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Abstract
Introduction Dural venous sinus stenosis (DVSS) may lead to increased intracranial pressure, sometimes requiring a stent if a high pressure gradient exists. Many neuroendovascular physicians use microcatheters to measure gradients, yet there are no studies comparing the accuracies and precisions of modern day microcatheters. We examined pressure recordings from five commonly used microcatheters in an experimental DVSS model.
Methods Using a programmable pump, dynamic flow was established in a closed circuit mimicking the venous sinus waveform. Microcatheters with 150 cm effective lengths were connected proximally to pressure transducers. Mean recording pressures were compared with a high fidelity microcatheter (HFM) in several configurations including no stenosis, proximal to a focal stenosis, and distal to a focal stenosis in opposing orientations.
Results All microcatheters recorded lower pressures than the HFM. Three of the five microcatheters successfully met intracranial pressure monitoring device standards in all conditions, while one did not meet standards in any configuration. The performance of the final microcatheter was variable, with inaccuracies occurring in unrestricted flow. All microcatheters demonstrated relatively high precision, but with variable accuracies. The larger diameter microcatheters displayed the least damping and therefore the greatest accuracies. Of the three smaller microcatheters, dimensions did not predict performance, suggesting that microcatheter construction may also play a role in pressure accuracy.
Conclusion The use of microcatheters to record dural venous sinus pressures must be done with an understanding of the inherent limitations and inaccuracies, especially if clinical decisions are made from the results.
- angiography
- blood pressure
- cathether
- intracranial pressure
- vein
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Introduction
Idiopathic intracranial hypertension (IIH) is a clinical syndrome affecting primarily obese women that is characterized by elevated intracranial pressure (ICP) in the absence of obvious causative etiologies such as hydrocephalus, intracranial mass or others.1 Characteristic signs and symptoms are in keeping with elevated ICP and include headache, papilledema, pulsatile tinnitus, transient visual obscurations, and possibly permanent loss of vision.1 Epidemiological studies have estimated that the prevalence of dural venous sinus stenosis (DVSS) with IIH ranges from 50% to 90%.2 3 While it is arguable whether the presence of a DVSS precludes the diagnosis of IIH, a potential treatment option for these patients is endovascular stenting of the stenosis, should it be deemed symptomatic. This important decision is often based on the recorded pressure gradient found across the stenosis using a microcatheter attached to a pressure transducer. While a gradient cut-off has not been universally agreed on, gradients above 10–12 mm Hg are generally classified as pathological in the literature, prompting intervention.4 5
In the current age of endovascular therapy, there is a plethora of microcatheters from which to select, each with varying dimensions, compliances, and chemical constructs. Therefore, it is not clear whether pressure readings are accurate to the actual venous pressures or reliable between recordings. Henkes et al analyzed the accuracy and reliability of pressure recordings from various microcatheters in 1999; however, the microcatheters examined are largely not in use today.6 The reliance on accurate pressure measurements across a DVSS to guide treatment for patients with IIH requires an understanding of the potential measurement errors associated with the microcatheter selected. As per the American National Standards Institute (ANSI)/Association for the Advancement of Medical Instrumentation (AAMI), ICP monitoring devices must be accurate to ±2 mm Hg when recording within the range of 0–20 mm Hg and <10% error within the range of 20–100 mm Hg.7 This study aims to provide an objective comparison under these guidelines between several commonly used microcatheters today using an experimental DVSS model. We hypothesize that microcatheters with larger inner diameters (ID) will decrease damping, resulting in higher accuracies.
Methods
Teflon tubing with ID of 0.7 cm and outer diameter (OD) of 1.0 cm was used to construct a closed circuit modeling a dural venous sinus in size.8 The circuit contained a mixture of two parts glycerol to three parts water, achieving a density of 1.02 g/cm3 and viscosity of 4.1 cP, similar to blood. A CompuFlow 1000 MR Control Unit with connected Pump Unit (Shelley Medical Imaging Technologies, London, Canada) was used to establish flow parameters. To mimic angiographic venous system convention, flow was denoted as proceeding from distal to proximal. The circuit contained an entry point on either side of the pressure recording site by way of T connectors, each connected to a Y connector to allow simultaneous access to the circuit by two devices either proximal or distal to the recording site (figure 1). Entry sites were placed sufficiently far from the recording site to prevent disruption of laminar flow.
