Background
Knowledge of lung volume is essential for monitoring and optimizing mechanical ventilation. The aim of this study was to compare lung volume measurements by multiple-breath heptafluoropropane (HFP) washout (MBW) and by respiratory gated computed tomography (CT) in ventilated newborn piglets.
Material and Method
In 6 ventilated piglets (age: <12 h, median weight: 945 g) blood gases, respiratory mechanics, and lung volumes were measured in both the supine and prone positions. The measurements were performed in random order. Functional residual capacity (FRC) was measured simultaneously by HFP MBW (FRC(HFP)) using a new infrared mainstream sensor and by CT (FRC(CT)) at the end of inspiration and expiration (multi-slice Toshiba Aquilon 16, Otawara, Japan). Tidal volume (V(T)) was measured both by the Dräger Babylog 8000 ventilator (V(T BL)) and the volume difference of the CT scans (V(T CT)).
Results
FRC(HFP) (25.2+/-8.5 ml) and FRC(CT) (24.9+/-7.6 ml) correlated strongly (r=0.97) without significant bias. Bland-Altman limits of agreement showed differences between the two methods that ranged from -19.7 to +19.5%. A similar strong correlation without statistically significant bias was found between V(T BL) (8.5+/-2.0 ml) and V(T CT) (9.0+/-2.4 ml) with r=0.91. The limits of agreement were -24.4 and +14.0%. Body position (prone vs. supine) had no significant effect on blood gases, respiratory mechanics, or lung volumes.
Conclusions
Lung volumes measured in small ventilated lungs by HFP washout and CT are highly correlated and independent of body position. However, the relatively large limits of agreement indicate differences in the two techniques.

Keywords: Tidal Volume - physiology, Sus scrofa - physiology, Respiration, Artificial, Respiration, Posture - physiology, Pressure, Lung Volume Measurements - methods, Hydrocarbons, Fluorinated - administration & dosage, Functional Residual Capacity - physiology, Animals, Newborn, Tomography, X-Ray Computed