In vitro assessment of equipment and software to assess tidal breathing parameters in infants.

The aim of this in vitro study was to compare the measurement accuracy of two currently available devices for measuring tidal breathing in infants. A mechanical model pump was used to generate flow profiles which simulated those observed in infants. A range of flows was applied simultaneously to two different devices, namely the commercially available SensorMedics 2600 (SM 2600) and more recently developed, custom-made equipment based on the flow-through technique (FTT). Automatically derived values from both devices were compared with one another and with manual calculations of printouts of the same breaths. There were no differences in the raw flow signal obtained from the two devices, nor between values calculated automatically or manually from the FTT. Similarly, the deviations between the FTT and SM 2600 were <3% for tidal volume, respiratory frequency and minute ventilation. However, when comparing either with manually calculated values or those derived automatically from the FTT, there was a systematic and highly significant underestimation of shape-dependent parameters, such as the time to peak tidal expiratory flow as a proportion of tidal expiratory time (tPTEF/tE), derived by the SM 2600. The lower the applied flow, the higher the observed deviations, the underestimation being up to 60% when flows simulating those observed in preterm neonates were applied. These errors appear to result from differences in signal processing such as the algorithms used for breath detection and can only be detected if appropriate nonsinusoidal flow profiles representing those seen in infants are used to evaluate equipment.

Effect of apparatus dead space on breathing parameters in newborns: "flow-through" versus conventional techniques.

Commercial devices for tidal breathing measurements in newborns allow only short-term measurements, due to the high apparatus dead space of the face mask and pneumotachometer. The flow-through technique (FTT) minimizes the dead space by a background flow, thereby allowing long-term measurements. The aim of this study was to investigate the comparability of tidal breathing parameters using both techniques. Paired measurements of tidal breathing were performed in 86 sleeping infants (median (range) body weight 2.8 kg (1.9-5.3 kg), age 65 days (3-150 days)), using the FTT and SensorMedics 2600 (SM 2600). There was a significant bias (p <0.001) in all tidal breathing parameters. Compared with the FTT, increases (95% confidence interval (CI)) in tidal volume (VT), respiratory frequency (fR), and minute ventilation (V'E) were 0.74 (0.5-1.0) mL.kg(-1), 9.0 (6.9-11.2).min(-1) and 92 (74-109) mL.min(-1).kg(-1) when measured with the SM 2600, representing average increases of 13, 17 and 30%, respectively, in response to the added dead space. By contrast, time to peak tidal expiratory flow as a proportion of expiratory time (tPTEF/tE) was changed by -0.09 (-0.11-0.08). The mean (95% CI) change in tPTEF/tE of -54 (-62-45)%, when measured in infants by the SM 2600, was remarkably similar to that observed during in vitro validation studies (-59 (-73-44)%), suggesting that the discrepancies in timing parameters may be largely attributable to differences in signal processing. In conclusion, differences in measurement technique and precision of the devices used can result in significant differences in tidal breathing parameters. This may impede the comparison of results within and between infants and the clinical interpretation of tidal breathing measurements in newborns.

Deadspace free ventilatory measurements in newborns during mechanical ventilation.

