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 (Vd/Vt) and gas exchange variables: Pao2, Paco2, Pao2/Fio2, arterial/alveolar oxygen tension ratio (Pao2/PAo2), alveolar-arterial oxygen tension difference/arterial oxygen tension ratio (P(A-a)o2/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 Vd/Vt, calculated using Spearman's correlation coefficient (rs): Pao2 (rs = -0.63, p < .001), Pao2/Fio2 (rs = -0.56, p = .002), Pao2/PAo2 (rs = -0.46, p = .012), P(A-a)o2/Pao2 (rs = -0.46, p = .012), Paco2 (rs = 0.51, p = .005), Vco2 (rs = -0,62, p < .01), oxygenation index (rs = 0.48, p = .009), and ventilation index (rs = -0.53, p = .003). A statistically significant association was found between an increase in Vd/Vt and severity of lung injury, defined as Pao2/Fio2 <200 (p = .03, Mann-Whitney).

CONCLUSIONS

In the study population, Vd/Vt 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 Vd/Vt as a marker of disease severity in severe acute bronchiolitis and other causes of respiratory failure.