Bedside waveforms interpretation as a tool to identify patient-ventilator asynchronies

Objective During assisted modes of ventilatory support the ventilatory output is the final expression of the interaction between the ventilator and the patients controller of breathing. This interaction may lead to patient-ventilator asynchrony, preventing the ventilator from achieving its goals, and may cause patient harm. Flow, volume, and airway pressure signals are significantly affected by patient-ventilator interaction and may serve as a tool to guide the physician to take the appropriate action to improve the synchrony between patient and ventilator. This review discusses the basic waveforms during assisted mechanical ventilation and how their interpretation may influence the management of ventilated patients. The discussion is limited on waveform eye interpretation of the signals without using any intervention which may interrupt the process of mechanical ventilation.Discussion Flow, volume, and airway pressure may be used to (a) identify the mode of ventilator assistance, triggering delay, ineffective efforts, and autotriggering, (b) estimate qualitatively patients respiratory efforts, and (c) recognize delayed and premature opening of exhalation valve. These signals may also serve as a tool for gross estimation of respiratory system mechanics and monitor the effects of disease progression and various therapeutic interventions.Conclusions Flow, volume, and airway pressure waveforms are valuable real-time tools in identifying various aspects of patient-ventilator interactionElectronic Supplementary Material Electronic supplementary material to this paper can be obtained by using the Springer Link server located at     

Splendors and miseries of expired CO2 measurement in the suspicion of pulmonary embolism

Capnography has been studied for decades as a potential diagnostic tool for suspected pulmonary embolism. Despite technological refinements and its combination with other non-invasive instruments, no evidence to date allows recommending the use of expired carbon dioxide measurement as a rule-out test for pulmonary embolism without additional radiological testing. Further investigations are, however, still warranted.In the previous issue of Critical Care, Rumpf and colleagues [1] evaluated the potential contribution of measuring end-tidal carbon dioxide (CO₂) for suspected pulmonary embolism (PE) in the prehospital setting. Capnography has been studied for decades as a potential diagnostic tool for patients with suspected PE. Indeed, PE is expected to create areas of reduced arterial flow with normal or increased alveolar ventilation, resulting in increased alveolar dead space volume and reduced global expired CO₂. This should create a difference between arterial and end-tidal CO₂ values, as first demonstrated by Robin and colleagues [2] in 1959. However, during the two following decades, several authors pointed out the numerous pitfalls and sources of errors in assessing the arterial to end-tidal CO₂ difference in the clinical suspicion of PE, and this test was finally abandoned until the nineties [3-5].Three elements explain the current resurgence of expired CO₂ measurement in the suspicion of PE. First, technical improvements now allow measuring CO₂ not only for monitoring purposes in intubated patients in operating rooms but also as a diagnostic tool in spontaneously breathing patients in the emergency department or even in the field. Second, volumetric capnography, which displays expired CO₂ as a function of the expired volume of the patient, did much to renew interest in capnography because of its potential for better performance in diagnosing PE than the arterial to endtidal CO₂ difference, even though that expectation could not be confirmed by recent results [6,7]. Finally, in the era of non-invasive strategies for PE combining several tests of various types, such as clinical evaluation, biological tests, and imaging, the evaluation of a potential role for CO₂ measurement in combination with those other instruments made sense. Numerous studies are available, and although none to date has been able to prove the safety of such a non-invasive strategy incorporating capnography with a high enough level of evidence to allow its recommendation in daily clinical practice, the venue remains interesting [7-11].Where then can we place the endeavor of Rumpf and colleagues? They included 131 consecutive patients suspected of PE who had an abnormal rapid point-of-care D-dimer result in a prehospital setting and evaluated them with a combination of clinical probability of PE (two-level Wells score) and measurement of the end-tidal partial pressure of CO₂ (PCO₂). PE was diagnosed in the emergency department by a positive spiral computed tomography, a high-probability V/Q scan, or a positive pulmonary angiogram. The combination of a normal end-tidal CO₂ value (defined as higher than 28 mm Hg based on a receiver operating characteristic analysis) and an unlikely probability of PE had a 100% sensitivity and 100% negative predictive value (95% confidence interval [CI] 90% to 100%) for ruling out PE. In contrast, the association of a low end-tidal CO₂ value (less than 28 mm Hg) and a high clinical probability had only an 86% positive predictive value for PE, and further tests would certainly be required in such patients. Clearly, those results are preliminary. This is a small series and it was designed to set the cutoff value for this particular capnography technique and assess its feasibility in the field. Moreover, as acknowledged by the authors themselves, the clinicians who established the diagnosis were not blinded to either clinical assessment or capnography results. Finally, the prevalence of PE is unusually high, although this would tend to bias the results toward lower, not higher, sensitivity. But the sheer simplicity of the technique used by Rumpf and colleagues [1] is appealing and certainly deserves validation in a large-scale prospective study. Indeed, it emphasizes the use of expired CO₂ alone without associated arterial PCO₂, and this is a pragmatic issue in modern emergency medicine [12]. Also, the use of capnography in the prehospital setting is interesting: there might be situations in which a rapid and rough evaluation of the patient''s expired CO₂ status would help emergency physicians in making vital decisions, such as starting thrombolysis for a suspected fulminant PE, as well as in monitoring the hemodynamic effect of thrombolysis in such patients [13].Finally, the merit of the article by Rumpf and colleagues [1] is to remind us that clinical applications of capnography are still growing, especially amongst spontaneously breathing patients. Physicians dealing with acute medicine should make every effort to become familiar with expired CO₂ measurement. Inconclusive capnographic results related to tachypneic or apprehensive patients do not overcome the potential for expired CO₂ to be placed inside the diagnostic algorithm of a challenging disease like PE.     