OBJECTIVE: To improve the accuracy of ventilatory measurements in ventilated newborns by means of a numerical correction when a deadspace free differential measuring method using two pneumotachographs (PNTs) is applied and to investigate the clinical usefulness of this correction procedure. DESIGN: In vitro study and prospective animal study. SETTING: Research laboratory of the Clinic of Neonatology and the Animal Research Laboratory, Charité Hospital Berlin. SUBJECTS: Ten newborn piglets, weighing 610-1340 g (median, 930 g), age <12 hrs. INTERVENTIONS: The accuracy of both the deadspace free method and the endotracheal flow measurements (conventional method) was investigated using mechanical lung models. A correction procedure for the deadspace free method was developed considering signal delay time and tube compliance between both PNTs. This method was applied to the piglets measured during partial liquid ventilation (PLV). Measurements were done before and after lung lavage and during 30 and 120 mins of PLV (30 mL/kg body weight perfluorocarbon). MEASUREMENTS AND MAIN RESULTS: In vitro measurements showed volume differences between both methods of 8%, 12%, 16%, and 17%, respectively, depending on the distance between the PNTs of 10, 60, 120, and 180 cm. After applying the correction algorithm, the differences decreased to 3%, 0%, -2%, and -8%, respectively. The piglets were measured with 120-cm tube length between the PNTs. The correction algorithm reduced the measured tidal volume before lavage by 7%, after lavage by 14%, 30-min PLV by 12%, and 120-min PLV by 10%, corresponding to the changes in respiratory compliance of 1.2, 0.6, 1.0, and 1.1 mL/cm H2O. CONCLUSIONS: The deadspace free method can be advantageously used for continuous measurements in newborns despite much higher technical expense. The correcting procedure improved the accuracy of the volume measurement remarkably, especially for lower respiratory compliance.

Novel technique to average breathing loops for infant respiratory function testing.

Breathing loops can be obtained by plotting two respiratory signals on an x-y diagram: the resulting loops represent a non-parametric description of the respiratory system. In infancy, loops are commonly measured during tidal breathing and their interpretation is hampered by high within-subject variability. Therefore a two-dimensional averaging technique for loops has been developed. The algorithm is based on segmentation of the loops and required two steps. First, the total length of the loop of every breathing cycle was divided into a specified number of equidistant intervals and the co-ordinates calculated by stepwise linear approximation of the curve. Second, averaged loops were calculated using the arithmetical mean (or the median if there were artifacts) of the x and y co-ordinates of the loops for all calculated points. To compare the new technique with averaging in the time domain a simulation study was performed using respiratory signals with a coefficient of variation (CV) of 5%, 10%, 15% and 20%. In contrast to the new technique, with increasing CV, averaging in the time domain led to increasing contortions in the averaged flow-volume loops. Mean errors of peak tidal expiratory flow were -3.3%, -13.9%, -21.3% and -34.2%, whereas errors with the new technique were considerably lower (0.5%, 0.4%, 0.5% and -0.1%) and independent of the level of CV.

Time of measurement influences the variability of tidal breathing parameters in healthy and sick infants.

The aim of this study was to investigate the influence of the time, when measuring tidal breathing parameters 1 min (epoch 1) and 5 min (epoch 2) after application of the facemask in healthy infants and infants with bronchopulmonary dysplasia (BPD), using the dead space free flow-through technique. In both patient groups, there were no statistically significant differences between epoch 1 and 2, in most of the tidal breathing parameters, except an increased VE and increased correlation dimension of the respiratory signal in the BPD infants in epoch 1. However, in nearly all parameters the coefficient of variation (CV) was significantly higher in epoch 1 compared with epoch 2, and in some infants, we found very high CVs (>50%) in epoch 1, which disappeared in epoch 2. The study shows that after having applied the facemask, a sufficient amount of adaptation time is necessary in order to reduce the within-subject variability and improve the reproducibility and interpretation of tidal breathing measurements in infants.

Accuracy of deadspace free ventilatory measurements for lung function testing in ventilated newborns: a simulation study.

OBJECTIVE: A deadspace free method based on simultaneous ventilatory measurements in the inspiratory and expiratory limb of the ventilator circuit was compared to the conventional endotracheal method where the flow is measured between ETT and Y-Piece. The aim of our study was to find out how the arrangement of this setup affects the measuring accuracy of 1) the ventilatory and 2) the lung mechanical parameters by means of a computer simulation. METHOD: The system consisting of ventilator tubes and lung was described in state space and the flow signals of endotracheal method, of deadspace free method and the pressure at the Y-piece were simulated in the time domain. To investigate the influence of the position of the pneumotachographs (PNTs) in the ventilator circuit on measuring accuracy, the distance d0 of the PNT from the Y-piece was varied between 0 and 900 mm. The respiratory compliance C, resistance R and inertance I were calculated by least square method using the simulated flow and pressure signals of both methods. RESULTS: Compared to the endotracheal method, with increasing d0, the tidal volume measured with the deadspace free method rose linearly, depending on the ratio between the compliance of the ventilator tubes to the respiratory compliance. The differences of C and R for both methods were acceptable (< 10%) if the distance between each PNT and the y-piece didn't exceed 200 mm and the shorter do the higher the measuring accuracy. The inertance could not be measured by this method with satisfactory accuracy if d0 was higher than 100 mm. IN CONCLUSION: The dead space free method can be used for accurate ventilatory measurements during mechanical ventilation. However, for lung mechanic measurements in very low birth weight infants the position of the PNTs must be as short as possible.

