Current limitations of volumetric capnography in surfactant-depleted small lungs

Relationship between ETCO2 and PaCO2 under Changing Capnogram in Ventilated Infants with NAVA: An Observational Study

This observational study evaluated the validity of end-tidal CO2 (ETCO2) as a surrogate for arterial PCO2 (PaCO2) in infants on neurally adjusted ventilatory assist (NAVA), particularly considering the influence of variable spontaneous breathing on capnography waveforms. The study involved 16 infants, analyzing 50 paired ETCO2 and PaCO2 values. Deming regression analysis highlighted a notably stronger correlation for maximum ETCO2 (r2 = 0.6783, p <0.0001) compared to mean ETCO2 (r2 = 0.5686, p <0.0001) and demonstrated a significantly weaker association for minimum ETCO2 (r2 = 0.1838). These findings emphasize the superior predictive value of maximum ETCO2 in estimating PaCO2, advocating its reliable use in clinical monitoring, especially given the dynamic capnography associated with NAVA's variable pressures. The results suggest ETCO2's potential to enhance noninvasive respiratory management, reduce the frequency of blood sampling, and improve overall care for infants requiring mechanical ventilation.

Development of a Novel Infant Volumetric Capnography Simulator: Making the Invisible Visible Improves Understanding and Safety

INTRODUCTION

Volumetric capnography depicts volumetric capnograms [ie, the plot of expired carbon dioxide (CO 2 ) over the tidal volume]. This bench study aimed to determine the reliability, accuracy, and precision of a novel infant simulator for volumetric capnography. This simulator would be clinically valuable for teaching purposes because it reflects the entire cardiopulmonary physiology within 1 breath.

METHODS

An infant lung simulator was fed with CO 2 supplied by a mass flow controller (VCO 2-IN ) and ventilated using standard settings. A volumetric capnograph was placed between the endotracheal tube and the ventilatory circuit. We simulated ventilated babies of different body weights (2, 2.5, 3, and 5 kg) with a VCO 2 ranging from 12 to 30 mL/min. The correlation coefficient ( r2 ), bias, coefficient of variation (CV = SD/ x × 100), and precision (2 × CV) between the VCO 2-IN and the elimination of CO 2 recorded by the capnograph (VCO 2-OUT ) were calculated. The quality of the capnogram's waveforms was compared with real ones belonging to anesthetized infants using an 8-point scoring system, where 6 points or greater meant that the simulated capnogram showed good, 5 to 3 points acceptable, and less than 3 points an unacceptable shape.

RESULTS

The correlation between VCO 2-IN and VCO 2-OUT was r2 = 0.9953 ( P < 0.001), with a bias of 0.16 (95% confidence intervals from 0.12 to 0.20) mL/min. The CV was 5% or less and the precision was 10% or less. All simulated capnograms showed similar shapes compared with real babies, scoring 6 points for 3 kg and 6.5 points for 2-, 2.5-, and 5-kg babies.

CONCLUSIONS

The simulator of volumetric capnograms was reliable, accurate, and precise for simulating the CO 2 kinetics of ventilated infants.

Effects of apparatus dead space on volumetric capnograms in neonates with healthy lungs: a simulation study

BACKGROUND

Volumetric capnography in healthy ventilated neonates showed deformed waveforms, which are supposedly due to technological limitations of flow and carbon dioxide sensors.

AIMS

This bench study analyzed the role of apparatus dead space on the shape of capnograms in simulated neonates with healthy lungs.

METHODS

We simulated mechanical breaths in neonates of 2, 2.5, and 3 kg of body weight using a neonatal volumetric capnography simulator. The simulator was fed by a fixed amount of carbon dioxide of 6 mL/kg/min. Such simulator was ventilated in a volume control mode using fixed ventilatory settings with a tidal volume of 8 mL/kg and respiratory rates of 40, 35, and 30 breaths per minute for the 2, 2.5 and 3 kg neonates, respectively. We tested the above baseline ventilation with and without an additional apparatus dead space of 4 mL.

RESULTS

Simulations showed that adding the apparatus dead space to baseline ventilation increased the amount of re-inhaled carbon dioxide in all neonates: 0.16 ± 0.01 to 0.32 ± 0.03 mL (2 kg), 0.14 ± 0.02 to 0.39 ± 0.05 mL (2.5 kg), and 0.13 ± 0.01 to 0.36 ± 0.05 mL (3 kg); (p < .001). Apparatus dead space was computed as part of the airway dead space, and therefore, the ratio of airway dead space to tidal volume increased from 0.51 ± 0.04 to 0.68 ± 0.06, from 0.43 ± 0.04 to 0.62 ± 0.01 and from 0.38 ± 0.01 to 0.60 ± 0.02 in the 2, 2.5 and 3 kg simulated neonates, respectively (p < .001). Compared to baseline ventilation, adding apparatus dead space decreased the ratio of the volume of phase III to VT size from 31% to 11% (2 kg), from 40% to 16% (2.5 kg) and from 50% to 18% (3 kg); (p < .001).