Monitoring pulmonary function with superimposed pulmonary gas exchange curves from standard analyzers

Objective.A repetitive graphic display of the single breath pulmonary function can indicate changes in cardiac and pulmonary physiology brought on by clinical events. Parallel advances in computer technology and monitoring make real-time, single breath pulmonary function clinically practicable. We describe a system built from a commercially available airway gas monitor and off the shelf computer and data-acquisition hardware. Methods.Analog data for gas flow rate, O₂, and CO₂ concentrations are introduced into a computer through an analog-to-digital conversion board. Oxygen uptake (VO₂) and carbon dioxide output (VCO₂) are calculated for each breath. Inspired minus expired concentrations for O₂ and CO₂ are displayed simultaneously with the expired gas flow rate curve for each breath. Dead-space and alveolar ventilation are calculated for each breath and readily appreciated from the display. Results.Graphs illustrating the function of the system are presented for the following clinical scenarios; upper airway obstruction, bronchospasm, bronchopleural fistula, pulmonary perfusion changes and inadequate oxygen delivery. Conclusions.This paper describes a real-time, single breath pulmonary monitoring system that displays three parameters graphed against time: expired flow rate, oxygen uptake and carbon dioxide production. This system allows for early and rapid recognition of treatable conditions that may lead to adverse events without any additional patient measurements or invasive procedures. Monitoring systems similar to the one described in this paper may lead to a higher level of patient safety without any additional patient risk.     

A sorbent containing pH-responsive chelating residues of aspartic and maleic acids for mitigation of toxic metal ions,cationic, and anionic dyes

