Response-time enhancement of a clinical gas analyzer facilitates measurement of breath-by-breath gas exchange

Dynamic body acceleration as a proxy to predict the cost of locomotion in bottlenose dolphins

Estimates of the energetic costs of locomotion (COL) at different activity levels are necessary to answer fundamental eco-physiological questions and to understand the impacts of anthropogenic disturbance to marine mammals. We combined estimates of energetic costs derived from breath-by-breath respirometry with measurements of overall dynamic body acceleration (ODBA) from biologging tags to validate ODBA as a proxy for COL in trained common bottlenose dolphins (Tursiops truncatus). We measured resting metabolic rate (RMR); mean individual RMR was 0.71-1.42 times that of a similarly sized terrestrial mammal and agreed with past measurements which used breath-by-breath and flow-through respirometry. We also measured energy expenditure during submerged swim trials, at primarily moderate exercise levels. We subtracted RMR to obtain COL, and normalized COL by body size to incorporate individual swimming efficiencies. We found both mass-specific energy expenditure and mass-specific COL were linearly related with ODBA. Measurements of activity level and cost of transport (the energy required to move a given distance) improve understanding of the costs of locomotion in marine mammals. The strength of the correlation between ODBA and COL varied among individuals, but the overall relationship can be used at a broad scale to estimate the energetic costs of disturbance, daily locomotion costs to build energy budgets, and investigate the costs of diving in free-ranging animals where bio-logging data are available. We propose that a similar approach could be applied to other cetacean species.

Test-retest variability of VO2max using total-capture indirect calorimetry reveals linear relationship of VO2 and Power

This study aimed to analyze the intra‐individual variation in VO_2max of human subjects using total‐capture and free‐flow indirect calorimetry. Twenty‐seven men (27 ± 5 year; VO_2max 49‐79 mL•kg⁻¹•min⁻¹) performed two maximal exertion tests (CPETs) on a cycle ergometer, separated by a 7 ± 2 day interval. VO₂ and VCO₂ were assessed using an indirect calorimeter (Omnical) with total capture of exhalation in a free‐flow airstream. Thirteen subjects performed a third maximal exertion test using a breath‐by‐breath calorimeter (Oxycon Pro). On‐site validation was deemed a requirement. For the Omnical, the mean within‐subject CV for VO_2max was 1.2 ± 0.9% (0.0%‐4.4%) and for ergometer workload P_max 1.3 ± 1.3% (0%‐4.6%). VO_2max values with the Oxycon Pro were significantly lower in comparison with Omnical (P < 0.001; t test) with mean 3570 vs 4061 and difference SD 361 mL•min⁻¹. Validation results for the Omnical with methanol combustion were −0.05 ± 0.70% (mean ± SD; n = 31) at the 225 mL•min⁻¹ VO₂ level and −0.23 ± 0.80% (n = 31) at the 150 mL•min⁻¹ VCO₂ level. Results using gas infusion were 0.04 ± 0.75% (n = 34) and −0.99 ± 1.05% (n = 24) over the respective 500‐6000 mL•min⁻¹ VO₂ and VCO₂ ranges. Validation results for the Oxycon Pro in breath‐by‐breath mode were ‐ 2.2 ± 1.6% (n = 12) for VO₂ and 5.7 ± 3.3% (n = 12) for VCO₂ over the 1000‐4000 mL•min⁻¹ range. On a Visual analog scale, participants reported improved breathing using the free‐flow indirect calorimetry (score 7.6 ± 1.2 vs 5.1 ± 2.7, P = 0.008). We conclude that total capturing free‐flow indirect calorimetry is suitable for measuring VO₂ even with the highest range. VO_2max was linear with the incline in P_max over the full range.

Correcting the dynamic response of a commercial esophageal balloon-catheter

It is generally recommended that an esophageal balloon-catheter possess an adequate frequency response up to 15 Hz, such that parameters of respiratory mechanics may be quantified with precision. In our experience, however, we have observed that some commercially available systems do not display an ideal frequency response (<8-10 Hz). We therefore investigated whether the poor frequency response of a commercially available esophageal catheter may be adequately compensated using two numerical techniques: 1) an exponential model correction, and 2) Wiener deconvolution. These two numerical techniques were performed on a commercial balloon-catheter interfaced with 0, 1, and 2 lengths of extension tubing (90 cm each), referred to as configurations L0, L90, and L180, respectively. The frequency response of the balloon-catheter in these configurations was assessed by empirical transfer function analysis, and its "working" range was defined as the frequency beyond which more than 5% amplitude and/or phase distortion was observed. The working frequency range of the uncorrected balloon-catheter extended up to only 10 Hz for L0, and progressively worsened with additional tubing length (L90 = 3 Hz, L180 = 2 Hz). Although both numerical methods of correction adequately enhanced the working frequency range of the balloon-catheter to beyond 25 Hz for all length configurations (L0, L90, and L180), Wiener deconvolution consistently provided more accurate corrections. Our data indicate that Wiener deconvolution provides a superior correction of the balloon-catheter's dynamic response, and is relatively more robust to extensions in catheter tube length compared with the exponential correction method.

