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

Abstract

SummaryThe 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 (N 2 ) washout to measure FRC, and a custom made system to monitor and record flow and airwayopening 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 N 2 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.

Comprehensive integrated spirometry using raised volume passive and forced expirations and multiple-breath nitrogen washout in infants

“…In addition to the FRC dyn , we reported measurements of the (passive) static FRC (FRC st ) by MBNW by inducing a brief post-hyperventilation apnea (PHA) in sedated, spontaneously breathing healthy infants. The FRC st was not different from the FRC dyn (Morris, 2009). Castile et al (2000) reported fractional lung volumes and TLC that were estimated from three measurements: (1) FVC by raised volume RTC from V 30 ; (2) FRC Pleth ; and (3) the expiratory reserve volume (ERV).…”

“…Attempts were made to assess total lung capacity (TLC) from measurements of crying vital capacity and residual volume (RV) but the (dynamic) (FRC dyn ) functional residual capacity (FRC) has been the lung volume most routinely and reliably measured. FRC has been measured by plethysmography (FRC Pleth ) and the multiple breath nitrogen washout (MBNW) technique and other inert marker gases such as sulfur hexafluoride and helium (Gaultier et al, 1979; Gappa et al, 1993; Quanjer and Stocks, 1995; Wauer et al, 1998; Morris, 1999b, 2009; Stocks, 1999, 2006; Morris et al, 2001; Castile et al, 2000, 2004; Hülskamp et al, 2006, 2009; Stocks et al, 2001, 2007; Robinson et al, 2009). Tidal breathing parameters neither reliably discriminated between healthy infants and the ones with lung disease nor distinguished types or levels of respiratory dysfunction.…”

“…CILF would be accomplished by combining in single maneuvers the volumes integrated from the pneumotachometer flow measurements with those measured indirectly by MBNW (Morris, 1999a, 1999b, 1999c, 2009; Morris et al, 2001; Robinson et al, 2009). Our main hypothesis was that CILF would enable the analysis and investigation of the relation between measurements, such as a dynamic and a static volume or two static volumes, not only intrasubject but also within a single maneuver.…”

“…The primary aims of this study were to present the methodological aspects of our novel physiologically-integrated approach, which included a refined raised volume RTC technique, and apply it to a group of healthy infants. The basic underlying concept of this investigation was the induction of a brief PHA (Eldridge, 1973; Mangin, 1982; Feher et al, 1996; Morris, 1999b, 2009; ATS/ERS Statement, 2005). Having the squeeze jacket unfastened, we hypothesized that during a PHA the passively exhaled gas volume from V 30 measured by a pneumotachometer (PNT) would be the inspiratory capacity (IC).…”

“…The IC is the volume at V 30 above the elastic equilibrium volume level of the infant’s respiratory system. The remaining gas volume is the FRC st measured by MBNW (Morris, 2009). Total lung capacity (TLC) would be the sum of the IC and FRC st measurements from a single maneuver.…”