Aspiration of airway dead space. A new method to enhance CO2 elimination

An appropriate inspiratory flow pattern can enhance CO2 exchange, facilitating protective ventilation of healthy lungs

BACKGROUND

In acute lung injury, CO2 exchange is enhanced by prolonging the volume-weighted mean time for fresh gas to mix with resident alveolar gas, denoted mean distribution time (MDT), and by increasing the flow rate immediately before inspiratory flow interruption, end-inspiratory flow (EIF). The objective was to study these effects in human subjects without lung disease and to analyse the results with respect to lung-protective ventilation of healthy lungs.

METHODS

During preparation for intracranial surgery, the lungs of eight subjects were ventilated with a computer-controlled ventilator, allowing breath-by-breath modification of the inspiratory flow pattern. The durations of inspiration (TI) and postinspiratory pause (TP) were modified, as was the profile of the inspiratory flow wave (i.e. constant, increasing, or decreasing). The single-breath test for CO2 was used to quantify airway dead space (VDaw) and CO2 exchange.

RESULTS

A long MDT and a high EIF augment CO2 elimination by reducing VDaw and promoting mixing of tidal gas with resident alveolar gas. A heat and moisture exchanger had no other effect than enlarging VDaw. A change of TI from 33 to 15% and of TP from 10 to 28%, leaving the time for expiration unchanged, would augment tidal elimination of CO2 by 14%, allowing a 10% lower tidal volume.

CONCLUSIONS

In anaesthetized human subjects without lung disease, CO2 exchange is enhanced by a long MDT and a high EIF. A short TI and a long TP allow significant reduction of tidal volume when lung-protective ventilation is required.

CLINICAL TRIAL REGISTRATION

NCT01686984.

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 (TP), 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.

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

Background

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.

Methods

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. Pressure–volume 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.

Results

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 ≈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.

Conclusions

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.

Re-inspiration of CO(2) from ventilator circuit: effects of circuit flushing and aspiration of dead space up to high respiratory rate

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 (V_T) 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 V_T 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 (VD_{aw, 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, VD_{aw, prox} was 7.6 ± 0.5% of V_T 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 (V_T), moderate positive end-expiratory pressure and low airway pressure. Aspiration of dead space (ASPIDS) allows a reduction in V_T by eliminating dead space in the tracheal tube and tubing. We hypothesized that, by applying goal-orientated ventilation based on iterative computer simulation, V_T 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 V_T, plateau pressure 30 cmH₂O and isocapnia, first by only increasing respiratory rate and then by using ASPIDS as well.

Results

V_T 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.

Heat and moisture exchangers and heated humidifiers in acute lung injury/acute respiratory distress syndrome patients. Effects on respiratory mechanics and gas exchange

OBJECTIVE

To compare, in acute lung injury/acute respiratory distress syndrome (ALI/ARDS) patients, the short-term effects of heat and moisture exchangers (HME) and heated humidifiers (HH) on gas exchange, and also on respiratory system mechanics when isocapnic conditions are met.

DESIGN

Prospective open clinical study.

SETTING

Intensive Care Service.

PATIENTS

Seventeen invasively ventilated ALI/ARDS patients.

INTERVENTION

The study was performed in three phases: (1) determinations were made during basal ventilatory settings with HME; (2) basal ventilatory settings were maintained and HME was replaced by an HH; (3) using the same HH, tidal volume (Vt) was decreased until basal PaCO2 levels were reached. FiO2, respiratory rate and PEEP were kept unchanged.

MEASUREMENTS AND RESULTS

Respiratory mechanics, Vdphys, gas exchange and hemodynamic parameters were obtained at each phase. By using HH instead of HME and without changing Vt, PaCO2 decreased from 46+/-9 to 40+/-8 mmHg (p<0.001) and Vdphys decreased from 352+/-63 to 310+/-74 ml (p<0.001). Comparing the first phase with the third, Vt decreased from 521+/-106 to 440+/-118 ml (p<0.001) without significant changes in PaCO2, Vd/Vt decreased from 0.69+/-0.11 to 0.62+/-0.12 (p<0.001), plateau airway pressure decreased from 25+/-6 to 21+/-6 cmH2O (p<0.001) and respiratory system compliance improved from 35+/-12 to 42+/-15 ml/cmH2O (p<0.001). PaO2 remained unchanged in the three phases.

