Lower limb ischemia is a serious complication in patients treated with peripheral venoarterial extracorporeal membrane oxygenation (VA-ECMO). This complication can be minimized by pacing a distal perfusion catheter (DPC) to the superficial femoral artery (SFA). DPC insertion on the posterior tibial artery was recently reported with successful outcome,1 but the SFA remained the standard sites for most centers. Surgical cut down allows direct visualization and proper placement of the DPC to the SFA. Very often, however, the attending doctor would prefer ultrasound guided percutaneous catheter insertion by seldinger technique to minimize patient transport, after taking into consideration the unstable condition of the patient who has a VA-ECMO just initiated. Theoretically, by this technique, one should identify the common femoral artery bifurcating into the superficial and the deep femoral artery by ultrasound before DPC insertion. The proper imaging of such anatomy is practically difficult due to a number of factors: First, the arterial ECMO catheter in-situ is exactly above the site where the ultrasound probe should be placed. Second, clear images are hindered by the soft-tissue swelling and hematoma formation after arterial ECMO cannulation. Third, even if the image is optimal, the size of the SFA shrinks, presumably due to vasospasm and reduced blood flow after ECMO cannulation. Lastly, even when the puncture needle is within the SFA, pulsatility, with is otherwise an important sign to confirm arterial puncture, is often absent in a patient supported by VA-ECMO. Therefore, accidental placement of the DPC to the venous rather than the arterial system is a serious but possible concern.
Confirmation of DPC position by ultrasound after DPC placement is technically even more demanding even for expert sonographers. Doppler distal to the DPC, for example, at popliteal artery, is often difficult due to low blood flow. Bunge et al.2 in their article described the fast confirmation of the DPC by pacing a 0.81 mm, 60 cm J tipped wire into the DPC and visualized the wire in the popliteal artery by ultrasound to confirm the right position. There are two potential risks of this technique. First, it requires circuit disconnection for the guide wire insertion. Risks of air embolism and infection are increased when the closed circuit is disturbed. Second, the blind insertion of the guide-wire, even though it is soft and J tipped, increases the risk of vascular trauma to the small distal arteries.
We in this how-to-do-it article describe the use of a micro-bubble ultrasound enhancing agent to confirm position of an inserted DPC. SonoVue (Bracco, Milan, Italy) is the micro-bubble agent used in our unit. The micro-bubbles in SonoVue is composed of sulfur hexafluoride, which is an inert, innocuous gas. The micro-bubbles have a mean diameter of about 2.5 μm, which are small enough to pass through the capillaries. The interface between the sulfur hexafluoride bubble and the aqueous medium acts as a reflector of the ultrasound beam thus enhancing blood echogenicity and increasing contrast between the blood and the surrounding tissues.3 The micro-bubble agent is prepared according to the product insert. Before injection, the popliteal artery and vein are visualized with ultrasound vascular probe. The side port of the DPC is disinfected. Few milliliters of blood is drawn from the side port to remove any blood clots, followed by saline flush to ensure its potency. With the vascular probe in position and the ultrasound machine ready, 1 ml of prepared SonoVue is injected to the side port of the DPC, followed by saline flush. If the DPC is in correct position and perfusing the distal circulation, in few seconds, the popliteal artery is first enhanced by the micro-bubbles (Figure 1, [see Video 1, Supplemental Digital Content, http://links.lww.com/ASAIO/A321]). The micro-bubbles are washed out, following blood flow to the distal leg arterioles, capillaries and then return through the popliteal vein (Figure 2 [see Video 2, Supplemental Digital Content, http://links.lww.com/ASAIO/A322]). The sequential appearance of the micro-bubbles in the popliteal artery and vein following injection through the DPC confirms the proper position of the DPC for distal perfusion.
Micro-bubble contrast enhancing agents are commonly employed for enhancing echocardiogram images. Less commonly, they can be used for better blood vessel assessment.3 This is the first article to describe the use of micro-bubbles through DPC injection to confirm its position in patients supported by peripheral VA-ECMO.
The need to confirm DPC position is common and is often triggered by signs of poor circulation of leg to which the arterial ECMO catheter is placed. These signs include pale limbs with slow capillary refill, decreased flow to the DPC as detected by the flow meter, drop in oximetry or signal tracing with Near-Infrared Spectroscopy4 or loss of Doppler signals in distal circulation. When compared with other imaging techniques including computer tomography and contrast x-ray to confirm DPC position, the main advantage of our method is avoidance of patient transport. The condition of a patient who has a recent peripheral VA ECMO set up is critical and patient transport is clumsy and risky. This method also precludes patient exposure to radiation and renal toxicity from iodine-based contrast agents. When compared with the method described by Bunge et al.2, our method does not disturb the closed ECMO circuit and avoids vascular injury by blind guide-wire insertion.
