Diminished spinal cord perfusion despite distal aortic perfusion: an acute translational large animal experiment with occluded segmental arteries

Abstract

OBJECTIVES

Neuroprotective measures have been established in open thoraco-abdominal aortic aneurysm repair to reduce the incidence of postoperative paraplegia. Distal aortic perfusion (DaP) is meant to increase blood flow to the abdominal organs and the spinal cord. Cerebrospinal fluid (CSF) drainage is part of peri- and postoperative clinical routine. We aimed to investigate the effect of both techniques on spinal cord perfusion in an acute large animal model with segmental artery occlusion.

METHODS

Eight pigs underwent minimally invasive segmental artery coil embolization prior to establishment of cardiopulmonary bypass with DaP. After initiation of DaP, CSF pressure was increased 3-fold by infusion of blood plasma. Collateral network near-infrared spectroscopy was used as an additional real-time monitoring method for indirect perfusion monitoring. Microspheres were injected for post-mortem regional spinal cord blood flow analysis.

RESULTS

DaP led to an increase in spinal cord perfusion limited to the very lower spinal cord (L3–S, up to 400% of baseline) and the corresponding paraspinous muscle area. The most vulnerable region between T8 and L2 was not reached by DaP (between 14% and 46% of baseline). After initiation of DaP, a 10% increase in oxygenation via collateral network near-infrared spectroscopy was observed for the low lumbar region. The increase in CSF pressure counteracted enhanced perfusion inflow leading to a decrease in net tissue perfusion.

CONCLUSIONS

DaP is effective in increasing blood flow to the distal spinal cord (effectively counteracting CSF pressure increase) and paraspinous muscles, despite occluded segmental arteries, resulting in hyperperfusion potentially leading to spinal cord oedema and delayed paraplegia postoperatively.

Open in new tabDownload slide

INTRODUCTION

Spinal cord ischaemia resulting in permanent paraplegia after thoraco-abdominal aortic aneurysm (TAA) repair is a tragic complication for the individual patient, restricting quality of life and increasing long-term mortality significantly [1, 2]. Rates of paraplegia in open and endovascular repair are still remarkably steady with up to 20%; however, the underlying patho-mechanisms are variable [3]. In open TAA repair, blood flow to the cord is suddenly interrupted by aortic cross-clamping [4]. Moreover, segmental arteries (SAs), usually providing spinal cord perfusion, are ligated—if not reimplanted—in the replaced aortic segment [5]. During the past decades, several neuroprotective adjuncts [e.g. drainage of cerebrospinal fluid (CSF), distal aortic perfusion (DaP), sequential clamping] have been implemented into clinical routine [6]. DaP provides retrograde inflow, potentially securing spinal cord perfusion limited to the distal part while not reaching the mid-thoracic level and thereby potentially endangering the spinal cord through regional hyperperfusion as previously shown in an experimental setting with open SAs [7].

In the current experimental series, we aimed to analyse exclusively the effect of DaP during aortic cross-clamping after SA occlusion augmented by iatrogenic CSF pressure increase on spinal cord perfusion.

MATERIAL AND METHODS

Ethics

The study was approved by the Institutional Animal Care and Use Committee and the local Veterinary Office (application number TVV18/01). Each experiment was performed in accordance with the principles of the National Society for Medical Research and the Guidelines for the Care and Use of Laboratory Animals [National Research Council (US) Committee for the Update of the Guide for the Care and Use of Laboratory Animals 2011] [8].

Anaesthesia

The experiments were performed in 8 juvenile German landrace pigs (mean weight 49.1 ± 5.6 kg, age from 15 to 18 weeks). After premedication using 0.5 mg/kg midazolam (Pfizer) and ketamine 15 mg/kg (Pfizer), the animals were transported to the experimental operating room. Two venous lines (a peripheral catheter in the ear vein and a central venous catheter in the external jugular vein) were placed. Afterwards, an arterial line was introduced into the right carotid artery, and a 7-Fr sheath was placed into the femoral artery (also allowing to draw blood and perform invasive pressure monitoring). The animal was positioned on its right side, and a lumbar puncture followed by the introduction of a cerebrospinal drain was performed between the L3 and L4 levels. In addition to the above-described monitoring, continuous ECG, rectal temperature measurements, fluid balance control, blood coagulation tests and blood gas analyses were performed during the experiment. Finally, paraspinous muscle’ oxygenation was evaluated by means of collateral network near-infrared spectroscopy using optodes placed at 4 levels: mid- and lower thoracic, and upper and lower lumbar (Fig. 1).

