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Is Dead Space a New Marker for Gravity on Pediatric Acute Hypoxemic Respiratory Failure?

To the Editor: In a recent issue of Critical Care Medicine, Bhalla et al (1) came in an interesting time, following the publication of The Pediatric Acute Lung Injury Consensus (2). On its supplement on definition, incidence, and epidemiology (3), the consensus authors addressed the important question of the influence of higher dead space on pediatric acute respiratory distress syndrome mortality and the possibility to include it on diagnostic criteria. Although there was a lack of evidence to recommend use of dead-space measurements as part of the diagnostic criteria, alveolar dead-space fraction (AVDSf) appeared to be useful in additional risk stratification. It is important to stress that depending on the clinical situation, AVDSf can be higher, because of “shunt-related” dead space (4). Kallet et al (5) reported that in adult patients with acute respiratory disease syndrome, there was an association between dead space/tidal volume ratio (Vd/Vt) and gas exchange variables. The increase in Vd/Vt can arise from a combination between shunt and alterations on ventilation/perfusion ratio (V/Q). Taking this into account, Bhalla et al (1) performed multivariate analysis of its sample in order to control the degree of shunt. After controlling for oxygenation index, Pao 2 /Fio 2 , inotrope use, and mean airway pressure, AVDSf was no longer associated with mortality, what reflected that dead space could be a consequence of different phenomena. Clinical available forms of capnography can be either time (TCap) or volume based (VCap). Volumetric-based capnography generates a capnogram that have a steeper slope of phaseIII (SIII). Tussman et al (6) have recently validated the use of the midportion of SIII as a way to accurately estimate mean alveolar partial pressure of Co 2 , setting the base for the use of Bohr dead space on a noninvasive monitoring. Thus, the use of VCap and the combination of Enghoff and Bohr dead-space Marin H. Kollef, MD, Washington University School of Medicine, Pulmonary and Critical Care Division, St. Louis, MO; Jason P. Burnham, MD, Division of Medical Education, Washington University School of Medicine, St. Louis, MO