t-Butyl hydroperoxide-initiated cycloterpolymerization of diallylaminoaspartic acid hydrochloride [(CH₂ CHCH₂)₂NH⁺CH(CO₂H)CH₂CO₂H Cl⁻] (I), maleic acid (HO₂CH CHCO₂H) (II) and cross-linker tetraallylhexane-1,6-diamine dihydrochloride [(CH₂ CHCH₂)₂NH⁺(CH₂)₆NH⁺ (CH₂CH CH₂)₂ 2Cl⁻] (III) afforded a new pH-responsive resin (IV), loaded with four CO₂H and a chelating motif of NH⁺⋯CO₂⁻ in each repeating unit. The removal of cationic methylene blue (MB) (3000 ppm) at pH 7.25 and Pb(ii) (200 ppm) at pH 6 by IV at 298, 313, and 328 K followed second-order kinetics with E_a of 33.4 and 40.7 kJ mol⁻¹, respectively. Both MB and Pb(ii) were removed fast, accounting for 97.7% removal of MB within 15 min at 313 K and 94% of Pb(ii) removal within 1 min. The super-adsorbent resin gave respective q_max values of 2609 mg g⁻¹ and 873 mg g⁻¹ for MB and Pb(ii). IV was also found to trap anionic dyes; it removed 91% Eriochrome Black T (EBT) from its 50 ppm solutions at pH 2. The resin was found to be effective in reducing priority metal contaminants (like Cr, Hg, Pb) in industrial wastewater to sub-ppb levels. The synthesis of the recyclable resin can be easily scaled up from inexpensive starting materials. The resin has been found to be better than many recently reported sorbents.

Cycloterpolymerization of diallylaminoaspartic acid hydrochloride (I), maleic acid (II) and a cross-linker (III) afforded a new pH-responsive resin (IV), loaded with four CO₂H and a chelating motif of NH⁺⋯CO₂⁻ in each repeating unit.     

The influence of physiotherapy and suction on respiratory deadspace in ventilated children

Objective To assess and compare the effects of respiratory physiotherapy and suction on deadspace volumes, carbon dioxide elimination (VCO₂), end tidal CO₂ (ETCO₂), and arterial partial pressure of carbon dioxide (PaCO₂) in ventilated infants and children.Design Randomised crossover study. Participants received both treatments with a washout interval of more than 90 min.Setting Intensive tertiary care units, Great Ormond Street Hospital, London.Patients Eighty-seven fully ventilated children, requiring physiotherapy, with arterial lines in situ. Paired measurements were obtained in 81 patients, of whom 6 were excluded because of tracheal tube leak greater than 20%.Interventions Respiratory physiotherapy and suction.Measurements and results Data were collected April 1998–March 2000. The CO₂SMO Plus respiratory monitor was used to calculate parameters before and 30 min after both interventions. Physiotherapy lasted longer and required more saline and catheters per treatment (p<0.005). There were significant increases in physiological deadspace (VD_phys)/kg (p<0.0001), alveolar deadspace (VD_alv)/kg (p<0.0001) and VD_phys/tidal volume (V_T) (p<0.05) following physiotherapy that were not observed following suction. There were no significant changes following either treatment with respect to airway deadspace (VD_airway), VCO₂ or PaCO₂. Comparison of the mean differences following treatments indicated significant differences between physiotherapy and suction in terms of VD_phys/kg (p<0.005), VD_alv/kg (p<0.005), expired tidal volumes (V_TE) (p<0.05), mixed expired CO₂ (PeCO₂) (p<0.04) and ETCO₂ (p<0.03).Conclusions Differences between physiotherapy and suction techniques probably accounted for their statistically distinguishable effects on deadspace. VD_phys and VD_alv may be more sensitive indicators of subtle changes in gas exchange and regional ventilation than VCO₂ or PaCO₂. However, interpretation of these outcomes is dependent on concurrent examination of the parameters from which they are derived.Electronic Supplementary Material Supplementary material is available in the online version of this article at Janet Stocks is supported by Portex PLC.This work was undertaken by Great Ormond Street Hospital for Children NHS Trust, which received a proportion of its funding from the NHS Executive; the views expressed in this publication are those of the authors and are not necessarily those of the NHS Executive.An editorial regarding this article can be found in the same issue ()     

Safety and applicability of a pre-stage public access ventilator for trained laypersons: a proof of principle study

Background

Contemporary resuscitation guidelines for basic life support recommend an immediate onset of cardiac compressions in case of cardiac arrest followed by rescue breaths. Effective ventilation is often omitted due to fear of doing harm and fear of infectious diseases. In order to improve ventilation a pre-stage of an automatic respirator was developed for use by laypersons.

