Relationship between physiologic deadspace/tidal volume ratio and gas exchange in infants with acute bronchiolitis on invasive mechanical ventilation

Objectives. To evaluate the association between deadspace/ tidal volume ratio (V-D/V-T) and gas exchange variables: PaO2, PacO(2), PaO2/FIO2, arterial/alveolar oxygen tension ratio (PaO2/PAO(2)), alveolar-arterial oxygen tension difference/arterial oxygen tension ratio (P(A-a)O-2/PaO2), carbon dioxide production (VCO2), ventilation index ([PaCO2 x peak inspiratory pressure x mechanical respiratory rate]/1000), and oxygenation index ([mean airway pressure x FIO2 x 100]/PaO2), all measured at an early stage in children with obstructive acute respiratory failure. Design: Prospective, cross-sectional, observational study. Setting: Pediatric intensive care unit, university hospital. Patients: Twenty-nine infants with acute bronchiolitis, defined according to clinical and radiologic criteria, Children with chronic pulmonary disease, neuromuscular disease, congenital cardiopathies, or hemodynamic instability were excluded. Interventions: Measurements were made between 24 and 72 hrs of mechanical ventilation using volumetric capnography and arterial blood gas analysis. Measurements and Main Results: The following variables significantly correlated with V-D/V-T, calculated using Spearman's correlation coefficient (r(s)): PaO2 (r(s) = -0.63, p < .001), PaO2/FIO2 (r(s) = -0.56, p = .002), PaO2/PAO(2) (r(s) = -0.46, p = .012), P(A-a)O-2/PaO2 (r(s) = 0.46, p =.012), PaCO2 (r(s) = 0.51, p = .005), VCO2 (r(s) = -0,62, p < .01), oxygenation index (r(s) = 0.48, p = .009), and ventilation index (r(s) = -0.53, p =.003). A statistically significant association was found between an increase in V-D/V-T and severity of lung injury, defined as PaO2/FIO2 < 200 (p =.03, Mann-Whitney). Conclusions: In the study population, V-D/V-T not only reflected ventilatory disorders, as is well recognized, but also was associated with disturbances of oxygenation. These results warrant further evaluation of the usefulness of serial measurement of V-D/V-T as a marker of disease severity in severe acute bronchiolitis and other causes of respiratory failure.8437237