Testing apparatus and configurations. (A) Portion of Teflon illustrating recording site (blue arrow) and two Y connectors (green arrows) used to access closed circuit through T connectors on either side of the recording site. (B) Unobstructed flow configuration with both microcatheter (MC) and high fidelity microcatheter (HFM) entering the closed circuit at the same location and oriented against the direction of flow. (C) Proximal to stenosis recording configuration with both devices accessing the stenotic region through the proximal access port. (D) Distal I configuration with the microcatheter accessing the circuit proximal to the stenosis, crossing the stenosis, and measuring the distal pressure. The HFM was introduced through the distal access port to avoid having to cross the stenosis. (E) Distal II configuration, with all devices recording the pressure distal to the stenosis by accessing the circuit through the distal port. All recordings in (C)–(E) were taken 1.5 cm from the edge of the stenosis.
Five different 150 cm microcatheters with varying constructs, commonly used in modern neuroendovascular surgery, were selected for comparison in the circuit: Echelon 10 (Medtronic, Dublin, Ireland), Excelsior SL-10 (Stryker, Kalamazoo, Michigan, USA), Excelsior 1018 (Stryker, Kalamazoo, Michigan, USA), Prowler Select Plus (Codman Neuro, Raynham, Massachusetts, USA), and Marksman 27 (Medtronic, Dublin, Ireland). Their properties are listed in table 1. Catheters were connected to a pressure transducer fixed at the level of the pressure recording site. Each microcatheter was compared with a 3F Mikro-Tip Pressure Catheter (Millar, Houston, Texas, USA) high fidelity microcatheter (HFM) with 130 cm length and 1 mm diameter. This HFM contains a side-mounted pressure sensor and no lumen, avoiding the potential issue of pressure damping. The HFM and pressure transducer were calibrated using a manometer to 0 mm Hg in air and 30 mm Hg prior to each test. Investigators were not blinded to the microcatheter used.
Properties of microcatheters used
The first objective was to determine the accuracy of each microcatheter compared with the HFM in an unobstructed circuit with dynamic flow, controlled by the CompuFlow 1000 MR Control Unit and Pump Unit using SimuFlow III Waveform Editing Software (Shelley Medical Imaging Technologies). Dynamic flow was programmed with a 558 ms period to approximate the average cardiac cycle in human adults. The peak flow was set at 0.5 mL/s as it corresponded to a peak pressure of ~10 mm Hg in our model, to mimic the supine resting ICP found in healthy adults, as cerebral venous pressure is always at least the ICP in healthy states.9 The remainder of the pressure curve was modeled after the superior sagittal sinus waveform recorded by Kim et al.10 All devices were inserted into the circuit through the proximal T connector and passed upstream to the recording site (figure 1B).
The second objective was to compare microcatheter and HFM pressure recordings both proximal and distal to a focal stenosis in the circuit. A standard binder clip was placed on the tubing in the recording site parallel to flow such that a 20 mm Hg pressure gradient was created, as verified by the HFM. All recordings were taken 1.5 cm from the edge of the stenosis. The proximal recording site was accessed through the proximal T connecter for both the microcatheters and the HFM. The distal recording site was accessed through the proximal T connector and across the stenosis, while the HFM was passed through the distal T connector to avoid crossing the stenosis and potentially affecting recordings (distal I configuration). Both proximal and distal I recordings were obtained in dynamic conditions as above (figure 1C,D).
Finally, we investigated the effect of microcatheter orientation on pressure recordings distal to a stenosis. Microcatheters were compared with the HFM in two configurations with opposite orientations. The first was the distal I configuration as described above. The opposing configuration involved accessing the circuit through the distal T connector, thereby not crossing the stenosis (distal II configuration). Therefore, the distal I configuration resulted in microcatheters facing into the direction of flow while the distal II configuration had them facing downstream (figure 1E). Devices were again parked 1.5 cm from the edge of the stenosis. Dynamic flow conditions were again used.
