Dorsal recruitment with flow-controlled expiration (FLEX): an experimental study in mechanically ventilated lung-healthy and lung-injured pigs.

BACKGROUND: Concepts for optimizing mechanical ventilation focus mainly on modifying the inspiratory phase. We propose flow-controlled expiration (FLEX) as an additional means for lung protective ventilation and hypothesize that it is capable of recruiting dependent areas of the lungs. This study investigates potential recruiting effects of FLEX using models of mechanically ventilated pigs before and after induction of lung injury with oleic acid. METHODS: Seven pigs in the supine position were ventilated with tidal volume 8 ml·kg- 1 and positive end-expiratory pressure (PEEP) set to maintain partial pressure of oxygen in arterial blood (paO2) at ≥ 60 mmHg and monitored with electrical impedance tomography (EIT). Two ventilation sequences were recorded - one before and one after induction of lung injury. Each sequence comprised 2 min of conventional volume-controlled ventilation (VCV), 2 min of VCV with FLEX and 1 min again of conventional VCV. Analysis of the EIT recordings comprised global and ventral and dorsal baseline levels of impedance curves, end-expiratory no-flow periods, tidal variation in ventral and dorsal areas, and regional ventilation delay index. RESULTS: With FLEX, the duration of the end-expiratory zero flow intervals was significantly shortened (VCV 1.4 ± 0.3 s; FLEX 0.7 ± 0.1 s, p < 0.001), functional residual capacity was significantly elevated in both conditions of the lungs (global: healthy, increase of 87 ± 12 ml, p < 0.001; injured, increase of 115 ± 44 ml, p < 0.001; ventral: healthy, increase of 64 ± 11 ml, p < 0.001; injured, increase of 83 ± 22 ml, p < 0.001; dorsal: healthy, increase of 23 ± 5 ml, p < 0.001; injured, increase of 32 ± 26 ml, p = 0.02), and ventilation was shifted from ventral to dorsal areas (dorsal increase: healthy, 1 ± 0.5%, p < 0.01; dorsal increase: injured, 6 ± 2%, p < 0.01), compared to conventional VCV. Recruiting effects of FLEX persisted during conventional VCV following FLEX ventilation mostly in the injured but also in the healthy lungs. CONCLUSIONS: FLEX shifts regional ventilation towards dependent lung areas in healthy and in injured pig lungs. The recruiting capabilities of FLEX may be mainly responsible for lung-protective effects observed in an earlier study.

The Equilibration of PCO2 in Pigs Is Independent of Lung Injury and Hemodynamics.

OBJECTIVES: In mechanical ventilation, normoventilation in terms of PCO2 can be achieved by titration of the respiratory rate and/or tidal volume. Although a linear relationship has been found between changes in respiratory rate and resulting changes in end-tidal cO2 (△PetCO2) as well as between changes in respiratory rate and equilibration time (teq) for mechanically ventilated patients without lung injury, it is unclear whether a similar relationship holds for acute lung injury or altered hemodynamics. DESIGN: We performed a prospective randomized controlled animal study of the change in PetCO2 with changes in respiratory rate in a lung-healthy, lung-injury, lung-healthy + altered hemodynamics, and lung-injury + altered hemodynamics pig model. SETTING: University research laboratory. SUBJECTS: Twenty mechanically ventilated pigs. INTERVENTIONS: Moderate lung injury was induced by injection of oleic acid in 10 randomly assigned pigs, and after the first round of measurements, cardiac output was increased by approximately 30% by constant administration of noradrenalin in both groups. MEASUREMENTS AND MAIN RESULTS: We systematically increased and decreased changes in respiratory rate according to a set protocol: +2, -4, +6, -8, +10, -12, +14 breaths/min and awaited equilibration of Petco2. We found a linear relationship between changes in respiratory rate and △PetCO2 as well as between changes in respiratory rate and teq. A two-sample t test resulted in no significant differences between the lung injury and healthy control group before or after hemodynamic intervention. Furthermore, exponential extrapolation allowed prediction of the new PetCO2 equilibrium and teq after 5.7 ± 5.6 min. CONCLUSIONS: The transition between PetCO2 equilibria after changes in respiratory rate might not be dependent on moderate lung injury or cardiac output but on the metabolic production or capacity of cO2 stores. Linear relationships previously found for lung-healthy patients and early prediction of PetCO2 equilibration could therefore also be used for the titration of respiratory rate on the PetCO2 for a wider range of pathologies by the physician or an automated ventilation system.

