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Reliability of dead-space measurements by volumetric capnography in ventilated small lungs

Presented at the Neonatal Society 2003 Summer Meeting (programme).

Roehr CC, Proquitté H, Krause S, Wauer RR, Schmalisch G

Clinic of Neonatology (CCM), Humboldt University Berlin (Charité), 10098 Berlin, Schumannstr. 20/21

Introduction: Recent studies have shown that the measurement of airway dead-space fractions by volumetric capnography in ventilated adults and older children has a high predictive value for successful extubation and clinical outcome (1,2). An essential prerequisite of this technique is the presence of alveolar gas at the end of expiration which is commonly indicated by a distinct decrease of the end-expiratory slope of the CO2-volume plot. The aim of this study was to investigate to which extent the prerequisite of alveolar gas is fulfilled in small stiff lungs in order to assess the reliability of this method in newborns.

Method: To simulate neonatal preterm lungs, 21 ventilated newborn piglets (age <12h, median weight 1000g (range 570-1950g)) were investigated. Surfactant depletion was induced by bronchoalveolar lavage (BAL) by repeated instillation of 30 mL / kg normal saline. Ventilatory, circulatory and lung mechanic parameters were measured before, immediately after BAL and after 60 min using the COSMO+ (Novametrix, USA). For each situation CO2-volume plots were recorded. The presence of alveolar gas was assumed if the end-expiratory slope was at least twofold less than the slope during the mid-expiratory part of the CO2-volume plot. This mid-expiratory part represents the mixed gas exhaled from the airways and is characterised by a steep slope.

Results: Before lavage in 76% of all records alveolar gas was seen in the CO2-volume plot. Immediately after BAL this incidence decreased to 50% and recovered slightly to 61% after 1h. Comparing the records with (group A) and without alveolar gas (group B) there were significant differences in the exhalation time (A: 254±73ms; B: 201±16ms, p<0.001) and the exhaled volume (Vex) (A: 9.6±2.5mL; B: 8.2±1.5mL, p<0.05). No significant differences were seen in the arterial pO2 and pCO2 between A and B. However, the evaluation of the CO2-volume plot did show that in records without alveolar gas both arterial-alveolar CO2 gradient (A: 9.4±7.5mmHg; B: 14.4±8.4mmHg, p<0.05) and the physiologic dead space fraction (VDphys/Vex) (A: 0.68±0.08; B: 0.84±0.06; p<0.0001) were significantly increased.

Conclusion: In contrast to adults and older children alveolar gas was not seen in every CO2-volume plot of ventilated small lungs: the lower the tidal volume and the lower the exhalation time the higher the incidence of records without alveolar gas. Furthermore, if there is no distinct decrease of the end-expiratory slope of the CO2-volume plot the measured end-expiratory CO2 does not necessarily reflect true alveolar CO2 tension. In these records arterial-alveolar CO2 gradient and calculated physiologic dead space fraction may thus have been overestimated. This limits the diagnostic value of volumetric capnography particularly in the preterm newborn.

Acknowledgements: The work was supported by the Grant 01ZZ9511 from the German Ministry of Education and Research.

1. Arnold, J. H., L. K. Bower, and J. E. Thompson. 1995. Respiratory deadspace measurements in neonates with congenital diaphragmatic hernia. Crit.Care Med. 23:371-375.
2. Nuckton, T. J., J. A. Alonso, R. H. Kallet, B. M. Daniel, J. F. Pittet, M. D. Eisner, and M. A. Matthay. 2002. Pulmonary dead-space fraction as a risk factor for death in the acute respiratory distress syndrome. N Engl J Med JID - 0255562 346:1281-1286.

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