Determination Of Functional Residual Capacity Research Articles

Novel methodology to perform sulfur hexafluoride (SF6)-based multiple-breath wash-in and washout in infants using current commercially available equipment.

Multiple-breath inert gas washout (MBW) is ideally suited for early detection and monitoring of serious lung disease, such as cystic fibrosis, in infants and young children. Validated commercial options for the MBW technique are limited, and suitability of nitrogen (N2)-based MBW is of concern given the detrimental effect of exposure to pure O2 on infant breathing pattern. We propose novel methodology using commercially available N2 MBW equipment to facilitate 4% sulfur hexafluoride (SF6) multiple-breath inert gas wash-in and washout suitable for the infant age range. CO2, O2, and sidestream molar mass sensor signals were used to accurately calculate SF6 concentrations. An improved dynamic method for synchronization of gas and respiratory flow was developed to take into account variations in sidestream sample flow during MBW measurement. In vitro validation of triplicate functional residual capacity (FRC) assessments was undertaken under dry ambient conditions using lung models ranging from 90 to 267 ml, with tidal volumes of 28-79 ml, and respiratory rates 20-60 per minute. The relative mean (SD, 95% confidence interval) error of triplicate FRC determinations by washout was -0.26 (1.84, -3.86 to +3.35)% and by wash-in was 0.57 (2.66, -4.66 to +5.79)%. The standard deviations [mean (SD)] of percentage error among FRC triplicates were 1.40 (1.14) and 1.38 (1.32) for washout and wash-in, respectively. The novel methodology presented achieved FRC accuracy as outlined by current MBW consensus recommendations (95% of measurements within 5% accuracy). Further clinical evaluation is required, but this new technique, using existing commercially available equipment, has exciting potential for research and clinical use.

Lung Volumes: Measurement, Clinical Use, and Coding

Measurement of lung volumes is an integral part of complete pulmonary function testing. Some lung volumes can be measured during spirometry; however, measurement of the residual volume (RV), functional residual capacity (FRC), and total lung capacity (TLC) requires special techniques. FRC is typically measured by one of three methods. Body plethysmography uses Boyle's Law to determine lung volumes, whereas inert gas dilution and nitrogen washout use dilution properties of gases. After determination of FRC, expiratory reserve volume and inspiratory vital capacity are measured, which allows the calculation of the RV and TLC. Lung volumes are commonly used for the diagnosis of restriction. In obstructive lung disease, they are used to assess for hyperinflation. Changes in lung volumes can also be seen in a number of other clinical conditions. Reimbursement for measurement of lung volumes requires knowledge of current procedural terminology (CPT) codes, relevant indications, and an appropriate level of physician supervision. Because of recent efforts to eliminate payment inefficiencies, the 10 previous CPT codes for lung volumes, airway resistance, and diffusing capacity have been bundled into four new CPT codes.

Determination of functional residual capacity by oxygen washin-washout: a validation study

To validate a new system for functional residual capacity (FRC) measurements using oxygen washin/washout in spontaneously breathing humans. The system (LUFU, Drägerwerk AG, Lübeck, Germany) consists of an unmodified EVITA 4 ventilator, a side-stream paramagnetic oxygen sensor and a dedicated software. Laboratory study and measurements in spontaneously breathing volunteers. Pulmonary function laboratory of a university hospital. 20 healthy and 15 lung diseased volunteers. FRC was measured by LUFU (LUFU-FRC) and by helium dilution (He-FRC); intra-thoracic gas volume (ITGV) was determined by body plethysmography. Each measurement cycle consisted of four independent LUFU-FRC determinations (step change of FiO(2) from 0.21 to 0.5 and back and from 0.21 to 1.0 and back), two helium-dilution runs and two body box measurements. Repeatability and agreement between methods were determined by comparing different measurements of one technique and by comparing different techniques among each other. Repeatability of LUFU-FRC was estimated by comparing washin to washout and the different FiO(2)steps. The difference of the means was 3.7% at the most. Agreement between methods resulted in the following differences (mean+/-standard deviation of differences) for healthy and lung-diseased volunteers, respectively: LUFU-FRC vs. He-FRC -0.40+/-0.50 L (0.02+/-0.95 L), LUFU-FRC vs. ITGV -0.43+/-0.54 L (-0.18+/-0.61 L) and He-FRC vs. ITGV -0.03+/-0.43 L (-0.20+/-0.98 L). LUFU is a non-invasive method for the determination of FRC that requires only minor additional equipment and no modification to the ventilator. It can be used in difficult conditions such as breathing patterns with variations from breath to breath. The results of this study show that LUFU is sufficiently reliable and repeatable to warrant its clinical application.