Pressure recordings were measured over a 10 s period, three times each, using SonoLab (Sonometrics, London, Canada), then converted and processed with CVWorks (AccuDAQ, Calgary, Canada) to obtain mean pressures. In compliance with the ANSI/AAMI standards for ICP monitoring devices, pressures <20 mm Hg were reported as the difference in mean pressures between the HFM and the microcatheters. For pressures of ≥20 mm Hg, mean pressures were reported as the error percentage of the HFM recordings. The targets were ±2 mm Hg and ±10%, respectively.
Results
For all catheters in all conditions, the mean pressure recorded by the HFM was higher than the microcatheter recordings. Furthermore, all microcatheters demonstrated pressure wave damping in dynamic flow conditions, with the degree of damping being inversely related to the ID.
As dynamic flow parameters with no focal stenosis established a maximal pressure of <20 mm Hg, target differences from the HFM recordings were less than 2 mm Hg as per ANSI/AAMI standards. The Echelon 10, Prowler Select Plus, and the Marksman 27 were all within the target pressure range. The Excelsior SL-10 and Excelsior 1018 were outside of this range, with mean pressure differences of 3.84±0.64 mm Hg and 2.37±0.66 mm Hg, respectively (figure 2A).
Mean differences in microcatheter pressure recordings from HFM pressure recordings in the following configurations: (A) no stenosis; (B) focal stenosis, proximal recording method; (C) focal stenosis, distal I recording method; (D) focal stenosis, distal II recording method. Each bar represents mean values obtained over three recordings, shown with SD. *Recordings that do not meet ANSI/AAMI intracranial pressure monitoring device standard of ±2 mm Hg when pressure is <20 mm Hg. +Recordings that do not meet ANSI/AAMI intracranial pressure monitoring device standard of ±10% when pressure is ≥20 mm Hg. HFM, high fidelity microcatheter; ANSI, American National Standards Institute; AAMI, Association for the Advancement of Medical Instrumentation.
The focal stenosis was then applied for the remainder of the experiment. The HFM recorded a mean pressure of 8.22 mm Hg in the proximal configuration, therefore a target of ±2 mm Hg was set. The Echelon 10, Excelsior 1018, Prowler Select Plus, and the Marksman 27 all reached this target, while the Excelsior SL-10 recorded a mean pressure difference of 2.84 mm Hg below the HFM (figure 2B).
In the distal I configuration, the HFM recorded a mean pressure of 30.69 mm Hg so the accuracy target was within 10%. Only the Excelsior SL-10 did not reach this target, which recorded a mean pressure difference 12.85% below the HFM (figure 2C). Similarly, in the distal II configuration, the HFM recorded a mean pressure of 23.71 mm Hg so microcatheters were targeting a difference of less than 10%. Again, the Exclesior SL-10 was the only microcatheter to exceed this target with a mean difference 12.79% below the HFM (figure 2D). The distal I and distal II configurations were then compared. All microcatheters recorded lower pressures in the distal II configuration than the distal I configuration (table 2).
Mean pressures for microcatheters in distal I and distal II configurations
Discussion
This study investigated the accuracy and precision of five commonly used microcatheters in recording flow pressures in a closed system. Recordings were compared with HFM pressure recordings in a circuit without stenosis, proximal and distal to a focal stenosis, and distal to a focal stenosis with opposing orientations, all in dynamic flow conditions aimed at replicating the dural venous sinus flow in humans. The ANSI/AAMI accuracy standards for ICP monitoring devices suggest an accuracy of within ±2 mm Hg for pressures <20 mm Hg and ±10% for pressures ≥20 mm Hg. We found that the Echelon 10, Prowler Select Plus, and Marksman 27 microcatheters met these requirements in all circuit configurations. Conversely, the Excelsior SL-10 failed to meet accuracy requirements in all scenarios. The Excelsior 1018 appeared reasonably accurate in stenotic configurations, but less so in the stenosis-free configuration.