Methods

Fifty-two healthy volunteers were ventilated by means of a prototype respirator via a full-face mask in a pilot study. The pre-stage public access ventilator (PAV) consisted of a low-cost self-designed turbine, with sensors for differential pressure, flow, FO₂, FCO₂ and 3-axis acceleration measurement. Sensor outputs were used to control the respirator and to recognize conditions relevant for efficiency of ventilation and patients’ safety. Different respiratory manoeuvres were applied: a) pressure controlled ventilation (PCV), b) PCV with controlled leakage and c) PCV with simulated airway occlusion. Sensor signals were analysed to detect leakage and airway occlusion. Detection based upon sensor signals was compared with evaluation based on clinical observation and additional parameters such as exhaled CO₂.

Results

Pressure controlled ventilation could be realized in all volunteers. Leakage was recognized with 93.5% sensitivity and 93.5% specificity. Simulated airway occlusion was detected with 91.8% sensitivity and 91.7% specificity.

Conclusion

The pre-stage PAV was able to detect potential complications relevant for patients’ safety such as leakage and airway occlusion in a proof of principle study. Prospectively, this device provides a respectable basis for the development of an automatic emergency respirator and may help to improve bystander resuscitation.

     

Highly efficient Co centers functionalized by nitrogen-doped carbon for the chemical fixation of CO2

CO₂, the main greenhouse gas, has received considerable attention due to environmental issues. From a scientific perspective, CO₂ as a cheap and abundant carbon source, could be applied in synthesizing more valuable chemicals such as urea, formic acid, and cyclic carbonates. However, the high bond energy of C O (750 kJ mol⁻¹) and the non-polarity property make CO₂ molecules difficult to activate. In this paper, we have carefully designed a low-cost, stable and, most importantly, highly efficient Co-based heterocatalyst Co@N_xC functionalized by nitrogen-doped carbon to activate CO₂ molecules and convert it into cyclic carbonates. The CO₂ conversion process could be triggered at very mild conditions (60 °C and 1 bar CO₂). We carefully adjusted the nitrogen content in the carbon support to enhance the catalytic performance of Co centers via the interface effect. Consequently, the optimal catalyst displayed extraordinary activity toward the cycloaddition of CO₂ with styrene oxide as high conversion (92%) and selectivity (>99%) were achieved in 4 h without byproducts.

A highly efficient Co@N_xC functionalized heterocatalyst has been prepared to activate CO₂ molecules and convert it into cyclic carbonates.     

Series dead space volume assessed as the mean value of a distribution function

Series dead space (VdS) is assumed to be represented by that volume exhaled until alveolar gas is observed. Phase II of the single breath CO₂-diagram contains the (flow, concentration and sequence weighted) distribution off all stationary interfaces (SI) expired before phase III. We describe a new method to estimate the mean value of VdS based on the differentiation of phase II. This approximation of VdS is called the Pre Interface Expirate (PIE) and is compared in this study with the integrative approach of Langley. Tidal volume (Vt) and PEEP were varied from 71 to 123% and from 0 to 6 cmH₂O respectively.The estimation of VdS by differentiation of phase II (PIE) shows excellent reproducibility and depends only on phase II — not on phase III and IV as does VdS-Langley. PIE does not depend on Vt and PEEP per se but reflects the distension of convective airways due to elevated end-inspiratory airway pressure.Our results confirm the predictions of Paiva's model calculations in that the size of VdS is determined by the distension of airways rather than by the altered position of the SI.     

