Distinct Alterations in Oxygenation, Ion Composition and Acid-Base Balance in Cerebral Collaterals During Large-Vessel Occlusion Stroke

We report a prospective observational single-center study conducted between 12 December 2018 and 31 August 2020, to investigate alterations in arterial blood gas (ABG) parameters within the collateral circulation (distal to the occlusion site) of LVO stroke patients. Endovascular sampling of ischemic blood from collateral blood vessels during the early phase of infarct formation was performed by microcatheter aspiration according to a prespecified protocol during emergency EVT [18‐23]. Systemic arterial blood which was collected from the ipsilateral cervical internal carotid artery (ICA) under physiological flow conditions served as an intraindividual control.

Inclusion criteria were defined as follows: (I) patients aged older than 18 years presenting with disabling first ever acute ischemic stroke which qualifies for EVT according to current guidelines and consensus recommendations [1, 24, 25] and (II) invasive angiographic confirmation of complete LVO of the following anterior circulation vessels: distal internal carotid artery (ICA-T), middle cerebral artery (MCA)-M1 segment or proximal MCA-M2 segment.

Patients were excluded based on the following criteria: (I) noninvasive or invasive angiographic confirmation of bilateral or multifocal vessel occlusions other than defined, (II) invasive angiographic confirmation of residual or restored antegrade CBF at the level of the EVT-qualifying lesion, (III) LVO in conjunction with either ≥ 50% cervical ICA stenosis or ICA dissection, (IV) intraprocedural percutaneous transluminal angioplasty (PTA) or stent implantation and (VI) principle deviations from the interventional protocol and sampling procedure [18‐23]. The flow of patient inclusion and exclusion is presented in Fig. 1.

All endovascular treatments were performed by board-certified neurointerventionalists or supervised neurointerventional fellows on a biplane angiography system (Axiom Artis Q, Siemens Healthcare, Erlangen, Germany). Endovascular access for EVT was established by means of a transfemoral approach through the common femoral artery (CFA) using the modified Seldinger technique. According to our in-house standard of practice, pressurized flush systems (0.9% saline solution with 1 IU/ml unfractionated heparin) were used to avoid thrombus formation on the inner surface of catheters. A detailed description of the standard procedure of mechanical thrombectomy has been published previously [26]. Recanalization of the EVT-qualifying lesion by stent-embolus retrieval was preceded by microcatheter navigation (Neuroslider 27 or 21; Acandis, Pforzheim, Germany) into the mid-insular MCA-M2 segment. The following arterial blood samples were obtained by microcatheter aspiration: (1) distal to the occlusive lesion under persistent ischemic conditions and (2) under systemic physiological blood flow conditions at the cervical ICA level. Specifically, the first sample was obtained as follows: before device deployment and immediately after microcatheter positioning, the precise microcatheter dead space volume was aspirated with a 3 ml Luer lock syringe and discarded. Then, the target sample of 1 ml of ischemic blood from within the collateral circulation was obtained. The procedure of cerebral arterial blood sampling is displayed in supplemental Fig. 1. After termination of the therapeutic steps of EVT, systemic blood samples were obtained from cervical ICA level using the exact same technique as described for ischemic sample material.

Arterial whole-blood samples were collected anaerobically in blood gas syringes containing Ca²⁺-balanced lithium heparin (S-Monovette, Sarstedt, Nümbrecht, Germany) and immediately analyzed (< 5 min) in a point-of-care blood gas analyzer (Rapidpoint 405, Siemens Healthcare Diagnostics, Erlangen, Germany). All syringes were carefully connected to the inlet of the analysis system without trapped air. A final sample volume of 200 µl was assessed for the following parameters: (1) pH, (2) arterial partial pressure of carbon dioxide (p_aCO₂), (3) arterial partial pressure of oxygen (p_aO₂), (4) base deviation in plasma (BE (B)), (5) standard bicarbonate (HCO₃⁻ std), (6) oxygen saturation of hemoglobin (sO₂), (7) Na⁺ ion concentration, (8) K⁺ ion concentration, (9) Na⁺:K⁺ ratio, (10) Ca²⁺ ion concentration, and (11) chloride (Cl⁻) ion concentration.

Prospective data collection included (but was not limited to) the following demographic, clinical, radiological, interventional, and sampling-related variables: age, sex, cerebrovascular risk factors, baseline drug treatment, preprocedural blood pressure and heart rate, previous alteplase administration, National Institute of Health Stroke Scale (NIHSS) at admission and at 48 h, time of non-invasive and angiographic image acquisition, occlusion location, leptomeningeal collateral status, Alberta Stroke Program Early CT Score (ASPECTS) at baseline and at 48 h, time of blood sampling and analysis, time of recanalization and reperfusion status, devices and technical management of EVT, peri-interventional and postinterventional complications, and short-term clinical outcome as assessed by the modified Rankin Scale (mRS) at hospital discharge.

