Lung Diffusing Capacities (DL ) for Nitric Oxide (NO) and Carbon Monoxide (CO): The Evolving Story

Long-term cognitive and pulmonary functions following a lower versus a higher oxygenation target in the HOT-ICU and HOT-COVID trials: A protocol update

BACKGROUND

The Handling Oxygenation Targets in the Intensive Care Unit (HOT-ICU) trial was a multicentre, randomised, parallel-group trial of a lower oxygenation target (arterial partial pressure of oxygen [PaO2 ] = 8 kPa) versus a higher oxygenation target (PaO2  = 12 kPa) in adult ICU patients with acute hypoxaemic respiratory failure; the Handling Oxygenation Targets in coronavirus disease 2019 (HOT-COVID) tested the same oxygenation targets in patients with confirmed COVID-19. In this study, we aim to evaluate the long-term effects of these oxygenation targets on cognitive and pulmonary function. We hypothesise that a lower oxygenation target throughout the ICU stay may result in cognitive impairment, whereas a higher oxygenation target may result in impaired pulmonary function.

METHODS

This is the updated protocol and statistical analysis plan of two pre-planned secondary outcomes, the long-term cognitive function, and long-term pulmonary function, in the HOT-ICU and HOT-COVID trials. Patients enrolled in both trials at selected Danish sites and surviving to 1 year after randomisation are eligible to participate. A Repeatable Battery for the Assessment of Neuropsychological Status score and a full-body plethysmography, including diffusion capacity for carbon monoxide, will be obtained. The last patient is expected to be included in the spring of 2024.

CONCLUSION

This study will provide important information on the long-term effects of a lower versus a higher oxygenation target on long-term cognitive and pulmonary functions in adult ICU patients with acute hypoxaemic respiratory failure.

Modeling of Red Blood Cells in Capillary Flow Using Fluid-Structure Interaction and Gas Diffusion

Red blood cell (RBC) distribution, RBC shape, and flow rate have all been shown to have an effect on the pulmonary diffusing capacity. Through this study, a gas diffusion model and the immersed finite element method were used to simulate the gas diffusion into deformable RBCs running in capillaries. It has been discovered that when RBCs are deformed, the CO flux across the membrane becomes nonuniform, resulting in a reduced capacity for diffusion. Additionally, when compared to RBCs that were dispersed evenly, our simulation showed that clustered RBCs had a significantly lower diffusion capability.

Long-term effects of lower versus higher oxygenation levels in adult ICU patients - a systematic review

Background

Oxygen therapy is a common treatment in the intensive care unit (ICU) with both potentially desirable and undesirable long‐term effects. This systematic review aimed to assess the long‐term outcomes of lower versus higher oxygenation strategies in adult ICU survivors.

Methods

We included randomised clinical trials (RCTs) comparing lower versus higher oxygen supplementation or oxygenation strategies in adults admitted to the ICU. We searched major electronic databases and trial registers. We included all non‐mortality long‐term outcomes. Prespecified co‐primary outcomes were the long‐term cognitive function measures, the overall score of any valid health‐related quality of life (HRQoL) evaluation, standardised 6‐min walk test, and lung diffusion capacity. The protocol was published and prospectively registered in the PROSPERO database (CRD42021223630).

Results

The review included 17 RCTs comprising 6592 patients, and six trials with 825 randomised patients reported one or more outcomes of interest. We observed no difference in cognitive evaluation via Telephone Interview for Cognitive Status (one trial, 409 patients) (mean score: 30.6 ± 4.5 in the lower oxygenation group vs. 30.4 ± 4.3 in the higher oxygenation group). The trial was judged at overall high risk of bias and the certainty of evidence was very low. Any difference was neither observed in HRQoL measured via EuroQol 5 dimensions 5 level questionnaire and EQ Visual Analogue Score (one trial, 499 patients) (mean score: 70.1 ± 22 in the lower oxygenation group vs. 67.6 ± 22.4 in the higher oxygenation group). The trial was judged as having high risk of bias, the certainty of evidence was very low. No trial reported neither the standardised 6‐min walk test nor lung diffusion test.

Conclusion

The evidence is very uncertain about the effect of a lower versus a higher oxygenation strategy on both the cognitive function and HRQoL. A lower versus a higher oxygenation strategy may have a little to no effect on both outcomes but the certainty of evidence is very low. No evidence was found for the effects on the standardised 6‐min walking test and diffusion capacity test.