Functional residual capacity and respiratory mechanics as indicators of aeration and collapse in experimental lung injury.

Increased functional residual capacity (FRC) and compliance are two desirable, but seldom measured, effects of positive end-expiratory pressure (PEEP) in mechanically ventilated patients. To assess how these variables reflect the morphological lung perturbations during the evolution of acute lung injury and the morphological changes from altered PEEP, we correlated measurements of FRC and respiratory system mechanics to the degree of lung aeration and consolidation on computed tomography (CT). We used a porcine oleic acid model with FRC determinations by sulfur hexafluoride washin-washout and respiratory system mechanics measured during an inspiratory hold maneuver. Within the first hour, during constant volume-controlled ventilation with PEEP 5 cm H(2)O, FRC decreased by 45% +/- 15% (P = 0.005) and compliance decreased by 35% +/- 12% (P = 0.005). Resistance increased by 60% +/- 62% (P = 0.005). Only the FRC changes correlated significantly to the decreased aeration (R(2) = 0.56; P = 0.01) and the increased consolidation (R(2) = 0.43; P = 0.04) on CT. When the PEEP was changed to either 10 or 0 cm H(2)O, there were larger changes in FRC than in compliance. We conclude that, in our model, FRC was a more sensitive indicator of PEEP-induced aeration and recruitment of lung tissue and that FRC may be a useful adjunct to PaO(2) monitoring. Lung injury was quantified on computed tomography and related to monitored values of functional residual capacity and mechanical properties of the respiratory system. We found the functional residual capacity to be a more sensitive marker of the lung perturbations than the compliance. It might be of value to include functional residual capacity in the monitoring of acute lung injury.

In vitro validation of an ultrasonic flowmeter in order to measure the functional residual capacity in newborns

Ultrasonic transit-time airflow meters (UFM) allow simultaneous measurements of volume flow V′(t) and molar mass MM(t) of the breathing gas in the mainstream. Consequently, by using a suitable tracer gas the functional residual capacity (FRC) of the lungs can be measured by a gas wash-in/wash-out technique. The aim of this study was to investigate the in vitro accuracy of a multiple-breath wash-in/wash-out technique for FRC measurements using 4% sulphur hexafluoride (SF6) in air. V′(t) and MM(t) were measured with a Spiroson SCIENTIFIC flowmeter (ECO Medics, CH) with 1.3 ml dead space. Linearity of airflow and MM were tested using different tidal volumes (VT and breathing gases with different O2 and SF6 concentrations. To determine the accuracy of FRC measurements SF6 wash-in and wash-out curves from four mechanical lung models (FRC of 22, 53, 102 and 153 ml) were evaluated by the Spiroson. For each model five measurements were performed with a physiological VT/FRC ratio of 0.3 and constant respiratory rate of 30 min−1. The error of measured VT (range 4–60 ml) was <2.5%. There was a strong correlation between the measured and calculated MM of different breathing gases (r = 0.989), and the measuring accuracy was better than 1%. The measured FRC of the four models were 20.3, 49.7, 104.3 and 153.4 ml with a coefficient of variation of 16.5%, 4.5%, 4.9% and 3%. Accordingly, for FRC < 100 ml the in vitro accuracy was better than 8% and for FRC > 100 ml better than 2.5%. The determination of FRC by MM measurements using the UFM is a simple and cost-effective alternative to conventionally used gas analysers with an acceptable accuracy for many clinical purposes.

Determination of functional residual capacity (FRC) by multibreath nitrogen washout in a lung model and in mechanically ventilated patients. Accuracy depends on continuous dynamic compensation for changes of gas sampling delay time.