Impact of Software Settings on Multiple-Breath Washout Outcomes

Background and Objectives

Multiple-breath washout (MBW) is an attractive test to assess ventilation inhomogeneity, a marker of peripheral lung disease. Standardization of MBW is hampered as little data exists on possible measurement bias. We aimed to identify potential sources of measurement bias based on MBW software settings.

Methods

We used unprocessed data from nitrogen (N₂) MBW (Exhalyzer D, Eco Medics AG) applied in 30 children aged 5–18 years: 10 with CF, 10 formerly preterm, and 10 healthy controls. This setup calculates the tracer gas N₂ mainly from measured O₂ and CO₂concentrations. The following software settings for MBW signal processing were changed by at least 5 units or >10% in both directions or completely switched off: (i) environmental conditions, (ii) apparatus dead space, (iii) O₂ and CO₂ signal correction, and (iv) signal alignment (delay time). Primary outcome was the change in lung clearance index (LCI) compared to LCI calculated with the settings as recommended. A change in LCI exceeding 10% was considered relevant.

Results

Changes in both environmental and dead space settings resulted in uniform but modest LCI changes and exceeded >10% in only two measurements. Changes in signal alignment and O₂ signal correction had the most relevant impact on LCI. Decrease of O₂ delay time by 40 ms (7%) lead to a mean LCI increase of 12%, with >10% LCI change in 60% of the children. Increase of O₂ delay time by 40 ms resulted in mean LCI decrease of 9% with LCI changing >10% in 43% of the children.

Conclusions

Accurate LCI results depend crucially on signal processing settings in MBW software. Especially correct signal delay times are possible sources of incorrect LCI measurements. Algorithms of signal processing and signal alignment should thus be optimized to avoid susceptibility of MBW measurements to this significant measurement bias.

Enhanced photoacoustic gas analyser response time and impact on accuracy at fast ventilation rates during multiple breath washout

BACKGROUND

The Innocor device contains a highly sensitive photoacoustic gas analyser that has been used to perform multiple breath washout (MBW) measurements using very low concentrations of the tracer gas SF6. Use in smaller subjects has been restricted by the requirement for a gas analyser response time of <100 ms, in order to ensure accurate estimation of lung volumes at rapid ventilation rates.

METHODS

A series of previously reported and novel enhancements were made to the gas analyser to produce a clinically practical system with a reduced response time. An enhanced lung model system, capable of delivering highly accurate ventilation rates and volumes, was used to assess in vitro accuracy of functional residual capacity (FRC) volume calculation and the effects of flow and gas signal alignment on this.

RESULTS

10-90% rise time was reduced from 154 to 88 ms. In an adult/child lung model, accuracy of volume calculation was -0.9 to 2.9% for all measurements, including those with ventilation rate of 30/min and FRC of 0.5 L; for the un-enhanced system, accuracy deteriorated at higher ventilation rates and smaller FRC. In a separate smaller lung model (ventilation rate 60/min, FRC 250 ml, tidal volume 100 ml), mean accuracy of FRC measurement for the enhanced system was minus 0.95% (range -3.8 to 2.0%). Error sensitivity to flow and gas signal alignment was increased by ventilation rate, smaller FRC and slower analyser response time.

CONCLUSION

The Innocor analyser can be enhanced to reliably generate highly accurate FRC measurements down at volumes as low as those simulating infant lung settings. Signal alignment is a critical factor. With these enhancements, the Innocor analyser exceeds key technical component recommendations for MBW apparatus.

Assessment of lung function using a non-invasive oscillating gas-forcing technique

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We propose a compact and non-invasive system for the measurement and monitoring of lung function.

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We develop a novel tidal ventilation model using a non-invasive oscillating gas-forcing technique.

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We compare a conventional continuous ventilation model with the proposed tidal ventilation model.

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The proposed technique has several advantages over conventional methods, and can be used to assess patient lung function.

