Abstract

Background: Volume-targeted ventilation is used in neonates to reduce volutrauma and inadvertent hyperventilation. Little is known about appropriate tidal volume (V_T) settings in extremely low birthweight (ELBW) infants who remain intubated for extended periods.

Hypothesis: The V_T required to maintain adequate partial pressure of carbon dioxide (Pco₂) levels changes as the underlying disease evolves in infants ventilated for prolonged periods.

Objective: To obtain normative data for V_T associated with normocapnia in ELBW infants ventilated with Volume Guarantee over the first 3 weeks of life.

Design/Methods: Set and measured V_T, peak pressure, respiratory rate and blood gas values were extracted from the records of babies <800 g born January 2003 to August 2005 and ventilated with Volume Guarantee. Data were collected at the time of each blood gas measurement during days 1–2, 5–7 and 14–21. Only the V_T corresponding to Pco₂ values within a defined normal range were included. Descriptive statistics were used to define the patient cohort. Mean V_T and Pco₂ for each patient during each epoch was calculated, and these values were analysed by repeated-measures analysis of variance.

Results: Twenty-six infants, mean (SD) birth weight 615 (104) g, were included. A total of 828 paired blood gas and V_T sets were analysed: days 1–2 = 251; days 5–7 = 185; days 14–17 = 216; days 18–21 = 176. Pco₂ values (mean (SD)) rose from 44.0 (5.4) mm Hg on days 1–2 to 46.3 (5.2) mm Hg on days 5–7 and remained stable during days 14–17 and 18–21 (53.9 (6.8) and 53.9 (6.2) mm Hg, respectively). Mean exhaled V_T rose from 5.15 (0.62) ml/kg on day 1 to 5.24 (0.71) ml/kg on days 5–7, 5.63 (1.0) ml/kg on days 14–17, and 6.07 (1.4) ml/kg on days 18–21 (p<0.05).

Conclusions: Despite permissive hypercapnia, V_T requirement rises with advancing postnatal age in ELBW infants. The increase is greatest during the third week of life, which is probably due to distension of the upper airways (acquired tracheomegaly) and increasing heterogeneity of lung inflation (increased alveolar dead space).