Evolution of tidal volume requirement during the first 3 weeks of life in infants <800 g ventilated with Volume Guarantee.

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 (P(CO2) 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 P(CO2) values within a defined normal range were included. Descriptive statistics were used to define the patient cohort. Mean V(T) and P(CO2) 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. P(CO2) 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).