Conventional methods for monitoring lung function can require complex, or special, gas analysers, and may therefore not be practical in clinical areas such as the intensive care unit (ICU) or operating theatre. The system proposed in this article is a compact and non-invasive system for the measurement and monitoring of lung variables, such as alveolar volume, airway dead space, and pulmonary blood flow. In contrast with conventional methods, the compact apparatus and non-invasive nature of the proposed method could eventually allow it to be used in the ICU, as well as in general clinical settings. We also propose a novel tidal ventilation model using a non-invasive oscillating gas-forcing technique, where both nitrous oxide and oxygen are used as indicator gases. Experimental results are obtained from healthy volunteers, and are compared with those obtained using a conventional continuous ventilation model. Our findings show that the proposed technique can be used to assess lung function, and has several advantages over conventional methods such as compact and portable apparatus, easy usage, and quick estimation of cardiopulmonary variables.

Lung function measurement with multiple-breath-helium washout system

Multiple-breath-washout (MBW) measurements are regarded as a sensitive technique which can reflect the ventilation inhomogeneity of respiratory airways. Typically nitrogen is used as the tracer gas and is washed out by pure oxygen in multiple-breath-nitrogen washout (MBNW) tests. In this study, instead of using nitrogen, (4)He is used as the tracer gas with smaller gas density which may be able to reach deeper into our lungs in a given time and the helium washout results may be more sensitive to the ventilation inhomogeneity in small airways. A multiple-breath-helium-washout (MBHW) system developed for the lung function study is also presented. Quartz tuning forks with a resonance frequency of 32,768Hz have been used for detecting the change of the respiratory gas density. The resonance frequency of the quartz tuning fork decreases linearly with increasing density of the surrounding gas. Knowing the CO2 concentration from the infrared carbon dioxide detector, the helium concentration can be determined. Results from 14 volunteers (3 mild asthmatics, 4 tobacco smokers, 1 with asthma history, 1 with COPD history, 5 normal) have shown that mild asthmatics have higher ventilation inhomogeneity in either conducting or acinar airways (or both). A feature has been found in washout curve of single breaths from 4 tobacco smokers with different length of smoking history which may indicate the early stage of respiratory ventilation inhomogeneity in acinar airways.

A realistic validation study of a new nitrogen multiple-breath washout system

BACKGROUND

For reliable assessment of ventilation inhomogeneity, multiple-breath washout (MBW) systems should be realistically validated. We describe a new lung model for in vitro validation under physiological conditions and the assessment of a new nitrogen (N(2))MBW system.

METHODS

The N(2)MBW setup indirectly measures the N(2) fraction (F(N2)) from main-stream carbon dioxide (CO(2)) and side-stream oxygen (O(2)) signals: F(N2) = 1-F(O2)-F(CO2)-F(Argon). For in vitro N(2)MBW, a double chamber plastic lung model was filled with water, heated to 37°C, and ventilated at various lung volumes, respiratory rates, and F(CO2). In vivo N(2)MBW was undertaken in triplets on two occasions in 30 healthy adults. Primary N(2)MBW outcome was functional residual capacity (FRC). We assessed in vitro error (√[difference](2)) between measured and model FRC (100-4174 mL), and error between tests of in vivo FRC, lung clearance index (LCI), and normalized phase III slope indices (S(acin) and S(cond)).

RESULTS

The model generated 145 FRCs under BTPS conditions and various breathing patterns. Mean (SD) error was 2.3 (1.7)%. In 500 to 4174 mL FRCs, 121 (98%) of FRCs were within 5%. In 100 to 400 mL FRCs, the error was better than 7%. In vivo FRC error between tests was 10.1 (8.2)%. LCI was the most reproducible ventilation inhomogeneity index.

CONCLUSION

The lung model generates lung volumes under the conditions encountered during clinical MBW testing and enables realistic validation of MBW systems. The new N(2)MBW system reliably measures lung volumes and delivers reproducible LCI values.

Gas exchange measurements within a magnetic environment: validation of a new system

Although simultaneous measurements of pulmonary oxygen uptake (VO2) and Phosphorus-31 magnetic resonance spectroscopy ((31)P MRS) is attractive to investigate muscular metabolism during exercise, the superconducting magnet requires the design of specific gas exchange analyser (GEA). Thus, this study aimed to assess the validity of a commercial GEA system (ZAN600) compatible with (31)P MRS measurements. Using nonmagnetic pneumotachograph and prolonged sampling line (from 2m, control condition, to 6.5m) did not alter the proper synchronisation between flow and gas concentration signals. Also, end-expiratory fraction of O2 (FETO2) and CO(2) (FETCO2), and finally the values of steady-state ventilation (V(E)), carbon dioxide production (VCO2) and VO2 kinetics during moderate knee-extension exercise were not significantly different between 2m and 6.5m conditions and between 6.5m condition inside and outside the magnet. These results showed that a prolonged sampling line used inside the superconducting magnet did not affect the accuracy of VO2 measurements of a commercial GEA system; the latter appears suitable for simultaneous measurements of VO2 and (31)P MRS.