Between 18 December, 2018, and 31 August, 2020, n = 366 consecutive patients who were treated by EVT were assessed for study eligibility according to a prespecified study protocol [18‐20]. N = 48 patients were excluded due to posterior circulation LVO and n = 11 patients due to bilateral or multifocal vessel occlusions. N = 50 patients did not qualify for inclusion upon invasive angiographic imaging either due to spontaneous recanalization or subocclusion with residual antegrade flow before EVT. Microcatheter-aspiration of ischemic blood samples was attempted in n = 257 patients with angiographically proven LVO of the following target sites: intracranial ICA‑T segment, MCA-M1, and proximal MCA-M2 segment, respectively. Microcatheter-aspiration of cerebral blood samples through blood gas syringes succeeded in n = 72 patients (28%). Out of this group, n = 51 consecutive patients (20%) met all a priori defined sampling and interventional criteria of inclusion and entered data analyses. The full patient flow without missing cases is given in Fig. 1. A detailed presentation of the ABG data including temporal, clinical functional and radiological structural correlations is given in Table 1, 2, 3, 4 and 5; a graphical summary is provided in the Supplementary Information.

The clinical, radiological, interventional, and sampling-related characteristics of the study population are presented in Table 1.

Among included patients, n = 34 (66.7%) were female and n = 17 (33.3%) were male. The median age of patients was 78 years (IQR 69–83 years). Arterial hypertension was the most prevalent comorbid disease (n = 45, 88.2%), followed by atrial fibrillation (n = 34, 66.7%). Hypercholesterolemia was found in n = 16 (31.4%) patients. Of the patients 10 (19.6%) had diabetes mellitus at the time of stroke diagnosis, 6 patients (11.8%) were current smokers, 8 patients (15.7%) were former smokers, and n = 37 patients (72.5%) had no smoking history. At the time of hospital admission, n = 45 (88.2%) patients received antihypertensive therapy, n = 13 (25.5%) patients received antiplatelet medication, and n = 20 (39.2%) patients received anticoagulant therapy. The median preprocedural systolic blood pressure was 161 mm Hg (IQR 150–178 mm Hg) and the median preprocedural diastolic blood pressure was 89 mm Hg (IQR 75–99 mm Hg). The median heart rate at baseline was 80 (IQR 71–96) beats per minute. Baseline clinical stroke severity as assessed by the NIHSS at hospital admission was 13 (IQR 8–17). The time of stroke onset was unknown in n = 17 patients (33.3%). Non-contrast computed tomography (NCCT) revealed a median baseline ASPECTS of 9 (IQR 8–9). A total of N = 34 (66.7%) patients were treated by EVT alone and n = 17 patients (33.3%) were treated by EVT with previous intravenous alteplase administration. The median time from symptom onset to groin puncture was 255 min (IQR 181–353 min). Invasive angiography confirmed the persistence of cerebral LVO in all patients and showed n = 15 (29.4%) intracranial ICA occlusions, n = 7 (13.7%) ICA‑T occlusions, n = 19 (37.3%) MCA-M1, and n = 17 (33.3%) MCA-M2 occlusions. The median invasive collateral score was 2 (IQR 0–3) [27]. Successful recanalization as assessed by the extended thrombolysis in cerebral infarction (eTICI) scale (eTICI ≥ 2b50) was achieved in 41 patients (80.4%) [28]. The median number of stent retriever passages required for recanalization was 2 (IQR 1–4). The median time interval from symptom onset to final recanalization was 336 min (IQR 274–462 min). No intraprocedural complications (e.g., thrombus fragmentation, vessel perforation or cerebral air embolism) were observed.

Median ASPECTS on follow-up imaging at 48 h after intervention was 8 (IQR 6–9) [29]. Follow-up CT imaging revealed intracranial hemorrhages as defined by the Heidelberg bleeding classification (HBC) in eight patients (15.7%) [30], three patients (5.9%) showed petechial hemorrhages (HBC classes 1a and 1b), one patient (2%) developed a large space-occupying intraparenchymal hematoma (HBC class 2), and subarachnoid hemorrhages (HBC class 3c) were observed in two patients (3.9%). Clinical stroke severity at 48 h after recanalization was reflected by a median NIHSS score of 9 (IQR 4–16). The median modified Rankin Scale (mRS) score at hospital discharge was 3 (IQR 1–4). Functional independence (mRS ≤ 0–2) was achieved in n = 20 (39.2%) patients and n = 7 patients (13.7%) died during hospital stay. All other patients (n = 44, 86.3%) were discharged either home, to a rehabilitation facility or to another hospital.

A total of n = 102 matched regional ischemic (intravascular sample from within the collateral circulation distal to the occlusion site) and systemic arterial blood samples (physiological flow conditions at cervical ICA level) were drawn from n = 51 LVO stroke patients. The data on ABG analysis of both sampling sites are presented in Table 2 and summarized in supplemental Fig. 1.