In vitro investigations of jet-pulses for the measurement of respiratory impedance in newborns.

The aim of this in vitro study was to investigate the measuring range and accuracy of a miniaturized equipment for respiratory impedance (Zrs) measurements in newborns using jet-pulses. Brief flow pulses (peak flow=16 L x min(-1), width=10 ms) were generated by a jet-generator consisting of a solenoid valve and an injector, situated between pneumotachograph and outflow resistance. Serially arranged resistance-inertance-compliance (R-I-C) lung models (RM=1.3-6.4 kPa x L(-1) x s, CM=7.4-36.9 mL x kPa(-1), IM=1.5 Pa x L(-1) x s2) were used to measure the real and imaginary part of Zrs between 4 and 50 Hz and to determine R, C and I by means of the method of least squares. The median errors for R, C and I were -0.1 kPa x L(-1) x s (-2%), 2.4 mL x kPa(-1)(13%) and -0.2 Pa x L(-1) x s2 (-13%) for measurements without breathing signals and 0.11 kPa x L(-1) -s (3%), 3 mL x kPa(-1) (16%) and 0.28 Pa x L (-1) x s2 (19%) in mechanically ventilated models. During spontaneous breathing the influence of the breathing flow on Zrs was negligible. The equipment did not show any nonlinearity when different pulse amplitudes were used (Vmax=13-22 L x min(-1)). The investigations have shown that jet-pulses allow reliable measurements of respiratory impedance and have the potential to provide valuable information about lung mechanics in spontaneously breathing and mechanically ventilated newborns. The developed measuring head has a low apparatus dead space, is easy to disinfect, has standard connections and can be used as the T-piece in a ventilator circuit.

Long term tidal breathing measurements in newborns: influence of a bias flow on ventilatory parameters.

Long-term pneumotachographic measurements in newborns are hampered by the apparatus dead space, which can be functionally eliminated by a constant bias flow (flow-through technique). In a clinical study the influence of this bias flow was investigated. - In 80 infants (group 1:20 healthy controls, group 2: 30 infants recovering from a pulmonary disease, group 3: 30 pulmonary ill infants) with a body weight 1060-9000g and an age 1-402d respiratory frequency (f) was measured pneumotachographically (PN) as well as with breathing belts (BB) in four periods: before any alteration (period 1, only BB), during the first calm interval after attaching the face mask, in which lung function tests are usually performed (period 2, PN and BB), in a defined steady state giving longer time for adaptation (period 3, PN and BB) and after removing of the mask (period 4, only BB). During periods 2 and 3 tidal breathing parameters were measured from the air flow signal. - In period 1 there were significant differences between the groups in f (1/min) (42.1 +/- 8.3, 44.2 +/- 9.3, 54.2 +/- 16.5, p <0.05) which also existed in periods 3 and 4, but not in period 2 (47.6 +/- 14.9, 44.8 +/- 16.3, 51.2 +/- 15.8). In periods 3 and 4 in all groups, f returned to the initial values. In contrast to group 3 we found significant differences in most of the tidal breathing parameters between periods 2 and 3 in group 1 and 2. - It was concluded that the bias flow has a negligible influence on tidal breathing measurements and allows a sufficient adaptation period necessary to recognise small deviations.

Comparative investigations of algorithms for the detection of breaths in newborns with disturbed respiratory signals.

