Reduction in minute alveolar ventilation causes hypercapnia in ventilated neonates with respiratory distress

Physiological dead space and alveolar ventilation in ventilated infants

BACKGROUND

Dead space is the volume not taking part in gas exchange and, if increased, could affect alveolar ventilation if there is too low a delivered volume. We determined if there were differences in dead space and alveolar ventilation in ventilated infants with pulmonary disease or no respiratory morbidity.

METHODS

A prospective study of mechanically ventilated infants was undertaken. Expiratory tidal volume and carbon dioxide levels were measured. Volumetric capnograms were constructed to calculate the dead space using the modified Bohr-Enghoff equation. Alveolar ventilation (VA) was also calculated.

RESULTS

Eighty-one infants with a median (range) gestational age of 28.7 (22.4-41.9) weeks were recruited. The dead space [median (IQR)] was higher in 35 infants with respiratory distress syndrome (RDS) [5.7 (5.1-7.0) ml/kg] and in 26 infants with bronchopulmonary dysplasia (BPD) [6.4 (5.1-7.5) ml/kg] than in 20 term controls with no respiratory disease [3.5 (2.8-4.2) ml/kg, p < 0.001]. Minute ventilation was higher in both infants with RDS or BPD compared to the controls. VA in infants with RDS or BPD was similar to that of the controls [p = 0.54].

CONCLUSION

Prematurely born infants with pulmonary disease have a higher dead space than term controls, which may influence the optimum level during volume-targeted ventilation.

IMPACT

Measurement of the dead space was feasible in ventilated newborn infants. The physiological dead space was a significant proportion of the delivered volume in ventilated infants. The dead space (per kilogram) was higher in ventilated infants with respiratory distress syndrome or evolving bronchopulmonary dysplasia compared to term controls without respiratory disease. The dead space volume should be considered when calculating the most appropriate volume during volume-targeted ventilation.

Large difference between Enghoff and Bohr dead space in ventilated infants with hypoxemic respiratory failure

BACKGROUND

Ventilated neonates with hypoxemic respiratory failure (HRF) may show a ventilation-perfusion (V/Q) mismatch.

OBJECTIVE

To evaluate the difference between the Bohr (V_{d, Bohr} ) and Enghoff (V_{d, Enghoff} ) dead spaces in infants by using volumetric capnography based on ventilator graphics and capnograms.

METHODS

This study enrolled 46 ventilated infants (mean birth weight, 2239 ± 640 g; mean gestational age, 35.5 ± 3.3 weeks). We performed volumetric capnography and calculated V_{d, Bohr} and V_{d, Enghoff} when arterial blood sampling was necessary for treatment. According to the oxygenation index (OI) based on the Montreux definition of neonatal acute respiratory distress syndrome, each measurement was classified into the HRF (OI ≥ 4) or control (OI < 4) group. Then, a regression analysis was performed to evaluate the correlation between the OI and the difference between V_{d, Enghoff} and V_{d, Bohr} .

RESULTS

The median V_{d, Enghoff} /tidal volume (V_T ) was significantly higher in the HRF group (0.55 [interquartile range, 0.47-0.68]) than in the control group (0.46 [0.37-0.57]). The HRF group showed a larger difference between V_{d, Enghoff} /V_T and V_{d, Bohr} /V_T than the control group (median, 0.22 [0.15-0.29] vs. 0.10 [0.06-0.14], respectively). Moreover, the regression analysis of the relationship between OI and V_{d, Enghoff} /V_T  - V_{d, Bohr} /V_T showed a positive correlation (r = .60, p < .001).

CONCLUSION

Ventilated neonates with hypoxemic respiratory failure showed a large difference between V_{d, Enghoff} and V_{d, Bohr} , possibly reflecting a low V/Q mismatch and right-to-left shunting.