The following ABG parameters were examined: pH, p_aCO₂, p_aO₂, BE (B), HCO₃⁻ std, and sO₂. Significantly more alkaline conditions were present within collateral blood vessels (pH_ischemic = 7.38 vs. pH_systemic = 7.37; p = 0.0019). Concomitantly, the oxygen partial pressure was significantly decreased under vascular occlusion as compared to systemic level (p_aO_2ischemic = 185.3 mm Hg vs. p_aO_2systemic = 193.6 mm Hg; p = 0.035). By contrast, there were no sampling site-related differences in carbon dioxide partial pressure (p_aCO_2ischemic = 36.94 mm Hg vs. p_aCO_{2 systemic} = 38.62 mm Hg; p = 0.16), standard bicarbonate (HCO₃⁻ std_ischemic = 22.1 mmol/L vs. HCO₃⁻ std_systemic = 21.82 mmol/L; p = 0.52), and buffer base concentrations (BE_ischemic = −3.04 mmol/L vs. BE_systemic = −3.28 mmol/L; p = 0.64). Likewise, we found no differences in oxygen saturation of hemoglobin (sO_2ischemic = 98.67% vs. sO_2systemic = 98.84%; p = 0.16).

We assessed the following ion parameters: Na⁺ ion concentration, K⁺ ion concentration, Na⁺:K⁺ ratio, iCa²⁺ ion concentration and Cl⁻ ion concentration. The were no significant differences in sodium concentrations between the sampling sites (Na⁺_ischemic = 138.5 mmol/L vs. Na⁺_systemic = 137.6 mmol/L; p = 0.11). By contrast, potassium concentrations were significantly decreased within the hemodynamically compromised collateral circulation as compared to systemic levels (K⁺_ischemic = 3.44 mmol/L vs. K⁺_systemic = 3.64 mmol/L; p = 0.0083). Consistently, the regional ischemic Na⁺:K⁺ ratios were significantly increased (Na⁺:K⁺ ratio_ischemic = 41.74 vs. Na⁺:K⁺ ratio_systemic = 40.38; p = 0.0048). There were no significant differences between ischemic and systemic ionized calcium (iCa²⁺_ischemic = 1.11 mmol/L vs. iCa²⁺_systemic = 1.12 mmol/L; p = 0.26) and chloride concentrations (Cl⁻_ischemic = 107.7 mmol/L vs. Cl⁻_systemic = 107.00 mmol/L; p = 0.42).

We used Spearman’s rank correlation coefficient to assess associations between ischemic ABG parameters and the time from stroke onset to ischemic sampling. The median time from stroke onset to distal sampling was 274 min (IQR 205–369 min). Regional ischemic pH values were found to be negatively associated with the duration of stroke, but the correlation was just above the threshold for statistical significance (r = −0.36; p = 0.055). There was also a borderline statistical insignificance for the association of local HCO₃⁻ std (r = −0.34; p = 0.074) and BE (B) (r = −0.34; p = 0.074) with occlusion duration. All other ischemic ABG parameters showed no correlation with the time from stroke onset to ischemic sampling (|r| < 0.3; Table 3 and supplemental Fig. 1).

Spearman’s rank correlation coefficient was used to investigate associations between the ischemic ABG parameters and clinical stroke severity at baseline and at 48 h after recanalization. These data are given in Table 4 and supplemental Fig. 1. Blood glucose levels within the collateral circulation were positively correlated with the NIHSS score at 48 h after recanalization (r = 0.37; p = 0.015); however, none of the other ischemic ABG parameters showed any association with either clinical stroke severity on admission or at 48 h.

Results of correlation analysis between regional ischemic ABG parameters and infarct extent before and after EVT are given in Table 5 and supplemental Fig. 1. Univariate analysis revealed significant associations between preserved tissue integrity (ASPECTS) at admission and intravascular Na⁺:K⁺ ratios (r = −0.32; p = 0.031), K⁺ concentrations (r = 0.3; p = 0.041), and Cl⁻ concentrations (r = −0.35; p = 0.018) distal to the occlusion site. No other associations between baseline infarct extent and ischemic ABG parameters were seen. Analysis of dichotomized data (more extensive infarcts, ASPECTS ≤ 7 vs. minor infarcts, ASPECTS ≥ 8; poor vs. moderate to good collaterals) did not show a difference between any of the ischemic ABG parameters (Supplementary Information).The Na⁺ concentrations within collateral blood vessels were found to be negatively correlated with postinterventional infarct extent 24–48 h after recanalization (r = −0.34, 95% CI: −0.58–−0.05; p = 0.02). There were no other correlations between the ischemic ABG parameters and tissue integrity on follow-up imaging.

We further analyzed the association between imaging defined preinterventional to postinterventional worsening of stroke (∆ ASPECTS) and regional ischemic ABG parameters. Infarct extent remained constant in n = 20 (39.2%) patients, whereas dynamic infarct progression equivalent to a median ∆ ASPECTS of 1 (IQR 1–2) was observed in n = 31 (60.8%) patients. Progressive infarction was positively correlated with the Na⁺ concentrations within the occluded vascular territory (r = 0.42; p = 0.0033). No other associations were found with respect to preinterventional to postinterventional infarct progression.