The correct detection of the beginning of inspiration and expiration in the respiratory signals is an essential prerequisite for accurate lung function testing in newborns. Five algorithms for breath detection using pneumotachographically measured flow and volume signals were investigated with regard to the error rate. To compare and to evaluate the reliability of these algorithms 12 minimally and 12 severely disturbed flow and volume signals from spontaneously breathing newborns were used. With the exception of an algorithm based on Walsh-transformed signals, all algorithms work reliably (error rate <1.1%) if disturbances are minimal. In severely disturbed signals there is a great difference between the algorithms. The most robust algorithm tested (trigger of the flow signal with an additional plausibility check of the recognized breath) resulted in an error rate of <3.4%. Not all algorithms tested are suitable for real-time applications because they differ considerably in delay time for breath detection.

Leak measurements in spontaneously breathing premature newborns by using the flow-through technique.

A new method for measuring and correcting air leaks during lung-function testing in infants has been validated in vitro and in vivo by using a flow-through system that measured the inflow and outflow of a face mask. An adjustable leak was quantified by using suction flow to validate the accuracy of leak measurements. To validate the leak correction, the volume of a pump was measured with different air leaks (0-30%). The method developed was tested in 67 infants breathing spontaneously. There was good agreement between measured and simulated leaks (r = 0.998, P < 0.001; 95% limits of agreement were -0.3 and 0.1%, respectively). The volume was generally underestimated because of leaks, and the volume error was up to 94% compared with the maximum error of 5% after leak correction. With continuous leak measurements in vivo, there were <4% actual leaks (median 2.6%), and we did not observe any leaks in >7% of cases. The leak correction improved the accuracy of ventilatory measurements. The monitoring of leaks is helpful for airtight placement of the face mask and for prevention of serious measurement errors caused by leaks.

Computer simulation of the measured respiratory impedance in newborn infants and the effect of the measurement equipment.

The forced oscillation technique (FOT) is a non-invasive method to investigate lung mechanics. FOT does not require active cooperation and therefore it seems to be useful for lung function measurements in newborn infants. The aims of this simulation study were to investigate the effects of development and growth of the lung, pulmonary inhomogeneities and the measurement equipment on the respiratory impedance (Zrs). The respiratory impedance was simulated by using four lung models with lumped parameters in the frequency range of 3-50 Hz considering resistive and elastic resistances of the respiratory system and the inertance of breathing air and tissue. The simulation has shown that the maturation of lungs produces only a parallel shifting of the real and imaginary part of the impedance curves whereas respiratory diseases change the course of the curves. Furthermore, a high influence of the measurement equipment (e.g. compliance of the face mask, endotracheal tube leaks) on Zrs was found. In conclusion, the simulation has shown that FOT offers a deeper insight in the structure of the respiratory system. However, the technical requirements for accurate measurements in newborns are very high.

Accuracy of volume measurements in mechanically ventilated newborns: a comparative study of commercial devices.

OBJECTIVE: Ventilatory measurements in ventilated newborns are increasingly used to monitor and to optimize mechanical ventilation. The aim of this study was to compare the accuracy of volume measurements by different instruments using standardized laboratory conditions. METHODS: The accuracy of displayed volume values of different commercial devices (Bicore CP-100, Ventrak 1500, Ventrak 1550, Babylog 8000, PEDS IV and SensorMedics 2600) was investigated using adjustable calibration syringes (volume range 2-60 ml, breathing rates 30/min-60/min) and humidified (>95%), heated (35 degrees C) breathing gas with adjustable FIO2 (0.21-1.0). The pneumotach and also the tubes were placed within an incubator (37 degrees C). RESULTS: The relative volume error of all devices was in conformity with clinically allowed tolerances (Bicore CP-100 6.4+/-0.5% (mean +/- SD), Ventrak 1500 3.6+/-4.2%, Ventrak 1550 6.5+/-2.7%, Babylog 8000 -5.5+/-1.5%, PEDS IV -4.0+/-1.4%, SensorMedics 2600 3.5+/-1.75%) for the measuring range studied (10 ml < V < 60 ml, rate 30-60/min, FIO2 = 0.21). Unacceptable errors were obtained for volumes lower than 10 ml with Bicore CP-100 (-28.5+/-26%) and PEDS IV (-10.3+/-3.4%). Changes in FIO2 had an important influence on volume measurements and only the SensorMedics 2600 and the PEDS IV corrected properly for FIO2 changes. CONCLUSION: Most of the currently available neonatal spirometry devices allow sufficiently accurate volume measurements in the range of 10-60 ml and at frequencies between 30-60/min provided that an increased FIO2 is taken into account.