CONCLUSIONS

Reducing dead space with the use of HH decreases PaCO2 and more importantly, if isocapnic conditions are maintained by reducing Vt, this strategy improves respiratory system compliance and reduces plateau airway pressure.

A prolonged postinspiratory pause enhances CO2 elimination by reducing airway dead space

BACKGROUND

CO2 elimination per breath (VCO2,T) depends primarily on tidal volume (VT). The time course of flow during inspiration influences distribution and diffusive mixing of VT and is therefore a secondary factor determining gas exchange. To study the effect of a postinspiratory pause we defined 'mean distribution time' (MDT) as the mean time given to inspired gas for distribution and diffusive mixing within the lungs. The objective was to quantify changes in airway dead space (VDaw), slope of the alveolar plateau (SLOPE) and VCO2,T as a function of MDT in healthy pigs.

METHODS

Ten healthy pigs were mechanically ventilated. Airway pressure, flow and partial pressure of CO2 were recorded during resetting of the postinspiratory pause from 10% (baseline) to, in random order, 0, 5, 20 and 30% of the respiratory cycle. The immediate changes in VDaw, SLOPE, VCO2,T, and MDT after resetting were analyzed.

RESULTS

VDaw in percent of VT decreased from 29 to 22%, SLOPE from 0.35 to 0.16 kPa per 100 ml as MDT increased from 0.51 to 1.39 s. Over the same MDT range, VCO2,T increased by 10%. All these changes were statistically significant.

CONCLUSION

MDT allows comparison of different patterns of inspiration on VDaw and gas exchange. Estimation of the effects of an altered ventilator setting on exchange of CO2 can be done only after about 30 minutes, while the transient changes in VCO2,T may give immediate information. MDT affects gas exchange to an important extent. Further studies in human subjects in health and in disease are needed.

Elimination of ventilator dead space during synchronized ventilation in premature infants

BACKGROUND

Mainstream airflow sensors used in neonatal ventilators to synchronize mechanical breaths with spontaneous inspiration and measure ventilation increase dead space and may impair carbon dioxide (CO(2)) elimination.

OBJECTIVE

To evaluate a technique consisting of a continuous gas leakage at the endotracheal tube (ETT) adapter to wash out the airflow sensor for synchronization and ventilation monitoring without CO(2) rebreathing in preterm infants.

DESIGN

Minute ventilation (V'(E)) by respiratory inductance plethysmography, end-inspiratory and end-expiratory CO(2) by side-stream microcapnography, and transcutaneous CO(2) tension (TcPCO(2)) were measured in 10 infants (body weight, 835+/-244 g; gestational age, 26+/-2 weeks; age, 19+/-9 days; weight, 856+/-206 g; ventilator rate, 21+/-6 beats/min; PIP, 16+/-1 centimeters of water (cmH(2)O); PEEP, 4.2+/-0.4 cmH(2)O; fraction of inspired oxygen (FIo(2)), 0.26+/-0.6). The measurements were made during four 30-minute periods in random order: IMV (without airflow sensor), IMV+Sensor, SIMV (with airflow sensor), and SIMV+Leak (ETT adapter continuous leakage).

RESULTS

Airflow sensor presence during SIMV and IMV+Sensor periods resulted in higher end-inspiratory and end-expiratory CO(2), Tcpco(2), and spontaneous V'(E) compared with IMV. These effects were not observed during SIMV+Leak.

CONCLUSIONS

The significant physiologic effects of airflow sensor dead space during synchronized ventilation in preterm infants can be effectively prevented by the ETT adapter continuous leakage technique.

Tracheal gas insufflation for the prevention of morbidity and mortality in mechanically ventilated newborn infants

BACKGROUND

Tracheal gas insufflation (TGI) is a technique where a continuous flow of gas is instilled into the lower trachea during conventional mechanical ventilation. TGI can improve carbon dioxide removal with lower ventilation pressures and smaller tidal volumes, potentially decreasing secondary lung injury and chronic lung disease (CLD).

OBJECTIVES

To assess whether, in mechanically ventilated neonates, the use of tracheal gas insufflation reduces mortality, CLD and other adverse clinical outcomes without significant side effects.

