Effect of cardiogenic oscillations on gas mixing during tracheal insufflation of oxygen

In a previous study using tracheal insufflation of O2 (TRIO) at a rate of 2 l/min, we showed that anesthetized paralyzed dogs could be adequately oxygenated for up to 5 h, albeit with hypercapnia (mean arterial PCO2 approximately 160 Torr). To examine the contribution of cardiogenic oscillations in producing this gas exchange, we studied seven anesthetized paralyzed dogs weighing between 19.6 and 25.5 kg and quantified gas transport by analyzing continuous N2-washout curves in vivo and postmortem. We found that cardiogenic oscillations increase gas mixing roughly fourfold and that this value was independent of insufflation flow rate (0.2-10.0 l/min). Our results lend indirect evidence that, with regard to gas exchange, there are two mechanistically different zones in the lung during TRIO. One zone, located in the more peripheral areas of the lung, is dominated by the effects of cardiac oscillations and molecular diffusion and accounts for the increase in gas mixing found in the alive vs. dead dog. A second zone, close to the insufflated jet of O2, uses convective streaming to produce greater gas mixing at higher flows.     

Constant-flow ventilation in pigs

Constant-flow ventilation (CFV) is a ventilatory technique in which physiological blood gases can be maintained in dogs by a constant flow of fresh gas introduced via two catheters placed in the main-stem bronchi (J. Appl. Physiol. 53: 483-489, 1982). High-velocity gas exiting from the catheters can create uneven pressure differences in adjacent lung segments, and these pressure differences could lead to gas flow through collateral channels. To examine this hypothesis, we studied CFV in pigs, animals known to have a high resistance to collateral ventilation. In three pigs we examined steady-state gas exchange, and in six others we studied unsteady gas exchange at three flow rates (20, 35, and 50 l/min) and three catheter positions (0.5, 1.5, and 2.5 cm distal to the tracheal carina). During steady-state runs we were unable to attain normocapnia; the arterial CO2 partial pressure (PaCO2) was approximately 300 Torr at all flow rates and all catheter positions, compared with 20-50 Torr at similar flows and positions in dogs studied previously. The initial unsteady gas-exchange experiments indicated no consistent effect of catheter position or flow rate on the rate of rise of PaCO2. In three other pigs, the rates of rise of PaCO2 were compared with the rates observed with apneic oxygenation (AO). At the maximum flow and deepest position, the rate of rise of PaCO2 was lower during CFV than during AO. These data suggest that flow through collateral channels might be important in producing adequate gas transport during CFV; however, other factors such as airway morphometry and the effects of cardiogenic oscillations may explain the differences between the results in pigs and dogs.     

Contributions of diffusion jet flow and cardiac activity to regional ventilation in CFV.

The magnitude and regional distribution of local gas transport during constant-flow ventilation (CFV) were quantified by imaging the washout of nitrogen 13 (13NN) from anesthetized and paralyzed mongrel dogs with positron emission tomography. Equal jet flows, through two 2-mm-ID bronchial catheters 1 cm distal to the carina, were adjusted to provide eucapnic CFV (total flow = 57.6 ml.s-1.kg-1). Basal, midheart, and apical transverse sections were studied in supine and prone anesthetized dogs. The ventilation per unit volume (sV) of selected areas was computed from local 13NN concentration vs. time curves during washout. To separate the regional contributions of CFV and cardiogenic oscillation to enhanced molecular diffusion, additional supine dogs were also studied during unilateral CFV. In this protocol the CFV jet flow was delivered to a single lung while the contralateral lung was left apneic. For each lung, washout data were obtained under CFV and apnea both living and postmortem animals. The local contributions of diffusion, CFV jet effects, and cardiac activity to gas transport were evaluated and tested for additive and multiplicative synergistic interactions. The regional distribution of gas transport during CFV was found to be highly nonuniform and characterized by higher ventilation to regions located close to the main bronchi and those located in the direction in which the CFV jet pointed. No major differences were observed between supine and prone positions. This regional pattern of ventilation distribution was found to be the result of complementary and nearly multiplicative interaction between the regional effects of the CFV jet, concentrated in the central airways, and the preferential cardiogenic gas transport enhancement in ventral regions close to the heart. The data were also analyzed with a model that divides the regional diffusive gas transport resistance into a central component, affected by the CFV jet, and a peripheral component, affected only by cardiac activity. This analysis showed substantial regional heterogeneities in the effects of the different gas transport mechanisms, which are consistent with the geometry of the bronchial tree and the location of the heart in the dog. The results indicate that regional nonuniformities must be considered when modeling gas transport in CFV.     

