Early in the course of Delta variant COVID-19, patients with respiratory failure have relatively normal lung compliance and elevated dead-space ventilation, a hallmark of diffuse pulmonary microvascular thrombosis (1–3). Autopsy studies of patients who died of COVID-19 have documented microthrombi in the pulmonary circulation (4–7). These alterations are driven by a systemic prothrombotic state compounded by fibrinolysis shutdown, leading to extensive small vessel thrombosis and major thromboembolic events (3,4). In hospitalized patients with COVID-19 who are not yet critically ill, therapeutic anticoagulation has been shown to improve outcomes (8), whereas if not initiated until the patients become critically, the benefit of therapeutic anticoagulation is lost, as such patients have already formed a significant clot burden (8–11). In patients with COVID-19, ventilation/perfusion (V/Q) mismatch is observed in patients without massive or submassive pulmonary embolism (PE), suggesting diffuse pulmonary small vessel/microthrombotic occlusion, consistent with the observations seen uniformly in COVID-19 autopsy specimens (12,13).
For these reasons, a proof-of-concept study was conducted using thrombolysis with catheter-directed tissue plasminogen activator (tPA), with pre- and post-tPA pulmonary angiography imaging and image analysis using the Syngo iFlow software to visualize perfusion deficits and reperfusion/therapeutic effects of tPA.
The study objective was to determine whether thrombolytic therapy would improve microvascular pulmonary perfusion due to microthrombosis in patients with severe COVID-19 infection requiring mechanical ventilation (MV).
The ethics and research committee at the Hospital General de México “Dr. Eduardo Liceaga” approved a prospective, open-label, compassionate 15-patient study, authorization number DI-222-2020, registered on ClinicalTrials.gov (NCT04926428). Due to the severity of the illness, the legally authorized representative signed informed consent in all patients enrolled in the study.
Selective catheter-directed thrombolysis with tPA (alteplase, Actilyse Boehringer Ingelheim Promeco Sa de CV, Mexico City, Mexico) was performed in each main pulmonary artery under fluoroscopic guidance using right common femoral vein access, and images were acquired and analyzed using Syngo iFlow. The procedure was executed in a hybrid operating room equipped with Siemens Artis Zeego equipment. A dose of 0.5 mg/kg of tPA was administered transcatheter divided into two doses, one for each pulmonary artery in a continuous infusion for 30 minutes (total time 1 hr). Immediate angiographic assessment was performed using the same protocol. The pressure and velocity of the contrast during the pre- and post-tPA angiography were controlled using an automated injector. If a macrovascular thrombus (segmental, subsegmental, or aortic thrombosis) was found during the procedure, the Syngo IFlow analysis was omitted.
Vascular reperfusion was evaluated using 2D perfusion angiography (2DPA), which creates a 2D color map and a time density curve (TDC) of the digital subtraction angiography (DSA) images. Image analysis was performed using the Siemens Healthineers Syngo WP post-processing iFlow software (Version VD20B; Siemens Healthcare, Erlangen, Germany). First, a 2DPA color map was created from the DSA images. Then, the TDC of the contrast input and output in a selected region of interest (ROI) was extracted from this color map. The ROI was located between the origin of each pulmonary artery and the subsegmental artery of the affected pulmonary segment. Finally, the ROI TDC was exported to a comma-separated value file and imported into MATLAB 2018a (Version 9.4; MathWorks, Natick, MA) for measurement extraction. Five measurements were calculated from the TDC: time of arrival, time to peak (TTP), mean transit time, area under the curve (AUC), and washout rate. The interpretation of the angiography and iFlow images was performed by two independent interventional radiologists; if there was a disagreement, a third interventional radiologist was consulted to obtain consensus. Coagulation status (partial thromboplastin time [PTT], prothrombin time [PT]/international normalized ratio [INR], thrombin time [TT], fibrinogen, d-dimer) and oxygenation assessment (Pao2/Fio2, oxygen saturation in arterial blood [Sao2]/Fio2, Pao2, Paco2, Fio2, ventilator settings including compliance) were measured to evaluate the clinical impact and its correlation with the acquired images.