Validation of an open-circuit multibreath nitrogen washout technique (MBNW) for measurement of functional residual capacity (FRC). The accuracy of FRC measurement with and without continuous viscosity correction of mass spectrometer delay time (TD) relative to gas flow signal and the influence of baseline FIO2 was investigated. Laboratory study and measurements in mechanically ventilated patients. Experimental laboratory and anesthesiological intensive care unit of a university hospital. 16 postoperative patients with normal pulmonary function (NORM), 8 patients with acute lung injury (ALI) and 6 patients with chronic obstructive pulmonary disease (COPD) were included. Change of FIO2 from baseline to 1.0. FRC was determined by MBNW using continuous viscosity correction of TD(TDdyn), a constant TD based on the viscosity of a calibration gas mixture (TD0) and a constant TD referring to the mean viscosity between onset and end of MBNW (TDmean). Using TDdyn, the mean deviation between 15 measurements of three different lung model FRCs (FRCmeasured) and absolute volumes (FRCmodel) was 0.2%. For baseline FIO2 ranging from 0.21 to 0.8, the mean deviation between FRCmeasured and FRCmodel was -0.8%. However, depending on baseline FIO2, the calculation of FRC using TDmean and TD0 increased the mean deviation between FRCmeasured and FRCmodel to 2-4% and 8-12%, respectively. In patients (n = 30) the average repeatability coefficient was 6.0%. FRC determinations with TDmean and TD0 were 0.8-13.3% and 4.2-23.9% (median 2.7% and 8.7%) smaller than those calculated with TDdyn. A dynamic viscosity correction of TD improves the accuracy of FRC determinations by MBNW considerably, when gas concentrations are measured in a sidestream. If dynamic TD correction cannot be performed, the use of constant TDmean might be suitable. However, in patient measurements this can cause an FRC underestimation of up to 13%.

Functional residual capacity and total respiratory system impedance in wheezing infants.

Airways obstruction has been demonstrated in acutely wheezing infants. The aim of the present study was to assess functional abnormalities as detected by measurement of total respiratory system resistance (Rrs) and functional residual capacity (FRC) in infants with a history of recurrent episodes of wheezing, while not acutely ill. In 30 such infants (mean age, 10 months; range, 4-17) and in 10 healthy infants (mean age, 6 months; range, 0-14) four Rrs measurements, performed with the forced pseudo-random noise (PRN) oscillation technique, and three FRC determinations, using the closed-circuit helium dilution technique, were averaged. A lower than predicted FRC was demonstrated in 20/30 (66%) patients. At 16 Hz, Rrs was significantly above predicted in 3/30 (10%) patients. Specific Rrs (Rrs x FRC) at 16 Hz was increased in 5/30 (17%) patients. In conclusion, the PRN oscillation technique combined with FRC measurement by helium dilution detects lung function abnormalities in a minority of wheezing infants during symptom-free intervals.

Renal extraction of endogenous gastrin in patients with normal renal function.

Renal extraction of gastrin in patients with normal clearance of inulin and p-aminohippuric acid was assessed by gastrin radioimmunoassay. In 23 fasting patients there was no significant difference in arterial gastrin (37.7 pg/ml serum +/- 25.1 S.D.) and the renal vein gastrin (35.9 +/- 24.6), resulting in a renal extraction ratio for gastrin not significantly different from zero (p greater than 0.5). The results in three patients indicated no extraction of gastrin for two consecutive periods after a protein-rich meal.

Effect of endotracheal tube leakage on functional residual capacity determination by nitrogen washout method in a small-sized lung model.

The determination of functional residual capacity (FRC) would be extremely helpful for the controlled adjustment of mechanical ventilation in sick neonates and infants. However, these patients have small lung volumes and usually have been intubated by uncuffed endotracheal tubes (ETT). Therefore, the open-circuit nitrogen washout technique (N2wo) may give false FRC values if the inspired oxygen concentration (FIO2) is high and leakage around the ETT is present. We evaluated the N2wo as supplied by the Pediatric Pulmonary System 2600 (Sensor-Medics) in a small-sized lung model by 570 measurements using five different ventilator settings, an FIO2 increasing up to 0.9, different bypass flows between 0 and 12 L/min, and various patterns of leakage, either during inspiration or exhalation, or both. We found the most reliable results (error, 0.6%; CV, 0.7%) with a bypass flow of 6 L/min. Absolute N2 volumes as small as 14 mL could be measured using an FIO2 as high as 0.9 with only slight loss of accuracy (error, 4%; CV, 2.8%). During leakage, FRC had been underestimated with a very strong correlation to the total amount of leakage over the measurement period, which was irrespective of the ventilatory parameters (r = 0.9, P < 0.001). The regression equation could, therefore, be used for FRC correction in the lung model. However, most of the miscalculation was due to N2 loss during expiratory leakage, which quite simply and reliably can be excluded by an end-inspiratory occlusion test.(ABSTRACT TRUNCATED AT 250 WORDS)

