linked to the channel proposed to funnel the produced superoxide species from NrdI to the di-manganese site in protein R2b. These specific features are coupled to further structural changes around the R2b-NrdI interaction surface. The mechanistic implications for the control of reactive oxygen species and radical generation in protein R2b are discussed.

Structural basis of mammalian complex IV inhibition by steroids

The mitochondrial electron transport chain maintains the proton motive force that powers adenosine triphosphate (ATP) synthesis. The energy for this process comes from oxidation of reduced nicotinamide adenine dinucleotide (NADH) and succinate, with the electrons from this oxidation passed via intermediate carriers to oxygen. Complex IV (CIV), the terminal oxidase, transfers electrons from the intermediate electron carrier cytochrome c to oxygen, contributing to the proton motive force in the process. Within CIV, protons move through the K and D pathways during turnover. The former is responsible for transferring two protons to the enzyme's catalytic site upon its reduction, where they eventually combine with oxygen and electrons to form water. CIV is the main site for respiratory regulation, and although previous studies showed that steroid binding can regulate CIV activity, little is known about how this regulation occurs. Here, we characterize the interaction between CIV and steroids using a combination of kinetic experiments, structure determination, and molecular simulations. We show that molecules with a sterol moiety, such as glyco-diosgenin and cholesteryl hemisuccinate, reversibly inhibit CIV. Flash photolysis experiments probing the rapid equilibration of electrons within CIV demonstrate that binding of these molecules inhibits proton uptake through the K pathway. Single particle cryogenic electron microscopy (cryo-EM) of CIV with glyco-diosgenin reveals a previously undescribed steroid binding site adjacent to the K pathway, and molecular simulations suggest that the steroid binding modulates the conformational dynamics of key residues and proton transfer kinetics within this pathway. The binding pose of the sterol group sheds light on possible structural gating mechanisms in the CIV catalytic cycle.

[Prognosis of patients with COVID-19 presenting acute ischaemic stroke and receiving interventional treatment]

Objetivo

Presentar nuestra experiencia y analizar el pronóstico de pacientes COVID-19 con ictus isquémico agudo por oclusión de grandes vasos tratados con neurointervencionismo (NIV) en la unidad de ictus.

Material y métodos

Se incluyeron todos los pacientes consecutivos con ictus isquémico agudo debido a oclusión de grandes vasos tratados por NIV en nuestra institución entre marzo y abril de 2020, durante el brote de COVID-19. Se realizó una comparación entre pacientes con COVID-19 y pacientes sin infección por coronavirus. Se comunican los resultados clínicos iniciales y a corto plazo.

Resultados

Del 1 de marzo al 30 de abril se realizaron 25 procedimientos de NIV por ictus isquémico agudo en nuestra institución. Ocho pacientes eran COVID-19 y 17 eran pacientes no COVID-19. La edad media de los pacientes con COVID-19 fue de 70,1 ± 12,23 años, y 7 fueron hombres (87,5%, p = 0,006). Mientras que todos los pacientes sin COVID procedían de urgencias, solo 5 pacientes con COVID-19 (62,5%) fueron atendidos desde urgencias por ictus (p = 0,01). Tres pacientes procedían de hospitalización. La tasa de mortalidad en pacientes sin COVID-19 fue del 5,8%, pero en pacientes con COVID-19 fue considerablemente alta (50%). Ningún parámetro analítico difirió entre ambos grupos. No se registraron hemorragias en esta serie.En comparación con el mismo período del año pasado, se observó una disminución de la actividad neurointervencionista del 39%.

Conclusiones

La mejor terapia médica y de NIV desembocó en malos resultados y una mortalidad dramática. La pandemia de COVID-19 dificultó significativamente el funcionamiento normal de los servicios de urgencias y la atención de estos pacientes con ictus.

Functional Water Wires Catalyze Long-Range Proton Pumping in the Mammalian Respiratory Complex I

The respiratory complex I is a gigantic (1 MDa) redox-driven proton pump that reduces the ubiquinone pool and generates proton motive force to power ATP synthesis in mitochondria. Despite resolved molecular structures and biochemical characterization of the enzyme from multiple organisms, its long-range (∼300 Å) proton-coupled electron transfer (PCET) mechanism remains unsolved. We employ here microsecond molecular dynamics simulations to probe the dynamics of the mammalian complex I in combination with hybrid quantum/classical (QM/MM) free energy calculations to explore how proton pumping reactions are triggered within its 200 Å wide membrane domain. Our simulations predict extensive hydration dynamics of the antiporter-like subunits in complex I that enable lateral proton transfer reactions on a microsecond time scale. We further show how the coupling between conserved ion pairs and charged residues modulate the proton transfer dynamics, and how transmembrane helices and gating residues control the hydration process. Our findings suggest that the mammalian complex I pumps protons by tightly linked conformational and electrostatic coupling principles.