Patients were eligible for inclusion in the study if they met the prespecified pulmonary and coagulation criteria. The pulmonary inclusion criteria were patients with severe acute respiratory syndrome coronavirus 2 infection, 18–75 years old, requiring endotracheal intubation, MV with a persistent Fio2 requirement of 70% or higher and Pao2/Fio2 ratio (or imputed ratio) less than 150 for more than 4 hours. The coagulation inclusion criteria were International Society on Thrombosis and Haemostasis score greater than 5, and presence of a d-dimer greater than 1,200, with viscoelastic testing using rotational thromboelastometry (Instrumentation Laboratories, Mexico City, Mexico) showing both hypercoagulability (EXTEM amplitude at 5 min > 65 FIBTEM > 30) and hypofibrinolysis (EXTEM maximum lysis < 8%).
Ischemic cardiovascular disease, presence of abnormal neurologic examination, active bleeding, myocardial infarction within the previous 3 weeks or cardiac arrest during hospitalization, cardiac tamponade, endocarditis, uncontrolled hypertension (systolic blood pressure > 185 mm Hg or diastolic blood pressure > 110 mm Hg), history of stage 4 cancer, history of brain tumor or cerebral arteriovenous malformation or ruptured aneurysm, major surgery or major trauma within the previous 2 weeks, pregnancy, fibrinogen less than 200 mg/dL, blood dyscrasias, or thrombocytopenia (platelet count < 30 × 10^3/uL).
Presence of macrovascular thrombosis (segmental or subsegmental pulmonary embolism or aortic thrombosis) precluded patients from iFlow analysis because the changes in perfusion areas using iFlow technology would cause bias in interpretation; however, these patients received in situ thrombolysis for outcome measures.
Descriptive statistics were presented as numbers and percentages for categorical variables and as means with sds for continuous variables. The Wilcoxon test was used to determine the difference between two-related samples (which did not meet normality criteria), while a t test was used for continuous variables (standard). One-way analysis of variance was used to compare the means of three samples that met the normality criteria, while the Friedman test was used for samples that did not meet this criterion. Cochrane’s Q test was used to compare the three groups with dichotomous variables. Statistical significance was set at p value of less than 0.05. Agreement between observations was evaluated using contingency tables and the Kappa Cohen index, while the overall agreement was evaluated using the Fleiss Kappa coefficient. For statistical analysis, Microsoft Excel (Version 2016; Microsoft Corporation, Mexico City, Mexico) (Computer Software) and IBM SPSS (Version 25; IBM, Mexico City, Mexico) (Computer Software) were used.
Descriptive Angiography Findings
Eighteen patients were consented for this study. Three patients died before the intervention, with the remaining 15 patients undergoing the study protocol. The characteristics of the patients are summarized in Table 1. Five patients (33.3%) had macrovascular thrombosis on initial angiography (one segmental and three subsegmental PE, one aortic thrombus) and therefore were excluded from the study and received standard of care for their macrothrombotic findings. The remaining 10 patients underwent angiography with catheter-directed tPA thrombolysis for severe COVID-19 respiratory failure. Of the 10 patients who underwent angiography and catheter-directed pulmonary tPA, seven completed the iFlow protocol for analysis due to technical issues with implementing use of the new technology at our institution.
TABLE 1. -
Characteristics of the Patients
||Total (n = 10)
|Sex = male
|Body mass index (kg/m2)
|Chronic obstructive pulmonary disease
|Number of comorbidities
|Acute kidney injury
|Renal replacement therapy
|Acute Physiology and Chronic Health Evaluation II
|Sequential Organ Failure Assessment
|International Society on Thrombosis and Haemostasis disseminated intravascular coagulation score
|Polymerase chain reaction (mg/L)
Categorical variables are expressed as n (%), while numerical variables are expressed as median (interquartile range).
From the pulmonary angiography reports of the 10 patients without macrothrombosis, nine (90%) of the subjects had peripheral filling defects suggestive of microvascular thrombosis. Bilateral involvement was evident in three (33.3%) of the nine subjects with peripheral filling defects. Eight patients (80%) had immediate imaging evidence of improved perfusion (reduced filling defects) after catheter-directed tPA, six patients (60%) had partial improvement of filling defects post-thrombolysis, and three (30%) had near-complete resolution of filling defects. One patient had no improvement in filling defects.
When the pre- and post-angiography values were compared with iFlow, a good agreement was found between the observations by both methods (ƙ = 0.714) (Figs. 1 and 2).