The pressure drop across the endotracheal tube in mechanically ventilated pediatric patients.

BACKGROUND: During mechanical ventilation, the airway pressure (Paw) is usually monitored. However, Paw comprises the endotracheal tube (ETT)-related pressure drop (∆PETT ) and thus does not reflect the pressure in the patients' lungs. Therefore, monitoring of mechanical ventilation should be based on the tracheal pressure (Ptrach ). We systematically investigated potential factors influencing ∆PETT in pediatric ETTs. METHODS: In this study, the flow-dependent pressure drop across pediatric ETTs from four manufacturers [2.0-4.5 mm inner diameter (ID)] was estimated in a physical model of the upper airways. Additionally, ∆PETT was examined with the ETTs shortened to 75% of their original length and at different curvatures. In nine healthy mechanically ventilated children (aged between 9 days and 29 months), Ptrach was compared to Paw . RESULTS: ∆PETT was nonlinearly flow dependent. Low IDs corresponded to high ∆PETT . Differences between ETTs from different manufacturers were identified. Shortening of the ETTs' length by 25% reduced ∆PETT on average by 14% of the value at original length. Ventilation frequency and tube curvature did not influence ∆PETT to a relevant extent. In the pediatric patients, the root mean square deviation between Paw and Ptrach was 2.3 cm H2O. CONCLUSION: Paw and Ptrach differ considerably (by ∆PETT ) during mechanical ventilation of pediatric patients. The ETTs' ID, tube length, and manufacturer type are significant factors for ∆PETT and should be taken into account when Paw is valuated. For this purpose, Ptrach can be continuously calculated with good precision by means of the Rohrer approximation.

Endoscopic imaging to assess alveolar mechanics during quasi-static and dynamic ventilatory conditions in rats with noninjured and injured lungs.

OBJECTIVES: Although global respiratory mechanics are usually used to determine the settings of mechanical ventilation, this approach does not adequately take into account alveolar mechanics. However, it should be expected that the ventilatory condition (quasi-static vs. dynamic) and lung condition (noninjured vs. injured) affect alveolar mechanics in a clinically relevant way. Accordingly, the aim of this study was to investigate alveolar mechanics during quasi-static and dynamic ventilatory maneuvers in noninjured and injured lungs. We hypothesized that alveolar mechanics vary with ventilatory and lung conditions. DESIGN: Prospective animal study. SETTING: Animal research laboratory. SUBJECTS: Male Wistar rats. INTERVENTIONS: Alveolar mechanics (derived from alveolar size and airway pressure) were determined in noninjured (n = 9) and in lungs lavaged with saline (n = 8) at quasi-static (low flow at a peak pressure of 40 cm H2O) and dynamic ventilatory maneuvers (increase and decrease in positive end-expiratory pressure from 0 to 15 and back to 0 cm H2O in steps of 3 cm H2O). Alveoli were recorded endoscopically and alveolar mechanics were extracted using automated tracking of alveolar contours. MEASUREMENTS AND MAIN RESULTS: The increase in alveolar size during quasi-static maneuvers was significantly greater than during dynamic maneuvers in noninjured (mean difference 18%, p < 0.001) but not in injured lungs (mean difference 3%, p = 0.293). During dynamic maneuvers, slope of the intratidal alveolar pressure/area curve (reflecting distensibility) decreased with increasing positive end-expiratory pressure (p = 0.001) independent of lung condition (noninjured and injured lungs). In contrast, independent of positive end-expiratory pressure but dependent on lung condition, the maximal tidal change in alveolar size was greater by an average of 40% in injured compared with noninjured lungs (p = 0.028). CONCLUSIONS: Alveolar mechanics during mechanical ventilation differed between quasi-static and dynamic conditions and varied with lung condition. Our data thus confirm that analysis of respiratory system mechanics under dynamic conditions is preferable to analysis during static conditions.

Inspiratory capacity at inflation hold in ventilated newborns: a surrogate measure for static compliance of the respiratory system.