CONCLUSIONS

The addition of a small apparatus dead space artificially deformed the volumetric capnograms in simulated neonates with healthy lungs.

Functional morphometry: non-invasive estimation of the alveolar surface area in extremely preterm infants

BACKGROUND

The main pathophysiologic characteristic of chronic respiratory disease following extremely premature birth is arrested alveolar growth, which translates to a smaller alveolar surface area (SA). We aimed to use non-invasive measurements to estimate the SA in extremely preterm infants.

METHODS

Paired measurements of the fraction of inspired oxygen and transcutaneous oxygen saturation were used to calculate the ventilation/perfusion ratio, which was translated to SA using Fick's law of diffusion. The SA was then adjusted using volumetric capnography.

RESULTS

Thirty infants with a median (range) gestational age of 26.3 (22.9-27.9) weeks were studied. The median (range) adjusted SA was 647.9 (316.4-902.7) cm2. The adjusted SA was lower in the infants who required home oxygen [637.7 (323.5-837.5) cm2] compared to those who did not [799.1 (444.2-902.7) cm2, p = 0.016]. In predicting the need for supplemental home oxygen, the adjusted SA had an area under the receiver operator characteristic curve of 0.815 (p = 0.017). An adjusted SA ≥688.6 cm2 had 86% sensitivity and 77% specificity in predicting the need for supplemental home oxygen.

CONCLUSIONS

The alveolar surface area can be estimated non-invasively in extremely preterm infants. The adjusted alveolar surface area has the potential to predict the subsequent need for discharge home on supplemental oxygen.

IMPACT

We describe a novel biomarker of respiratory disease following extremely preterm birth. The adjusted alveolar surface area index was derived by non-invasive measurements of the ventilation/perfusion ratio and adjusted by concurrent measurements of volumetric capnography. The adjusted alveolar surface area was markedly reduced in extremely preterm infants studied at 7 days of life and could predict the need for discharge home on supplemental oxygen. This method could be used at the bedside to estimate the alveolar surface area and provide an index of the severity of lung disease, and assist in monitoring, clinical management and prognosis.

Volumetric capnography pre- and post-surfactant during initial resuscitation of premature infants

BACKGROUND

Volumetric capnography allows for continuous monitoring of expired tidal volume and carbon dioxide. The slope of the alveolar plateau of the capnogram (S_III) could provide information regarding ventilation homogeneity. We aimed to assess the feasibility of measuring S_III during newborn resuscitation and determine if S_III decreased after surfactant indicating ventilation inhomogeneity improvement.

METHODS

Respiratory function traces of preterm infants resuscitated at birth were analysed. Ten capnograms were constructed for each infant: five pre- and post-surfactant. If a plateau was present S_III was calculated by regression analysis.

RESULTS

Thirty-six infants were included, median gestational age of 28.7 weeks and birth weight of 1055 g. Average time between pre- and post-surfactant was 3.2 min. Three hundred and sixty capnograms (180 pre and post) were evaluated. There was adequate slope in 134 (74.4%) capnograms pre and in 100 (55.6%) capnograms post-surfactant (p = 0.004). Normalised for tidal volume S_III pre-surfactant was 18.89 mmHg and post-surfactant was 24.86 mmHg (p = 0.006). An increase in S_III produced an up-slanting appearance to the plateau indicating regional obstruction.

CONCLUSION

It was feasible to evaluate the alveolar plateau pre-surfactant in preterm infants. Ventilation inhomogeneity increased post-surfactant likely due to airway obstruction caused by liquid surfactant present in the airways.

IMPACT

Volumetric capnography can be used to assess homogeneity of ventilation by S_III analysis. Ventilation inhomogeneity increased immediately post-surfactant administration during the resuscitation of preterm infants, producing a characteristic up-slanting appearance to the alveolar plateau. The best determinant of alveolar plateau presence in preterm infants was the expired tidal volume.