Pre-treatment was compared with the post-treatment of healthy and diseased lung tissue to identify differences in the PEAK ROI/PEAK reference (REF), TTP, and AUC ROI/AUC REF in phase 1 and phase 2. A statistically significant difference was identified in the TTP of healthy tissue in phase 1, medium density (MD) –4.18 ± 0.685 (p = 0.092) (Supplemental Material 1, https://links.lww.com/CCX/A959), and in the AUC ROI/AUC REF of healthy tissue in phase 2, MD –0.09 ± 0.166 (p = 0.003). When we compared the same values grouped by tissue filling defect status, phase, and affected lung segment, statistically significant differences were identified in the AUC ROI/AUC REF of tissue with filling defects when compared with tissue without filling defects in phase 2 of the middle (p = 0.041) and lower (p = 0.013) segments and in the PEAK ROI/PEAK REF of healthy tissue in phase 2 of the lower segment (p = 0.041). No statistically significant differences were found in the other comparisons (Supplemental Material 2, https://links.lww.com/CCX/A960).
No statistically significant difference was found when the MD of the PEAK ROI/PEAK REF, TTP, and AUC ROI/AUC REF values of the pre-treatment and post-treatment, tissue, with filling defects and tissue without filling defects were compared.
The Pao2/Fio2 values immediately after the procedure and after 48 hours were significantly higher than the values before the procedure (p = 0.001 and p = 0.005, respectively). In addition, statistically significant differences were found in d-dimer (p = 0.007), Fio2 (p = 0.002), and Sao2/Fio2 (p = 0.045) taken before the procedure, immediately after the procedure, and at 48 hours post-procedure, as well as in the number of patients who required prone positioning before the procedure, immediately after the procedure, and at 48 hours after the procedure (p = 0.002). No statistically significant differences were found in the values of PTT, PT, INR, TT, fibrinogen, Pao2, Sao2, and Paco2 (Table 2). Six (60%) of the study patients died.
TABLE 2. -
Clinical or Biochemical Characteristics Before and After the Procedure
|Clinical or Biochemical Characteristic
||Pre, Mean (sd)
||Post, Mean (sd)
||48 hr, Mean (sd)
|Partial thromboplastin time
|International normalized ratio
|Dilute thrombin time
Sao2 = oxygen saturation in arterial blood.
A key clinical feature of severe COVID-19 is a highly prothrombotic state linked to excess arterial and venous microvascular and macrovascular thromboses (13–21). Patients who present with COVID-19 respiratory failure have preserved lung compliance early despite profound respiratory failure, consistent with V/Q mismatch from pulmonary microvascular thromboses. The high mortality of critically ill patients with COVID-19 infection, the presence of microvascular thrombosis on autopsy, the efficacy of anticoagulation in treating moderate disease but lack of efficacy once severe disease has occurred, collectively suggest that small pulmonary vessel thrombosis is contributory, if not the predominant reason, for the V/Q mismatch observed in COVID-19 patients (14).
Our study shows in severe COVID-19 respiratory failure, using pulmonary angiography and iFlow technology, that there is imaging evidence of pulmonary microvascular perfusion defects that can be reversed, at least partially, through thrombolysis with tPA, and this corresponds with clinical improvement in oxygenation. These findings, in the context of the above literature on thrombotic phenomena in COVID patients, are highly suggestive that the peripheral filling defects seen in our series are due to microvascular thrombosis, rather than some other phenomena (e.g., vasoconstriction).
Our findings are consistent with the recently published STudy of Alteplase for Respiratory failure in SARS-Cov2 COVID-19 trial, which was the first prospective randomized controlled trial of fibrinolytic therapy in COVID-19 respiratory failure. This study showed significantly improved oxygenation, half the mortality and 12 more ventilator-free days in the tPA bolus group (underpowered for significance), with an acceptable risk profile when patients were carefully selected with no intracranial hemorrhages (22).
This study has several limitations. First, pulmonary arterial infusion is resource intensive and the dynamics of pulmonary perfusion have unique characteristics that require consideration of multiple complex variables in order to translate our findings obtained by iFlow before and after noncatheter-directed thrombolysis (23–26). However, delivery of tPA via a central venous catheter where the tip is near the cavoatrial junction, just proximal to the pulmonary vessels, may provide similar effects. Second, this was an uncontrolled study with respect to patients, although the pre- and post-tPA nature of the imaging served as an internal control. Finally, the impact on clinical outcomes was not feasible due to the lack of a matched control group.
In sum, our observational study provides novel insight with direct imaging evidence of the presence of perfusion alterations that improve with the application of catheter-directed thrombolysis via tPA in patients with severe COVID-19 respiratory failure.
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