OBJECTIVE: To study appropriateness of respiratory system compliance calculation using an inflation hold and compare it with ventilator readouts of pressure and tidal volume as well as with measurement of compliance of the respiratory system with the single-breath-single-occlusion technique gained with a standard lung function measurement. DESIGN: Prospective clinical trial. SETTING: Level III neonatal unit of a university hospital. SUBJECTS: Sixty-seven newborns, born prematurely or at term, ventilated for a variety of pathologic conditions. INTERVENTIONS: A standardized sigh maneuver with a predefined peak inspiratory pressure of 30 cm H2O, termed inspiratory capacity at inflation hold, was applied. Using tidal volume, exhaled from inspiratory pause down to ambient pressure, as displayed by the ventilator, and predefined peak inspiratory pressure, compliance at inspiratory capacity at inflation hold conditions could be calculated as well as ratio of tidal volume and ventilator pressure using tidal volume and differential pressure at baseline ventilator settings: peak inspiratory pressure minus positive end-expiratory pressure. MEASUREMENTS AND MAIN RESULTS: For the whole cohort, the equation for the regression between tidal volume at inspiratory capacity at inflation hold and compliance of the respiratory system was: compliance of the respiratory system = 0.052 * tidal volume at inspiratory capacity at inflation hold - 0.113, and compliance at inspiratory capacity at inflation hold conditions was closely related to the standard lung function measurement method of compliance of the respiratory system (R = 0.958). In contrast, ratio of tidal volume and ventilator pressure per kilogram calculated from the ventilator readouts and displayed against compliance of the respiratory system per kilogram yielded a broad scatter throughout the whole range of compliance; both were only weakly correlated (R = 0.309) and also the regression line was significantly different from the line of identity (p < .05). Peak inspiratory pressure at study entry did not affect the correlation between compliance at inspiratory capacity at inflation hold conditions and compliance of the respiratory system. CONCLUSIONS: After a standard sigh maneuver, inspiratory capacity at inflation hold and the derived quantity compliance at inspiratory capacity at inflation hold conditions can be regarded as a valid, accurate, and reliable surrogate measure for standard compliance of the respiratory system in contrast to ratio of tidal volume and ventilator pressure calculated from the ventilator readouts during ongoing mechanical ventilation at respective ventilator settings.

Analysis of dynamic intratidal compliance in a lung collapse model.

BACKGROUND: For mechanical ventilation to be lung-protective, an accepted suggestion is to place the tidal volume (V(T)) between the lower and upper inflection point of the airway pressure-volume relation. The drawback of this approach is, however, that the pressure-volume relation is assessed under quasistatic, no-flow conditions, which the lungs never experience during ventilation. Intratidal nonlinearity must be assessed under real (i.e., dynamic) conditions. With the dynamic gliding-SLICE technique that generates a high-resolution description of intratidal mechanics, the current study analyzed the profile of the compliance of the respiratory system (C(RS)). METHODS: In 12 anesthetized piglets with lung collapse, the pressure-volume relation was acquired at different levels of positive end-expiratory pressure (PEEP: 0, 5, 10, and 15 cm H(2)O). Lung collapse was assessed by computed tomography and the intratidal course of C(RS) using the gliding-SLICE method. RESULTS: Depending on PEEP, C(RS) showed characteristic profiles. With low PEEP, C(RS) increased up to 20% above the compliance at early inspiration, suggesting intratidal recruitment; whereas a profile of decreasing C(RS), signaling overdistension, occurred with V(T) > 5 ml/kg and high PEEP levels. At the highest volume range, C(RS) was up to 60% less than the maximum. With PEEP 10 cm H(2)O, C(RS) was high and did not decrease before 5 ml/kg V(T) was delivered. CONCLUSIONS: The profile of dynamic C(RS) reflects nonlinear intratidal mechanics of the respiratory system. The SLICE analysis has the potential to detect intratidal recruitment and overdistension. This might help in finding a combination of PEEP and V(T) level that is protective from a lung-mechanics perspective.

PEEP titration guided by ventilation homogeneity: a feasibility study using electrical impedance tomography.

