A novel physiological investigation of the functional residual capacity by the bias flow nitrogen washout technique in infants

Abstract

The dynamic functional residual capacity (FRC_dyn), the lung volume most routinely measured in infants, is an unreliable volume landmark. In addition to the FRC_dyn, we measured the (passive) static FRC (FRC_st) by inducing a brief post-hyperventilation apnea (PHA) in 33 healthy infants aged 7.4–127.2 weeks. A commercial system for nitrogen (N₂) washout to measure FRC, and a custom made system to monitor and record flow and airway opening pressure signals in real-time were used in unison. Infants were manually hyperventilated to induce a PHA. After the last passive expiration, FRC_st was estimated by measuring the volume of N₂ expired after end-passive expiratory switching of the inspired gas from room air to 100% oxygen during the post-expiratory apneic pause. Repeatable intrasubject FRC_st and FRC_dyn measurements overlapped in most infants including the younger ones (P = 0.2839). Mean (95% confidence interval [CI]) FRC_st was 21.1 (20.0–22.3), and error-corrected FRC_dyn was 21.4 (20.4–22.4) ml/kg. Mean (washout time [t]) tFRC_st was longer than tFRC_dyn 60 sec (95% CI 55–65) versus 47 sec (95% CI 43–51) (P < 0.0001). The FRC and washout time were dependent on body length, weight and age. We conclude that the FRC_st is not different from the FRC_dyn in infants. The FRC_st is a reliable volume landmark because the PHA stabilizes the end-expiratory level by potentially abolishing the sedated infant's breathing strategies. The FRC_st lacks potential sources of errors and disadvantages associated with measuring the FRC_dyn. The findings cast significant doubt on the traditional physiology of air trapping in healthy infants' lungs. Pediatr Pulmonol. 2009; 44:683–692. © 2009 Wiley-Liss, Inc.