Non-invasive carbon dioxide monitoring in neonates: methods, benefits, and pitfalls

Wide fluctuations in partial pressure of carbon dioxide (PaCO2) can potentially be associated with neurological and lung injury in neonates. Blood gas measurement is the gold standard for assessing gas exchange but is intermittent, invasive, and contributes to iatrogenic blood loss. Non-invasive carbon dioxide (CO2) monitoring has become ubiquitous in anesthesia and critical care and is being increasingly used in neonates. Two common methods of non-invasive CO2 monitoring are end-tidal and transcutaneous. A colorimetric CO2 detector (a modified end-tidal CO2 detector) is recommended by the International Liaison Committee on Resuscitation (ILCOR) and the American Academy of Pediatrics to confirm endotracheal tube placement. Continuous CO2 monitoring is helpful in trending PaCO2 in critically ill neonates on respiratory support and can potentially lead to early detection and minimization of fluctuations in PaCO2. This review includes a description of the various types of CO2 monitoring and their applications, benefits, and limitations in neonates.

Carbon dioxide monitoring in the newborn infant

Carbon dioxide (CO₂ ) monitoring is vital during mechanical ventilation of newborn infants, as morbidity increases when CO₂ levels are inappropriate. Our aim was to review the uses and limitations of such noninvasive monitoring methods. Colorimetry is primarily utilized during resuscitation to determine whether successful intubation has occurred. False negative and positive results can however lead to delays in detecting tracheal versus esophageal intubation. Transcutaneous carbon dioxide sensors have limited use during resuscitation, but can be utilized to provide continuous trend data during on-going ventilation. End-tidal capnography can provide clinicians with quantitative end-tidal CO₂ (EtCO₂ ) values and a continuous real-time capnogram waveform trace. These devices are becoming more widely accepted for use in the neonatal population as the new devices are lightweight with minimal additional dead space. Nevertheless, they have been reported to have variable accuracy when compared to arterial CO₂ measurements, however, divergence of results may be related to disease severity rather than technological limitations. During resuscitation EtCO₂ can be detected by capnography more rapidly than by colorimetry. Furthermore, capnography can be currently utilized in neonatal research settings to determine the physiological dead space and ventilation inhomogeneity, and thus has potential to be beneficial to clinical care. In conclusion, novel modes of noninvasive carbon dioxide monitoring can be safely and reliably utilized in newborn infants during mechanical ventilation. Future randomized trials should aim to address which device provides the most optimal form of monitoring in different clinical contexts.

Continuous Noninvasive Carbon Dioxide Monitoring in Neonates: From Theory to Standard of Care

Ventilatory support may affect the short- and long-term neurologic and respiratory morbidities of preterm infants. Ongoing monitoring of oxygenation and ventilation and control of adequate levels of oxygen, pressures, and volumes can decrease the incidence of such adverse outcomes. Use of pulse oximetry became a standard of care for titrating oxygen delivery, but continuous noninvasive monitoring of carbon dioxide (CO₂) is not routinely used in NICUs. Continuous monitoring of CO₂ level may be crucial because hypocarbia and hypercarbia in extremely preterm infants are associated with lung and brain morbidities, specifically bronchopulmonary dysplasia, intraventricular hemorrhage, and cystic periventricular leukomalacia. It is shown that continuous monitoring of CO₂ levels helps in maintaining stable CO₂ values within an accepted target range. Continuous monitoring of CO₂ levels can be used in the delivery room, during transport, and in infants receiving invasive or noninvasive respiratory support in the NICU. It is logical to hypothesize that this will result in better outcome for extremely preterm infants. In this article, we review the different noninvasive CO₂ monitoring alternatives and devices, their advantages and disadvantages, and the available clinical data supporting or negating their use as a standard of care in NICUs.

Physiological and anatomical dead space in mechanically ventilated newborn infants

OBJECTIVES

To compare the anatomical (V_D-Ana ) and alveolar dead space (V_D-Alv ) in term and prematurely born infants and identify the clinical determinants of those indices.

WORKING HYPOTHESIS

V_D-Ana and V_D-Alv will be higher in prematurely born compared to term born infants.

STUDY DESIGN

Retrospective analysis of data collected at King's College Hospital NHS Foundation Trust, London, UK.

PATIENT SELECTION

Fifty-six infants (11 term, 45 preterm) were studied at a median age of 8 (IQR 2-33) days.

METHODOLOGY

V_D-Ana was determined using Fowler's method of volumetric capnography. V_D-Alv was determined by subtracting V_D-Ana from the physiological dead space which was determined by the Bohr-Enghoff equation. V_D-Ana and V_D-Alv were related to body weight at the time of study.

