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

Nasal versus oronasal raised volume forced expirations in infants--a real physiologic challenge

Raised volume rapid thoracoabdominal compression (RTC) generates forced expiration (FE) in infants typically from an airway opening pressure of 30 cm H(2)O (V(30)). We hypothesized that the higher nasal than pulmonary airway resistance limits forced expiratory flows (FEF(%)) during (nasal) FE(n), which an opened mouth, (oronasal) FE(o), would resolve. Measurements were performed during a brief post-hyperventilation apnea on 12 healthy infants aged 6.9-104 weeks. In two infants, forced expiratory (FEFV) flow volume (FV) curves were generated using a facemask that covered the nose and a closed mouth, then again with a larger mask with the mouth opened. In other infants (n = 10), the mouth closed spontaneously during FE. Oronasal passive expiration from V(30) generated either the inspiratory capacity (IC) or by activating RTC before end-expiration, the slow vital capacity ((j) SVC). Peak flow (PF), FEF(25), FEF(50), FEF(25-75), FEV(0.4), and FEV(0.5) were lower via FE(n) than FE(o) (P < 0.05), but the ratio of expired volume at PF and forced vital capacity (FVC) as percent was higher (P < 0.05). FEF(75), FEF(85), FEF(90), FVC as well as the applied jacket pressures were not different (P > 0.05). FEFV curves generated via FE(o) exhibited higher PF than FV curves of IC (P < 0.05); PF of those produced via FE(n) were not different from FV curves of IC (P > 0.05) but lower than those of (j) SVC (P < 0.05). In conclusion, the higher nasal than pulmonary airways resistance unequivocally affects the FEFV curves by consistently reducing PF and decreases mid-expiratory flows. A monitored slightly opened mouth and a gentle anterior jaw thrust are physiologically integral for raised volume RTC in order to maximize the oral and minimize nasal airways contribution to FE so that flow limitation would be in the pulmonary not nasal airways.

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

With the rapid somatic growth and development in infants, simultaneous accurate measurements of lung volume and airway function are essential. Raised volume rapid thoracoabdominal compression (RTC) is widely used to generate forced expiration from an airway opening pressure of 30 cmH(2)O (V(30)). The (dynamic) functional residual capacity (FRC(dyn)) remains the lung volume most routinely measured. The aim of this study was to develop comprehensive integrated spirometry that included all subdivisions of lung volume at V(30) or total lung capacity (TLC(30)). Measurements were performed on 17 healthy infants aged 8.6-119.7 weeks. A commercial system for multiple-breath nitrogen washout (MBNW) to measure lung volumes and a custom made system to perform RTC were used in unison. A refined automated raised volume RTC and the following two novel single maneuvers with dual volume measurements were performed from V(30) during a brief post-hyperventilation apneic pause: (1) the passive expiratory flow was integrated to produce the inspiratory capacity (IC) and the static (passive) FRC (FRC(st)) was estimated by initiating MBNW after end-passive expiration; (2) RTC was initiated late during passive expiration, flow was integrated to produce the slow vital capacity ((j)SVC) and the residual volume (RV) was measured by initiating MBNW after end-expiration while the jacket (j) was inflated. Intrasubject FRC(dyn) and FRC(st) measurements overlapped (p=0.6420) but neither did with the RV (p<0.0001). Means (95% confidence interval) of FRC(dyn), IC, FRC(st), (j)SVC, RV, forced vital capacity and tidal volume were 21.2 (19.7-22.7), 36.7 (33.0-40.4), 21.2 (19.6-22.8), 40.7 (37.2-44.2), 18.1 (16.6-19.7), 40.7 (37.1-44.2) and 10.2 (9.6-10.7)ml/kg, respectively. Static lung volumes and capacities at V(30) and variables from the best forced expiratory flow-volume curve were dependent on age, body length and weight. In conclusion, we developed a comprehensive physiologically integrated approach for in-depth investigation of lung function at V(30) in infants.