Background & objective ARDS is characterised by bilateral pulmonary infiltrates and refractory hypoxemia attributed to V/Q mismatch. We used dynamic CT to characterise changes in lung composition, regional perfusion and tissue distribution in patients with ARDS in comparison to healthy subjects. Methods The Fick principle was applied to serial attenuation measurements constructed from sequential CT images acquired during the passage of a bolus of iodinated contrast medium in healthy subjects (n=3) and patients with ARDS (n=11). Perfusion was calculated by the Mullani-Gould method and mapped throughout both lungs. Gradients of perfusion and tissue density against vertical height were constructed. Results In comparison to normal individuals, the tissue component of lungs from patients with ARDS was significantly increased (p<0.05). Blood fraction was unchanged. There was a discernable gradient in tissue density from non dependent to dependent regions in the patients with ARDS that was significantly different from controls. The proportion of perfusion applied to consolidated areas (ie shunt) correlated significantly (p<0.05) with the severity of hypoxaemia. Conclusions In patients with ARDS there are changes in both lung composition and the distribution of tissue and perfusion that may account in part for the physiological changes that define the syndrome.

Rationale and Objectives Pulmonary vascular control mechanisms are complex and likely to differ between species. We wish to quantify regional perfusion and the effects of gravity using computed tomography. Materials and Methods Sequential density measurements following the administration of a bolus of iodinated contrast medium were acquired from four healthy human subjects and four dogs. Results In humans, perfusion (Q) was linear throughout most of the range of vertical height, with an overall gradient of −2.6% cm−1. However, when perfusion was normalized to “tissue” density (blood plus tissue: sQt), maximum perfusion occurred around the mid-range of vertical height, being 9% (range 1–22%) greater than either the dorsal or ventral extreme. Within discrete transverse axial sections, concentric zones of perfusion centered on blood vessels were demonstrated. The relationship between sQt and vertical height in dogs was distinctly linear, with a gradient of −7.2% cm−1. In dogs, the median gradient of Q was −13.6% cm−1 (range −9.7 to −17.1%). Conclusions Differences in regional pulmonary perfusion, particularly the vertical gradient observed in humans and dogs, may in part reflect anatomic differences between the symmetric dichotomous branching structure of the human pulmonary vasculature and the more asymmetrical structure found in dogs.

Aim: COVID-19 is a known risk factor for pulmonary embolism (PE). In this retrospective, multicenter study, we aimed to determine an optimal D-dimer cutoff to predict PE in hospitalized patients with COVID-19. Materials & methods: A total of 193 patients underwent computerized tomographic pulmonary angiography imaging and were classified into PE positive and negative groups. Physiological, radiological and biochemical parameters were compared and receiver operator curve analysis was conducted to determine a predictive D-dimer threshold. Results: An optimal D-dimer cutoff of 2494 ng/ml was selected (Youden index: 0.906), giving a sensitivity of 100% (95% CI: 100–100) and specificity of 90.62% (95% CI: 90.5–90.8) for predicting PE. Conclusion: We propose that in the absence of other clinical signs, a D-dimer threshold of 2495 ng/ml could be used with high sensitivity and specificity to predict PE in hospitalized patients with COVID-19.

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