Minimally invasive segmental artery coil embolization

Using selective angiography (Artis Zee floor-mounted system with PURE, Siemens, Germany), sequential coil embolization of all the SAs from the T4 to the L5/S level was performed. At 1st, a 7-Fr sheath was placed into the femoral artery and a standard internal mammary artery catheter (5.2 Fr, 100 cm in length) was inserted. Diluted (1:2) contrast medium (Ultravist^® 370, Bayer Vital, Germany) was injected while the catheter was moved along the aorta in caudal direction. Once a SA was detected, a microcatheter (Codman, Johnson & Johnson, MA, USA) was advanced through the internal mammary artery catheter into the artery. In some cases, a guiding wire was necessary in order to stabilize the microcatheter for appropriate positioning in the right or left SA. Pushable platinum coils (3–7 mm) were inserted into each SA (or if technically not possible—into the SA trunk). Angiographic control was performed after coiling of each SA pair and again after coiling of all SAs. Appropriate occlusion of the artery was confirmed when no contrast agent was visualized distally to the inserted coils. A total of 53 ± 15 coils of various sizes were required for complete coil embolization in each animal.

Surgical approach

Surgical exposure of the heart and the great vessels was performed via upper left thoracotomy in the 3rd intercostal space (Fig. 1). A 4-Fr catheter for microsphere (MS) injections was placed into the left atrium using the Seldinger’s technique. A 400 IU/kg loading dose of heparin was injected intravenously and was followed by cannulation of the descending thoracic aorta (DTA) (above the level of the 4th SA in cranial fashion) and the pulmonary trunk. Finally, a 2nd incision was performed, exposing the retroperitoneal space. The abdominal aorta was mobilized and cannulated with adequate distance above its trifurcation with the tip of the cannula being directed caudally. Both of the arterial lines (placed in DTA and the abdominal aorta) were 12-Fr in diameter and a three-way valve placed at the same distance for simultaneous MS injections. The cardiopulmonary circuit consisted of a membrane oxygenator with a venous reservoir and was primed with gelofusine and heparin.

Experimental sequence

After SA coil embolization and cannulation of the great vessels, non-pulsatile cardiopulmonary bypass (CPB) with 50–70 mmHg mean arterial pressure was initiated (Fig. 2). After 10 min of stable CPB, the 1st MS injection (T1) was administered into the left atrium. The proximal DTA was cross-clamped below the T4 SA level: 5 min later, the second MS injection (T2) was completed, followed by cross-clamping of the abdominal aorta and initiation of DaP with 60 mmHg target perfusion pressure. After 5 min of stable DaP, a simultaneous injection of 2 MS colours (T3) was administered into the proximal and distal arterial lines of the CPB. Finally, evaluation of DaP counteraction to CSF pressure increase was initiated by means of plasma injection into the intrathecal space so that stable 3-fold increase of CSF pressure values compared to the individual baseline was achieved. Afterwards, the last MS injection (2 colours simultaneously) was performed (T4).

Tissue harvesting and microspheres quantification

Once injected, MS are travelling with the blood stream from the location of the injection towards the capillary bed where they can be measured post-mortem. Regional perfusion at different time points can be distinguished by the different MS colours. The spinal cord was removed en bloc and 2 m³ samples of paraspinous muscles were harvested from the mid-part of each collateral network near-infrared spectroscopy level. Regional perfusion analysis was performed using absorbance-dyed polystyrene MS (Dye-Track VII+ System, Triton Technology Inc., USA). These small coloured particles were injected into the arterial flow: 1 colour into the left atrium during the T1 and T2 time points and 2 colours simultaneously into the arterial CPB lines at T3 and T4 time points. Before and during MS injection, reference blood was withdrawn from the arterial lines. Thus, using reference blood and wet weight of the tissue samples, it was possible to calculate the regional perfusion of the tissue relative to cardiac output at each time point (T1–T4). MS recovery was performed using a standard filtration technique. One of the 7 available colours was used as process control during microsphere quantification. A detailed description of sample handling and flow assessment has been described previously [9]. Colour analysis was performed using Synergy^TM H1 Plate Reader and Gen5 Software (BioTek, Winooski, VT, USA). Measurements were made at the wavelengths corresponding to the respective MS colours.