Water-Gated Proton Transfer Dynamics in Respiratory Complex I

The respiratory complex I transduces redox energy into an electrochemical proton gradient in aerobic respiratory chains, powering energy-requiring processes in the cell. However, despite recently resolved molecular structures, the mechanism of this gigantic enzyme remains poorly understood. By combining large-scale quantum and classical simulations with site-directed mutagenesis and biophysical experiments, we show here how the conformational state of buried ion-pairs and water molecules control the protonation dynamics in the membrane domain of complex I and establish evolutionary conserved long-range coupling elements. We suggest that an electrostatic wave propagates in forward and reverse directions across the 200 Å long membrane domain during enzyme turnover, without significant dissipation of energy. Our findings demonstrate molecular principles that enable efficient long-range proton–electron coupling (PCET) and how perturbation of this PCET machinery may lead to development of mitochondrial disease.

Quantum Chemical and QM/MM Models in Biochemistry

Quantum chemical (QC) calculations provide a basis for deriving a microscopic understanding of enzymes and photobiological systems. Here we describe how QC models can be used to explore the electronic structure, dynamics, and energetics of biomolecules. We introduce the hybrid quantum mechanics/classical mechanics (QM/MM) approach, where a quantum mechanically described system of interest is embedded in a classically described force field representation of the biochemical surroundings. We also discuss the QM cluster model approach, as well as embedding theories, that provide complementary methodologies to model quantum mechanical effects in biomolecules. The chapter also provides some practical guides for building quantum biochemical models using the quinone reduction catalysis in respiratory complex I and a model reaction in solution as examples.

Redox-coupled proton pumping drives carbon concentration in the photosynthetic complex I

Photosynthetic organisms capture light energy to drive their energy metabolism, and employ the chemical reducing power to convert carbon dioxide (CO2) into organic molecules. Photorespiration, however, significantly reduces the photosynthetic yields. To survive under low CO2 concentrations, cyanobacteria evolved unique carbon-concentration mechanisms that enhance the efficiency of photosynthetic CO2 fixation, for which the molecular principles have remained unknown. We show here how modular adaptations enabled the cyanobacterial photosynthetic complex I to concentrate CO2 using a redox-driven proton-pumping machinery. Our cryo-electron microscopy structure at 3.2 Å resolution shows a catalytic carbonic anhydrase module that harbours a Zn2+ active site, with connectivity to proton-pumping subunits that are activated by electron transfer from photosystem I. Our findings illustrate molecular principles in the photosynthetic complex I machinery that enabled cyanobacteria to survive in drastically changing CO2 conditions.

Electric field modulated redox-driven protonation and hydration energetics in energy converting enzymes

Biological energy conversion is catalysed by proton-coupled electron transfer (PCET) reactions that form the chemical basis of respiratory and photosynthetic enzymes. Despite recent advances in structural, biophysical, and computational experiments, the mechanistic principles of these reactions still remain elusive. Based on common functional features observed in redox enzymes, we study here generic mechanistic models for water-mediated long-range PCET reactions. We show how a redox reaction within a buried protein environment creates an electric field that induces hydration changes between the proton acceptor and donor groups, and in turn, lowers the reaction barrier and increases the thermodynamic driving forces for the water-mediated PCET process. We predict linear free energy relationships, and discuss the proposed mechanism in context of PCET in cytochrome c oxidase.

Molecular mechanism of polyketide shortening in anthraquinone biosynthesis of Photorhabdus luminescens

Anthraquinones, produced by a type II polyketide synthase in Photorhabdus luminescens, are derived from polyketide chain shortening.