Combination of constant-flow and continuous positive-pressure ventilation in canine pulmonary edema

Constant-flow ventilation (CFV) maintains alveolar ventilation without tidal excursion in dogs with normal lungs, but this ventilatory mode requires high CFV and bronchoscopic guidance for effective subcarinal placement of two inflow catheters. We designed a circuit that combines CFV with continuous positive-pressure ventilation (CPPV; CFV-CPPV), which negates the need for bronchoscopic positioning of CFV cannula, and tested this system in seven dogs having oleic acid-induced pulmonary edema. Addition of positive end-expiratory pressure (PEEP, 10 cmH2O) reduced venous admixture from 44 +/- 17 to 10.4 +/- 5.4% and kept arterial CO2 tension (PaCO2) normal. With the innovative CFV-CPPV circuit at the same PEEP and respiratory rate (RR), we were able to reduce tidal volume (VT) from 437 +/- 28 to 184 +/- 18 ml (P less than 0.001) and elastic end-inspiratory pressures (PEI) from 25.6 +/- 4.6 to 17.7 +/- 2.8 cmH2O (P less than 0.001) without adverse effects on cardiac output or pulmonary exchange of O2 or CO2; indeed, PaCO2 remained at 35 +/- 4 Torr even though CFV was delivered above the carina and at lower (1.6 l.kg-1.min-1) flows than usually required to maintain eucapnia during CFV alone. At the same PEEP and RR, reduction of VT in the CPPV mode without CFV resulted in CO2 retention (PaCO2 59 +/- 8 Torr). We conclude that CFV-CPPV allows CFV to effectively mix alveolar and dead spaces by a small bulk flow bypassing the zone of increased resistance to gas mixing, thereby allowing reduction of the CFV rate, VT, and PEI for adequate gas exchange.     

Cardiogenic oscillations in He and SF6 expirograms during airway and venous loading

Cardiogenic oscillations in the expired partial pressure profiles of two inert gases (He and SF6) were monitored in seven anesthetized paralyzed mechanically ventilated dogs. He and SF6 were administered either intravenously by a membrane oxygenator and partial arteriovenous bypass [venous loading (VL)] or by washin into lung gas [airway loading (AL)]. The single-breath expirograms obtained during constant-flow expiration after inspiration of test gas-free air displayed distinct and regular cardiogenic oscillations. The relative oscillation amplitude (ROA), calculated as oscillation amplitude divided by mixed expired-inspired partial pressure difference, was in the range of 1-8%. The ROA for both He and SF6 was approximately 4.2 times higher in VL than in AL, which indicated that among lung units that emptied sequentially in the cardiac cycle, the effects of alveolar ventilation-perfusion (VA/Q) inequality were more pronounced than those of alveolar ventilation-alveolar volume (VA/VA) inequality. In AL, He and SF6 oscillations were 180 degrees out of phase compared with CO2 and O2 oscillations and with He and SF6 oscillations in VL, which suggests that regions with low VA/VA had high VA/Q and very low Q/VA. The ROA was practically unaffected by breath holding in both AL and VL, which indicates that there was little diffusive or convective (cardiogenic) mixing between the lung units that were responsible for cardiogenic oscillations. The ROA was consistently higher for He than for SF6, and the He-to-SF6 ratio was independent of route of test gas loading, averaging 1.6 in both AL and VL. This result may be explained by laminar Taylor dispersion, whereby oscillations generated in peripheral lung regions are dissipated in inverse proportion to diffusion coefficient during transit through the proximal (larger) airways.     