RESULTS

The median V_D-Ana /kg was higher in prematurely born infants [3.7 (IQR: 3.0-4.5) mL/kg] compared to term infants [2.4 (IQR: 1.9-2.9) mL/kg, adjusted P = 0.001]. The median V_D-Alv /kg was not higher in prematurely born infants [0.3 (IQR: 0.1-0.5)] compared to term infants [0.1 (IQR: 0.0-0.2) mL/kg] after adjusting for differences in respiratory rate and days of ventilation (P = 0.482). V_D-Ana /kg was related to postmenstrual age (r = -0.388, P < 0.001), birth weight (r = -0.397, P < 0.001), and weight at measurement (r = -0.476, P < 0.001). V_D-Alv /kg was related to postmenstrual age (r = -0.254, P < 0.001), birth weight (r = -0.291, P = 0.002), and weight at measurement (r = -0.281, P = 0.003) and related to days of ventilation (r = 0.194, P = 0.044).

CONCLUSIONS

V_D-Ana /kg and V_D-Alv /kg increased with decreasing weight and gestation. V_D-Alv was higher in infants that have undergone prolonged mechanical ventilation.

Basic principles of respiratory function monitoring in ventilated newborns: A review

Respiratory monitoring during mechanical ventilation provides a real-time picture of patient-ventilator interaction and is a prerequisite for lung-protective ventilation. Nowadays, measurements of airflow, tidal volume and applied pressures are standard in neonatal ventilators. The measurement of lung volume during mechanical ventilation by tracer gas washout techniques is still under development. The clinical use of capnography, although well established in adults, has not been embraced by neonatologists because of technical and methodological problems in very small infants. While the ventilatory parameters are well defined, the calculation of other physiological parameters are based upon specific assumptions which are difficult to verify. Incomplete knowledge of the theoretical background of these calculations and their limitations can lead to incorrect interpretations with clinical consequences. Therefore, the aim of this review was to describe the basic principles and the underlying assumptions of currently used methods for respiratory function monitoring in ventilated newborns and to highlight methodological limitations.

Current methodological and technical limitations of time and volumetric capnography in newborns

Although capnography is a standard tool in mechanically ventilated adult and pediatric patients, it has physiological and technical limitations in neonates. Gas exchange differs between small and adult lungs due to the greater impact of small airways on gas exchange, the higher impact of the apparatus dead space on measurements due to lower tidal volume and the occurrence of air leaks in intubated patients. The high respiratory rate and low tidal volume in newborns, especially those with stiff lungs, require main-stream sensors with fast response times and minimal dead-space or low suction flow when using side-stream measurements. If these technical requirements are not fulfilled, the measured end-tidal CO2 (P et CO 2 ), which should reflect the alveolar CO2 and the calculated airway dead spaces, can be misleading. The aim of this survey is to highlight the current limitations of capnography in very young patients to avoid pitfalls associated with the interpretation of capnographic parameters, and to describe further developments.

Impact of Continuous Capnography in Ventilated Neonates: A Randomized, Multicenter Study

OBJECTIVE

To compare the time spent within a predefined safe range of CO2 (30-60 mmHg) during conventional ventilation between infants who were monitored with distal end-tidal CO2 (dETCO2, or capnography) and those who were not.

STUDY DESIGN

For this randomized, controlled multicenter study, ventilated infants with a double-lumen endotracheal tube were randomized to 1 of 2 groups: the open (monitored) group, in which data from the capnograph were recorded, displayed to the medical team, and used for patient care, and the masked group, in which data from the capnograph were recorded. However, the measurements were masked and not available for patient care. dETCO2 was compared with PaCO2 measurements recorded for patient care.

RESULTS

Fifty-five infants (25 open, 30 masked) participated in the study (median gestational age, 28.6 weeks; range, 23.5-39.0 weeks). The 2 groups were comparable. dETCO2 was in good correlation (r = 0.73; P < .001) and adequate agreement (mean ± SD of the difference, 3.0 ± 8.5 mmHg) with PaCO2. Compared with infants in the masked group, those in the monitored group had significantly (P = .03) less time with an unsafe dETCO2 level (high: 3.8% vs 8.8% or low: 3.8% vs 8.9%). The prevalence of intraventricular hemorrhage or periventricular leukomalacia rate was lower in the monitored group (P = .02) and was significantly (P < .05) associated with the independent factors dETCO2 monitoring and gestational age.

CONCLUSION

Continuous dETCO2 monitoring improved control of CO2 levels within a safe range during conventional ventilation in a neonatal intensive care unit.

TRIAL REGISTRATION

ClinicalTrials.gov: NCT01572272.