Anthraquinones, a widely distributed class of aromatic natural products, are produced by a type II polyketide synthase system in the Gram-negative bacterium Photorhabdus luminescens. Heterologous expression of the antABCDEFGHI anthraquinone biosynthetic gene cluster in Escherichia coli identified AntI as an unusual lyase, catalysing terminal polyketide shortening prior to formation of the third aromatic ring. Functional in vitro and in vivo analysis of AntI using X-ray crystallography, structure-based mutagenesis, and molecular simulations revealed that AntI converts a defined octaketide to the tricyclic anthraquinone ring via retro-Claisen and Dieckmann reactions. Thus, AntI catalyses a so far unobserved multistep reaction in this PKS system.

Energetics and Dynamics of Proton-Coupled Electron Transfer in the NADH/FMN Site of Respiratory Complex I

Complex I functions as an initial electron acceptor in aerobic respiratory chains that reduces quinone and pumps protons across a biological membrane. This remarkable charge transfer process extends ca. 300 Å and it is initiated by a poorly understood proton-coupled electron transfer (PCET) reaction between nicotinamide adenine dinucleotide (NADH) and a protein-bound flavin (FMN) cofactor. We combine here large-scale density functional theory calculations and quantum/classical models with atomistic molecular dynamics simulations to probe the energetics and dynamics of the NADH-driven PCET reaction in complex I. We find that the reaction takes place by concerted hydrogen atom (H^•) transfer that couples to an electron transfer (eT) between the aromatic ring systems of the cofactors and further triggers reduction of the nearby FeS centers. In bacterial, Escherichia coli-like complex I isoforms, reduction of the N1a FeS center increases the binding affinity of the oxidized NAD⁺ that prevents the nucleotide from leaving prematurely. This electrostatic trapping could provide a protective gating mechanism against reactive oxygen species formation. We also find that proton transfer from the transient FMNH^• to a nearby conserved glutamate (Glu97) residue favors eT from N1a onward along the FeS chain and modulates the binding of a new NADH molecule. The PCET in complex I isoforms with low-potential N1a centers is also discussed. On the basis of our combined results, we propose a putative mechanistic model for the NADH-driven proton/electron-transfer reaction in complex I.

Molecular dynamics and structural models of the cyanobacterial NDH-1 complex

NDH-1 is a gigantic redox-driven proton pump linked with respiration and cyclic electron flow in cyanobacterial cells. Based on experimentally resolved X-ray and cryo-EM structures of the respiratory complex I, we derive here molecular models of two isoforms of the cyanobacterial NDH-1 complex involved in redox-driven proton pumping (NDH-1L) and CO2-fixation (NDH-1MS). Our models show distinct structural and dynamic similarities to the core architecture of the bacterial and mammalian respiratory complex I. We identify putative plastoquinone-binding sites that are coupled by an electrostatic wire to the proton pumping elements in the membrane domain of the enzyme. Molecular simulations suggest that the NDH-1L isoform undergoes large-scale hydration changes that support proton-pumping within antiporter-like subunits, whereas the terminal subunit of the NDH-1MS isoform lacks such structural motifs. Our work provides a putative molecular blueprint for the complex I-analogue in the photosynthetic energy transduction machinery and demonstrates that general mechanistic features of the long-range proton-pumping machinery are evolutionary conserved in the complex I-superfamily.

How inter-subunit contacts in the membrane domain of complex I affect proton transfer energetics

The respiratory complex I is a redox-driven proton pump that employs the free energy released from quinone reduction to pump protons across its complete ca. 200 Å wide membrane domain. Despite recently resolved structures and molecular simulations, the exact mechanism for the proton transport process remains unclear. Here we combine large-scale molecular simulations with quantum chemical density functional theory (DFT) models to study how contacts between neighboring antiporter-like subunits in the membrane domain of complex I affect the proton transfer energetics. Our combined results suggest that opening of conserved Lys/Glu ion pairs within each antiporter-like subunit modulates the barrier for the lateral proton transfer reactions. Our work provides a mechanistic suggestion for key coupling effects in the long-range force propagation process of complex I.

