Re-inspiration of CO2 from ventilator circuit: effects of circuit flushing and aspiration of dead space up to high respiratory rate

Abstract Introduction: Dead space negatively influences carbon dioxide (CO2) elimination, particularly at high respiratory rates (RR) used at low tidal volume ventilation in acute respiratory distress syndrome (ARDS). Aspiration of dead space (ASPIDS), a known method for dead space reduction, comprises two mechanisms activated during late expiration: aspiration of gas from the tip of the tracheal tube and gas injection through the inspiratory line - circuit flushing. The objective was to study the efficiency of circuit flushing alone and of ASPIDS at wide combinations of RR and tidal volume (VT) in anaesthetized pigs. The hypothesis was tested that circuit flushing and ASPIDS are particularly efficient at high RR. Methods: In Part 1 of the study, RR and VT were, with a computer-controlled ventilator, modified for one breath at a time without changing minute ventilation. Proximal dead space in a y-piece and ventilator tubing (VDaw,prox) was measured. In part two, changes in CO2 partial pressure (PaCO2) during prolonged periods of circuit flushing and ASPIDS were studied at RR 20, 40 and 60 minutes-1. Results: In Part 1, VDaw,prox was 7.6 ± 0.5% of VT at RR 10 minutes 1 and 16 ± 2.5% at RR 60 minutes 1. In Part 2, circuit flushing reduced PaCO2 by 20% at RR 40 minutes 1 and by 26% at RR 60 minutes 1. ASPIDS reduced PaCO2 by 33% at RR 40 minutes 1 and by 41% at RR 60 minutes 1. Conclusions: At high RR, re-breathing of CO2 from the y-piece and tubing becomes important. Circuit flushing and ASPIDS, which significantly reduce tubing dead space and PaCO2, merit further clinical studies

Computer simulation allows goal-oriented mechanical ventilation in acute respiratory distress syndrome

Introduction To prevent further lung damage in patients with acute respiratory distress syndrome ( ARDS), it is important to avoid overdistension and cyclic opening and closing of atelectatic alveoli. Previous studies have demonstrated protective effects of using low tidal volume ( VT), moderate positive end-expiratory pressure and low airway pressure. Aspiration of dead space ( ASPIDS) allows a reduction in VT by eliminating dead space in the tracheal tube and tubing. We hypothesized that, by applying goal-orientated ventilation based on iterative computer simulation, VT can be reduced at high respiratory rate and much further reduced during ASPIDS without compromising gas exchange or causing high airway pressure. Methods ARDS was induced in eight pigs by surfactant perturbation and ventilator-induced lung injury. Ventilator resetting guided by computer simulation was then performed, aiming at minimal VT, plateau pressure 30 cmH(2)O and isocapnia, first by only increasing respiratory rate and then by using ASPIDS as well. Results VT decreased from 7.2 +/- 0.5 ml/kg to 6.6 +/- 0.5 ml/kg as respiratory rate increased from 40 to 64 +/- 6 breaths/min, and to 4.0 +/- 0.4 ml/kg when ASPIDS was used at 80 +/- 6 breaths/min. Measured values of arterial carbon dioxide tension were close to predicted values. Without ASPIDS, total positive end-expiratory pressure and plateau pressure were slightly higher than predicted, and with ASPIDS they were lower than predicted. Conclusion In principle, computer simulation may be used in goal-oriented ventilation in ARDS. Further studies are needed to investigate potential benefits and limitations over extended study periods

Dead space and CO2 elimination related to pattern of inspiratory gas delivery in ARDS patients