A rate-based transcutaneous CO2 sensor for noninvasive respiration monitoring

The pain and risk of infection associated with invasive blood sampling for blood gas measurements necessitate the search for reliable noninvasive techniques. In this work we developed a novel rate-based noninvasive method for a safe and fast assessment of respiratory status. A small sampler was built to collect the gases diffusing out of the skin. It was connected to a CO2 sensor through gas-impermeable tubing. During a measurement, the CO2 initially present in the sampler was first removed by purging it with nitrogen. The gases in the system were then recirculated between the sampler and the CO2 sensor, and the CO2 diffusion rate into the sampler was measured. Because the measurement is based on the initial transcutaneous diffusion rate, reaching mass transfer equilibrium and heating the skin is no longer required, thus, making it much faster and safer than traditional method. A series of designed experiments were performed to analyze the effect of the measurement parameters such as sampler size, measurement location, subject positions, and movement. After the factor analysis tests, the prototype was sent to a level IV NICU for clinical trial. The results show that the measured initial rate of increase in CO2 partial pressure is linearly correlated with the corresponding arterial blood gas measurements. The new approach can be used as a trending tool, making frequent blood sampling unnecessary for respiratory status monitoring.

Non invasive monitoring in mechanically ventilated pediatric patients

Cardiopulmonary monitoring is a key component in the evaluation and management of critically ill patients. Clinicians typically rely on a combination of invasive and non-invasive monitoring to assess cardiac output and adequacy of ventilation. Recent technological advances have led to the introduction: of continuous non-invasive monitors that allow for data to be obtained at the bedside of critically ill patients. These advances help to identify hemodynamic changes and allow for interventions before complications occur. In this manuscript, we highlight several important methods of non-invasive cardiopulmonary monitoring, including capnography, transcutaneous monitoring, pulse oximetry, and near infrared spectroscopy.

Effect of intubation and mechanical ventilation on exhaled nitric oxide in preterm infants with and without bronchopulmonary dysplasia measured at a median postmenstrual age of 49 weeks

Background

Exhaled nitric oxide (eNO) is a marker of established airway inflammation in adults and children, but conflicting results have been reported in preterm infants when postnatal eNO is measured during tidal breathing. This study investigated the extent to which intubation and mechanical ventilation (MV) affect eNO and NO production (V’_NO) in preterm infants with and without bronchopulmonary dysplasia (BPD).

Patients and methods

A total of 176 very low birth weight (VLBW) infants (birth weight <1500 g), including 74 (42%) with and 102 (58%) without BPD, were examined at a median postmenstrual age of 49 weeks. Of the 176 infants, 84 (48%) did not require MV, 47 (27%) required MV for <7 days and 45 (26%) required MV for ≥7 days. Exhaled NO and tidal breathing parameters were measured in sleeping infants during tidal breathing, respiratory mechanics were assessed by occlusion tests, and arterialized capillary blood gas was analyzed.

Results

eNO was significantly correlated with tidal breathing parameters, while V’_NO was correlated with growth parameters, including age and body length (p < 0.001 each). Infants who were intubated and received MV for <7 days had significantly lower eNO (p < 0.01) and V’_NO (p < 0.01) than non-ventilated infants. In contrast, eNO and V’_NO did not differ significantly in non-ventilated infants and those receiving MV for ≥7 days. Multivariate analysis showed that independent on the duration of MV eNO (p = 0.003) and V’_NO (p = 0.018) were significantly increased in BPD infants comparable with the effects of intubation and MV on eNO (p = 0.002) and V’_NO (p = 0.017).

Conclusions

Preterm infants with BPD show only weak postnatal increases in eNO and V’_NO, but these changes may be obscured by the distinct influences of breathing pattern and invasive respiratory support. This limits the diagnostic value of postnatal eNO measurements in the follow-up of BPD infants.

Dead space reduction by Kolobow's endotracheal tube does not justify the waiving of volume monitoring in small, ventilated lungs

In ventilated preterm infants the flow sensor contributes significantly to the total apparatus dead space, which may impair gas exchange. The aim of the study was to quantify to which extent a dead space reduced Kolobow tube (KB) without flow sensor improves the gas exchange compared with a conventional ventilator circuit with flow sensor [Babylog 8000 (BL)]. In a cross-over trial in 14 tracheotomized, surfactant-depleted (saline lavage) and mechanically ventilated newborn piglets (age <12 h; body weight 705-1200 g) BL and KB was applied alternately for 15 min and blood gases were recorded. The inner diameter of the endotracheal tube was 3.6 mm and the apparatus dead space of BL and KB including the endotracheal tube were 3.0 and 1.34 mL. Despite a 50 % apparatus dead space reduction with KB compared to BL statistically significant improvements were only observed for body weights <900 g. In this weight group median paCO2 was decreased by 5 mmHg (p < 0.01), whereas the improvement decreased with decreasing baseline paCO2. Furthermore, median paO2 was increased by 4 mmHg (p < 0.05) and O2 saturation was increased by 2.5 % (p < 0.05). No significant changes were seen in the circulatory parameters. In very small, ventilated lungs the use of KB improved the gas exchange; however, the improvement was moderate and does not justify the waiving of volume monitoring.

