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

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.

THAM reduces CO2-associated increase in pulmonary vascular resistance - an experimental study in lung-injured piglets

INTRODUCTION

Low tidal volume (VT) ventilation is recommended in patients with acute respiratory distress syndrome (ARDS). This may increase arterial carbon dioxide tension (PaCO2), decrease pH, and augment pulmonary vascular resistance (PVR). We hypothesized that Tris(hydroxymethyl)aminomethane (THAM), a pure proton acceptor, would dampen these effects, preventing the increase in PVR.

METHODS

A one-hit injury ARDS model was established by repeated lung lavages in 18 piglets. After ventilation with VT of 6 ml/kg to maintain normocapnia, VT was reduced to 3 ml/kg to induce hypercapnia. Six animals received THAM for 1 h, six for 3 h, and six serving as controls received no THAM. In all, the experiment continued for 6 h. The THAM dosage was calculated to normalize pH and exhibit a lasting effect. Gas exchange, pulmonary, and systemic hemodynamics were tracked. Inflammatory markers were obtained at the end of the experiment.

RESULTS

In the controls, the decrease in VT from 6 to 3 ml/kg increased PaCO2 from 6.0±0.5 to 13.8±1.5 kPa and lowered pH from 7.40±0.01 to 7.12±0.06, whereas base excess (BE) remained stable at 2.7±2.3 mEq/L to 3.4±3.2 mEq/L. In the THAM groups, PaCO2 decreased and pH increased above 7.4 during the infusions. After discontinuing the infusions, PaCO2 increased above the corresponding level of the controls (15.2±1.7 kPa and 22.6±3.3 kPa for 1-h and 3-h THAM infusions, respectively). Despite a marked increase in BE (13.8±3.5 and 31.2±2.2 for 1-h and 3-h THAM infusions, respectively), pH became similar to the corresponding levels of the controls. PVR was lower in the THAM groups (at 6 h, 329±77 dyn∙s/m(5) and 255±43 dyn∙s/m(5) in the 1-h and 3-h groups, respectively, compared with 450±141 dyn∙s/m(5) in the controls), as were pulmonary arterial pressures.

CONCLUSIONS

The pH in the THAM groups was similar to pH in the controls at 6 h, despite a marked increase in BE. This was due to an increase in PaCO2 after stopping the THAM infusion, possibly by intracellular release of CO2. Pulmonary arterial pressure and PVR were lower in the THAM-treated animals, indicating that THAM may be an option to reduce PVR in acute hypercapnia.

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.

Update: adjuncts to mechanical ventilation

PURPOSE OF REVIEW

To describe the most recent advances and clinical applications of adjunctive techniques in mechanical ventilation, focusing on their overall impact on mortality and their potential indications in critically ill patients.

RECENT FINDINGS

The modern variants of extracorporeal membrane oxygenation are not only rescue alternatives but also therapeutic options for patients with severe but potentially reversible acute respiratory distress syndrome. Prone positioning returns as a desirable therapeutic option for patients with severe acute respiratory distress syndrome. Recent reports suggest that permissive hypercapnia, therapeutic paralysis, sedation, and controlled hypothermia could potentially improve important clinical outcomes. Although more clinical trials are clearly needed to support the use of inhaled prostacyclins in severe respiratory failure, encouraging results have been described in recent publications.

SUMMARY

Giving the complexity and dynamism of acute lung injury, timing, severity, and pathophysiologic pertinence are mandatory components of decision-making when considering the application of adjunctive measures to support mechanical ventilation.

Year in review 2010: Critical Care--Respirology

In this review, 21 original papers published last year in the respirology and critical care sections of Critical Care are classified and analyzed in the following categories: mechanical ventilation, lung recruitment maneuvers, and weaning; the role of positive end-expiratory pressure in acute lung injury models; animal models of ventilator-induced lung injury; diaphragmatic dysfunction; the role of mechanical ventilation in heart-lung interaction; and miscellanea.