Physiological and anatomical dead space in mechanically ventilated newborn infants.

OBJECTIVES: To compare the anatomical (VD-Ana ) and alveolar dead space (VD-Alv ) in term and prematurely born infants and identify the clinical determinants of those indices.

WORKING HYPOTHESIS: VD-Ana and VD-Alv will be higher in prematurely born compared to term born infants.

STUDY DESIGN: Retrospective analysis of data collected at King's College Hospital NHS Foundation Trust, London, UK.

PATIENT SELECTION: Fifty-six infants (11 term, 45 preterm) were studied at a median age of 8 (IQR 2-33) days.

METHODOLOGY: VD-Ana was determined using Fowler's method of volumetric capnography. VD-Alv was determined by subtracting VD-Ana from the physiological dead space which was determined by the Bohr-Enghoff equation. VD-Ana and VD-Alv were related to body weight at the time of study.

RESULTS: The median VD-Ana /kg was higher in prematurely born infants [3.7 (IQR: 3.0-4.5) mL/kg] compared to term infants [2.4 (IQR: 1.9-2.9) mL/kg, adjusted P = 0.001]. The median VD-Alv /kg was not higher in prematurely born infants [0.3 (IQR: 0.1-0.5)] compared to term infants [0.1 (IQR: 0.0-0.2) mL/kg] after adjusting for differences in respiratory rate and days of ventilation (P = 0.482). VD-Ana /kg was related to postmenstrual age (r = -0.388, P < 0.001), birth weight (r = -0.397, P < 0.001), and weight at measurement (r = -0.476, P < 0.001). VD-Alv /kg was related to postmenstrual age (r = -0.254, P < 0.001), birth weight (r = -0.291, P = 0.002), and weight at measurement (r = -0.281, P = 0.003) and related to days of ventilation (r = 0.194, P = 0.044).

CONCLUSIONS: VD-Ana /kg and VD-Alv /kg increased with decreasing weight and gestation. VD-Alv was higher in infants that have undergone prolonged mechanical ventilation.