Functional residual capacity and absolute lung volume

Effect of prone positioning on end-expiratory lung volume, strain and oxygenation change over time in COVID-19 acute respiratory distress syndrome: A prospective physiological study

BACKGROUND

Prone position (PP) is a recommended intervention in severe classical acute respiratory distress syndrome (ARDS). Changes in lung resting volume, respiratory mechanics and gas exchange during a 16-h cycle of PP in COVID-19 ARDS has not been yet elucidated.

METHODS

Patients with severe COVID-19 ARDS were enrolled between May and September 2021 in a prospective cohort study in a University Teaching Hospital. Lung resting volume was quantitatively assessed by multiple breath nitrogen wash-in/wash-out technique to measure the end-expiratory lung volume (EELV). Timepoints included the following: Baseline, Supine Position (S1); start of PP (P0), and every 4-h (P4; P8; P12) until the end of PP (P16); and Supine Position (S2). Respiratory mechanics and gas exchange were assessed at each timepoint.

MEASUREMENTS AND MAIN RESULTS

40 mechanically ventilated patients were included. EELV/predicted body weight (PBW) increased significantly over time. The highest increase was observed at P4. The highest absolute EELV/PBW values were observed at the end of the PP (P16 vs S1; median 33.5 ml/kg [InterQuartileRange, 28.2-38.7] vs 23.4 ml/kg [18.5-26.4], p < 0.001). Strain decreased immediately after PP and remained stable between P4 and P16. PaO2/FiO2 increased during PP reaching the highest level at P12 (P12 vs S1; 163 [138-217] vs 81 [65-97], p < 0.001). EELV/PBW, strain and PaO2/FiO2 decreased at S2 although EELV/PBW and PaO2/FiO2 were still significantly higher as compared to S1. Both absolute values over time and changes of strain and PaO2/FiO2 at P16 and S2 versus S1 were strongly associated with EELV/PBW levels.

CONCLUSION

In severe COVID-19 ARDS, EELV steadily increased over a 16-h cycle of PP peaking at P16. Strain gradually decreased, and oxygenation improved over time. Changes in strain and oxygenation at the end of PP and back to SP were strongly associated with changes in EELV/PBW. Whether the change in EELV and oxygenation during PP may play a role on outcomes in COVID-ARDS deserves further investigation.

CLINICAL TRIAL REGISTRATION

[www.ClinicalTrials.gov], identifier [NCT04818164].

Bias field correction in hyperpolarized 129 Xe ventilation MRI using templates derived by RF-depolarization mapping

PURPOSE

To correct for RF inhomogeneity for in vivo 129 Xe ventilation MRI using flip-angle mapping enabled by randomized 3D radial acquisitions. To extend this RF-depolarization mapping approach to create a flip-angle map template applicable to arbitrary acquisition strategies, and to compare these approaches to conventional bias field correction.

METHODS

RF-depolarization mapping was evaluated first in digital simulations and then in 51 subjects who had undergone radial 129 Xe ventilation MRI in the supine position at 3T (views = 3600; samples/view = 128; TR/TE = 4.5/0.45 ms; flip angle = 1.5; FOV = 40 cm). The images were corrected using newly developed RF-depolarization and templated-based methods and the resulting quantitative ventilation metrics (mean, coefficient of variation, and gradient) were compared to those resulting from N4ITK correction.

RESULTS

RF-depolarization and template-based mapping methods yielded a pattern of RF-inhomogeneity consistent with the expected variation based on coil architecture. The resulting corrected images were visually similar, but meaningfully distinct from those generated using standard N4ITK correction. The N4ITK algorithm eliminated the physiologically expected anterior-posterior gradient (-0.04 ± 1.56%/cm, P < 0.001). These 2 newly introduced methods of RF-depolarization and template correction retained the physiologically expected anterior-posterior ventilation gradient in healthy subjects (2.77 ± 2.09%/cm and 2.01 ± 2.73%/cm, respectively).

CONCLUSIONS

Randomized 3D 129 Xe MRI ventilation acquisitions can inherently be corrected for bias field, and this technique can be extended to create flip angle templates capable of correcting images from a given coil regardless of acquisition strategy. These methods may be more favorable than the de facto standard N4ITK because they can remove undesirable heterogeneity caused by RF effects while retaining results from known physiology.