Time and volume dependence of dead space in healthy and surfactant-depleted rat lungs during spontaneous breathing and mechanical ventilation

Volumetric capnography is a standard method to determine pulmonary dead space. Hereby, measured carbon dioxide (CO2) in exhaled gas volume is analyzed using the single-breath diagram for CO2. Unfortunately, most existing CO2 sensors do not work with the low tidal volumes found in small animals. Therefore, in this study, we developed a new mainstream capnograph designed for the utilization in small animals like rats. The sensor was used for determination of dead space volume in healthy and surfactant-depleted rats ( n = 62) during spontaneous breathing (SB) and mechanical ventilation (MV) at three different tidal volumes: 5, 8, and 11 ml/kg. Absolute dead space and wasted ventilation (dead space volume in relation to tidal volume) were determined over a period of 1 h. Dead space increase and reversibility of the increase was investigated during MV with different tidal volumes and during SB. During SB, the dead space volume was 0.21 ± 0.14 ml and increased significantly at MV to 0.39 ± 0.03 ml at a tidal volume of 5 ml/kg and to 0.6 ± 0.08 ml at a tidal volume of 8 and 11 ml/kg. Dead space and wasted ventilation during MV increased with tidal volume. This increase was mostly reversible by switching back to SB. Surfactant depletion had no further influence on the dead space increase during MV, but impaired the reversibility of the dead space increase.

A comparison of conventional surfactant treatment and partial liquid ventilation on the lung volume of injured ventilated small lungs

As an alternative to surfactant therapy (ST), partial liquid ventilation (PLV) with perfluorocarbons (PFC) has been considered as a treatment for acute lung injury (ALI) in newborns. The instilled PFC is much heavier than the instilled surfactant and the aim of this study was to investigate whether PLV, compared to ST, increases the end-expiratory volume of the lung (VL). Fifteen newborn piglets (age <12 h, mean weight 678 g) underwent saline lung lavage to achieve a surfactant depletion. Thereafter animals were randomized to PLV (n = 8), receiving PFC PF5080 (3M, Germany) at 30 mL kg(-1), and ST (n = 7) receiving 120 mg Curosurf®. Blood gases, hemodynamics and static compliance were measured initially (baseline), immediately after ALI, and after 240 min mechanical ventilation with either technique. Subsequently all piglets were killed; the lungs were removed in toto and frozen in liquid N2. After freeze-drying the lungs were cut into lung cubes (LCs) with edge lengths of 0.7 cm, to calculate VL. All LCs were weighed and the density of the dried lung tissue was calculated. No statistically significant differences between treatment groups PLV and ST (means ± SD) were noted in body weight (676 ± 16 g versus 679 ± 17 g; P = 0.974) or lung dry weight (1.64 ± 0.29 g versus 1.79 ± 0.48 g; P = 0.48). Oxygenation index and ventilatory efficacy index did not differ significantly between both groups at any time. VL (34.28 ± 6.13 mL versus 26.22 ± 8.1 mL; P < 0.05) and the density of the dried lung tissue (48.07 ± 5.02 mg mL(-1) versus 69.07 ± 5.30 mg mL(-1); P < 0.001), however, differed significantly between the PLV and ST groups. A 4 h PLV treatment of injured ventilated small lungs increased VL by 30% and decreased lung density by 31% compared to ST treatment, indicating greater lung distension after PLV compared to ST.

Advances in monitoring and management of pediatric acute lung injury

This article focuses on the respiratory management and monitoring of pediatric acute lung injury (ALI) as a specific cause for respiratory failure. Definitive, randomized, controlled trials in pediatrics to guide optimal ventilatory management are few. The only adjunct therapy that has been proved to improve clinical outcome is low tidal volume ventilation, but only in adult patients. Careful monitoring of the patient's respiratory status with airway graphic analysis and capnography can be helpful. Definitive data are needed in the pediatric population to assist in the care of infants, children, and adolescents with ALI to improve survival and functional outcome.

