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Updated: Jul 14, 2026

Combining Volumetric Capnography And Barometric Plethysmography To Measure The Lung Structure-function Relationship
Published on: January 8, 2019
Relationship between physiologic deadspace/tidal volume ratio and gas exchange in infants with acute bronchiolitis on
Armando A Almeida-Junior1, Marcos T Nolasco da Silva, Celize C B Almeida
1Pediatric Intensive Care Unit, Center for Investigation in Pediatrics, State University of Campinas Medical School, Campinas, SP, Brazil. armandoalmeida@directnet.com.br
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.
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