Introduction: The inspiratory flow pattern influences CO2 elimination by affecting the time the tidal volume remains resident in alveoli. This time is expressed in terms of mean distribution time (MDT), which is the time available for distribution and diffusion of inspired tidal gas within resident alveolar gas. In healthy and sick pigs, abrupt cessation of inspiratory flow (that is, high end-inspiratory flow (EIF)), enhances CO2 elimination. The objective was to test the hypothesis that effects of inspiratory gas delivery pattern on CO2 exchange can be comprehensively described from the effects of MDT and EIF in patients with acute respiratory distress syndrome (ARDS). Methods: In a medical intensive care unit of a university hospital, ARDS patients were studied during sequences of breaths with varying inspiratory flow patterns. Patients were ventilated with a computer-controlled ventilator allowing single breaths to be modified with respect to durations of inspiratory flow and postinspiratory pause (T-P), as well as the shape of the inspiratory flow wave. From the single-breath test for CO2, the volume of CO2 eliminated by each tidal breath was derived. Results: A long MDT, caused primarily by a long TP, led to importantly enhanced CO2 elimination. So did a high EIF. Effects of MDT and EIF were comprehensively described with a simple equation. Typically, an efficient and a less-efficient pattern of inspiration could result in +/- 10% variation of CO2 elimination, and in individuals, up to 35%. Conclusions: In ARDS, CO2 elimination is importantly enhanced by an inspiratory flow pattern with long MDT and high EIF. An optimal inspiratory pattern allows a reduction of tidal volume and may be part of lung-protective ventilation

Computer simulation - a tool for optimisation of ventilator setting in critical lung disease

Increasing attention is paid to mechanical ventilation as one cause behind aggravation of lung injury. Lung protective ventilation can be achieved e.g. by minimising tidal lung collapse and re-expansion and by the use of small tidal volume allowing some degree of permissive hypercapnia. The ventilator and the sick lung comprise a very complex system. Identification of a ventilator setting that with respect to respiratory rate, tidal volume, I:E ratio, PEEP, etc is optimal with respect to desired physiological effects is therefore difficult. The main objective behind this thesis was to develop and validate a system for computer simulation of respiratory mechanics and gas exchange that allows prediction of physiological effects of resetting the ventilator. To reach this objective, methods for characterisation of lung physiology, which require little or no disturbance of the breathing pattern, were developed. Complementary studies of how gas exchange is affected by ventilator setting were performed. Longer time for equilibration between inspired and alveolar gas decreased airway dead space. When, in acute lung injury, PaO2 increased in response to PEEP, alveolar dead space decreased and vice versa. The hypothesis that mechanical behaviour and CO2 elimination after resetting respiratory rate and tidal volume could be predicted by simulation in healthy pigs was confirmed. Further, the hypothesis that immediate effects of moderate PEEP increments on mechanics and CO2 elimination in patients with acute lung injury could be predicted by simulation was also confirmed. Future development of the lung model used for simulation was outlined so as to include how time for gas equilibration affects gas exchange and to account for non-linear elastic properties. Iterative simulation may be a tool in future goal-oriented ventilation strategies

Pattern of inspiratory gas delivery affects CO(2) elimination in health and after acute lung injury.

Objective To avoid ventilator induced lung injury, tidal volume should be low in acute lung injury (ALI). Reducing dead space may be useful, for example by using a pattern of inspiration that prolongs the time available for gas distribution and diffusion within the respiratory zone, the mean distribution time (MDT). A study was conducted to investigate how MDT affects CO2 elimination in pigs at health and after ALI. Design and setting Randomised crossover study in the animal laboratory of Lund University Biomedical Center. Subjects and intervention Healthy pigs and pigs with ALI, caused by surfactant perturbation and lung-damaging ventilation were ventilated with a computer-controlled ventilator. With this device each breath could be tailored with respect to insufflation time and pause time (T I and T P) as well as flow shape (square, increasing or decreasing flow). Measurements and results The single-breath test for CO2 allowed analysis of the volume of expired CO2 and the volume of CO2 re-inspired from Y-piece and tubes. With a long MDT caused by long T I or T P, the expired volume of CO2 increased markedly in accordance with the MDT concept in both healthy and ALI pigs. High initial inspiratory flow caused by a short T I or decreasing flow increased the re-inspired volume of CO2. Arterial CO2 increased during a longer period of short MDT and decreased again when MDT was prolonged. Conclusions CO2 elimination can be enhanced by a pattern of ventilation that prolongs MDT. Positive effects of prolonged MDT caused by short T I and decreasing flow were attenuated by high initial inspiratory flow. Electronic supplementary material The online version of this article (doi:10.1007/s00134-007-0840-7) contains supplementary material, which is available to authorized users

Effects of positive end-expiratory pressure on dead space and its partitions in acute lung injury