Using hyperpolarized 129Xe gas-exchange MRI to model the regional airspace, membrane, and capillary contributions to diffusing capacity

Hyperpolarized ¹²⁹Xe MRI has emerged as a novel means to evaluate pulmonary function via 3D mapping of ventilation, interstitial barrier uptake, and RBC transfer. However, the physiological interpretation of these measurements has yet to be firmly established. Here, we propose a model that uses the three components of ¹²⁹Xe gas-exchange MRI to estimate accessible alveolar volume (V_A), membrane conductance, and capillary blood volume contributions to DL_CO. ¹²⁹Xe ventilated volume (VV) was related to V_A by a scaling factor k_V = 1.47 with 95% confidence interval [1.42, 1.52], relative ¹²⁹Xe barrier uptake (normalized by the healthy reference value) was used to estimate the membrane-specific conductance coefficient k_B = 10.6 [8.6, 13.6] mL/min/mmHg/L, whereas normalized RBC transfer was used to calculate the capillary blood volume-specific conductance coefficient k_R = 13.6 [11.4, 16.7] mL/min/mmHg/L. In this way, the barrier and RBC transfer per unit volume determined the transfer coefficient K_CO, which was then multiplied by image-estimated V_A to obtain DL_CO. The model was built on a cohort of 41 healthy subjects and 101 patients with pulmonary disorders. The resulting ¹²⁹Xe-derived DL_CO correlated strongly (R² = 0.75, P < 0.001) with the measured values, a finding that was preserved within each individual disease cohort. The ability to use ¹²⁹Xe MRI measures of ventilation, barrier uptake, and RBC transfer to estimate each of the underlying constituents of DL_CO clarifies the interpretation of these images while enabling their use to monitor these aspects of gas exchange independently and regionally.NEW & NOTEWORTHY The diffusing capacity for carbon monoxide (DL_CO) is perhaps one of the most comprehensive physiological measures used in pulmonary medicine. Here, we spatially resolve and estimate its key components-accessible alveolar volume, membrane, and capillary blood volume conductances-using hyperpolarized ¹²⁹Xe MRI of ventilation, interstitial barrier uptake, and red blood cell transfer. This image-derived DL_CO correlates strongly with measured values in 142 subjects with a broad range of pulmonary disorders.

Lung diffusing capacity for nitric oxide and carbon monoxide following mild-to-severe COVID-19

A decreased lung diffusing capacity for carbon monoxide (DL_CO ) has been reported in a variable proportion of subjects over the first 3 months of recovery from severe coronavirus disease 2019 (COVID-19). In this study, we investigated whether measurement of lung diffusing capacity for nitric oxide (DL_NO ) offers additional insights on the presence and mechanisms of gas transport abnormalities. In 94 subjects, recovering from mild-to-severe COVID-19 pneumonia, we measured DL_NO and DL_CO between 10 and 266 days after each patient was tested negative for severe acute respiratory syndrome coronavirus 2. In 38 subjects, a chest computed tomography (CT) was available for semiquantitative analysis at six axial levels and automatic quantitative analysis of entire lungs. DL_NO was abnormal in 57% of subjects, independent of time of lung function testing and severity of COVID-19, whereas standard DL_CO was reduced in only 20% and mostly within the first 3 months. These differences were not associated with changes of simultaneous DL_NO /DL_CO ratio, while DL_CO /V_A and DL_NO /V_A were within normal range or slightly decreased. DL_CO but not DL_NO positively correlated with recovery time and DL_CO was within the normal range in about 90% of cases after 3 months, while DL_NO was reduced in more than half of subjects. Both DL_NO and DL_CO inversely correlated with persisting CT ground glass opacities and mean lung attenuation, but these were more frequently associated with DL_NO than DL_CO decrease. These data show that an impairment of DL_NO exceeding standard DL_CO may be present during the recovery from COVID-19, possibly due to loss of alveolar units with alveolar membrane damage, but relatively preserved capillary volume. Alterations of gas transport may be present even in subjects who had mild COVID-19 pneumonia and no or minimal persisting CT abnormalities. TRIAL REGISTRY: ClinicalTrials.gov PRS: No.: NCT04610554 Unique Protocol ID: SARS-CoV-2_DLNO 2020.