Mutagenesis of Sequence Determinants of Truncated Porcine ALOX15 Induces Changes in the Reaction Specificity by Altering the Catalytic Mechanism of Initial Hydrogen Abstraction

The reaction specificity of lipoxygenases is of physiological relevance since the various oxygenation products exhibit different biological activities. Among mammalian ALOX15 orthologs there are arachidonic acid 12- and 15-lipoxygenating enzymes and recent studies suggested an evolutionary switch in that reaction specificity during late primate development. Previous reports showed that 12-lipoxygenating ALOX15 orthologs can be converted to 15-lipoxygenating enzymes by site-directed mutagenesis of some sequence determinants. Unfortunately, the molecular basis for those alterations are not well understood. Here, the arachidonic acid 12-lipoxygenating N-terminal truncation variant of pig ALOX15, for which a crystal structure is available, was used to explore the catalytic mechanism of the specificity switch induced by mutagenesis of Val418 and Val419 sequence determinants. We found that Val418Ile+Val419Met double mutant is dominantly 15-lipoxygenating. Docking and MD simulations, and quantum mechanics/molecular mechanics calculations indicated that the wildtype energy barrier for arachidonic acid 15-lipoxygenation is 3.4 kcal mol^-1 higher than for 12-lipoxygenation. In contrast, for the Val418Ile+Val419Met double mutant the energy barrier for 12-lipoxygenation is 6.0 kcal mol^-1 higher than for 15-lipoxygenation. Our data suggest that enzyme-substrate complex geometries determine the value of these energy barriers and, as a consequence, the reaction specificity of ALOX15 orthologs.

Computational insight into the catalytic implication of head/tail-first orientation of arachidonic acid in human 5-lipoxygenase: consequences for the positional specificity of oxygenation

Using a multi-scale approach to search for the arachidonic acid binding modes that determine the catalytic specificity of human 5-LOX.

Endovascular treatment of intracranial aneurysms using the Pipeline Flex embolization device: a case series of 30 consecutive patients

BACKGROUND

The Pipeline Flex embolization device has some peculiarities in comparison with the previous generation device. Despite recent reports of the modified delivery system, its safety is still unknown.

OBJECTIVE

To illustrate the intraprocedural and periprocedural complication rate with this new device in 30 consecutive patients.

MATERIAL AND METHODS

Clinical, procedural, and angiographic data, including aneurysm size and location, device or devices used, angiographic and clinical data were analyzed.

RESULTS

30 patients harboring 30 aneurysms were analyzed. 39 devices were placed properly. Multiple Pipeline embolization devices (PEDs) were used in 7 cases. In 28 devices the distal end opened fully from the beginning with a complete wall apposition. In the remaining 11 devices, distal-end opening of the devices was instant but partial, but fully opened easily after recapture. Among the 30 procedures, recapture and reposition of the Pipeline Flex was performed four times owing to proximal migration/malposition of the device during delivery. Four intraprocedural/periprocedural complications occurred, of which 2 resulted in major complications, with neurologic deficits persisting for longer than 7 days. The 30-day morbidity rate was 6.6%, with no deaths. No aneurysm rupture or parenchymal hemorrhage was seen.

CONCLUSIONS

The Pipeline Flex embolization device allows more precise and controlled deployment than the first-generation device. The number of devices and the complication rate during the learning curve are lower than reported with the first-generation PED. The new delivery system and the resheathing maneuvers do not seem to increase the intraprocedural complication rate in comparison with the first-generation PED.

Y-stent-assisted coil embolization of anterior circulation aneurysms using two Solitaire AB devices: a single center experience

Wide-neck intracranial aneurysms remain a challenge to endovascular treatment. We describe our experience in repairing wide-neck aneurysms of the anterior circulation located at arterial branch points using coil embolization assisted by Y-stenting using two Solitaire(®) stents.Six wide-neck intracranial aneurysms located on the middle cerebral artery bifurcation( 3), pericallosal artery( 1), and anterior communicating artery( 2) were repaired by Y-stent-assisted coil embolization using two Solitaire(®) stents. Four cases were incidental findings of aneurysm and two cases were previously treated ruptured aneurysms that had undergone recanalization. All the cases were successfully treated without complications. Follow-up by digital subtraction angiography and magnetic resonance angiography at six months showed the stents to be patent with no recanalization of the aneurysm sacs. Repairing wide-neck aneurysms of the anterior circulation by Y-stent-assisted coil embolization using two Solitaire(®) stents is a simple and safe method of treating complex aneurysms. While the results are promising, larger series with longer term follow-ups are needed to corroborate that this treatment method is superior to other techniques.

[Utility of the detection of nasal flaring in the assessment of severity of dyspnea]

OBJECTIVE

To determine if the presence of nasal flaring is indicative of severe respiratory insufficiency.

METHODS

Prospective observational study of patients consulting in the Emergency Department because of dyspnea whose triage level is II or III in the Spanish Triage System (MAT-SET). Vital signs, SpO2, arterial blood gases and nasal flaring presence were recorded, as well as the need for hospital admission and length of hospital stay. Data are presented as median (25-75th percentile).