Statistical analysis

Power analysis for adequate sample size was calculated using G*Power (Version 3.1.9.6 for Mac OS, Heinrich-Heine University, Düsseldorf, Germany) prior to the experiments. Calculations were performed for spinal cord perfusion with effect size estimated based on previous works published by the group, leading to 7 animals per group required for a repeated measures ANOVA (Cohen’s f 0.75, alpha error 0.05, Power 0.9 and number of repeated measurement 4). Data analysis was performed using GraphPad Prism Software for MacOS, Version 8.00 (San Diego, CA, USA). In continuous variables, the distribution was evaluated using the D’Agostino-Pearson, the Kolmogorov–Smirnov tests and Q–Q plots. For the time-dependent repeated measurements, the repeated measures analysis of variance (one-way repeated measures ANOVA) was performed using absolute values of spinal cord (at each level) and paraspinous muscle (at each level as well) regional perfusion, and % from baseline collateral network NIRS values. The changes in these values were analysed during 4 experimental time points for each level separately (within-subjects ANOVA). This was followed by the Tukey’s post hoc test for within-subject multiple comparisons of the values measured at different time points at each level. The ANOVA model was built at each level of spinal cord or paraspinous muscle The values used for ANOVA analysis were normally, or near-to-normally (based on Q–Q plots) distributed. The violation of the sphericity assumption was corrected using the Greenhouse–Geisser method. The Friedman non-parametric test was used in the paraspinous muscle perfusion analysis at the mid-thoracic level. Statistical significance was assumed at a P-value of <0.05 for two-tailed testing.

RESULTS

Spinal cord regional perfusion (microsphere measurements)

The measurements of regional spinal cord perfusion throughout the 6 different time points are presented in Table 1 and Fig. 3.

Proximal aortic cross-clamp (T2)

The cross-clamping of DTA resulted in an overall increase of perfusion values in the C1-T7 region (to 122% of baseline, IQR 92–190%). The spinal cord perfusion at the mid-level of T8 to the caudal level (L2, L3 down to sacral) both decreased (T8-L2: 56% of baseline, IQR 42–70%; L3–S level: 24% of baseline, IQR 17–37%) with the lowest amount of residual tissue perfusion in the most distal part of the spinal cord.

Distal aortic cross-clamping and initiation of distal aortic perfusion (T3)

Marked hyperperfusion of the upper spinal cord (C1-T7: 281% of baseline; IQR 275–472%) was measured with proximal MS injection after distal aortic cross clamping and start of DaP. By injection of MS into the distal arterial line, nearly no perfusion (originating from downstream inflow vessels) could be detected with the spinal perfusion nadir at 3% of baseline. The spinal cord tissue perfusion in region T8-L2 decreased further compared to T2, leading to tissue perfusion values between 47% (measured via proximally injected MS) and 14% (measured via distally injected MS). Almost no changes at the other 2 levels were observed when analysing the proximal injections. The area of L3 to S was only slightly reached by MS in the proximal arterial line, leading to spinal cord perfusion values of 34% of baseline. With the distal injections, on the contrary, a hyperperfusion was detected (to 395% of baseline, IQR 286–656%) due to perfusion consequential to DaP.

Three-fold CSF pressure increase, while on distal aortic perfusion (with proximally and distally clamped aorta)

Deliberate 3-fold elevation of CSF pressure did not impact hyperperfusion in C1-T7 (329% of baseline, measure by proximally injected MS). Distal injections of MS did not reach the upper spinal cord region. Spinal cord tissue perfusion of T8–L2 even diminished further. With proximal injection showing only 32% of baseline perfusion and distally injected MS, revealed 13% of baseline spinal cord perfusion. Further changes in spinal cord perfusion were detected at L3–S region. The analysis of the proximally injected MS showed an increase in perfusion up to 120% of baseline, whereas the distally injected MS demonstrated a decrease in perfusion from 395% to 101% of baseline values—altering hyperperfusion to baseline perfusion. Detailed results of ANOVA analyses are presented in Supplementary Material, Tables S1 and S2.