INTRODUCTION: Lung protective ventilation requires low tidal volume and suitable positive end-expiratory pressure (PEEP). To date, few methods have been accepted for clinical use to set the appropriate PEEP. The aim of this study was to test the feasibility of PEEP titration guided by ventilation homogeneity using the global inhomogeneity (GI) index based on electrical impedance tomography (EIT) images. METHODS: In a retrospective study, 10 anesthetized patients with healthy lungs mechanically ventilated under volume-controlled mode were investigated. Ventilation distribution was monitored by EIT. A standardized incremental PEEP trial (PEEP from 0 to 28 mbar, 2 mbar per step) was conducted. During the PEEP trial, "optimal" PEEP level for each patient was determined when the air was most homogeneously distributed in the lung, indicated by the lowest GI index value. Two published methods for setting PEEP were included for comparison based on the maximum global dynamic compliance and the intra-tidal compliance-volume curve. RESULTS: No significant differences in the results were observed between the GI index method (12.2 +/- 4.6 mbar) and the dynamic compliance method (11.4 +/- 2.3 mbar, P > 0.6), or between the GI index and the compliance-volume curve method (12.2 +/- 4.9 mbar, P > 0.6). CONCLUSIONS: According to the results, it is feasible and reasonable to use the GI index to select the PEEP level with respect to ventilation homogeneity. The GI index may provide new insights into the relationship between lung mechanics and tidal volume distribution and may be used to guide ventilator settings.

A lung area estimation method for analysis of ventilation inhomogeneity based on electrical impedance tomography.

PURPOSE: To evaluate a novel method for lung area estimation (LAE method) in electrical impedance tomography (EIT) images as a prerequisite of quantitative analysis of ventilation inhomogeneity. METHODS: The LAE method mirrors the lung regions in the functional EIT (fEIT) image and subsequently subtracts the cardiac related areas. In this preliminary study, 51 mechanically ventilated patients were investigated, including 39~patients scheduled for thoracic surgery (test group); 10 patients scheduled for orthopedic surgery without pulmonary disease (control group) and 2 ICU patients undergoing chest computed tomography (CT) examination. EIT data was recorded in all groups. The results of the LAE method were compared to those obtained with the fEIT method and to CT images. RESULTS: The lung area size determined with fEIT in control group is S(C,fEIT) = 361 +/- 35 (mean +/- SD) and in test group S(T,fEIT) = 299 +/- 61 (p< 0.01). The sizes estimated with the LAE method in control group S(C,LAE) = 353 +/- 27 and in test group S(T,LAE) = 353 +/- 61 (p=0.41). The result demonstrates that the novel LAE method improves the identification of lung region in EIT images, from which the analysis of ventilation distribution will benefit. The preliminary comparison with CT images exemplary indicates a closer match of the lung area shapes after the LAE than after the fEIT-based analysis. CONCLUSION: The LAE method is a robust lung area determination method, suitable for patients with healthy or seriously injured lungs.

Pressure-dependent stress relaxation in acute respiratory distress syndrome and healthy lungs: an investigation based on a viscoelastic model.

INTRODUCTION: Limiting the energy transfer between ventilator and lung is crucial for ventilatory strategy in acute respiratory distress syndrome (ARDS). Part of the energy is transmitted to the viscoelastic tissue components where it is stored or dissipates. In mechanically ventilated patients, viscoelasticity can be investigated by analyzing pulmonary stress relaxation. While stress relaxation processes of the lung have been intensively investigated, non-linear interrelations have not been systematically analyzed, and such analyses have been limited to small volume or pressure ranges. In this study, stress relaxation of mechanically ventilated lungs was investigated, focusing on non-linear dependence on pressure. The range of inspiratory capacity was analyzed up to a plateau pressure of 45 cmH2O. METHODS: Twenty ARDS patients and eleven patients with normal lungs under mechanical ventilation were included. Rapid flow interruptions were repetitively applied using an automated super-syringe maneuver. Viscoelastic resistance, compliance and time constant were determined by multiple regression analysis using a lumped parameter model. This same viscoelastic model was used to investigate the frequency dependence of the respiratory system's impedance. RESULTS: The viscoelastic time constant was independent of pressure, and it did not differ between normal and ARDS lungs. In contrast, viscoelastic resistance increased non-linearly with pressure (normal: 8.4 (7.4-11.9) [median (lower - upper quartile)] to 35.2 (25.6-39.5) cmH2O.sec/L; ARDS: 11.9 (9.2-22.1) to 73.5 (56.8-98.7)cmH2O.sec/L), and viscoelastic compliance decreased non-linearly with pressure (normal: 130.1(116.9-151.3) to 37.4(34.7-46.3) mL/cmH2O; ARDS: 125.8(80.0-211.0) to 17.1(13.8-24.7)mL/cmH2O). The pulmonary impedance increased with pressure and decreased with respiratory frequency. CONCLUSIONS: Viscoelastic compliance and resistance are highly non-linear with respect to pressure and differ considerably between ARDS and normal lungs. None of these characteristics can be observed for the viscoelastic time constant. From our analysis of viscoelastic properties we cautiously conclude that the energy transfer from the respirator to the lung can be reduced by application of low inspiratory plateau pressures and high respiratory frequencies. This we consider to be potentially lung protective.

