The impact of instrumental dead-space in volume-targeted ventilation of the extremely low birth weight (ELBW) infant

Volume-targeted ventilation is increasingly used in neonatal ventilation to reduce the risk of volutrauma and inadvertent hyperventilation. However, normative data for appropriate tidal volume (V(T)) settings are lacking, especially in extremely low birth weight (ELBW) infants in whom the added dead space (DS) of the flow sensor may be important.To quantify the effect of instrumental dead-space (IDS) on ventilation and to obtain normative data for initial V(T) associated with normocapnia in ELBW infants ventilated with volume guarantee (VG) ventilation.Set and measured V(T), respiratory rate (RR) and arterial blood gas values (ABG) were extracted from charts of babies800 g born between January 2003 and August 2005, who were ventilated with VG. Data were collected at the time of each ABG during the 1st 48 hr of life. Theoretical alveolar minute ventilation (AMV) was calculated as (V(T) - DS) x RR. IDS was measured by filling with water a 2.5 mm endotracheal tube cut to 10 cm with attached hub of the inline suction catheter and flow sensor. We added 0.5 mL/kg to this value to account for distal tracheal/mainstem bronchi DS (anatomical dead space). Descriptive statistics and linear regression were used for analysis.The measured IDS was 2.7 mL. Mean combined DS (instrumental + anatomical) was 3.01 mL. There were 344 paired observations of V(T) and ABG with PaCO(2) in the normocapnic range in 38 infants (mean birth weight 625 g +/- 115 g SD, range 400-790 g) during the study period. The mean pH was 7.30 +/- 0.06 (SD), mean PaCO(2) 43.4 +/- 5.4 Torr. The mean target V(T) was 3.11 +/- 0.64 mL and the measured V(T) was 3.17 +/- 0.73 mL. Despite normocapnia, 47% of the V(T) were equal to or less than estimated DS. Mean theoretical AMV was only 8.7 mL/kg/min. The V(T)/kg needed for normocapnia was inversely related to weight (r = -0.70, P0.01), indicating some effect of the fixed IDS. Mean V(T)/kg of infants500 g was 5.9 +/- 0.3 mL, compared to 4.7 +/- 0.5 mL for those700 g (P0.001).Effective alveolar ventilation occurs with V(T) at or below calculated DS. This can be explained by the fact that at the high flow rates seen in these tiny infants who have extremely short inspiratory times, fresh gas penetrates through the dead space gas, rather than pushing it ahead. Therefore there is no need to forego synchronized and volume targeted ventilation because of dead space concerns. In infants800 g, initial V(T) of 5-6 mL/kg was associated with normocapnia when using assist/control or pressure support ventilation.