Myths and Misconceptions of Airway Pressure Release Ventilation: Getting Past the Noise and on to the Signal

In the pursuit of science, competitive ideas and debate are necessary means to attain knowledge and expose our ignorance. To quote Murray Gell-Mann (1969 Nobel Prize laureate in Physics): “Scientific orthodoxy kills truth”. In mechanical ventilation, the goal is to provide the best approach to support patients with respiratory failure until the underlying disease resolves, while minimizing iatrogenic damage. This compromise characterizes the philosophy behind the concept of “lung protective” ventilation. Unfortunately, inadequacies of the current conceptual model–that focuses exclusively on a nominal value of low tidal volume and promotes shrinking of the “baby lung” - is reflected in the high mortality rate of patients with moderate and severe acute respiratory distress syndrome. These data call for exploration and investigation of competitive models evaluated thoroughly through a scientific process. Airway Pressure Release Ventilation (APRV) is one of the most studied yet controversial modes of mechanical ventilation that shows promise in experimental and clinical data. Over the last 3 decades APRV has evolved from a rescue strategy to a preemptive lung injury prevention approach with potential to stabilize the lung and restore alveolar homogeneity. However, several obstacles have so far impeded the evaluation of APRV’s clinical efficacy in large, randomized trials. For instance, there is no universally accepted standardized method of setting APRV and thus, it is not established whether its effects on clinical outcomes are due to the ventilator mode per se or the method applied. In addition, one distinctive issue that hinders proper scientific evaluation of APRV is the ubiquitous presence of myths and misconceptions repeatedly presented in the literature. In this review we discuss some of these misleading notions and present data to advance scientific discourse around the uses and misuses of APRV in the current literature.

Accuracy of the end-expiratory lung volume measured by the modified nitrogen washout/washin technique: a bench study

Background

The functional residual capacity (FRC) determines the oxygenating capacity of the lung and is heavily affected in the clinical context of the acute respiratory distress syndrome. Nitrogen-wash-in/wash-out methods have been used to measure FRC. These methods have rarely been validated against exactly known volumes. The aim of the study was to assess the accuracy and precision of the N₂ washout/washin method in measuring FRC, by comparing it with set volumes in a lung simulator.

Methods

We conducted a diagnostic bench study in the Intensive Care Unit and Radiology Department of a tertiary hospital in Switzerland. Using a fully controllable high fidelity lung simulator (TestChest®), we set the functional residual capacity at 1500 ml, 2000 ml and 2500 ml and connected to the GE Carestation respirator, which includes the nitrogen washout/washin technique (INview™ tool). FRC was then set to vary by different levels of PEEP (5, 8, 12 and 15 cmH₂O). The main outcome measures were bias and precision of the TestChest® when compared to the results from the washout/washin technique, according to the results of a Bland Altman Analysis. We verified our findings with volumetric computed tomography.

Results

One hundred and thirty-five nitrogen-wash-in/wash-out measurements were taken at three levels of F_IO₂ (0.4, 0.5, 0.6). The CT volumetry reproduced the set end-expiratory volumes at the Simulator with a bias of 4 ml. The nitrogen-wash-in/wash-out method had a bias of 603 ml with acceptable limits of agreement (95% CI 252 to − 953 ml). Changes were detected with a concordance rate of 97%.

Conclusions

We conclude that the TestChest® simulator is an accurate simulation tool, concerning the simulation of lung volumes. The nitrogen wash-in/wash out method correlated positively with FRC changes, despite a relatively large bias in absolute measurements. The reference volumes in the lung simulator verified with CT volumetry were very close to their expected values. The reason for the bias could not be determined.

Effect of positive end-expiratory pressure on functional residual capacity in two experimental models of acute respiratory distress syndrome

OBJECTIVE

Measurement of positive end-expiratory pressure (PEEP)-induced recruitment lung volume using passive spirometry is based on the assumption that the functional residual capacity (FRC) is not modified by the PEEP changes. We aimed to investigate the influence of PEEP on FRC in different models of acute respiratory distress syndrome (ARDS).

METHODS

A randomized crossover study was performed in 12 pigs. Pulmonary (n = 6) and extra-pulmonary (n = 6) ARDS models were established using an alveolar instillation of hydrochloric acid and a right atrium injection of oleic acid, respectively. Low (5 cmH2O) and high (15 cmH2O) PEEP were randomly applied in each animal. FRC and recruitment volume were determined using the nitrogen wash-in/wash-out technique and release maneuver.

RESULTS

FRC was not significantly different between the two PEEP levels in either pulmonary ARDS (299 ± 92 mL and 309 ± 130 mL at 5 and 15 cmH2O, respectively) or extra-pulmonary ARDS (305 ± 143 mL and 328 ± 197 mL at 5 and 15 cmH2O, respectively). The recruitment volume was not significantly different between the two models (pulmonary, 341 ± 100 mL; extra-pulmonary, 351 ± 170 mL).

CONCLUSIONS

PEEP did not influence FRC in either the pulmonary or extra-pulmonary ARDS pig model.

Clinical and experimental validation of a capnodynamic method for end-expiratory lung volume assessment

INTRODUCTION

Lung protective ventilation can decrease post-operative pulmonary complications. The aim of this study was to evaluate a capnodynamic method estimating effective lung volume (ELV) as a proxy for end-expiratory lung volume in response to PEEP changes in patients, healthy subjects and a porcine model.

METHODS

Agreement and trending ability for ELV in anaesthetized patients and agreement in awake subjects were evaluated using nitrogen multiple breath wash-out/in and plethysmography as a reference respectively. Agreement and trending ability were evaluated in pigs during PEEP elevations with inert gas wash-out as reference.