There are few complications and contraindications for using micro-bubbles ultrasound enhancing agent. Sulfur allergy and large intracardiac shunts are the absolute contraindications of using SonoVue.3 Risk of systemic air embolism is one of the major concerns of intra-arterial micro-bubble injection. However, intra-arterial injection for diagnostic purpose is proven safe and supported by international guidelines.3 As described, the small sizes of the micro-bubbles allow their passage through capillaries. When a small amount of the micro-bubbles are injected to the end artery of the leg through the DPC, they pass through the arterioles, the capillaries and flow back to the leg venous system. They will be exhaled when they reach the lung through the pulmonary circulation. The lack of triggering of the bubble detection alarm by our ECMO machine confirms the absence of systemic leak of micro-bubbles. However, we would also recommend close monitoring of the patient by another ECMO specialist during this test, as there has been report of ECMO circuit cut-flow due to activation of bubble alarm when performing micro-bubble echocardiography.5 If there are residual micro-bubbles when the test is finished, complete destruction of the remaining micro-bubbles by ultrasound waves of high mechanical index is recommended.3
Another major concern is the lack of micro-bubble imaging time in patients on VA-ECMO, due to potential destruction of micro-bubbles by the ECMO oxygenator6 and the ultrasound waves of the vascular probe with high mechanical index.3 In our method, despite only 1 mL of SonoVue use, the ultrasound image is good enough for our diagnostic purpose, despite possible swirling artifact from micro-bubble destruction. The flow to the DPC is low. The low flow allows stasis of micro-bubbles and thus adequate imaging time. Moreover, the injection of the micro-bubbles through the DPC means that the micro-bubbles would pass through the leg circulation for our diagnostic purpose before possible destruction by the ECMO oxygenator.
While a positive test by our micro-bubble technique reassures the correct position and functioning of DPC, there are potential other reasons for a false negative test. Causes of limb ischemia are not limited to malfunction of DPC alone. Peripheral vascular disease and acute arterial thrombosis or embolization, for example, can result in negative micro-bubble tests. We would recommend other diagnostic imaging when a negative micro-bubble test could not help to confirm proper DPC position. Besides, a positive test does not guarantee adequate perfusion to the distal circulation and the current methods for monitoring including Doppler and Near-infrared spectroscopy remained nonreplaceable. It is however possible that the timing of detection of microbubbles from the popliteal artery to the vein might correlate with the lower limb perfusion. Further studies on this might shed light on the application of this test to monitor distal limb perfusion.
Finally, confirming DPC position by SonoVue is an unlabeled use of this ultrasound image enhancing product.
In patients supported by VA-ECMO, DPC position confirmation by micro-bubble injection through side-port of DPC and imaging of the popliteal vessels by ultrasound is a safe, simple and feasible bed-side technique.
1. Spurlock DJ, Toomasian JM, Romano MA, Cooley E, Bartlett RH, Haft JW. A simple technique to prevent limb ischemia during veno-arterial ECMO using the femoral artery: The posterior tibial approach. Perfusion 2012.27: 141–145.
2. Bunge JJH, Mahtab EAF, Caliskan K, Reis Miranda D. Fast confirmation of correct position of distal perfusion cannula during venoarterial extracorporeal membrane oxygenation. Intensive Care Med 2018.44: 658–660.
3. Porter TR, Mulvagh SL, Abdelmoneim SS, et al. Clinical applications of ultrasonic enhancing agents in echocardiography: 2018 American Society of Echocardiography guidelines update. J Am Soc Echocardiogr 2018.31: 241–274.
4. Wong JK, Smith TN, Pitcher HT, Hirose H, Cavarocchi NC. Cerebral and lower limb near-infrared spectroscopy in adults on extracorporeal membrane oxygenation. Artif Organs 2012.36: 659–667.
5. Bennett CE, Tweet MS, Michelena HI, Schears GJ, Mulvagh SL. Safety and feasibility of contrast echocardiography for ECMO evaluation. JACC Cardiovasc Imaging 2017.10: 603–604.
6. Platts DG, McDonald C, Shekar K, et al. Quantification of perflutren microsphere contrast destruction during transit through an ex vivo extracorporeal membrane oxygenation circuit. Intensive Care Med Exp 2016.4: 7.