The Precise Mechanisms of a High-Speed Ultrasound Gas Sensor and Detecting Human-Specific Lung Gas Exchange

In this paper, we propose and develop a new real-time human respiration process analysis method using a high-time-sampling gas concentration sensor based on ultrasound. A unique point about our proposed gas concentration sensor is its 1 kHz gas concentration sampling speed. This figure could not have been attained by previously proposed gas concentration measurement methods such as InfraRed, semiconductor gas sensors, or GC-MS, because the gas analysis speeds were a maximum of a few hundred milliseconds. First, we describe the proposed new ultrasound sound speed measurement method and the signal processing, and present the measurement circuit diagram. Next, we analyse the human respiration gas variation patterns of five healthy subjects using a newly developed gas-mask-type respiration sensor. This reveals that the rapid gas exchange from H2O to CO2 contains air specific to the human being. In addition, we also measured medical symptoms in subjects suffering from asthma, hyperventilation and bronchial asthma. The millisecond level high-speed analysis of the human respiration process will be useful for the next generation of healthcare, rehabilitation and sports science technology.

A novel method of distal end-tidal CO2 capnography in intubated infants: comparison with arterial CO2 and with proximal mainstream end-tidal CO2

OBJECTIVE

The objective of this study was to evaluate a novel method of distal end-tidal CO2 capnography by comparison with PaCO2 and with the more standard method that measures mainstream proximal end-tidal CO2 in intubated infants.

METHODS

Included in the study were all infants who were ventilated with conventional mechanical ventilation and intubated with a double-lumen endotracheal tube in our NICU during the study period. Data were collected prospectively from 2 capnographs simultaneously and compared with PaCO2. Sidestream distal end-tidal CO2 was measured by a Microstream capnograph via the extra port of a double-lumen endotracheal tube. Mainstream proximal end-tidal CO2 was measured via capnograph connected to the endotracheal tube.

RESULTS

Twenty-seven infants (median [range] birth-weight: 1835 [490-4790] g; gestational age: 32.5 [24.8-40.8] weeks) participated in the study. We used for analysis 222 and 212 measurements of distal end-tidal CO2 and proximal end-tidal CO2, respectively. Distal compared with proximal end-tidal CO2 had a better correlation with PaCO2 and a better agreement with PaCO2. The accuracy of distal end-tidal CO2 decreased, but it remained a useful measure of PaCO2 in the high range of PaCO2 (>or=60 mmHg) or in conditions of severe lung disease. A subanalysis for infants who weighed<1500 g (13 infants, 84 observations) revealed a good correlation and agreement between distal end-tidal CO2 and PaCO2 and poor correlation and agreement for proximal end-tidal CO2.

CONCLUSIONS

Distal end-tidal CO2 measured via a double-lumen endotracheal tube was found to have good correlation and agreement with PaCO2, remained reliable in conditions of severe lung disease, and was more accurate than the standard mainstream proximal end-tidal CO2.

Evaluation of a Reduced Dose Protocol for Respiratory Gated Lung Computed Tomography in an Animal Model

OBJECTIVE

We sought to evaluate and validate a low-dose protocol for respiratory-gated multislice computed tomography (CT) for volume calculations in small ventilated neonatal animals as a model for the ventilated human neonatal lung.

MATERIALS AND METHODS

Five mechanically ventilated newborn piglets were imaged in a multislice CT scanner (0.5-mm slice thickness, 4:16 pitch, 0.5 seconds rotation time, 120 kV) using a normal (100 mAs) and a reduced (10 mAs) dose protocol. All animals were scanned twice (at 100 and 10 mAs) at each of 3 different ventilator settings. Complete volume datasets were reconstructed throughout the respiratory cycle in increments of 10% using retrospective half-scan reconstruction. End-inspiratory volumes and volumes during maximal expiration (functional residual capacity) were calculated by a customized software and values for normal and reduced dose protocols were compared using Kolmogorov-Smirnov test and Bland-Altman plots.

RESULTS

Two volume datasets (one normal and one reduced dose protocol) showed artifacts on the axial images, which could not be analyzed by the software. Those values were determined after manual segmentation and excluded from final analysis. The mean (+/-SD) end-inspiratory volumes and functional residual capacity were 34.3 +/- 10.1 mL and 25.3 +/- 8.0 mL for the normal-dose protocol versus 33.1 +/- 10.0 mL and 24.7 +/- 8.1 mL for the reduced-dose protocol, respectively. There was no statistically significant difference between normal and reduced dose protocol (KS-Test: D = 0.14 < Dmax).

CONCLUSION

Lung volume calculation in ventilated newborn piglets (end-inspiratory volumes and functional residual capacity) can be performed using respiratory-gated multislice CT even at a substantially reduced dose (eg, to 10 mAs). This makes the technique a candidate for future pediatric use.

The effect of changing ventilator settings on indices of ventilation inhomogeneity in small ventilated lungs

Background

In ventilated newborns the use of multiple breath washout (MBW) techniques for measuring both lung volume and ventilation inhomogeneity (VI) is hampered by the comparatively high dead space fraction. We studied how changes in ventilator settings affected VI indices in this particular population.