John Caffey Award. Determination of functional residual capacity from digital radiographs of the normal neonatal chest: studies in a rabbit model.

Spirometric measurement of lung volume in an infant or uncooperative child is often a difficult and time-consuming procedure. Previous investigators have successfully estimated total lung capacity in adults and older children by using chest radiography. The purpose of this study was to assess the accuracy of functional residual capacity (FRC) determination from chest radiographs in an animal model of the normal neonatal chest. FRC was determined with the helium dilution (HeD) method in 13 anesthetized, intubated, and paralyzed rabbits (weight range, 1.5-3.9 kg). FRC was then estimated from digital frontal and lateral chest radiographs. The radiographic estimate was made by applying measurements taken from the radiographic film pair to a geometric model of the chest. The best-fit model assumed an elliptical cross section for the intrathoracic cavity, heart, paraspinal structures, and diaphragm in the axial plane. Linear regression on independently determined HeD and radiographically determined FRC yielded a slope of 1.02 (p less than .001), an intercept of -3.02 cm3 (p = .565), and a correlation coefficient of 0.96 (p less than .001). We conclude that radiographic FRC estimates closely approximate HeD estimates in a rabbit model of the normal neonatal chest.

A multiuser system for whole body plethysmographic measurements and interpretation

A multiuser system for whole body plethysmographic measurements and interpretation which has been developed under clinical conditions is described. The following measurements can be carried out in a rapid way and in one session with the patient: specific airway resistance during spontaneous breathing, determination of functional residual capacity, static lung volumes, and maximal forced expiratory data. Each section is normally measured twice and can be repeated up to ten times. The final results are displayed and printed together with a consistent system of normal reference values. All values and selected original curves are stored automatically in an integrated data base system. Obstructive patients are measured again after the inhalation of a bronchodilator. All results are evaluated by an automatic interpretation program. This analyzes and graduates airway obstruction, lung volumes, and pharmacological airway reversibility using standardized texts which are written below all numerical printouts and graphical plots. The interpretation algorithm is tree structured and uses the normal reference values as a knowledge base. The system supports up to four online laboratories with their own A/D converter and up to 20 video terminals, printers, plotters, and modems. Our laboratory performs 8,297 such complete measurements on 4,671 different patients per year with one body box.

An Automated Bedside Method for Measuring Functional Residual Capacity by N2 Washout in Mechanically Ventilated Children

Beside measurement of functional residual capacity (FRC) in ventilated children is impractical. Using a simple technique based on open circuit N2 washout, we measured FRC in ventilated children. The system was evaluated in the laboratory and in patients. Using a mechanical lung, the reproducibility of 200 studies over a range of 100-500 mL at each of four different flow rates (10 determinations at each level) was very high with a mean coefficient of variation of 2.3% (range 0.5-5.1%). Linearity of the integrated N2 signal for volumes of 100-500 mL washed out at different flow rates was excellent (range 7.4-17.9 L/min), r = 0.99. The mean difference between measured and preset mechanical lung volumes was 2.4% (range 0-4.6%). In vivo, reproducibility of six to 10 FRC determinations in each of 30 children gave a mean coefficient of variation of 2.7%. Comparison to the conventional Douglas bag collection method showed a high correlation (r = 0.97). We conclude that this is an easy, highly reproducible, and accurate method for FRC determination suitable to ventilated infants and children.

A correction formula for computing specific airway resistance from a single-step measurement.