Expiratory automatic endotracheal tube compensation reduces dynamic hyperinflation in a physical lung model.

INTRODUCTION: The effect of expiratory endotracheal tube (ETT) resistance on dynamic lung inflation is unknown. We hypothesized that ETT resistance causes dynamic lung hyperinflation by impeding lung emptying. We further hypothesized that compensation for expiratory ETT resistance by automatic tube compensation (ATC) attenuates dynamic lung hyperinflation. METHODS: A ventilator equipped with the original ATC mode and operating in a pressure-targeted mode was connected to a physical lung model that consists of four equally sized glass bottles filled with copper wool. Inspiratory pressure, peak expiratory flow, trapped lung volume and intrinsic positive end-expiratory pressure (PEEP) were assessed at combinations of four inner ETT diameters (7.0, 7.5, 8.0 and 8.5 mm), four respiratory rates (15, 20, 25 and 30/minute), three inspiratory pressures (3.0, 4.5 and 6.0 cmH2O) and four lung compliances (113, 86, 58 and 28 ml/cmH2O). Intrinsic PEEP was measured at the end of an expiratory hold manoeuvre. RESULTS: At a given test lung compliance, inspiratory pressure and ETT size, increasing respiratory rates from 15 to 30/minutes had the following effects: inspiratory tidal volume and peak expiratory flow were decreased by means of 25% (range 0% to 51%) and 11% (8% to 12%), respectively; and trapped lung volume and intrinsic PEEP were increased by means of 25% (0% to 51%) and 26% (5% to 45%), respectively (all P < 0.025). At otherwise identical baseline conditions, introduction of expiratory ATC significantly attenuated (P < 0.025), by approximately 50%, the respiratory rate-dependent decreases in inspiratory tidal volume and the increases in trapped lung volume and intrinsic PEEP. CONCLUSIONS: In a lung model of pressure-targeted ventilation, expiratory ETT resistance caused dynamic lung hyperinflation during increases in respiratory rates, thereby reducing inspiratory tidal volume. Expiratory ATC attenuated these adverse effects.

Pressure loss caused by pediatric endotracheal tubes during high-frequency-oscillation-ventilation.

In a physical model of a pediatric respiratory system we measured the pressure drop across ETTs of 3 and 4mm inner diameter (ID) when we varied frequency, mean airway pressure and pressure amplitude of high-frequency-oscillation-ventilation (HFOV). Depending on ventilator settings the relative loss of mean pressure amplitude caused by the ETT ranged from 3.3% to 24.7% for ETT 4mm ID, respectively, from 23.8% to 51.8% for 3mm ID. In addition to the well-described flow dependency, ventilation frequency affected ETT resistance. Due to this frequency dependence, calculation of the pressure drop across the ETT using Rohrer's or Blasius-Itos' approach underestimated the true pressure drop significantly (p<0.001). Based on the experimental results, nomograms for graphical determination of the pressure drop across the ETT during HFOV were developed. We conclude that the pressure drop across the ETT during HFOV is dependent on ETT size, pressure amplitude and ventilation frequency. Calculation of this pressure drop with conventional methods is inaccurate. The high-frequency-resistance of the ETT might protect the lungs from excessive pressure amplitudes during HFOV.