RESULTS

A total of 43 patients were analyzed (70% men, aged 77 (67-82) years), 7 of whom showed nasal flaring. Those having flaring had higher respiratory rate (36 (34-40) vs. 25 (20-28) vs., p=0.001) and were more acidotic (pH 7.34 [7.23-7.40] vs. 7.42 [7.39-7.46] vs., p=0.03) than patients without this sign. There were no differences between groups in SpO2, PaCO2, heart rate and arterial pressure. There were no differences in the rate of hospital admission-(6 patients [85.7%] in nasal flaring group vs 29 patients [80.5%] in the non nasal flaring group [p=0,06], or in the length of the hospital stay-3 days [1-16] in nasal flaring group vs. 6 days [1-10] in the non nasal flaring group, p=0.6). All patients with nasal flaring had tachypnea.

CONCLUSION

In our study, nasal flaring does not indicate severity in dyspneic patients in spite of its association with tachypnea and acidosis.

Multicenter study of the multiple organ dysfunction syndrome in intensive care units: the usefulness of Sequential Organ Failure Assessment scores in decision making

OBJECTIVE

This study examined the incidence and mortality of multiple organ dysfunction syndrome (MODS) in intensive care units, evaluated the limitation of life support in these patients, and determined whether daily measurement of the Sequential Organ Failure Assessment (SOFA) is useful for decision making.

DESIGN AND SETTING

Prospective, observational study in 79 intensive care units.

PATIENTS AND PARTICIPANTS

Of the 7,615 patients admitted during a 2-month period we found 1,340 patients to have MODS.

MEASUREMENTS AND RESULTS

We recorded mortality and length of stay in the intensive care unit and the hospital and the maximum and minimum total SOFA scores during MODS. Limitation of life support in MODS patients was also evaluated. Stepwise logistic regression was used to determine the factors predicting mortality. The in-hospital mortality rate in patients with MODS was 44.6%, and some type of limitation of life support was applied in 70.6% of the patients who died. The predictive model maximizing specificity included the following variables: maximum SOFA score, minimum SOFA score, trend of the SOFA for 5 consecutive days, and age over 60 years. The model diagnostic yield was: specificity 100%, sensitivity 7.2%, positive predictive value 100%, and negative predictive value 57.3%; the area under the receiver operating characteristic curve was 0.807.

CONCLUSIONS

This model showed that in our population with MODS those older than 60 years and with SOFA score higher than 9 for at least 5 days were unlikely to survive.

Effect of acute moderate changes in PaCO2 on global hemodynamics and gastric perfusion

OBJECTIVE

To describe global hemodynamics and splanchnic perfusion changes in response to acute modifications in Paco2 in hemodynamically stable patients.

DESIGN

Prospective, randomized crossover study.

SETTING

Medical-surgical intensive care unit at a community hospital (400,000 inhabitants).

PATIENTS

Ten critically ill patients who were sedated, paralyzed, and mechanically ventilated.

INTERVENTIONS

Hypercapnia and hypocapnia were obtained by increasing and reducing instrumental deadspace in random order. After each intervention, patients returned to the basal condition. Each period lasted 80 min: 20 min to achieve stable Paco2 and 60 min for tonometer equilibration. In each period, global hemodynamic variables and tonometric data were collected. The periods were compared using analysis of variance.

MEASUREMENTS AND MAIN RESULTS

Acute hypercapnia (Paco2 from 40+/-3 to 52+/-3 torr, p<.05) increased cardiac index (3.43+/-0.37 vs. 3.97+/-0.43 mL/min/m2, p<.05), heart rate (95+/-6 vs. 105+/-3 beats/min, p<.05), and mean pulmonary artery pressure (21+/-1 vs. 24+/-1 mm Hg, p<.05) and reduced systemic vascular resistance (992+/-98 vs. 813+/-93 dyne x sec/ cm5, p<.05) and oxygen extraction ratio (27+/-3% vs. 22+/-2%, p<.05). Standardized intramucosal Pco2 increased from 49+/-2 to 61+/-3 torr (p<.05) with an associated decrease in calculated intramucosal pH ([pHi] 7.35+/-0.03 vs. 7.25+/-0.02, p<.05), but the gastro-arterial Pco2 gradient (deltaPco2) did not change. Acute hypocapnia (Paco2 from 41+/-3 to 34+/-3 torr, p<.05; pH 7.41+/-0.01 to 7.47+/-0.02, p<.05) induced slight increments in systemic vascular resistance (995+/-117 vs. 1088 +/- 160 dyne x sec/cm5, p<.05) and oxygen extraction ratio (28+/-2% vs. 30+/-2%, p<.05). Standardized intramucosal Pco2 decreased (50+/-4 vs. 44+/-3 torr, p<.05), pHi increased (7.33+/-0.03 vs. 7.36+/-0.02; p<.05), but deltaPco2 did not change.