Methods

Using a computer simulation of a uniformly ventilated volume the interaction between VI indices (lung clearance index (LCI), moment ratios (M₁/M₀, M₂/M₀, AMDN₁, AMDN₂) of the washout curve) and tidal volume (V_T), dead space (V_D) and functional residual capacity (FRC) were calculated. The theoretical results were compared with measurements in 15 ventilated piglets (age <12 h, median weight 1135 g) by increasing the peak inspiratory pressure (PIP). FRC and VI indices were measured by MBW using 0.8% heptafluoropropane as tracer gas.

Results

The computer simulation showed that the sensitivity of most VI indices to changes in V_D/V_T and V_T/FRC increase, in particular for V_D/V_T > 0.5. In piglets, the raised PIP caused a significant increase of V_T from 15.4 ± 9.5 to 21.9 ± 14.7 (p = 0.003) and of the FRC from 31.6 ± 14.7 mL to 35.0 ± 15.9 mL (p = 0.006), whereas LCI (9.15 ± 0.75 to 8.55 ± 0.74, p = 0.019) and the moment ratios M₁/M₀, M₂/M₀ (p < 0.02) decreased significantly. No significant changes were seen in AMDN₁ and AMDN₂. The within-subject variability of the VI indices (coefficient of variation in brackets) was distinctly higher (LCI (9.8%), M₁/M₀ (6.6%), M₂/M₀ (14.6%), AMDN₁ (9.1%), AMDN₂ (16.3%)) compared to FRC measurements (5.6%). Computer simulations showed that significant changes in VI indices were exclusively caused by changes in V_T and FRC and not by an improvement of the homogeneity of alveolar ventilation.

Conclusion

In small ventilated lungs with a high dead space fraction, indices of VI may be misinterpreted if the changes in ventilator settings are not considered. Computer simulations can help to prevent this misinterpretation.

Functional residual capacity measurement by heptafluoropropane in ventilated newborn lungs: in vitro and in vivo validation

OBJECTIVE

Heptafluoropropane is an inert gas commercially used as propellant for inhalers. Since heptafluoropropane can be detected in low concentrations, it could also be used as a tracer gas to measure functional residual capacity and ventilation homogeneity. The aim of the present study was to validate functional residual capacity measurements by heptafluoropropane wash-in/wash-out (0.8%) during mechanical ventilation in small, surfactant-depleted lungs using a newborn piglet model.

DESIGN

Prospective laboratory and animal trial.

SETTING

Animal laboratory in a university setting.

SUBJECTS

Sixteen newborn piglets (age<12 hrs, median weight 1390 g [705-4200 g]) before and after surfactant depletion (Pao2<100 torr in Fio2=1.0) by lung lavage.

INTERVENTIONS

Heptafluoropropane was measured with a new infrared mainstream sensor connected with the flow sensor of the Dräger Babylog 8000. Accuracy and precision of the measurement technique were tested in a mechanical lung model with a volume range from 11 to 35 mL. Reproducibility of the method and its sensitivity to detect changes of functional residual capacity were assessed in vivo by variation of ventilatory variables.

MEASUREMENTS AND MAIN RESULTS

In vitro the absolute error of functional residual capacity was <1 mL (relative errors<3%) with a coefficient of variation<4%. The coefficient of variation of consecutive in vivo measurements was only slightly higher (<5.1%). Measurement of heptafluoropropane concentrations in blood showed no significant accumulation for repeated functional residual capacity measurements within short time periods. After lung lavage, the functional residual capacity decreased from 20.9 mL/kg to 14.5 mL/kg (p<.05) despite increased ventilatory pressures, and lung clearance index (p<.001) and moment ratios (p<.01) increased significantly due to uneven alveolar ventilation. In healthy lungs, the increase in peak inflation pressure and positive end-expiratory pressure by 3-4 cm H2O had only a moderate effect on functional residual capacity (20.9+/-8.6 vs. 26.0+/-11.9 mL/kg, p=.17) and no effect on ventilatory homogeneity, whereas in surfactant-depleted lungs the functional residual capacity increased from 14.5+/-6.7 mL/kg to 29.9+/-12.6 mL/kg (p<.001) and lung clearance index and moment ratios decreased significantly (p < .01).

CONCLUSIONS

Heptafluoropropane is a suitable tracer gas for precise functional residual capacity measurements tested in vitro and allows for reproducible measurements in ventilated small lungs without any adverse effects on mechanical ventilation. The sensitivity of the method is sufficiently high to demonstrate the effect of changes in ventilatory settings on the functional residual capacity and ventilation homogeneity.