The Prowler Select Plus and Marksman 27 have the largest IDs of the five microcatheters and displayed the least amount of damping, regardless of circuit configuration, consistent with previous literature.6 As all microcatheter pressure recordings were lower than HFM recordings, this likely played a significant role in their ability to meet ANSI/AAMI standards. However, the smallest ID belongs to the Echelon 10, which performed much better than the Excelsior SL-10. Since the microcatheter lengths were all identical, the construction of the microcatheter appears to be an important factor, and the Echelon 10 probably has a lower compliance than the Excelsior SL-10.
For all microcatheters we found that the SD generally increased as pressure increased. However, the magnitudes of these variances were considerably small, indicating that the pressure recordings from each microcatheter were relatively precise, despite their variable accuracies. While not all microcatheters met ANSI/AAMI standards, the use of any of these microcatheters for recording venous pressures requires an awareness of their inherent limitations, especially when making clinical decisions based on pressure recordings.
The configurations selected for this study were designed to mimic clinical scenarios encountered by neuroendovascular physicians. In some cases of IIH a focal stenosis is not encountered, yet venous pressure recordings may still be elevated.3 In the case of a focal stenosis, the approach is usually from the ipsilateral internal jugular vein, thus the distal recording site is approached by crossing the stenosis, as replicated in our distal I method. However, in the case of extensive stenosis involving the internal jugular vein, an approach from the contralateral side may be pursued to record the distal pressure. Thus, we have compared the distal I method with the distal II method and found that the distal II configuration will result in lower pressure recordings. This may be due to the downstream orientation in distal II creating an area of lower pressure immediately distal to the tip. Alternatively, the pressure gradient may be lower in distal II as the microcatheter is not crossing the stenosis and contributing to the reduction of cross-sectional area, unlike the distal I configuration.
The highest mean pressure recorded in our model under restricted flow was 30.69 mm Hg, therefore higher instantaneous pressures were recorded. While the pressures encountered in IIH are often less than this, dural venous sinus pressures are known to be elevated to over 80 mm Hg in certain pathological states.11 Therefore the pressure range recorded in this study is clinically relevant beyond just IIH.
This study has several limitations to report. First, only five microcatheters were selected, with many others in production that may be used for measuring dural venous sinus pressures. We selected what we believe are five of the most commonly used products. Next, the Teflon tubing used to create the circuit does not have the same properties as the dural venous sinuses. The tubing was not fixed against a rigid structure, unlike the dural venous sinuses, and their compliances are not identical. While this would potentially affect the pressures recorded by the HFM, we feel that this does not significantly impact the implications of the results. Furthermore, the focal stenosis model was relatively crude in that it is impossible to create identical stenoses for each of the trials. However, we attempted to correct for this in two ways. First, we ensured the HFM-recorded pressure gradients in each trial for every microcatheter were as close to 20 mm Hg as possible. Second, every trial was conducted simultaneously with an HFM recording, with differences being calculated between the microcatheter and HFM of the same trial. A final limitation is that the dynamic flow rate waveform had a peak velocity of 0.5 mL/s, while greater flow rates may be observed in the dural venous sinuses. For example, the superior sagittal sinus flow rate ranges from approximately 0.7 mL/s to 4 mL/s.12 Our target for this study was to replicate physiological venous pressures. The peak flow rate corresponded to a pressure of ~10 mm Hg in our model without a focal stenosis, thus approximating normal physiological pressures.
Conclusion
Many microcatheters are being used for diagnostic pressure recordings in the setting of IIH and other pathologies. Our study demonstrates that these microcatheters have varying properties that translate into variable pressure recording accuracies or inaccuracies, but with relatively high precision. Therefore, while some do not meet ANSI/AAMI ICP monitoring device accuracy standards under all conditions, neuroendovascular physicians who choose to use these devices should understand their limitations, especially when making clinical decisions.
Footnotes
Contributors MBA planned and conducted the statistical analysis of the data, drafted and revised the manuscript. SS contributed to the overall research design and conducted the data acquisition. JKE contributed to the research design and assisted with data acquisition. ME contributed to the research design, monitored data collection and processing, and revised the manuscript. APM contributed to the research design, monitored data collection and processing, assisted with statistical analysis of the data and revised the manuscript.
Competing interests APM has received grant funding from Stryker Corporation, outside of the submitted work.
Provenance and peer review Not commissioned; externally peer reviewed.