CONCLUSIONS

In this small group of stable patients, moderate acute variations in Paco2 had a significant effect on global hemodynamics, but splanchnic perfusion, assessed by deltaPco2, did not change. In these conditions, the use of pHi to evaluate gastric perfusion appears unreliable.

Effect of ranitidine on gastric intramucosal pH in critically ill patients

OBJECTIVE

To determine whether ranitidine a) increases the values of gastric intramucosal pH (pHi) in critically ill patients, as determined by tonometry; b) reduces the variability of these measurements.

DESIGN

Prospective, double blind, randomized, placebo-controlled study.

SETTING

General Intensive Care Unit of a teaching hospital.

PATIENTS

Twenty-five critically ill, mechanically ventilated patients requiring arterial catheter and nasogastric tube.

INTERVENTIONS

Tonometer placement; blind, random administration of intravenous ranitidine (50 mg) or placebo.

MEASUREMENTS AND MAIN RESULTS

Tonometer saline PCO2 (PCO2i), arterial blood gases, gastric juice pH and pHi were determined immediately before, and 2, 4, 6 and 8 h after, ranitidine (12 patients) or placebo (13 patients). Ranitidine significantly increased gastric juice pH, but did not affect PCO2i or pHi; pHi was 7.34 +/- 0.14 before ranitidine, and 7.30 +/- 0.12, 7.31 +/- 0.11, 7.31 +/- 0.14 and 7.31 +/- 0.12-2, 4, 6 and 8 h, respectively, after ranitidine administration (p = 0.55). Ranitidine did not modify the coefficients of variation of PCO2i or pHi, either. No significant changes in gastric juice pH, PCO2i or pHi were observed in the placebo group.

CONCLUSIONS

In critically ill patients, ranitidine has no effect on pHi values, and does not increase the reproducibility of pHi measurements.

Effect of sucralfate on gastric intramucosal pH in critically ill patients

OBJECTIVE

To determine whether sucralfate administration affects the tonometric measurement of gastric intramucosal pH (pHi).

DESIGN

Non-randomized observational study.

SETTING

General intensive care unit of a teaching hospital.

PATIENTS

Twenty critically ill, mechanically ventilated, consecutively admitted patients requiring an arterial catheter and nasogastric tube.

INTERVENTIONS

Tonometer placement and sucralfate administration.

MEASUREMENTS AND MAIN RESULTS

We simultaneously determined tonometer saline PCO2 (PCO2i), arterial blood gases, pH of gastric juice and pHi. These parameters were evaluated immediately before sucralfate administration, and 2 h and 4 h after. We did not detect any change in either PCO2i or pHi after sucralfate administration (PCO2i: basal 6.4 +/- 1.7, 2 h 6.3 +/- 1.7, 4 h 6.3 +/- 1.7; pHi: basal 7.35 +/- 0.13, 2 h 7.36 +/- 0.12, 4 h 7.36 +/- 0.12).

CONCLUSIONS

Sucralfate does not affect the tonometric measurement of PCO2i and pHi.

Clinical consequences of the implementation of a weaning protocol

OBJECTIVE

To analyze the clinical and economic consequences of the implementation of a weaning protocol in patients mechanically ventilated (MV) for more than 48 h.

DESIGN

Comparative study.

SETTING

General intensive care unit (ICU) in a county hospital covering 360000 inhabitants.

PATIENTS

51 patients weaned by a fixed protocol were studied prospectively and compared with 50 retrospective controls.

MEASUREMENTS

The following variables were assessed: Acute Physiology and Chronic Health Evaluation (APACHE) II score, age, cause of respiratory failure, type of extubation (direct extubation or extubation using a weaning technique), number of days on MV before the weaning trial, weaning time, total duration of MV, complications (reintubations and tracheostomies), length of ICU stay, and mortality.