Introducing hydrophilic ultra-thin ZIF-L into mixed matrix membranes for CO2/CH4 separation

Mixed matrix membranes (MMMs) were developed by mixing hydrophilically modified two-dimensional (2D) imidazole framework (named as hZIF-L) flakes into a Pebax MH 1657 (Pebax) matrix, and designed to separate carbon dioxide/methane (CO₂/CH₄) mixtures. The hZIF-L flakes were important for increasing the effectiveness of the MMMs. First, the tannic acid (TA) etched hZIF-L flakes have a large number of microporous (1.8 nm) and two-dimensional anisotropic transport channels, which offered convenient gas transport channels and improved the permeability of CO₂. Second, the TA molecules provide the surface of the ZIF-L flakes with more hydrophilic functional groups such as carbonyl groups (C O) and hydroxyl groups (–OH), which could effectively prevent non-selective interfacial voids and filler agglomeration in the Pebax matrix, and also presented strong binding ability to water and CO₂ molecules. The satisfactory interface compatibility and affinity with the CO₂ molecule promoted its permeability, solubility, and selectivity. As a result, the MMMs exhibited the highest performance of gas separation with the hZIF-L flake weight content of 5%, at which the CO₂ permeability and CO₂/CH₄ selectivity were 502.44 barrer and 33.82 at 0.2 MPa and 25 °C, respectively.

Schematic diagram of CO₂ transfer in Pebax/hZIF-L mixed matrix membranes.     

A Flow-Through Capnometer for Obstructive Sleep Apnea

Introduction

Capnogram is often distorted due to aspiration of expired gas when a sidestream capnometer is used for non-intubated, spontaneously breathing condition. The purpose of this study was to make a flow-through capnometer without aspiration and to check if this capnometer precisely detected apnea during obstructive sleep apnea (OSA).

Methods

(1) Flow-through capnometer The capnometer consisted of a flow-through etCO₂ sensor, cap-ONE^®, and an accompanying capnometer. The size of cap-ONE^® was small enough to be fitted under the nose where gas expired from the nose and the mouth passed through. Thus, the expired gas to be measured is directly blown into the cap-ONE^®. (2) The cap-ONE^® using a spontaneously breathing model Capnograms obtained by the cap-ONE^® and sidestream capnometers during nasal and oral breathing under normal and reduced ventilation were compared with a reference capnogram. (3) Clinical study with OSA patients With nineteen OSA patients capnograms during apnea events diagnosed as OSA by polysomnography were examined using the cap-ONE^®. (4) Simulation study with an OSA model Apnea in which inspiratory flow was zero and small expiratory flows repeated was produced. Capnograms and apnea detection were compared between the cap-ONE^® and sidestream capnometers.

Results

In the spontaneouly breathing model capnograms and etCO₂ of the cap-ONE^® during nasal and oral breathing were almost identical with the reference capnogram but those of sidetream capnometers during oral breathing were significantly reduced. In the clinical study 41% of total OSA events showed capnograms with prolonged and elevated phase with small ripples. In a simulation study reduction of CO₂ tension during no-inspiration was small and apnea was successfully detected with the cap-ONE^®. However, with sidestream capnometers the reduction of CO₂ tension was large and apnea was not detected.

Conclusions

We concluded that the cap-ONE^® can record capnograms with minimum distortion and detect apnea reliably during OSA.

     

Measurement of CO2 response with the breath-by-breath automatic acquisition of the breathing pattern and occlusion pressure

