Measurement of Functional Residual Capacity in Rabbits and Children Using an Ultrasonic Flow Meter

Abstract

A sulfur hexafluoride (SF 6 ) washin/washout technique was developed using an ultrasonic flowmeter to measure functional residual capacity (FRC) during mechanical ventilation. The ultrasonic flowmeter measures simultaneously flow and molar mass of the mainstream gas. Ventilation distribution was studied using moment ratios analysis (alveolar-based mean dilution number). Accuracy and precision of the measurement technique were tested in a mechanical lung model, and the method's sensitivity to changes of FRC was assessed in seven ventilated rabbits and six children. In the mechanical lung model with a volume range from 10 to 60 mL, the mean error of FRC measurement was 0.096 Ϯ 0.9 mL (range, 0 -2 mL). In seven rabbits (mean body weight, 3.6 kg), measurements of FRC and alveolar-based mean dilution number were made at positive end-expiratory pressures (PEEP) of 0, 3, and 6 cm H 2 O. The mean coefficient of variation of 66 FRC-measurements was 5.5% (range, 0 -15.3%). As the applied PEEP increased, mean FRC per kilogram body weight increased from 13.3 Ϯ 3.4 mL/kg (PEEP of 0 cm H 2 O) to 16.7 Ϯ 3.6 mL/kg (PEEP of 3 cm H 2 O) and to 20.8 Ϯ 4.3 mL/kg (PEEP of 6 cm H 2 O). Alveolar-based mean dilution number decreased accordingly from 1.94 Ϯ 0.42 (PEEP ϭ 0; mean Ϯ SD), to 1.91 Ϯ 0.45 (PEEP ϭ 3) and to 1.59 Ϯ 0.35 (PEEP ϭ 6). In the six children, as applied PEEP increased, mean FRC per kilogram increased from 21.1 Ϯ 4.51 mL/kg (PEEP ϭ 0), to 22.4 Ϯ 1.8 mL/kg (PEEP ϭ 5) and 27.2 Ϯ 3.4 mL/kg (PEEP ϭ 10). FRC measurement using the ultrasonic flowmeter is accurate and simple to use in ventilated and spontaneously breathing children. The monitoring of FRC is an important tool for interpreting volume-dependent pulmonary mechanics (e.g. lung elastic recoil pressure or airway resistance). Since the open-circuit MBNW was first described by Darling et al. in 1940 (1), several investigators have used gas washout techniques to measure FRC in spontaneously breathing (2, 3) and mechanically ventilated patients (4 -6). Other gas washout techniques involve helium dilution (7), argon washout (8), or washout of SF 6 (9). Each of these techniques has its own limitations. The MBNW can only be accurately performed if the patient's oxygen requirement does not exceed a certain amount (4 -6).Although MBNW can be performed easily, the considerable changes in gas viscosity during the washout maneuver significantly affect the accuracy of the gas flow measurement by pneumotachography (10, 11). The helium dilution technique is based on a closed circuit, and a leak in the measuring system reduces its accuracy (7). SF 6 washout is a potentially suitable technique for children, including those who require near 100% oxygen to maintain normal Hb saturation. There are a few studies using SF 6 as a washout gas (12-14) and a mass spectrometer or an infrared analyzer to measure SF 6 concentration. Neither technique is commercially available for routine measurement. The infrared analyzer has a low signal-to-noise ratio and a response time of 20 ms or more. Ai...

Diaphragm Electrical Activity During Expiration in Mechanically Ventilated Infants

“…Although we did not measure the EELV in our patients, several studies report a positive correlation between increasing PEEP and EELV in mechanically ventilated infants (24,37), although the amount of change in EELV per cm H 2 O of PEEP varies among the studies. Regardless of the magnitude, we believe that it is reasonable to assume that the EELV was reduced by the PEEP removal in the present study.…”

Functional residual capacity measurements in healthy infants: ultrasonic flow meterversusa mass spectrometer

“…Flow was computed from the difference in the time of flight for ultrasonic pulses emitted from each transducer, whilst molar mass is a function of the absolute time of flight [3], thereby permitting estimation of the concentration of a tracer gas. Only washout data was used for comparison with the mass spectrometer FRC measurements.…”

“…Recorded flows and volumes were converted to body temperature and pressure, saturated (BTPS) conditions. The analysis of the ultrasonic washouts included correction for a side chamber reservoir effect and the effects of within-cycle changes in temperature on molecular mass were accounted for using a temperature model as previously described [3].…”

“…Initially reported in 1986 by BUESS et al [2], studies using a prototype system for measurement of lung volume and indices of ventilation inhomogeneity have recently been published in experimental animal models [3] and in both nonventilated [4] and ventilated [3] infants and children. Whilst accuracy of the measured volumes has been confirmed in vitro [4,5], validation of this technique has not been reported in a healthy infant population.…”

Lung Function Tests in Neonates and Infants with Chronic Lung Disease of Infancy: Functional Residual Capacity

“…The ultrasonic flow-meter (USFM), which measures flow by assessing differences in transit time between two ultrasonic signals through the respiratory gas flow, was modified recently for use in infants. [26][27][28] As molar mass influences the transit time of the ultrasonic signal, inert gases with a molar mass sufficiently different from normal inspired gas can be detected and used as a tracer gas for multiple-breath washout (MBW). This equipment is now available for research use in infants breathing without ventilatory support.…”