Simulation of gas transport due to cardiogenic oscillations

We simulated gas transport due to cardiogenic oscillations (CO) using a model developed to quantify the gas mixing due to high-frequency ventilation (16). The basic components of the model are 1) gas mixing by augmented transport, 2) symmetrical lung morphometry, and 3) a Lagrangian (moving) reference frame. The theoretical predictions of the model are in general agreement with published experimental studies that have examined the effect of CO on the nitrogen concentration obtained by intrapulmonary gas sampling and the effect of CO on regional and total anatomical dead space. Further, the model predicts that augmentation of gas transport due to CO is less, nearer to the alveolar regions of the lung, and that the effect of CO during normal tidal breathing is negligible, but that CO may contribute up to approximately 10% of the alveolar ventilation in patients with severe hypoventilation. The agreement between experimental and theoretical results suggests that it may not be necessary to invoke gas transport mechanisms specific to an asymmetrical bronchial tree to explain the major proportion of gas transport due to CO.     

Contribution of cardiogenic oscillations to gas exchange in constant-flow ventilation

The contribution of cardiogenic oscillations to gas exchange during constant-flow ventilation was examined in 11 dogs. With the use of two variations of cardiopulmonary bypass to maintain the systemic and pulmonary circulation, the influence of cardiogenic oscillations was removed by arresting the heart. Cardiac arrest by ventricular fibrillation was associated with a mean decrease in alveolar ventilation of 43% in five dogs on right and left heart bypass. However, successful defibrillation and return of the prearrest level of alveolar ventilation could not be achieved; thus we studied six dogs on left heart bypass. Alveolar ventilation decreased an average of 37% with cardiac arrest, and defibrillation resulted in a return of alveolar ventilation to 81% of the prearrest value. These results are consistent with previous predictions that cardiogenic oscillations are an important mechanism of gas transport during constant-flow ventilation.     

长白山阔叶红松林二氧化碳湍流交换特征

采用开路式涡度相关技术,研究了长白山阔叶红松林森林-大气界面的CO₂湍流交换特征.结果表明,在近中性大气层结条件下,冠层上方垂直风速和CO₂浓度功率谱在惯性子区基本符合-2/3定律,垂直方向主导湍涡尺度约为40 m.湍流通量贡献区主要在0.01～2 Hz频率范围内,冠层上方低频传输的湍涡贡献了更多的CO₂通量.这说明开路式涡度相关仪器系统可以满足冠层上方湍流通量观测的基本要求.但通过涡度相关法实测获得的森林-大气CO₂通量仍存在夜间低估现象,非湍流过程的增加是涡度相关技术应用的主要制约因素.因此,需要对弱湍流条件下的CO₂通量做相应的修订.     

Gas exchange and intrapulmonary distribution of ventilation during continuous-flow ventilation

In 12 anesthetized paralyzed dogs, pulmonary gas exchange and intrapulmonary inspired gas distribution were compared between continuous-flow ventilation (CFV) and conventional mechanical ventilation (CMV). Nine dogs were studied while they were lying supine, and three dogs were studied while they were lying prone. A single-lumen catheter for tracheal insufflation and a double-lumen catheter for bilateral endobronchial insufflation [inspired O2 fraction = 0.4; inspired minute ventilation = 1.7 +/- 0.3 (SD) 1.kg-1.min-1] were evaluated. Intrapulmonary gas distribution was assessed from regional 133Xe clearances. In dogs lying supine, CO2 elimination was more efficient with endobronchial insufflation than with tracheal insufflation, but the alveolar-arterial O2 partial pressure difference was larger during CFV than during CMV, regardless of the type of insufflation. By contrast, endobronchial insufflation maintained both arterial PCO2 and alveolar-arterial O2 partial pressure difference at significantly lower levels in dogs lying prone than in dogs lying supine. In dogs lying supine, the dependent lung was preferentially ventilated during CMV but not during CFV. In dogs lying prone, gas distribution was uniform with both modes of ventilation. The alveolar-arterial O2 partial pressure difference during CFV in dogs lying supine was negatively correlated with the reduced ventilation of the dependent lung, which suggests that increased ventilation-perfusion mismatching was responsible for the increase in alveolar-arterial O2 partial pressure difference. The more efficient oxygenation during CFV in dogs lying prone suggests a more efficient matching of ventilation to perfusion, presumably because the distribution of blood flow is also nearly uniform.     

