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

Edoardo De Robertis,Björn Jonson,Leif Uttman

doi:10.1186/cc8986

Abstract

IntroductionDead 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.MethodsIn 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.ResultsIn 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.ConclusionsAt 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.

Highlights

Dead space negatively influences carbon dioxide (CO2) elimination, at high respiratory rates (RR) used at low tidal volume ventilation in acute respiratory distress syndrome (ARDS)
The volume of CO2 eliminated per breath (VTCO2) corresponds to the area within the loop, Figure 3
Part 1 At increasing RR, airway dead space (VDaw), prox decreased from 31 ± 2 ml at RR10 to 11 ± 2 ml at RR60 tightly according a logarithmic equation (Figure 4)

Summary

Introduction

Dead space negatively influences carbon dioxide (CO2) elimination, 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 hypothesis was tested that circuit flushing and ASPIDS are efficient at high RR. In acute respiratory distress syndrome, severe obstructive lung disease, and at increased intracranial pressure it may be important to maintain adequate CO2 exchange at low tidal volume ventilation (LTVV). At high RR, when dead space as a fraction of tidal volume increases, dead space reduction may be called for.

Objectives

Methods

Results

Discussion

Conclusion