RESULTS

The groups were comparable in terms of age, APACHE II score, and main cause of acute respiratory failure. Number of days on MV up to the weaning trial were similar in the two groups (8.4 +/- 7.7 in the protocol group vs 7.5 +/- 5.5 in the control group, NS). Most of the patients (80%) in the protocol group were directly extubated without a weaning technique, unlike the control group (10%) (p < 0.01). When a weaning technique was used, the weaning time was similar in both groups (3.5 +/- 3.9 days vs 3.6 +/- 2.2 days in the control group). Duration of MV was shorter in the protocol group (10.4 +/- 11.6 days) than in the control group (14.4 +/- 10.3 days) (p < 0.05). As a result, the ICU stay was reduced by using the weaning protocol (16.7 +/- 16.5 days vs 20.3 +/- 13.2 days in the control group, p < 0.05). We found no differences in reintubation rate (17 vs 14% in the control group) and need for tracheostomies (2 vs 8% in the control group).

CONCLUSION

The implementation of a weaning protocol decreased the duration of MV and ICU stay by increasing the number of safe, direct extubations.

Use of capnography to detect hypercapnic episodes during weaning from mechanical ventilation

OBJECTIVE

To evaluate the relationship between PaCO2 and end-tidal CO2 tension (PetCO2) before weaning and during a weaning trial and to determine the ability of PetCO2 to identify clinically relevant episodes of hypercapnia.

DESIGN

Open, prospective study.

SETTING

General intensive care unit.

PATIENTS

30 critically ill patients (mean age 63 +/- 2 years; Acute Physiology And Chronic Health Evaluation (APACHE) II of 18.4 +/- 3) who underwent a weaning trial during the recovery phase of acute respiratory failure requiring mechanical ventilation (MV) (8.9 +/- 1 days on MV).

INTERVENTIONS

Weaning trial consisted of 2 h breathing on 5 cmH2O of Continuous Positive Airway Pressure (CPAP).

MEASUREMENTS AND RESULTS

Arterial blood gas values, PetCO2 register and pulse oximetry determinations were recorded in assist/control ventilation before CPAP, after 1 h on CPAP and after 2 h on CPAP (immediately before extubation) or immediately before returning to assist/control mode in patients who failed the weaning trial. Clinically relevant hypercapnic episodes were described as: (1) an increment in PaCO2 > 42 mm Hg in previously normocapnic patients and (2) an increment of > 8 mm Hg from previous PaCO2 in previously hypercapnic patients. Changes in PaCO2 and changes in PetCO2 between MV and the first and second hour of CPAP showed a significant correlation (r = 0.74; p < 0.01). Clinically relevant hypercapnic episodes were detected by increments of > 3 mm Hg in PetCO2 with a sensitivity of 82%, a specificity of 76% and a negative predictive value of 94%. The area under the receiver operating characteristic curve for increments in PetCO2 was 0.90.

CONCLUSIONS

Capnography provided good assessment of hypercapnic episodes during weaning, although the high number of false positives may result in arterial blood sampling in patients who do not present with ventilation failure.

Impact of using an indwelling introducer on diagnosis of Swan-Ganz pulmonary artery catheter colonization

A prospective study was conducted to determine how the use of an indwelling introducer influences the diagnosis of pulmonary artery catheter (PAC) colonization. Sixty-six consecutive PACs and introducers were aseptically removed over a 15-month period. Two segments of the catheter and the catheter tip (a proximal segment from the portion of the catheter beneath the introducer) and two segments of the introducer (a proximal intradermal segment and the introducer tip) were cultured using a semiquantitative technique. Nineteen of 66 (28.7%) PACs showed colonization, representing an incidence of 5.6 episodes per 100 catheterization-days. Catheter tip cultures identified only 68% of colonized PACs; this yield rose to 91% when introducer tip cultures were added. These results indicate a need to evaluate both introducer tip cultures and catheter tip cultures for an accurate diagnosis of PAC colonization when an indwelling introducer is used.

[Selective pulmonary vasodilator effect of inhaled nitric oxide in a patient with pulmonary thromboembolism]

The increase in pulmonary vascular resistences in acute pulmonary thromboembolism (APT) is the consequence of anatomical obstruction and pulmonary artery constriction. The administration of inhaled nitric oxide (NO) may be therapeutically useful in acute pulmonary hypertension by APT given its limited vasodilator effect on pulmonary circulation. A patient with APT in whom this selective vasodilator effect was observed is presented. The authors suggest that the administration of inhaled NO may be a potentially beneficial coadjuvant therapy in acute pulmonary hypertension induced by APT.