Mechanisms of gas exchange with different gases during constant-flow ventilation

To investigate the mechanisms responsible for the difference in gas exchange during constant-flow ventilation (CFV) when using gases with different physical properties, we used mixtures of 70% N2-30% O2 (N2-O2) and 70% He-30% O2 (He-O2) as the insufflating gases in 12 dogs. All dogs but one had higher arterial PCO2 (PaCO2) with He-O2 compared with N2-O2. At a flow of 0.37 +/- 0.12 l/s, the mean PaCO2's with N2-O2 and He-O2 were 41.3 +/- 13.9 and 53.7 +/- 20.3 Torr, respectively (P less than 0.01); at a flow rate of 0.84 +/- 0.17 l/s, the mean PaCO2's were 29.1 +/- 11.3 and 35.3 +/- 13.6 Torr, respectively (P less than 0.01). The chest was then opened to alter the apposition between heart and the lungs, thereby reducing the extent of cardiogenic oscillations by 58.4 +/- 18.4%. This intervention did not significantly alter the difference in PaCO2 between N2-O2 and He-O2 from that observed in the intact animals, although the individual PaCO2 values for each gas mixture did increase. When the PaCO2 was plotted against stagnation pressure (rho V2), the difference in PaCO2 between N2-O2 and He-O2 was nearly abolished in both the closed- and open-chest animals. These findings suggest that the different PaCO2's obtained by insufflating gases with different physical properties at a fixed flow rate, catheter position, and lung volume result mainly from a difference in the properties of the jet.     

Observation of nonlinear coupling between drift and ion-acoustic oscillations in low-frequency plasma turbulence

It is shown experimentally that the characteristics of structural ion-acoustic turbulence in a plasma are governed primarily by the development of density gradient-driven drift oscillations. The cyclicity of appearance and disappearance of drift wave packets and ensembles of ion-acoustic solitons in a steady-state turbulent plasma, as well as the correlation between them, is determined.     

Gas dispersion in volume-cycled tube flow. II. Tracer bolus experiments.

We present a new method for rapid measurement of local gas dispersion in volume-cycled tube flow. After a small bolus of tracer gas (argon) was injected into the oscillating flow, the time-averaged effective diffusion coefficient (mean value of Deff/D) for axial transport of a tracer gas is evaluated from local argon concentration measurements taken by a mass spectrometer. Two methods are presented for the evaluation of mean value of Deff/D from the concentration measurements: one uses all the sampled data, and the other uses only the local peaks of the concentration. Experiments were conducted in two tubes (radius = 0.85 or 1.0 cm) over a range of frequencies (0.42 less than or equal to f less than or equal to 8.5 Hz) and tidal volumes (7 less than or equal to VT less than or equal to 48 ml). The experimental results show very good agreement with the theoretical predictions of Elad et al. (J. Appl. Physiol. 72: 312-320, 1992). In the absence of oscillations (static fluid), the resulting mean value of Deff/D converges to that of molecular diffusion. We also show that concentration data may be acquired at any radial or axial position, not necessarily at the tracer gas injection point, and the resulting mean value of Deff/D is independent of the spatial position of the sampling catheter. This method is of similar accuracy and is substantially faster than previous methods for measuring gas dispersion in oscillatory flows. The rapidity of these measurements may permit this method to be used for the in vivo assessment of gas transport properties within the pulmonary system.     

How do sinking phytoplankton species manage to persist?

Phytoplankton require light for photosynthesis. Yet, most phytoplankton species are heavier than water and therefore sink. How can these sinking species persist? Somehow, the answer should lie in the turbulent motion that redisperses sinking phytoplankton over the vertical water column. Here, we show, using a reaction-advection-diffusion equation of light-limited phytoplankton, that there is a turbulence window sustaining sinking phytoplankton species in deep waters. If turbulent diffusion is too high, phytoplankton are mixed to great depths, and the depth-averaged light conditions are too low to allow net positive population growth. Conversely, if turbulent diffusion is too low, sinking phytoplankton populations end up at the ocean floor and succumb in the dark. At intermediate levels of turbulent diffusion, however, phytoplankton populations can outgrow both mixing rates and sinking rates. In this way, the reproducing population as a whole can maintain a position in the well-lit zone near the top of the water column, even if all individuals within the population have a tendency to sink. This theory unites earlier classic results by Sverdrup and Riley as well as our own recent findings and provides a new conceptual framework for the understanding of phytoplankton dynamics under the influence of mixing processes.     