Collateral network regional perfusion and collateral network near-infrared spectroscopy monitoring

Perfusion

The results of paraspinous muscle’ perfusion analysis are presented graphically in Fig. 4. Absolute values are given in Supplementary Material, Table S3. A decrease of muscle perfusion was observed after proximal aortic cross-clamping. Additional distal systemic inflow after distal aortic cross-clamping and start of DaP, however, leads to a return of muscle perfusion values to baseline in the lower thoracic and lower lumbar region. The mid-thoracic region increased, but not to baseline values. Nearly no changes could be displayed in the upper lumbar segment.

Oxygenation

The collateral network NIRS values during SA coil embolization and the entire experimental set-up are presented in Fig. 5. Oxygenation values decreased continuously at mid-thoracic level over the course of the experiment, most pronounced after DTA cross-clamping, followed by another drop after distal aortic cross-clamping and DaP initiation at this level (baseline: 59.5 ± 7 versus proximal clamp: 40.6 ± 8 versus distal clamp + DaP: 29.0 ± 5). At the upper lumbar level, a comparable decrease of collateral network near-infrared spectroscopy values was observed simultaneously (baseline: 63.3 ± 5 versus proximal clamp: 42.0 ± 4 versus distal clamp + DaP: 38.9 ± 6), see Supplementary Material, Table S4 and Supplementary Material, Fig. S1.

Vital parameters

Proximal and distal aortic cross-clamping was associated with some reactive changes of mean arterial pressure presented in Table 2 and Supplementary Material, Fig. S2. These changes reflected the clinical perioperative course. Central venous pressure did not change significantly throughout the course of the experiment. CSF pressure ranged from 9 to 12 mmHg and was then increased by factor 3, always according to the individual starting CSF pressure of each animal.

DISCUSSION

We investigated the influence of DaP on spinal cord perfusion in an acute large animal model with previously occluded SAs mimicking modern open TAA repair without reimplantation of SAs. Additionally, the interaction of DaP and CSF pressure elevation, a common clinical scenario, was analysed. We found that DaP led to a significant increase in spinal cord perfusion as well as blood flow within the paraspinous muscles, hence the collateral network. However, this increase in perfusion was limited to the lowest spinal cord segments (L3 downwards) and similarly to the lumbar CN region of the paraspinous muscles. Consequently, blood flow to the most vulnerable region of the spinal cord located between T8 and L2 cannot be sufficiently secured by utilization of DaP. However, compared to cross-clamping without DaP, the perfusion decline in C1–T7 and L3–S is less pronounced if DaP is established (see Fig. 3).

It is known that spinal cord perfusion is compromised in open and endovascular TAA repair resulting in spinal cord ischaemia and permanent paraplegia, by far the most devastating complication for the individual patient, their next of kin and the medical team involved. Several attempts have been made during the past decades to improve spinal cord protection by pre-, peri- and postoperative measures. In case of elective treatment, staging the procedure to allow the paraspinous collateral network to develop seems one of the most promising strategies [9, 10]. However, in urgent or emergent cases, the time frame for staging is limited, and extensive open or endovascular aortic repair remains the only option to save the patients’ life. In open repair, spinal cord perfusion is negatively impacted by (i) the intraoperative interruption of blood flow during aortic cross-clamping and (ii) the sacrifice of SAs due to the repair itself. For the latter, it has been attempted to mitigate this complication by SA reimplantation; however, results on patency are divergent, and CPB time is significantly prolonged [11, 12]. In 1983, Gelman et al. [13] could demonstrate a decrease in blood flow to the lower spinal cord by 85–94% once the aorta was clamped. In our current experiment, aortic cross-clamping also led to a decline in spinal cord perfusion to 56% (T8–L2) and 24% (L3–S) of baseline perfusion, whereas the upper part of the spinal cord (C1–T7) showed an increased perfusion up to 122%, mainly due to the pronounced collateralization via the subclavian arteries, further explaining the hyperperfusion from C1 to T7 throughout the entire experiment, augmented by the increase in blood pressure after proximal aortic cross-clamping. In a previous experimental series of aortic cross-clamping (without CPB), von Aspern et al. [14] also demonstrated a marked increase in (carotid) MAP after cross-clamping up to 186% of baseline. Apart from the increased perfusion of the upper spinal cord, we found extensive hyperperfusion of the lower spinal cord close to 400% of baseline after initiation of DaP. In our previous experimental series, hyperperfusion of L3 downwards was even more pronounced with up to 480% of baseline, probably due to the presence of patent SAs [7]. A recent study by Pasrija et al. [15] on spinal cord infarction in patients with femoral venoarterial ECMO suggested hyperperfusion and spinal cord oedema as the origin of spinal cord infarction in the lower thoracic spinal cord (T7–T12) downwards to the cauda equina. In 5% of patients, spinal cord infarction was seen, and magnetic resonance imaging detected hyperintensity between T7 and T11, which progressed distally with most severe spinal cord infarction in the caudal region [15]. In 1996, an experimental study on dogs also demonstrated that hyperperfusion of the spinal cord measured by laser Doppler flowmeter correlated with adverse neurologic outcome [16].