Hemodynamic responses to external counterbalancing of auto-positive end-expiratory pressure in mechanically ventilated patients with chronic obstructive pulmonary disease

OBJECTIVE

To study the effect of positive end-expiratory pressure (PEEP) on right ventricular hemodynamics and ejection fraction in patients with chronic obstructive pulmonary disease and positive alveolar pressure throughout expiration by dynamic hyperinflation (auto-PEEP).

DESIGN

Open, prospective, controlled trial.

SETTING

General intensive care unit of a community hospital.

PATIENTS

Ten patients sedated and paralyzed with an acute exacerbation of chronic obstructive pulmonary disease undergoing mechanical ventilation.

INTERVENTIONS

Insertion of a pulmonary artery catheter modified with a rapid response thermistor and a radial arterial catheter. PEEP was then increased from 0 (PEEP 0) to auto-PEEP level (PEEP = auto-PEEP) and 5 cm H2O above that (PEEP = auto-PEEP +5).

MEASUREMENTS

At each level of PEEP, airway pressures, flow and volume, hemodynamic variables (including right ventricular ejection fraction by thermodilution technique), and blood gas analyses were recorded.

MAIN RESULTS

The mean auto-PEEP was 6.6 +/- 2.8 cm H2O and the total PEEP reached was 12.2 +/- 2.4 cm H2O. The degree of lung inflation induced by PEEP averaged 145 +/- 87 mL with PEEP = auto-PEEP and 495 +/- 133 mL with PEEP = auto-PEEP + 5. The PEEP = auto-PEEP caused a right ventricular end-diastolic pressure increase, but there was no other significant hemodynamic change. With PEEP = auto-PEEP + 5, there was a significant increase in intravascular pressures; this amount of PEEP reduced cardiac output (from 4.40 +/- 1.38 L/min at PEEP 0 to 4.13 +/- 1.48 L/min; p < .05). The cardiac output reduction induced by PEEP = auto-PEEP + 5 was > 10% in only five cases and this group of patients had significantly lower right ventricular volumes than the group with less cardiac output variation (right ventricular end-diastolic volume: 64 +/- 9 vs. 96 +/- 26 mL/m2; right ventricular end-systolic volume: 38 +/- 6 vs. 65 +/- 21 mL/m2; p < .05) without significant difference in the other variables that were measured. Neither right ventricular ejection fraction nor right ventricle volumes changed as PEEP increased, but there were marked interpatient differences and also pronounced changes in volume between stages in individual patients.

CONCLUSIONS

In the study conditions, PEEP application up to values approaching auto-PEEP did not result in the impairment of right ventricular hemodynamics, while higher levels reduced cardiac output in selected patients.

Relationship between expired capnogram and respiratory system resistance in critically ill patients during total ventilatory support

To examine the relationship of expired capnograms and respiratory system resistance (Rrs) in intubated critically ill patients, we consecutively studied 41 mechanically ventilated patients to (1) analyze the association between expired CO2 slope and auto-positive end-expiratory pressure (auto-PEEP), between Rrs and auto-PEEP, between Rrs and expired CO2 slope, and between Rrs and arterial minus end-tidal PCO2 gradient (PaCO2-PETCO2 gradient) and (2) to investigate the capacity of the expired CO2 slope and PaCO2-PETCO2 gradient to predict Rrs during mechanical ventilation. Regression analysis found a close correlation between Rrs and expired CO2 slope (r = 0.86; p < 0.001), between Rrs and auto-PEEP (r = 0.75; p < 0.001), and between auto-PEEP and expired CO2 slope (r = 0.74; p < 0.001). Weak correlation was found between Rrs and PaCO2-PETCO2 gradient (r = 0.48; p < 0.01). Prediction interval limits at 95 percent confidence level for Rrs are approximately +/- 7.39 cm H2O/L/s from the predicted value obtained by the regression equation, where Rrs = 11.42 + 2.28 expired CO2 slope. These observations suggest that CO2 elimination in critically ill patients is strongly modulated by lung, airway, endotracheal tube, and ventilator equipment resistances. Although continuous capnogram waveform monitoring at the bedside might be useful to assess Rrs, very accurate predictions could be done only in determinate patients.