Ventilation-perfusion inequality during constant-flow ventilation

Previous work by Lehnert et al. (J. Appl. Physiol. 53:483-489, 1982) has demonstrated that adequate alveolar ventilation can be maintained during apnea in anesthetized dogs by delivering a continuous stream of inspired ventilation through cannulas aimed down the main-stem bronchi. Because an asymmetric distribution of ventilation might introduce ventilation-perfusion (VA/Q) inequality, we compared gas exchange efficiency in nine anesthetized and paralyzed dogs during constant-flow ventilation (CFV) and conventional ventilation (intermittent positive-pressure ventilation, IPPV). Gas exchange was assessed using the multiple inert gas elimination technique. During CFV at 3 l X kg-1 X min-1, lung volume, retention-excretion differences (R-E*) for low- and medium-solubility gases, and the log standard deviation of blood flow (log SD Q) increased, compared with the findings during IPPV. Reducing CFV flow rate to 1 l X kg-1 X min-1 at constant lung volume improved R-E* and log SD Q, but significant VA/Q inequality compared with that at IPPV remained and arterial PCO2 rose. Comparison of IPPV and CFV at the same mean lung volume showed a similar reversible deterioration in gas exchange efficiency during CFV. We conclude that CFV causes significant VA/Q inequality which may be due to nonuniform ventilation distribution and a redistribution of pulmonary blood flow.     

Convective and diffusive gas transport in canine intrapulmonary airways.

The significance of convective and diffusive gas transport in the respiratory system was assessed from the response of combined inert gas and particle boluses inhaled into the conducting airways. Particles, considered as "nondiffusing gas," served as tracers for convection and two inert gases with widely different diffusive characteristics (He and SF6) as tracers for convection and diffusion. Six-milliliter boluses labeled with monodisperse di-2-ethylhexyl sebacate droplets of 0.86-microns aerodynamic diameter, 2% He, and 2% SF6 were inspired by three anesthetized mechanically ventilated beagle dogs to volumetric lung depths up to 170 ml. Mixing between inspired and residual air caused dispersion of the inspired bolus, which was quantified in terms of the bolus half-width. Dispersion of particles increased with increasing lung depth to which the boluses were inhaled. The increase followed a power law with exponents less than 0.5 (mean 0.39), indicating that the effect of convective mixing per unit volume was reduced with depth. Within the pulmonary dead space, the behavior of the inert gases He and SF6 was similar to that of the particles, suggesting that gas transport was almost solely due to convection. Beyond the dead space, dispersion of He and SF6 increased more rapidly than dispersion of particles, indicating that diffusion became significant. The gas and particle bolus technique offers a suitable approach to differential analysis of gas transport in intrapulmonary airways of lungs.     

Catheter position and blood gases during constant-flow ventilation

We studied the effect of catheter position and flow rate on gas exchange during constant-flow ventilation (CFV) in eight anesthetized, paralyzed dogs. The distal tips of the insufflation catheters were positioned 0.5, 2.0, 3.5, and 5.0 cm from the tracheal carina. Flow rates were varied between 10 and 55 l/min and steady-state arterial blood gases were measured. At a given flow rate, arterial CO2 pressure (PaCO2) decreased as CFV was administered further into the lung up to a distance of 3.5 cm from the carina; there were no significant differences in PaCO2 at 3.5 and 5.0 cm. For a given catheter position, PaCO2 decreased with increasing flow rate up to a flow rate of 40 l/min. Further increases in flow rate had no significant effect on PaCO2. Arterial O2 pressure (PaO2) was relatively constant at all flow rates and catheter positions. We conclude that, up to a point, CO2 elimination can be improved by positioning the catheters further into the lung; advancing the catheters further than 3.5 cm from the carina may cause over-ventilation of specific lung regions resulting in a relative plateau in CO2 elimination and relatively constant PaO2's. Positioning the catheters further into the lung permits the use of lower flow rates, thus potentially minimizing the risk of barotrauma.     