Detection of partial endotracheal tube obstruction by forced pressure oscillations.

Rapid airway occlusions during mechanical ventilation are followed immediately by high-frequency pressure oscillations. To answer the question if the frequency of forced pressure oscillations is an indicator for partial obstruction of the endotracheal tube (ETT) we performed mathematical simulations and studies in a ventilated physical lung model. Model-derived predictions were evaluated in seven healthy volunteers. Partial ETT obstruction was mimicked by decreasing the inner diameter (ID) of the ETT. In the physical model ETTs of different ID were used. In spontaneously breathing volunteers viscous fluid was applied into the ETT's lumen. According to the predictions derived from mathematical simulations, narrowing of the ETT's ID from 9.0 to 7.0mm decreased the frequency of the pressure oscillations by 11% while changes of the respiratory system's compliance had no effect. In volunteers, a similar reduction (10.9%) was found when 5 ml fluid were applied. We conclude that analysis of pressure oscillations after flow interruption offers a tool for non-invasive detection of partial ETT obstruction.

Optical clearing: impact of optical and dielectric properties of clearing solutions on pulmonary tissue mechanics.

Optical clearing allows tissue visualization under preservation of organ integrity. Optical clearing of organs with a physiological change in three-dimensional geometry (such as the lung) would additionally allow visualization of macroscopic and microscopic tissue geometry. A prerequisite, however, is the preservation of the native tissue mechanics of the optically cleared lung tissue. We investigated the impact of optical and dielectric properties of clearing solutions on biomechanics and clearing potency in porcine tissue strips of healthy lungs. After fixation, bleaching, and rehydration, four methods of optical clearing were investigated using eight different protocols. The mechanical and optical properties of the cleared lung tissue strips were investigated by uniaxial tensile testing and by analyzing optical transparency and translucency for red, green, and blue light before, during, and after the biochemical optical clearing process. Fresh tissue strips were used as controls. Best balance between efficient clearing and preserved mechanics was found for clearing with a 1:1 mixture of dimethyl sulfoxide (DMSO) and aniline. Our findings show that 1) the degree of tissue transparency and translucency correlated with the refractive index of the clearing solution index (r = 0.976, P = 0.0004; and r = 0.91, P = 0.0046, respectively), 2) tissue mechanics were affected by dehydration and the type of clearing solution, and 3) tissue biomechanics and geometry correlated with the dielectric constant of the clearing solution (r = -0.98, P < 0.00001; and r = 0.69, P = 0.013, respectively). We show that the lower the dielectric constant of the clearing solutions, the larger the effect on tissue stiffness. This suggests that the dielectric constant is an important measure in determining the effect of a clearing solution on lung tissue biomechanics. Optimal tissue transparency requires complete tissue dehydration and a refractive index of 1.55 of the clearing solution.NEW & NOTEWORTHY Investigating optical clearing in porcine lung tissue strips, we found that refractive index and dielectric constant of the clearing solution affected tissue clearing and biomechanics. By documenting the impact of the composition of the clearing solution on clearing potency and preservation of tissue mechanics, our results help to compose optimal clearing solutions. In addition, the results allow conclusions on the molecular interaction of solvents with collagen fibers in tissue, thereby consolidating existing theories about the functionality of collagen.

Change in expiratory flow detects partial endotracheal tube obstruction in pressure-controlled ventilation.

Only extreme degrees of endotracheal tube (ETT) narrowing can be detected with monitoring of tidal volume (V(T)) during pressure-controlled ventilation (PCV). To assess the degree of ETT obstruction in PCV and to compare it to V(T) monitoring, we produced 3 levels of partial ETT obstruction in 11 healthy anesthetized piglets using ETTs of 4 different inner diameters (IDs 9.0, 8.0, 7.0, and 6.0 mm). An expiratory flow over volume ((e)-V) curve was plotted and the time constant (tau(e)) at 15% of expiration time (T(e)) was calculated. We also calculated the fractional volume expired during the first 15% of T(e) (V(ex fract,15)) and compared those variables to full expiratory V(T) for each of the 3 obstructions. V(T) monitoring failed to detect ETT narrowing. By contrast, V(ex fract,15) decreased and tau(e) increased significantly with increasing ETT narrowing (for IDs 9.0, 8.0, 7.0, and 6.0, mean V(ex fract,15) was 195, 180, 146, and 134 mL respectively and mean tau(e) was 380, 491, 635, 794 ms for IDs 9.0, 8.0, 7.0, and 6.0 respectively). We conclude that when the elastic recoil that drives (e) is appropriately considered, analysis of (e) and V(ex fract,15) detects partial ETT obstruction during PCV.