[Effect of background flow on the accuracy of respiratory flow and respiratory volume measurement in newborn infants]. / Einfluss eines Hintergrundflows auf die Genauigkeit der Atemflow- und Atemvolumenmessung bei Neugeborenen.

Measurement of ventilation, in particular in preterm infants, is greatly impaired by equipment dead space, with its significant effect on the ventilatory pattern and gas exchange. For patients of this age, therefore, dead-spacefree methods are needed for long-term measurements. Rebreathing can be avoided if the pneumotachograph (PNT) and face mask are flushed with a continuous background flow. The effect of this on the measurements has not yet been investigated in detail. A measuring system comprising two identical baby PNTs (Jaeger/Germany) permitting a background flow of between 0 and 7 l per min was used. Spontaneous breathing was simulated with a 100 ml calibration syringe employing volumes of 20, 40, 60 and 100 ml (Rudolph/USA) and a frequency of 30 min-1. The measurements were carried out with a T-piece from a respirator circuit, a hand mask (50 ml) and a face chamber having a volume of 850 ml (Siemens-Elema/Sweden). To investigate the dynamic properties of the equipment, we employed flow jumps generated with a magnetic valve (response time < 2 ms) and analysed the responses using Fourier analysis. We were unable to find any significant relationship between the accuracy of volume measurement and tidal volume for any of the measured volumes. An increase in background flow resulted in an underestimation of the volume with an error < 3%. We found no influence of the background flow or type of face mask on the frequency response.(ABSTRACT TRUNCATED AT 250 WORDS)

[Effect of physical properties of respiratory gas on pneumotachographic measurement of ventilation in newborn infants]. / Einfluss der physikalischen Eigenschaften des Atemgases auf die pneumotachographische Ventilationsmessung bei Neugeborenen.

UNLABELLED: The measurement of ventilation in neonates has a number of specific characteristics; in contrast to lung function testing in adults, the inspiratory gas for neonates is often conditioned. In pneumotachographs (PNT) based on Hagen-Poiseuille's law, changes in physical characteristics of respiratory gas (temperature, humidity, pressure and oxygen fraction [FiO2]) produce a volume change as calculated with the ideal gas equation p*V/T = const; in addition, the viscosity of the gas is also changed, thus leading to measuring errors. In clinical practice, the effect of viscosity on volume measurement is often ignored. The accuracy of these empirical laws was investigated in a size 0 Fleisch-PNT using a flow-through technique and variously processed respiratory gas. Spontaneous breathing was simulated with the aid of a calibration syringe (20 ml) and a rate of 30 min-1. RESULTS: The largest change in viscosity (11.6% at 22 degrees C and dry gas) is found with an increase in FiO2 (21...100%). A rise in temperature from 24 to 35 degrees C (dry air) produced an increase in viscosity of 5.2%. An increase of humidity (0...90%, 35 degrees C) decreased the viscosity by 3%. A partial compensation of these viscosity errors is thus possible. Pressure change (0...50 mbar, under ambient conditions) caused no measurable viscosity error. With the exception of temperature, the measurements have shown good agreement between the measured volume measuring errors and those calculated from viscosity changes. CONCLUSIONS: If the respiratory gas differs from ambient air (e.g. elevated FiO2) or if the PNT is calibrated under BTPS conditions, changes in viscosity must not be neglected when performing accurate ventilation measurements. On the basis of the well-known physical laws of Dalton, Thiesen and Sutherland, a numerical correction of adequate accuracy is possible.