Objective. Our objective is to present a methodology for the automated acquisition and storage of BP and P_0.1 during a CO₂ rebreathing test.Methods. The system consists of a microcomputer with additional circuits and an automatic electronically controlled valve to occlude the inspiratory airway. Data collection and data processing are separate programs. Airway pressure and flow are digitized at a 100-Hz rate, whilePetCO₂ is determined and P_0.1 is measured on a breath-by-breath basis. Off-line processing calculates the BP variables, generates a correlation matrix (Ve/PetCO₂,Ttot/PetCO₂,Ti/PetCO₂,Te/PetCO₂, [Vt/Ti]/PetCO₂, [Ti/Ttot]/PetCO₂, P_0.1/PetCO₂), and edits graphic data. The accuracy of the volume and pressure measurements was tested by comparing known volumes provided by a syringe (n=100) and a series of pressures controlled by a water manometer (n=41) on the one hand, with volumes and pressures measured by the device. The accuracy of the time intervals and P_0.1 was assessed by comparing in 10 healthy subjects the values measured manually on a graphic recording with those provided by the device (n=170).Results. Volumes: V_measured=0.99×V_controlled,r=0.99,p<0.001. Pressures: P_measured=0.97×P_controlled+0.09,r=0.98,p<0.001. Inspiratory time:Ti _automatic=0.91×Ti _graphic+0.22,r=0.93,p<0.001. Expiratory time:Te _automatic=0.93×Te _graphic+0.34,r=0.95,p<0.001. Occlusion pressure: P_{0.1 automatic}=0.95×P_{0.1 graphic}+0.62,r=0.94,p<0.001. Reproducibility was assumed to be represented by the intraindividual coefficient of variation of the CO₂ response. The comparison of an automatic breath-to-breath method with a graphic manual recording revealed significantly less variability with the former (Ve/PetCO₂: 15.2±4.5% vs 22.5±6.3%,p<0.01; P_0.1/PetCO₂: 8.3±4.3% vs 19.7±7.2%,p<0.001; [Vt/Ti]/PetCO₂: 9.1±3.5% vs 14.5±5.3%,p<0.05).Conclusion. Our automated acquisition and storage of waveforms and breath-by-breath determination of BP and P_0.1 provide an easy and thorough analysis of the respiratory response to CO₂ and decrease the variability of the results.     

A comparison of the I-gel supraglottic airway as a conduit for tracheal intubation with the intubating laryngeal mask airway: A manikin study

Background

Insertion of a supraglottic airway and tracheal intubation through it may be indicated in resuscitation scenarios where conventional laryngoscopy fails. Various supraglottic devices have been used as conduits for tracheal intubation, including the intubating laryngeal mask airway (ILMA), the Ctrach™ laryngeal mask and the I-gel supraglottic airway.

Methods

A prospective study with 25 participants evaluated the success rate of blind intubation (using a gum-elastic bougie, an Aintree intubating catheter (AIC) and designated tracheal tube) and fibrescope-guided tracheal intubation (through the intubating laryngeal mask airway and the I-gel supraglottic airway) on three different airway manikins.

Results

Twenty-five anaesthetists performed three intubations with each method on each of three manikins. The success rate of the fibrescope-guided technique was significantly higher than blind attempts (P < 0.0001) with both devices. For fibreoptic techniques, there was no difference found between the ILMA and I-gel (P > 0.05). All blind techniques were significantly more successful in the ILMA group compared to the I-gel (P < 0.0001 for bougie, Aintree catheter and tracheal tube, respectively).

Conclusions

The results of this study show that, in manikins, fibreoptic intubation through both ILMA and I-gel is a highly successful technique. Blind intubation through the I-gel showed a low success rate and should not be attempted.     

Predicting the lack of ROSC during pre-hospital CPR: Should an end-tidal CO2 of 1.3 kPa be used as a cut-off value?

Aim

The aim of this study was to investigate if an initial ETCO₂ value at or below 1.3 kPa can be used as a cut-off value for whether return of spontaneous circulation during pre-hospital cardio-pulmonary resuscitation is achievable or not.

Materials and methods

We prospectively registered data according to the Utstein-style template for reporting data from pre-hospital advanced airway management from February 1st 2011 to October 31st 2012. Included were consecutive patients at all ages with pre-hospital cardiac arrest treated by eight anaesthesiologist-staffed pre-hospital critical care teams in the Central Denmark Region.

