A tip for assessing blood flow in distal perfusion catheter during veno-arterial extracorporeal membrane oxygenation

Dear Editor,

In a recent release in the journal Intensive Care Medicine, Saura et al. presented a systematic echo checklist for managing veno-arterial extracorporeal membrane oxygenation (VA-ECMO) [1]. They proposed nine key challenges requiring ultrasound assessment throughout the entire VA-ECMO course. We have read this article with great interest and found the comprehensive insights highly beneficial. However, we would like to add a general comment regarding the assessment of blood flow in a distal perfusion catheter (DPC) during VA-ECMO using ultrasound.

In the last decades, V-A ECMO has been increasingly used to provide temporary cardiopulmonary support for potentially reversible cardiac diseases or as a bridge therapy to transplantation or ventricular assist device for unrecoverable cardiac illnesses. Peripheral cannulation represents the primary catheterization approach of VA-ECMO, usually accompanied by a high incidence of vascular complications [2]. Lower limb ischemia is a common and clinically important complication of VA-ECMO, often attributed to femoral arterial cannulation and vasospasm. Implanting a DPC into the superficial femoral artery effectively alleviates lower limb ischemia and is increasingly used for prophylactic purposes. According to the Extracorporeal Life Support Organization guidelines, maintaining a blood flow of at least 100 mL/min in the DPC is necessary to ensure adequate limb perfusion [3]. However, monitoring the blood flow in a DPC remains a challenge in the clinical management of VA-ECMO. While ultrasonic flowmeters offer continuous DPC flow assessment, it is typically limited to ¼-inch connecting tubes and is unavailable in many ECMO centers, particularly in resource-limited settings [4]. To address these limitations, we propose a method for quantitatively estimating the DPC flow using conventional ultrasound, without relying on flowmeters or specific tube diameters.

In our ECMO center, a 6-Fr introducer sheath is routinely used as a DPC to prevent limb ischemia in VA-ECMO patients, with dialysis tubing commonly serving as the connecting tube between the DPC and the arterial return cannula (Fig. 1A). First, a linear array probe (4–12 MHz) is placed on the connecting tube to obtain a long-axis view (Fig. 1B), allowing measurement of the internal diameter of the connecting tube (D_CT) (Fig. 1C), while tilting the probe to form a certain angle with the connecting tube to reduce the angle between the ultrasound beam and the blood flow (≤ 60°). Next, pulsed-wave Doppler ultrasound is used to measure the blood flow velocity. To accurately measure the time-averaged mean velocity (TAMEAN), the sampling volume size is adjusted to cover the diameter of the tube and a conventional angle correction line is regulated (Fig. 1D). Given the non-pulsatile and laminar nature of ECMO blood flow, the DPC flow can be calculated as: π × (D_CT/2)² × TAMEAN × 60 (mL/min). For example, with an ECMO flow of 3.0 L/min at a pump speed of 3500 RPM, the measured TAMEAN was 24.6 cm/s, resulting in a calculated DPC flow of 167 mL/min (Fig. 1D). When the pump speed was reduced to generate an ECMO flow of 2.0 L/min, the measured TAMEAN decreased to 17.8 cm/s, and the calculated DPC flow was 121 mL/min (Fig. 1E). The decrease in the DPC flow measured by conventional ultrasound is proportional to the reduction of ECMO flow, indicating the sensitivity of this method in measuring the DPC flow during VA-ECMO support. However, we have to realize that the flow measurement based on pulsed-wave Doppler ultrasound is not error-free. Multiple factors influencing the insonation characteristics could contribute to the measurement error, such as ultrasonic beam profile and vessel characteristics [5]. An in vitro model validation study demonstrated that the flow measurement error based on TAMEAN seems acceptable, indicating the possibility of improving the measurement accuracy through repeated measurements [6].

Fig. 1

A A 6-Fr introducer sheath was implanted as a DPC to prevent low limb ischemia, and a small-diameter dialysis tubing was used to connect the DPC and the arterial return cannula. B A linear array probe was placed on the connecting tube to obtain a long-axis ultrasonic view while tilting the probe to form a certain angle with the connecting tube to reduce the angle between the ultrasound beam and the blood flow (red arrow). C The internal diameter of the connecting tube was measured (0.38 cm) in the long-axis view D The measured TAMEAN was 24.6 cm/s and the calculated flow was 167 mL/min, with an ECMO blood flow of 3.0 L/min. E The measured TAMEAN was 17.8 cm/s and the calculated flow was 121 mL/min, with an ECMO blood flow of 2.0 L/min. DPC distal perfusion catheter; TAMEAN time-averaged mean velocity; ECMO extracorporeal membrane oxygenation

Overall, our experience suggests the feasibility and practicability of conventional ultrasound in quantitatively estimating the DPC flow during VA-ECMO. This non-invasive and real-time method enables dynamic DPC flow monitoring and should be considered in the echo checklist, particularly in resource-limited ECMO centers.