DaP is believed to maintain perfusion of the spinal cord (and the renals) during completion of the proximal anastomosis, sequential clamping and can be continued during reimplantation of the visceral vessels. However, all existing clinical studies on DaP are retrospective or observational; no randomized controlled trials have been realized yet [17]. Nevertheless, experimental studies have demonstrated a relevant increase in CSF pressure during aortic cross-clamping (from 3% up to 100% of baseline values) [16, 18–20] linking DaP and CSF drainage, underscoring the need for a thorough understanding of the impact of both strategies and their interaction on spinal cord perfusion.

The clinical combination of DaP and CSF drainage has been retrospectively investigated in 1004 patients with descending thoracic or thoraco-abdominal aortic replacement by Safi et al. [21] 20 years ago. In 74% of patients with neuroprotective adjuncts (DaP and CSF drainage), the rate of immediate neurologic deficit was significantly lower with 2.4% compared to 6.8% in patients without neuroprotective adjuncts (P < 0.01). This effect, however, has to be interpreted with caution, since 42.4% of patients also received intercostal artery reattachment with uncertain impact on spinal cord perfusion. Two years later, the same group published a subgroup analysis solely focusing on DaP and CSFD as neuroprotective adjuncts in 238 patients concluding a significant reduction in the incidence of neurological deficits by factor 4.5 (1.3% vs 6.5%) compared to a group of patients with simple aortic cross-clamp without any neuroprotective adjuncts [22]. As our large animal experiment was only an acute model, no statement on neurological impairment can be made. However, we can conclude that DaP led to an improved perfusion of the lower spinal cord and paraspinous muscles. Furthermore, this improved perfusion in the distal cord was counteracted by (artificial) increase in CSF pressure lowering spinal cord perfusion between T8 and L2 to levels similar to perfusion without DaP (roughly 13% of baseline perfusion). A previous experimental series of our group could demonstrate the same vulnerable spinal cord region when DaP is applied with open SAs [7]. As we did not occlude the SAs prior to initiation of CPB and cross-clamping of the aorta in our previous experiment, the decline in spinal cord perfusion was not as pronounced with lowest values in Th8-L2 of 24% of baseline perfusion [7]—compared to 13% in the current experimental series. A study by Zoli et al. [23] in 2010 could also demonstrate a decrease in spinal cord perfusion pressure to 38% of baseline after occlusion of all SAs. Von Aspern et al. [24] detected in a chronic animal study comparing different patterns of SA occlusion an acute decrease in regional perfusion of the lumbar spinal cord to roughly 59% of baseline when every other SA was occluded (alternating pattern). However, lowest perfusion was detected 24 h after the 1st stage of SA occlusion (Th 12–L2, watershed area) in the lumbar region, reaching 22% (without additional increase in CSF pressure).