Specific airway resistance (SRaw) is conventionally determined by multiplying the plethysmographically measured values of airway resistance and functional residual capacity (FRC). An alternative single-step method, which avoids the need for airway occlusion during determination of FRC, has been described by Dab and Alexander. The single-step method provides no correction for resistance or dead space of the apparatus and, as a result, systematically overestimates SRaw. Using 1,000 paired measurements, it was possible to compute a formula for correcting the single-step measurement. This correction formula can be adjusted and applied to measurements made in any laboratory. The unlimited applicability of the proposed correction has been demonstrated by 234 plethysmographic measurements made in the Pediatric Clinic in Rome.

Helium wash-in time as a critical factor in the determination of functional residual capacity in children

The wash-in time (twi), defined as time until gaseous equilibrium between lungs and spirometer is reached during helium dilution rebreathing, and cumulative inspired volume (CIV) ventilated during this time were measured in 64 healthy children (30 boys and 34 girls, aged 6.5-14.7 years). The interrelationship between twi, CIV, respiratory frequency, tidal volume and lung growth was studied. Functional residual capacity (FRC) was compared with the plethysmographically determined thoracic gas volume. CIV is linearly related to standing height, and the ratio CIV/FRC is an age-independent constant (5.05 +/- 0.75). twi was found to be multi-linearly dependent of standing height and minute ventilation. A close relationship was found between FRC and thoracic gas volume (r = 0.94). It is concluded that FRC determination in children with lung disease should be standardized with respect to the predicted age-dependent twi for helium.

A simple method for measuring functional residual capacity by N2 washout in small animals and newborn infants.

An open circuit N2 washout technique is described for the determination of functional residual capacity in infants. Either 100% O2 or any oxygen/helium mixture can be used as the washing gas. The subject breathes the washing gas through a T-tube and the washed out nitrogen is mixed with this gas in a mixing chamber, placed into the exhalation part of the circuit. The N2 concentration of the mixed gas is analyzed continuously, and the concentration signal is electronically integrated over time. Calibration of the system is accomplished by injecting known amounts of nitrogen or room air into the circuit. The gas flow through the system must remain constant and is adjusted to approximate peak inspiratory flow of the infant. In vitro testing of the system showed that the technique gives reproducible values (coefficient of variance less than 1.0%) and that the integrated signal output has a close linear correlation with the amount of N2 washed out (r = 0.99). In vivo measurements in 10 cats confirmed the accuracy and reproducibility of the method when compared with N2 collection. The technical advantages of the system are simplicity of components, absence of valves, easy calibration, low dead space, and no need to collect or measure expired gases. For the infant this means no added resistance during washout and no risk of hypoxia, hyperoxia, or hypercapnea. In the presence of pulmonary disease and poor gas mixing the washout period can be prolonged as needed. There is no lower limit of weight or size for functional residual capacity measurements in small infants or animals.

A practical procedure for measuring functional residual capacity during mechanical ventilation with or without PEEP.

The measurement of functional residual capacity (FRC) in patients receiving mechanical ventilation may provide valuable data in the assessment and management of acute respiratory failure. Previous descriptions of apparatus and techniques for FRC measurement have either been inapplicable to patients receiving positive end-expiratory pressure (PEEP), or insufficiently detailed to allow convenient duplication in the clinical setting. The authors describe a helium rebreathing method for bedside determination of FRC which can be performed during ventilation with PEEP and which is applicable in patients with prolonged equilibration times. The method is both reproducible in patients (variation from mean FRC: +/- 2.2%) and accurate (coefficient of variation from in vitro FRC of 3000 ml: +/- 1.7%). The apparatus and assembly are described in detail and require only components which are readily available commercially, so that they may be applicable to clinical use in a general hospital.

A system to measure functional residual capacity in critically ill patients.