Short-term effects of positive end-expiratory pressure on breathing pattern: an interventional study in adult intensive care patients.

INTRODUCTION: Positive end-expiratory pressure (PEEP) is used in mechanically ventilated patients to increase pulmonary volume and improve gas exchange. However, in clinical practice and with respect to adult, ventilator-dependent patients, little is known about the short-term effects of PEEP on breathing patterns. METHODS: In 30 tracheally intubated, spontaneously breathing patients, we sequentially applied PEEP to the trachea at 0, 5 and 10 cmH2O, and then again at 5 cmH2O for 30 s each, using the automatic tube compensation mode. RESULTS: Increases in PEEP were strongly associated with drops in minute ventilation (P < 0.0001) and respiratory rate (P < 0.0001). For respiratory rate, a 1 cmH2O change in PEEP in either direction resulted in a change in rate of 0.4 breaths/min. The effects were exclusively due to changes in expiratory time. Effects began to manifest during the first breath and became fully established in the second breath for each PEEP level. Identical responses were found when PEEP levels were applied for 10 or 60 s. Post hoc analysis revealed a similar but stronger response in patients with impaired respiratory system compliance. CONCLUSION: In tracheally intubated, spontaneously breathing adult patients, the level of PEEP significantly influences the resting short-term breathing pattern by selectively affecting expiratory time. These findings are best explained by the Hering-Breuer inflation/deflation reflex.

Simultaneous monitoring of intratidal compliance and resistance in mechanically ventilated piglets: A feasibility study in two different study groups.

Compliance measures the force counteracting parenchymal lung distension. In mechanical ventilation, intratidal compliance-volume (C(V))-profiles therefore change depending on PEEP, tidal volume (VT), and underlying mechanical lung properties. Resistance counteracts gas flow through the airways. Due to anatomical linking between parenchyma and airways, intratidal resistance-volume (R(V))-profiles are hypothesised to change in a non-linear way as well. We analysed respiratory system mechanics in fifteen piglets with lavage-induced lung injury and nine healthy piglets ventilated at different PEEP/VT-settings. In healthy lungs, R(V)-profiles remained mostly constant and linear at all PEEP-settings whereas the shape of the C(V)-profiles showed an increase toward a maximum followed by a decrease (small PEEP) or volume-dependent decrease (large PEEP). In the lavage group, a large drop in resistance at small volumes and slow decrease toward larger volumes was found for small PEEP/VT-settings where C(V)-profiles revealed a volume-dependent increase (small PEEP) or a decrease (large PEEP and large VT). R(V)-profiles depend characteristically on PEEP, VT, and possibly whether lungs are healthy or not. Curved R(V)-profiles might indicate pathological changes in the underlying mechanical lung properties and/or might be a sign of derecruitment.

Decreasing size of cardiogenic oscillations reflects decreasing compliance of the respiratory system during long-term ventilation.

Part of the energy produced by the heartbeat is transferred to the lung and promotes intrapulmonary gas mixing. It is likely that this transmission in the form of local mechanical disturbances affects and reflects respiratory mechanics. The effects of the cardiogenic oscillations were studied in seven piglets during 7 h of monotonous mechanical ventilation. During the 1st h of ventilation, every heartbeat triggered a noticeable transient increase in lung volume of 14 ml (95% confidence interval = 10-17 ml). After 7 h, the increase in lung volume due to heartbeat significantly decreased to 7 ml (95% confidence interval = 2-9 ml, P < 0.05). During the course of ventilation, overall lung compliance and gas exchange were progressively compromised. We conclude that 1) sufficient mechanical energy is transferred from the beating heart to the lung to increase lung volume, and 2) the ability of the heartbeats to help increase lung volume is reduced during long-term ventilation, which reflects the changes in lung compliance.