As CSF pressure is directly contributing to spinal cord perfusion pressure [25], CSF drainage is 1 major component of peri- and postoperative spinal cord protection, especially in open thoracoabdominal aortic aneurysm (TAAA) repair. As demonstrated by the herein presented data, an increase in CSF pressure can counteract the positive effects generated by DaP. The clinical importance of CSF drainage has been demonstrated by Coselli et al. [26] in 2002 in a randomized controlled study leading to a 5-fold decrease in paraplegia or paraparesis in the group with CSF drainage. Our group could also show that even in healthy pigs with open SAs, an increase in CSF pressure by factor 3 led to a significant reduction in spinal cord perfusion [27].

Limitations

The presented acute experimental study naturally has several limitations. First, we only investigated 2 neuroprotective measures. In clinical practice, neuroprotection is regularly augmented, e.g., by motor-evoked potentials (MEP)/ somato-sensory evoked potential (SSEP) measurements, hypothermia and sequential clamping. However, to gain a thorough understanding of the isolated effects, we opted for investigating solely the impact of DaP and CSF pressure increase. Second, although the pig is a widely accepted large animal model for cardiovascular diseases, it has to be kept in mind that (i) a few anatomical differences remain between humans and pig and (ii) all pigs are juvenile and healthy in contrast to the typical cardiovascular patient with relevant co-morbidities such as arterial hypertension, diabetes mellitus, atherosclerosis and many more. Third, we limited our experiment to 1 DaP pressure mode, as previously reported [7], with a target pressure of 60 mmHg. The target pressure remained unadjusted during the whole experiment. Fourth, our study was performed in a relatively small number of animals. However, statistical significance was reached. In addition, no pre-experimental assessment of each animal’s spinal cord was performed, so we cannot exclude any possible presence of anatomical collateral network variations. Moreover, no instrumental evaluation of the spinal cord condition after the procedure was performed, such as magnetic resonance imaging for oedema detection.

This study is an acute translational large animal experiment: it would be interesting to perform a chronic animal model to gain more information about these neuroprotective techniques in the long term.

CONCLUSIONS

DaP is effective in improving spinal cord perfusion of the most distal part, also increasing lumbar paraspinous muscle perfusion, leading to better spinal cord perfusion in case of preoperative priming of the collateral network. However, the vulnerable region around T8 to L2 is not positively affected by DaP, and the lower spinal cord is endangered by pronounced hyperperfusion. An increase in CSF pressure can counteract this effect in the distal spinal cord, reversing the increase in perfusion. In clinical routine, protocols need to be developed and established to avoid pronounced alterations between hyper- and hypoperfusion, risking the development of spinal oedema and irreversible spinal cord damage.

SUPPLEMENTARY MATERIAL

Supplementary material is available at EJCTS online.

FUNDING

This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement 733203 and from the German Research Foundation under grant number 127/2-1.

Conflict of interest: At time of the experiment, Professor Christian D. Etz held the Heisenberg-Professorship for Aortic Surgery of the German Research Foundation (DFG) (Project-No: 278040814). The other authors report no conflicts of interest.

DATA AVAILABILITY

The data underlying this article will be shared on reasonable request to the corresponding author.

Author contributions

Josephina Haunschild: Conceptualization; Data curation; Formal analysis; Investigation; Methodology; Project administration; Validation; Visualization; Writing—original draft. Zara Dietze: Investigation; Methodology; Visualization; Writing—review & editing. Konstantin von Aspern: Conceptualization; Investigation; Methodology; Writing—review & editing. Susann Ossmann: Investigation; Methodology; Writing—review & editing. Jörg Naumann: Investigation; Methodology. Michael A Borger: Funding acquisition; Supervision. Christian D. Etz: Conceptualization; Funding acquisition; Methodology; Project administration; Resources; Software; Supervision; Writing—review & editing.

Reviewer information

European Journal of Cardio-Thoracic Surgery thanks Peng Zhu, Zinar Apaydın, David Barilla and the other anonymous reviewers for their contribution to the peer review process of this article.

REFERENCES

ABBREVIATIONS

ABBREVIATIONS
 
• CPB

  Cardiopulmonary bypass

 
• CSF

  Cerebrospinal fluid

 
• DaP

  Distal aortic perfusion

 
• DTA

  Descending thoracic aorta

 
• MS

  Microspheres

 
• SA

  Segmental artery

 
• TAA

  Thoraco-abdominal aortic aneurysm

Author notes