SEARCH STRATEGY

Searches were made of MEDLINE 1966 to December 2001, CINAHL 1982 to December 2001, the Cochrane Controlled Trials Register (Cochrane Library, Issue 4, 2001) and conference and symposia proceedings.

SELECTION CRITERIA

Randomised controlled trials (RCT) that include newborn infants who are mechanically ventilated, and compare TGI during conventional mechanical ventilation (CMV) with CMV alone. Primary outcomes - mortality, CLD and neurodevelopmental outcome; secondary outcomes - air leak, intraventricular haemorrhage, periventricular leukomalacia, duration of mechanical ventilation, duration of respiratory support, duration of oxygen therapy, duration of hospital stay, retinopathy of prematurity, immediate adverse effects.

DATA COLLECTION AND ANALYSIS

Each reviewer assessed eligibility, trial quality and extracted data separately. Study authors were contacted for additional information if necessary.

MAIN RESULTS

Only one small study was found to be eligible. This study found no evidence of effect on mortality, CLD or age at first extubation. The total duration of ventilation was 9.3 days shorter in the TGI group (95% CI from 15.7 to 2.9 days shorter). The age at complete weaning from ventilation was 26 days shorter in the TGI group (95% CI from 46 to 6 days shorter). There was no evidence of effect on the total duration of respiratory support, oxygen therapy or hospital stay.

REVIEWER'S CONCLUSIONS

There is evidence from a single RCT that TGI may reduce the duration of mechanical ventilation in preterm infants - although the data from this small study do not give sufficient evidence to support the introduction of TGI into clinical practice. The technical requirements for performing TGI (as performed in the single included study) are great. There is no statistically significant reduction in the total duration of respiratory support or hospital stay. TGI cannot be recommended for general use at this time.

Continuous tracheal gas insufflation in preterm infants with hyaline membrane disease. A prospective randomized trial

In mechanically ventilated neonates, the instrumental dead space is a major determinant of total minute ventilation. By flushing this dead space, continuous tracheal gas insufflation (CTGI) may allow reduction of the risk of overinflation. We conducted a randomized trial to evaluate the efficacy of CTGI in reducing airway pressure over the entire period of mechanical ventilation while maintaining oxygenation. A total of 34 preterm newborns, ventilated in conventional pressure-limited mode, were enrolled in two study arms, to receive or not receive CTGI. Transcutaneous Pa(CO(2)) (tcPa(CO(2))) was maintained at 40 to 46 mm Hg in both groups to ensure comparable alveolar ventilation. Respiratory data were collected several times during the first day and daily until Day 28. Both groups were similar at the time of inclusion. During the first 4 d of the study, the difference between peak pressure and positive end-expiratory pressure was significantly lower in the CTGI group by 18% to 35%, with the same tcPa(CO(2)) level and with no difference in the ratio of tcPa(O(2)) to fraction of inspired oxygen (245 +/- 29 versus 261 +/- 46 mm Hg [mean +/- SD] over the first 4 d). Extubation occurred sooner in the CTGI group (p < 0.05), and the duration of mechanical ventilation was shorter (median: 3.6 d; 25th to 75th quartiles: 1.5 to 12.0 d; versus median: 15.6 d; 25th to 75th quartiles: 7.9 to 22.2; p < 0.05) than in the non-CTGI group. CTGI allows the use of low-volume ventilation over a prolonged period and reduces the duration of mechanical ventilation.

cDNA cloning of ovine pulmonary SP-A, SP-B, and SP-C: isolation of two different sequences for SP-B

Pulmonary surfactant promotes alveolar stability by lowering the surface tension at the air-liquid interface in the peripheral air spaces. The three surfactant proteins SP-A, SP-B, and SP-C contribute to dynamic surface properties involved during respiration. We have cloned and sequenced the complete cDNAs for ovine SP-A and SP-C and two distinct forms of ovine SP-B cDNAs. The nucleotide sequence of ovine SP-A cDNA consists of 1,901 bp and encodes a protein of 248 amino acids. Ovine SP-C cDNA contains 809 bp, predicting a protein of 190 amino acids. Ovine SP-B is encoded by two mRNA species, which differ by a 69-bp in-frame deletion in the region coding for the active airway protein. The larger SP-B cDNA comprises 1,660 bp, encoding a putative protein of 374 amino acids. With the sequences reported, a more complete analysis of surfactant regulation and the determination of their physiological function in vivo will be enabled.