Effects of lung time constant, gas analyser delay and rise time on measurements of respiratory dead-space

This study evaluated effects of mechanical time constants (tau(m)) of the respiratory system, delays between flow and CO(2) partial pressure (P(CO)(2)) signals and rise time of the CO(2) analyser on dead-space measurements. A computer model simulated low alveolar dead-space, high alveolar dead-space, 0.2 <or= tau(m) <or= 3.6 s and varying ventilation-perfusion ratios (V/Q). CO(2) expirograms were recorded from the model under each condition and from 22 anaesthetized intubated patients. P(CO)(2) signals were shifted with respect to flow to produce varying time delays and anatomic and physiological dead-spaces were calculated. The CO(2) analyser was simulated as a critically damped second-order system with 10-90% rise times of 25-400 ms. The error in measured dead-space increases approximately 2.5% per 10 ms signal delay for normal lungs (tau(m) = 1 s), but has low sensitivity (0.58% per 10 ms) to the rise time of the CO(2) analyser. Sensitivity of physiological dead-space, but not anatomic dead-space to delay is decreased in high alveolar dead-space and abnormal V/Q distribution. Shorter tau(m) increase the error sensitivity of both physiological and anatomic dead-spaces to both delay and rise time. P(CO)(2) and flow should be well synchronized, particularly when tau(m) are short, to avoid dead-space errors.

Current limitations of volumetric capnography in surfactant-depleted small lungs

OBJECTIVE

To investigate the suitability of volumetric capnography for assessing alveolar gas exchange in very small, surfactant-depleted lungs.

DESIGN

Prospective animal trial.

SETTINGS

Animal laboratory in a university setting.

SUBJECTS

Twenty-one ventilated newborn piglets (age <12 hrs; median weight, 890 g; range, 560-1435 g).

INTERVENTIONS

Bronchoalveolar lavage with instillation of 30 mL/kg normal saline. Ventilatory, circulatory, and lung mechanic variables were measured before and 0, 30, and 60 mins after bronchoalveolar lavage.

MEASUREMENTS AND MAIN RESULTS

The alveolar deadspace fraction calculated by the Bohr and the Bohr/Enghoff equations increased three-fold (p<.001) after bronchoalveolar lavage in capnograms with distinct alveolar plateau, whereas in capnograms without alveolar plateau no statistical significant difference was seen. The main problem of capnography in small and especially stiff lungs was the high number of discarded records exclusively caused by a missing alveolar plateau. Rates of discarded records of capnography were 9.5% before lavage and increased (p<.01) to 52.4%, 47.6%,42.8% after bronchoalveolar lavage (0, 30, and 60 mins). With decreasing exhalation time, the number of discarded records increased significantly. No plateau was seen in >75% of recorded files with exhalation times <200 msecs. The effect of bronchoalveolar lavage on all variables measured was quite different, with the highest impact on required ventilatory settings, calculated oxygenation variables, and compliance. The effect of bronchoalveolar lavage on arterio-alveolar CO2 difference, CO2 production, and alveolar deadspace was much lower and statistically significant only in capnograms with alveolar plateau.

CONCLUSIONS

Volumetric capnography is a useful tool to detect impaired alveolar gas exchange in surfactant-depleted small lungs. However, the method failed if there was no alveolar plateau in the volumetric capnogram especially in stiff lungs with short exhalation times.

A method of reconstruction of clinical gas-analyzer signals corrupted by positive-pressure ventilation

The use of sidestream infrared and paramagnetic clinical gas analyzers is widespread in anesthesiology and respiratory medicine. For most clinical applications, these instruments are entirely satisfactory. However, their ability to measure breath-by-breath volumetric gas fluxes, as required for measurement of airway dead space, oxygen uptake, and so on, is usually inferior to that of the mass spectrometer, and this is thought to be due, in part, to their slower response times. We describe how volumetric gas analysis with the Datex Ultima analyzer, although reasonably accurate for spontaneous ventilation, gives very inaccurate results in conditions of positive-pressure ventilation. We show that this problem is a property of the gas sampling system rather than the technique of gas analysis itself. We examine the source of this error and describe how cyclic changes in airway pressure result in variations in the flow rate of the gas within the sampling catheter. This results in the phenomenon of "time distortion," and the resultant gas concentration signal becomes a nonlinear time series. This corrupted signal cannot be aligned or integrated with the measured flow signal. We describe a method to correct for this effect. With the use of this method, measurements required for breath-by-breath gas-exchange models can be made easily and reliably in the clinical setting.