Impact of different inspiratory flow patterns on arterial C02-tension

Pattern of inspiratory gas delivery affects CO2 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 (TI and TP) 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 TI or TP, 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 TI 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 TI and decreasing flow were attenuated by high initial inspiratory flow.

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.

Innovative practices of ventilatory support with pediatric patients

OBJECTIVES

The recognition that alveolar overdistension rather than peak inspiratory airway pressure is the primary determinant of lung injury has shifted our understanding of the pathogenesis of ventilator-induced side effects. In this review, contemporary ventilatory methods, supportive treatments, and future developments relevant to pediatric critical care are reviewed.

DATA SYNTHESIS

A strategy combining recruitment maneuvers, low-tidal volume, and higher positive end-expiratory pressure (PEEP) decreases barotrauma and volutrauma. Given that appropriate tidal volumes are critical in determining adequate alveolar ventilation and avoiding lung injury, volume-control ventilation with high PEEP levels has been proposed as the preferable protective ventilatory mode. Pressure-related volume control ventilation and high-frequency oscillatory ventilation (HFOV) have taken on an important role as protective lung strategies. Further data are required in the treatment of children, confirming the preliminary results in specific lung pathologies. Spontaneous breathing supported artificially during inspiration (pressure support ventilation) is widely used to maintain or reactivate spontaneous breathing and to avoid hemodynamic variation. Volume support ventilation reduces the need for manual adaptation to maintain stable tidal and minute volume and can be useful in weaning. Prone positioning and permissive hypercapnia have taken on an important role in the treatment of patients undergoing artificial ventilation. Surfactant and nitric oxide have been proposed in specific lung pathologies to facilitate ventilation and gas exchange and to reduce inspired oxygen concentration. Investigation of lung ventilation using a liquid instead of gas has opened new vistas on several lung pathologies with high mortality rates.

RESULTS

The conviction emerges that the best ventilatory treatment may be obtained by applying a combination of types of ventilation and supportive treatments as outlined above. Early treatment is important for the overall positive final result. Lung recruitment maneuvers followed by maintaining an open lung favor rapid resolution of pathology and reduce side effects.

CONCLUSIONS

The methods proposed require confirmation through large controlled clinical trials that can assess the efficacy reported in pilot studies and case reports and define the optimal method(s) to treat individual pathologies in the various pediatric age groups.