Alveolar pressure inhomogeneity and gas exchange during constant-flow ventilation in dogs

Analysis of momentum transfer between inflow jets and resident gas during constant-flow ventilation (CFV) predicts inhomogeneity of alveolar pressures (PA) and volume, which might account for specific ventilation-variance in the lung. Using alveolar needles to measure pressures (PA) during CFV in eight anesthetized dogs with wide thoracotomy, we observed random dispersion of PA among lobes of up to 12.5 cmH2O. Within each lobe, the PA dispersion was up to 10 cmH2O at CFV of 90 l/min; when flow decreased, PA at all sites decreased, as did the intralobar dispersion. These pressure differences were not observed during conventional mechanical ventilation (CMV). During CFV with room air, dogs were hypoxemic [arterial PO2 (Pao2) 54 +/- 15 Torr] and the venous admixture (Qva/QT) was 50 +/- 15%. When inspiratory O2 fraction was increased to 0.4, Pao2 increased to 172 +/- 35 Torr and Qva/QT dropped to 13.5 +/- 8.4%, confirming considerable ventilation-perfusion (VA/Q) variance not observed during CMV. We conclude that momentum transfer between the inflow stream and resident gas caused inhomogeneities of alveolar pressures, volumes, and ventilation responsible for VA/Q variance and hypoxemia during CFV. Conceivably, the abnormal ventilation distribution is minimized by collateral ventilation and forces of interdependence between regions of high and low alveolar pressures. Momentum transfer also predicted the mucosal damage observed on histological evaluation of the bronchial walls near the site of inflow jet impact.     

The effect of a turbulent jet on gas transport during oscillatory flow

Axial mass transport due to the combined effects of flow oscillation and a turbulent jet was studied both experimentally and with a simple theoretical model. The experiments show that the distance over which turbulence enhances transport is greatly increased by flow oscillation, and is particularly sensitive to tidal volume. The jet flow rate and jet configuration are relatively less important. To analyze the results, the region influenced by the jet is divided into two zones: a near field in which the time-mean flow velocities are larger than the turbulent fluctuations, and a far field where the time-mean flow is essentially zero. In the far field, axial mass transport is increased due to the turbulence which decays in strength away from the jet. When oscillatory flow is superimposed upon the steady jet flow, the turbulence in the far field interacts with the flow oscillations to augment the transport of turbulence energy and of mass. This transport enhancement is modeled by introducing an effective axial diffusivity analogous to that used in laminar oscillatory flow.     

Effect of cardiogenic gas mixing on arterial O2 and CO2 tensions during breath holding

To examine the effect of cardiogenic gas mixing on gas exchange we measured arterial tension of O2 (PaO2) and arterial tension of CO2 (PaCO2) during 3- to 5-min breath holds (BH) before and after infusing 50 ml of saline into the pericardial space (PCF) of seven anesthetized, paralyzed, mechanically ventilated dogs. During BH the ventilator was disconnected and a bias flow of 50% O2 at 4-5 l/min was delivered through the side ports of a small catheter whose tip was positioned 1 cm cephalad of the carina. Paired runs, alternately with and without PCF, were performed in triplicate in each dog. Initial PaO2 was similar for control runs [81 +/- 3 mmHg (SE)] and PCF runs (78 +/- 3 mmHg; P greater than 0.1). After 3-min BH, PaO2 in PCF runs (33 +/- 3 mmHg) was less than that in control runs (58 +/- 4 mmHg) (P less than 0.001). In contrast, the pattern of PaCO2 during BH did not differ with PCF. After 3-min BH, PaCO2 was 49 +/- 3 mmHg with PCF and 49 +/- 2 mmHg in the control runs (P greater than 0.7). In two dogs, repeated 50-ml reductions in lung volume, produced by rib cage compression, did not alter the time course of PaO2 during BH. Although cardiac output decreased slightly with PCF, hemodynamic changes due to PCF were unlikely to account for the observed fall in PaO2. Our results indicate a substantial effect of cardiogenic gas mixing on O2 uptake when tracheal gas is O2 enriched during breath holding.(ABSTRACT TRUNCATED AT 250 WORDS)