Positive Bubble Study in Severe COVID-19 Indicates the Development of Anatomical Intrapulmonary Shunts in Response to Microvascular Occlusion

Contribution of intrapulmonary shunt to the pathogenesis of profound hypoxaemia in viral infection: a mechanistic discussion with an illustrative case

Background

The formation of anastomoses between the pulmonary arteries and pulmonary veins, or the pulmonary and the bronchial circulation, is part of normal foetal lung development. They persist in approximately 30% of adults at rest, and open in almost all adults during exertion. Blood flowing through these anastomoses bypasses the alveolar surface and increases in such shunting can thus cause hypoxaemia. This is now known to contribute to the pathogenesis of hypoxaemia in COVID-19 disease. We here provide evidence to support a similar role in influenza A infection.

Illustrative case presentation

We describe a case of influenza A infection associated with severe hypoxaemia, poorly responsive to supplemental oxygen and which worsened following the application of continuous positive airway pressure (CPAP), despite the presence of a normal physical examination, chest radiograph and echocardiogram. This combination suggests a significant intrapulmonary (extra-alveolar) shunt as a cause of the severe hypoxaemia. The shunt fraction was estimated to be approximately 57%.

Discussion and conclusion

Intrapulmonary vascular shunts can contribute substantially to hypoxaemia in viral infection. Seeking to understand the pathogenesis of observed hypoxaemia can help guide respiratory therapy. Mechanistic research may suggest novel therapeutic targets which could assist in avoiding intubation and mechanical ventilatory support.

Echocardiographic detection of transpulmonary bubble transit during coronavirus-2019 disease (COVID-19)

Background

Transpulmonary bubble transit (TPBT) detected with contrast echocardiography is reported as a sign of intrapulmonary arteriovenous shunt. However, its pathological meaning is not clear during coronavirus-2019 disease (COVID-19) related acute respiratory distress syndrome (ARDS). Our aim was to determine the prevalence and clinical significance of TPBT detection during COVID-19 related ARDS.

Methods

We carried out a prospective observational study performed in a high complexity intensive care unit from Argentina. Patients with COVID-19 related ARDS underwent transthoracic echocardiography with saline contrast. Moderate-to-large TPBT was defined as right-to-left passage of at least twelve bubbles to left chambers after at least three cardiac cycles and complete opacification of the right atrium.

Results

We analyzed the results of 28 patients (24 men and 4 women). Seventy-five percent of the patients received invasive mechanical ventilation. Moderate-to-large TPBT was detected in 1 patient (3.5%). Among the 27 patients without significant TPBT, 23 had no TPBT and 4 had a minor TPBT. TPBT was not associated with invasive mechanical ventilation requirement (p  = 0.5737) nor in-hospital mortality (p  = 1).

Conclusions

TPBT was not associated with severe hypoxemia or invasive mechanical ventilation requirement, although more studies are needed to further clarify its contributing role in COVID-19 hypoxemia.

Perspective: The Case for Acute Large Vessel Ischemic Stroke in COVID-19 Originating Within Thrombosed Pulmonary Venules