RESULTS

In anaesthetized patients bias (95% limits of agreement [LoA]) and percentage error (PE) at PEEP 0 cm H₂ O were 133 mL (-1049 to 1315) and 71%, at PEEP 5 cm H₂ O 161 mL (-1291 to 1613 mL) and 66%. In healthy subjects: 21 mL (-755 to 796 mL) and 26%. In porcines, at PEEP 5-20 cm H₂ O bias decreased from 223 mL to 136 mL LoA (34-412) to (-30 to 902) and PE 29%-49%. Trending abilities in anaesthetized patients and porcines were 100% concordant.

CONCLUSION

The ELV-method showed low bias but high PE in anaesthetized patients. Agreement was good in awake subjects. In porcines, agreement was good at lower PEEP levels. Concordance related to PEEP changes reached 100% in all settings. This method may become a useful trending tool for monitoring lung function during mechanical ventilation, if findings are confirmed in other clinical contexts.

Pre-extubation functional residual capacity and risk of extubation failure among patients with hypoxemic respiratory failure

Hypoxemic respiratory failure is usually accompanied with a certain extent of consolidation and alveolar derecruitment, which may still be present even after the patients have achieved the status of readiness to extubate. Functional residual capacity (FRC) is an indicator of lung aeration. This study aimed to evaluate whether pre-extubation FRC is associated with the risk of extubation failure in patients with hypoxemic respiratory failure. We prospectively included 92 patients intubated for hypoxemic respiratory failure. We used a technique based on a nitrogen multiple breath washout method to measure FRC before the planned extubation. The median FRC before extubation was 25 mL/kg (Interquartile range, 20–32 mL/Kg) per predicted body weight (pBW). After extubation, 20 patients (21.7%) were reintubated within 48 hours. The median FRC was higher in the extubation success group than in the extubation failure group (27 versus 21 mL/Kg, p < 0.001). Reduced FRC was associated with higher risk of extubation failure (odds ratio, 1.14 per each decreased of 1 mL/Kg of FRC/pBW, 95% CI, 1.05–1.23, p = 0.002). In conclusion, pre-extubation FRC is associated with the risk of extubation failure. Reduced FRC may be incorporated into the traditional risk factors to identify patients at high risk for extubation failure.

Functional residual capacity in beagle dogs with and without acute respiratory distress syndrome

BACKGROUND

Traditionally, the choice of tidal volume for mechanical ventilation was based on body weight (BW) and usually, predicted BW was used to correct actual BW inter-individual variations in obesity and muscle weight. The method of selecting tidal volume depended on the fact that normal lung volumes, especially functional residual capacity (FRC), were mainly determined by height (indirectly by predicted BW), sex and age in healthy persons. However, FRCs in patients with acute respiratory distress syndrome (ARDS) might not abide by the same rule and be significantly different from each other in patients with the same height and sex. We hypothesized that FRC was determined by body length (surrogate for predicted BW) and age in healthy male beagle dogs but not in lung injured ones.

METHODS

A total of 24 dogs were recruited and ARDS model was induced by intravenous injection of oleic acid. FRC was measured by chest computer tomography. Blood gas analysis, extra vascular lung water and respiratory system mechanics were tested at baseline and post-lung injury. Age, body length and actual BW were also recorded before experiments.

RESULTS

After lung injury, FRC decreased sharply from baseline (414±84) to (214±70) mL. For healthy lungs, FRC could be estimated by the following formula: FRC =21.86 × age (months) + 20.55 × body length (cm) - 1,337.98 (P<0.05), while for injured lungs, the formula of multiple linear regression was invalid (P=0.305).

CONCLUSIONS

FRC was linearly related to body length in healthy dogs but not in lung injured ones. The traditional view of setting tidal volume based on predicted BW should be challenged cautiously.

The influence of obturators on the respiration of patients with maxillary defects: a clinical study

The study evaluated the effects of obturators on respiratory function by analyzing the changes in nasal anatomic structures and physiologic function in maxillectomy patients with and without obturators. Twenty-six patients who underwent maxillectomy were chosen and rehabilitated with obturators by a single maxillofacial prosthodontist. The geometric shape of the nasal cavity, the nasal airway resistance, and the ratio of residual volume to total lung capacity (RV/TLC) were evaluated using acoustic rhinometry, rhinomanometry, and a pulmonary function test apparatus, respectively. All patients were tested twice, with and without their obturators. The results were statistically analyzed with a paired t-test. The nasal cavities (0–7 cm to the anterior nostril) of the patients with obturators had a significantly smaller volume ([-8.92, -0.60], P = 0.027), smaller effective nasal cross-sectional area MCA2 ([-3.80, -1,81], P<0.0001), increased airflow in the nasal cavity ([17.76, 147.39], P = 0.015), reduced nasal airway resistance ([-0.11, -0.02], P = 0.009), and reduced RV/TLC ([-5.32, -1.30], P = 0.004) compared with the patients without obturators. According to the results of this study, obturators can improve respiratory function by effectively decreasing the volume of enlarged nasal cavities as well as the nasal air resistance and volume of anatomical dead space after maxillectomy.