Results

We registered data from 595 cardiac arrest patients; in 60.2% (n = 358) of these cases the pre-hospital critical care teams performed pre-hospital advanced airway management beyond bag-mask ventilation. An initial end-tidal CO₂ measurement following pre-hospital advanced airway management were available in 75.7% (n = 271) of these 358 cases. We identified 22 patients, who had an initial end-tidal CO₂ at or below 1.3 kPa. Four of these patients achieved return of spontaneous circulation.

Conclusion

Our results indicates that an initial end-tidal CO₂ at or below 1.3 kPa during pre-hospital CPR should not be used as a cut-off value for the achievability of return of spontaneous circulation.     

Pressure modes of mechanical ventilation: the good, the bad, and the ugly

Numerous pressure modes are currently available on ventilators. The application of microprocessor technology has resulted in sophisticated mode options that are very responsive to patient-initiated efforts, yet little is known about how to use the modes or their effect on patient outcomes. This article describes a wide variety of pressure modes including traditional modes such as pressure support and pressure-controlled ventilation in addition to less traditional new modes such as airway pressure release ventilation, biphasic positive airway pressure, Pressure Augmentation (Bear 1000, Viasys Healthcare, Yorba Linda, California), Volume Support (Maquet, Bridgewater, New Jersey), Pressure Regulated Volume Control (Maquet, Bridgewater, New Jersey), Volume Ventilation Plus (Puritan Bennett, Boulder, Colorado), Adaptive Support Ventilation (Hamilton Medical, Switzerland), and Proportional Assist Ventilation (Dr?ger Medical, Richmond Hill, Ontario, Canada). The "good, the bad, and the ugly" issues surrounding the application, evaluation, and outcomes of the modes are discussed.     

Bench test assessment of mainstream capnography during high frequency oscillatory ventilation

To assess the feasibility, stability and predictability of pCO₂ measurement (PetCO₂) using a main stream capnograph in a high frequency oscillatory ventilation circuit. A commercially available capnograph was mounted into a high frequency oscillatory ventilator patient circuit, adjustable CO₂ flow was introduced into an artificial lung and the output of the CO₂ sensor assessed under varying ventilator settings. Influence of oxygen content, pressures, heat and moisture were recorded. A linear relationship between CO₂ flow rate and PetCO₂ was found. Varying ventilator settings influenced the measurements, but the results for PetCO₂ remained within a range of 1.5 mmHg above or under then mean measurement value. Measurements remained stable despite humidification, heat, pressure amplitudes or mean airway pressure changes. From this bench test, we conclude it is feasible to measure PetCO₂ using a main stream capnograph during high frequency oscillatory conditions, these measurements were stable during the experiment. Changes in CO₂ production or output can be detected. The system may prove to be of clinical value, but further in vivo measurements are warranted.     

Comparison of six methods to calculate airway resistance during mechanical ventilation in adults

Objective. A variety of methods are used to calculate indices of lung mechanics. We conducted this study to compare 6 methods of calculating airway resistance.Methods. Data were recorded from 20 adult mechanically ventilated patients. All were relaxed and breathing in synchrony with the ventilator, and an end-inspiratory pause sufficient to produce a pressure plateau (0.5–1.5 s) was used. Pressure and flow rate were measured at the proximal airway using a calibrated lung mechanics analyzer (VenTrak, Med Science, St Louis, MO). Flow rate, pressure, and volume were printed simultaneously. Airway resistance was calculated using 6 methods: Suter, Krieger, Neergard, Bergman, Comroe, and Jonson.Results. Mean calculated resistances (± SD) (cm H₂O/L/s) were 11.7 ± 4.8 (Suter method), 13.3 ± 5.0 (Krieger method), 14.9 ± 5.3 (Neergard method), 25.0 ± 6.6 (Bergman method), 24.7 ± 6.4 (Comroe method), and 26.9 ± 4.8 (Jonson method). By repeated measures analysis of variance, these differences were significant (p < 0.001). Using Scheffe analysis, no difference was found between the calculations using the Bergman, Comroe, and Jonson methods; these were significantly greater than the other 3 methods (p < 0.05).Conclusions. Methods that evaluate expiratory resistance (Comroe, Bergman, and Jonson) produce higher values than methods that evaluate inspiratory resistance (Suter and Neergard) or a combination of inspiratory and expiratory resistance (Krieger). Because of these differences, investigators should clearly describe their calculations when reporting airway resistance values.     