Objective: A large tidal volume (VT) and lung collapse and re-expansion may cause ventilator-induced lung injury (VILI) in acute lung injury (ALI). A low VT and a positive end-expiratory pressure (PEEP) can prevent VILI, but the more VT is reduced, the more dead space (VD) compromises gas exchange. We investigated how physiological, airway and alveolar VD varied with PEEP and analysed possible links to respiratory mechanics. Setting: Medical and surgical intensive care unit (ICU) in a university hospital. Design: Prospective, non-randomised comparative trial. Patients: Ten consecutive ALI patients. Intervention: Stepwise increases in PEEP from zero to 15 cmH(2)O. Measurements and results: Lung mechanics and VD were measured at each PEEP level. Physiological VD was 41-64% of VT at zero PEEP and increased slightly with PEEP due to a rise in airway VD. Alveolar VD was 11-38% of VT and did not vary systematically with PEEP. However, in individual patients a decrease and increase of alveolar VD paralleled a positive or negative response to PEEP with respect to oxygenation (shunt), respectively. VD fractions were independent of respiratory resistance and compliance. Conclusions: Alveolar VD is large and does not vary systematically with PEEP in patients with various degrees of ALI. Individual measurements show a diverse response to PEEP. Respiratory mechanics were of no help in optimising PEEP with regard to gas exchange

Protective ventilation in experimental acute respiratory distress syndrome after ventilator-induced lung injury: a randomized controlled trial

Low tidal volume (V-T), PEEP, and low plateau pressure (P-PLAT) are lung protective during acute respiratory distress syndrome (ARDS). This study tested the hypothesis that the aspiration of dead space (ASPIDS) together with computer simulation can help maintain gas exchange at these settings, thus promoting protection of the lungs. ARDS was induced in pigs using surfactant perturbation plus an injurious ventilation strategy. One group then underwent 24 h protective ventilation, while control groups were ventilated using a conventional ventilation strategy at either high or low pressure. Pressurevolume curves (P-el/V), blood gases, and haemodynamics were studied at 0, 4, 8, 16, and 24 h after the induction of ARDS and lung histology was evaluated. The P-el/V curves showed improvements in the protective strategy group and deterioration in both control groups. In the protective group, when respiratory rate (RR) was approximate to 60 bpm, better oxygenation and reduced shunt were found. Histological damage was significantly more severe in the high-pressure group. There were no differences in venous oxygen saturation and pulmonary vascular resistance between the groups. The protective ventilation strategy of adequate pH or Pa-CO2 with minimal V-T, and high/safe P-PLAT resulting in high PEEP was based on the avoidance of known lung-damaging phenomena. The approach is based upon the optimization of V-T, RR, PEEP, I/E, and dead space. This study does not lend itself to conclusions about the independent role of each of these features. However, dead space reduction is fundamental for achieving minimal V-T at high RR. Classical physiology is applicable at high RR. Computer simulation optimizes ventilation and limiting of dead space using ASPIDS. Inspiratory P-el/V curves recorded from PEEP or, even better, expiratory P-el/V curves allow monitoring in ARDS

CO2 elimination at varying inspiratory pause in acute lung injury

Previous studies have indicated that, during mechanical ventilation, an inspiratory pause enhances gas exchange. This has been attributed to prolonged time during which fresh gas of the tidal volume is present in the respiratory zone and is available for distribution in the lung periphery. The mean distribution time of inspired gas (MDT) is the mean time during which fractions of fresh gas are present in the respiratory zone. All ventilators allow setting of pause time, T-P, which is a determinant of MDT. The objective of the present study was to test in patients the hypothesis that the volume of CO2 eliminated per breath, VTCO2, is correlated to the logarithm of MDT as previously found in animal models. Eleven patients with acute lung injury were studied. When T-P increased from 0% to 30%, MDT increased fourfold. A change of T-P from 10% to 0% reduced VTCO2 by 14%, while a change to 30% increased VTCO2 by 19%. The relationship between VTCO2 and MDT was in accordance with the logarithmic hypothesis. The change in VTCO2 reflected to equal extent changes in airway dead space and alveolar PCO2 read from the alveolar plateau of the single breath test for CO2. By varying T-P, effects are observed on VTCO2, airway dead space and alveolar PCO2. These effects depend on perfusion, gas distribution and diffusion in the lung periphery, which need to be further elucidated