The main burden of SARS-CoV-2 falls on the lungs but neurological manifestations, the most disabling of which are strokes and which correlate with disease severity, are common. We proffer a novel mechanism for acute COVID-19 stroke whereby pulmonary vein clots developing within the characteristic pulmonary intravascular thrombotic lesions can embolize to the brain. Appreciation of this mechanism requires an understanding of the tricompartmental model of lung parenchyma oxygenation (the alveolus, the bronchial artery, and the pulmonary artery), all of which are compromised in COVID-19. Of these 3 sources, the bronchial artery plays a crucial role in COVID-19 stroke because the unique collaterals from bronchial artery to pulmonary vein which exist under normal physiological conditions (and which maintain venous patency when the pulmonary artery is blocked by embolus) are occluded, thus leading to venular thrombosis in the presence of hypercoagulability. Dislodgement of clots from this source translocates the pathology to the brain and is a disease mechanism, formerly rare, which may account for many cases of large vessel occlusion stroke in COVID-19. This mechanism extends the concept of cardioembolic stroke from endocardium retrogradely into the pulmonary circulation with which the left cardiac chambers lie in direct continuity, and which is an accepted stroke mechanism under other circumstances such as lung lobectomy, where surgical ligation of the pulmonary vein creates a blind sac from which thrombi can embolize. The proposed model is supported by postmortem studies which have demonstrated venular thrombosis and by case reports of pulmonary vein thrombosis in COVID-19. This concept provides a more plausible cause for COVID-19 associated large vessel occlusion stroke than other putative mechanisms, such as cerebral endotheliitis, cytokine storm, and hypercoagulopathy, although it is acknowledged that the latter mechanism contributes to the genesis of pulmonary vein clots. Recognizing that extrapulmonary manifestations including stroke arise within thrombosed pulmonary veins is key to understanding of neurological manifestations of SARS-CoV-2 infection.

Pulmonary surfactant and COVID-19: A new synthesis

Chapter 1

COVID-19 pathogenesis poses paradoxes difficult to explain with traditional physiology. For instance, since type II pneumocytes are considered the primary cellular target of SARS-CoV-2; as these produce pulmonary surfactant (PS), the possibility that insufficient PS plays a role in COVID-19 pathogenesis has been raised. However, the opposite of predicted high alveolar surface tension is found in many early COVID-19 patients: paradoxically normal lung volumes and high compliance occur, with profound hypoxemia. That 'COVID anomaly' was quickly rationalised by invoking traditional vascular mechanisms-mainly because of surprisingly preserved alveolar surface in early hypoxemic cases. However, that quick rejection of alveolar damage only occurred because the actual mechanism of gas exchange has long been presumed to be non-problematic, due to diffusion through the alveolar surface. On the contrary, we provide physical chemical evidence that gas exchange occurs by an process of expansion and contraction of the three-dimensional structures of PS and its associated proteins. This view explains anomalous observations from the level of cryo-TEM to whole individuals. It encompasses results from premature infants to the deepest diving seals. Once understood, the COVID anomaly dissolves and is straightforwardly explained as covert viral damage to the 3D structure of PS, with direct treatment implications. As a natural experiment, the SARS-CoV-2 virus itself has helped us to simplify and clarify not only the nature of dyspnea and its relationship to pulmonary compliance, but also the fine detail of the PS including such features as water channels which had heretofore been entirely unexpected.

Chapter 2

For a long time, physical, colloid and surface chemistry have not intersected with physiology and cell biology as much as we might have hoped. The reasons are starting to become clear. The discipline of physical chemistry suffered from serious unrecognised omissions that rendered it ineffective. These foundational defects included omission of specific ion molecular forces and hydration effects. The discipline lacked a predictive theory of self-assembly of lipids and proteins. Worse, theory omitted any role for dissolved gases, O2, N2, CO2, and their existence as stable nanobubbles above physiological salt concentration. Recent developments have gone some way to explaining the foam-like lung surfactant structures and function. It delivers O2/N2 as nanobubbles, and efflux of CO2, and H2O nanobubbles at the alveolar surface. Knowledge of pulmonary surfactant structure allows an explanation of the mechanism of corona virus entry, and differences in infectivity of different variants. CO2 nanobubbles, resulting from metabolism passing through the molecular frit provided by the glycocalyx of venous tissue, forms the previously unexplained foam which is the endothelial surface layer. CO2 nanobubbles turn out to be lethal to viruses, providing a plausible explanation for the origin of 'Long COVID'. Circulating nanobubbles, stable above physiological 0.17 M salt drive various enzyme-like activities and chemical reactions. Awareness of the microstructure of Pulmonary Surfactant and that nanobubbles of (O2/N2) and CO2 are integral to respiratory and circulatory physiology provides new insights to the COVID-19 and other pathogen activity.