Reflection of differential pulmonary perfusion in polytrauma patients on differential lung ventilation (DLV)

Seventeen polytrauma patients with asymmetric pulmonary contusion were treated with differential lung ventilation (DLV). The ratios of differential values of end-tidal CO₂ concentration (ETCO₂) and CO₂ excretion ml/min ( ) were compared as indirect parameters for differential pulmonary perfusion. Both CO₂-derived methods indicated asymmetry after starting DLV suggesting asymmetric pulmonary perfusion as a consequence of contusion. Prior to stopping DLV a significant improvement in asymmetry was indicated by the differential ratios of ETCO₂ and values. The ETCO₂ ratio increased from 0.74±0.17 to 0.88±0.10, the ratio from 0.57±0.23 to 0.86±0.11. In two patients with very severe contusion who underwent bilobectomies a marked difference between the ratios of ETCO₂ and was observed. It is concluded that differential measurement of CO₂-derived variables may be useful in indicating differential perfusion in clinical practice on DLV. In very severe asymmetric contusion ETCO₂ ratios may underestimate the differential perfusion ratio.     

Study of the thermal decomposition mechanism of FOX-7 by molecular dynamics simulation and online photoionization mass spectrometry

The thermal decomposition mechanism of energetic materials is important for analyzing the combustion mechanisms of propellants and evaluating the safety of propellants during transport and storage. 1,1-Diamino-2,2-dinitroethylene (FOX-7) is an important insensitive energetic material that can be used as an oxidizer in propellants. However, the initial decomposition mechanism of FOX-7 is not clear to date. The ReaxFF molecular dynamics method is widely used in the investigation of the thermal decomposition mechanisms of energetic materials. Meanwhile, the combination of thermogravimetry with online photoionization time-of-flight mass spectrometry (TG-PI-TOF-MS) and online single-photon ionization time-of-flight mass spectrometry (SPI-TOF-MS) can reveal the decomposition products, which may be integrated with the results of the simulation. In this study, the primary thermal decomposition mechanism of 1,1-diamino-2,2-dinitroethylene (FOX-7) was studied by the ReaxFF molecular dynamics simulations and online photoionization mass spectrometry. The results of the molecular dynamics simulations showed that the primary decomposition step of FOX-7 is C–NO₂ cleavage; after this, C O formation occurs via a three-membered ring transition state, followed by NO elimination. The remaining structure loses NH₂ and H, resulting in the formation of the NHC C O structure, which finally breaks down into HNC and CO. NH₂ reacts with an H atom to produce NH₃. A reversible intramolecular hydrogen transfer was also observed at 2500 K; however, it failed to dominate the decomposition reaction. During the decomposition of FOX-7, the major products are N₂, NH₃, CO₂, and H₂N₂ and the minor products are H₂O, HN₂, and H₂. The TG-PI-TOF-MS spectrum shows three signals, i.e., m/z = 18, 28, and 30, which can be assigned to H₂O, CO, and NO, respectively. Moreover, four signals at m/z = 72.72, 55.81, 45.79, and 29.88 corresponding to the products (NH₂)₂C C O, (NH₂)C C O, NO₂, and NO have been obtained in the SPI-TOF-MS spectrum. The experimental data obtained via online photoionization mass spectrometry further validated the results of the molecular dynamics simulations.

In this work, the primary thermal decomposition mechanism of 1,1-diamino-2,2-dinitroethylene (FOX-7) was studied by ReaxFF molecular dynamics simulations and online photoionization mass spectrometry.