The use of continuous positive airway pressure (CPAP) and intermittent mandatory ventilation (IMV) in spontaneously breathing, intubated patients has prompted the development of new procedures for measuring functional residual capacity (FRC). The authors have developed a system for measuring FRC by the multiple breath nitrogen washout technique, which is suitable for use on intubated patients breathing with CPAP, IMV, or intermittent positive pressure ventilation (CONTROL) and on nonintubated patients. This system uses a pair of synchronized volume ventilators to permit a step change in inspired N2 fraction while providing therapeutic ventilatory support. A rapid-response nitrogen analyzer and a modified bellows spirometer are used for continuous measurement of airway nitrogen concentration and expired gas flow rate. FRC is calculated on-line by a digital computer. The system accuracy was tested on a mechanical lung simulator in the CPAP and CONTROL modes. The measured volume was found to agree within 58 +/- 52 ml of the actual volume in the CONTROL mode and within 104 +/- 22 ml in the CPAP mode. The system was also tested for repeatability by making duplicate FRC determinations in patients with respiratory insufficiency. In the 18 patients studied, the correlation coefficient of these duplicate measurements was r = 0.987 and the mean difference between measurements was 49 +/- 24 ml. This noninvasive system also provides data used to calculate anatomical deadspace by Fowler's method (VSDS) and uniformity of ventilation (V/V) for multicompartment lung models.

Determination of functional residual capacity with 133-xenon radiospirometry. Comparison with body plethysmography and helium spirometry. Effect of body position

This study was undertaken to estimate the accuracy of 133-xenon radiospirometry for determination of FRC in healthy subjects. Forty healthy volunteers, both smokers and non-smokers, were examined. The FRC of each subject was concurrently determined with radiospirometric, He-dilution in closed circuit, and body plethysmographic methods. The radiospirometric and He-dilution measurements were done in supine and in sitting positions, the body plethysmography on sitting subjects, only. The mean FRC measured by radiospirometry (FRCRS) was 0.72 1 larger than that measured by helium spirometry (FRCHe) in sitting position (P < 0.01). In supine position the FRCRS was 0.65 1 larger than the FRCHe (P < 0.01). The body plethysmography gave FRC (TGV) 0.35 1 larger than the FRCHe sitting (P < 0.01). The FRCHe and the FRCRS in the sitting position were 0.48 and 0.55 1 larger than in the supine position (P < 0.01), respectively. Trapped air correlated significantly (P < 0.01) with the difference FRCRS-FRCHe, when sitting. The results indicate that the FRC determined radiospirometrically is significantly larger than the FRC determined with He-spirometry. The difference is systematic, suggesting that it is caused by 133-xenon dissolved in blood and accumulated in tissues of the thoracic cage and by dissimilar representation of trapped air in FRCRS and FRCHe. With eventual correction of the systematic error, the FRC obtained as a by-product of radiospirometry may be used, e.g. for clinical purposes.

Effects of Endotracheal Tube Leaks on Functional Residual Capacity Determination in Intubated Neonates

The present study evaluates a new closed circuit helium (He) dilution technique for determination of endotracheal (ET) tube leakage and functional residual capacity (FRC) in neonates with ET tubes. By analytically relating the fall in He concentration due to mixing with that due to leakage, it is possible to predict the final equilibration concentration of He and, therefore, correct for ET tube leaks. The system (120 ml) contains an air pump, He meter, breathing bag in cyclinder, a strip chart readout, and solenoid valve. Continuous positive airway pressure (CPAP) or ventilator pressure can be applied during testing. FRC measurements were performed on 13 neonates (799--4500 g) on CPAP with ET tubes. Leak rates were significantly higher (P less than 0.001) on 3 cm H2O CPAP compared to O cm H2O CPAP. The mean measured FRC was 53.5 ml at 3 cm H2O and 46.3 ml at 0 cm H2O CPAP. If gas leakage had not been considered in FRC calculations, the error in FRC could have been as high as 39% at 3 cm H2O CPAP and 18% at 0 cm H2O CPAP.

A method for measuring functional residual capacity in neonates with endotracheal tubes.

This study evaluates a new 60-s closed circuit helium (He) dilution technique for determination of functional residual capacity (FRC) in intubated neonates independent of snall gas leaks present around uncuffed endotracheal (ET) tubes. By analyticaly relating the fall in He concentration due to mixing with that due to leakage it is possible to predict the final equflibration concentration of He and, therefore, correct for ET tube leaks. The system (120 ml) contains an air pump, He meter, breathing bag in cylinder, a strip chart readout, and solenoid value. Continuous positive airway pressure (CPAP) or ventilator pressure can be applied during testing. One hundred in vitro measurements of FRC ranging from 5-100 cc in both leak and nonleak models were performed and were accurate to within ±7.8% standard deviation. Functional residual capacity measurements were also performed in 30 infants (weight 600-4400 gm).