A large patient population persists that requires complete respiratory support either for recovery from ARDS or as a bridge to lung transplantation. During the last three decades, ECMO, utilizing a modified heart-lung machine with a bulky, relatively high resistance silicone membrane gas exchanger has been used in select circumstances for prolonged respiratory support (weeks). Recently, a PAL has been developed for long term ambulatory respiratory support but currently major surgery is necessary to anastomose the PAL to the heart and main pulmonary artery.17,18,20–22 A DLC was developed two decades ago for infant/pediatric VV ECMO and proved the concept of respiratory support with a less invasive, single cannula venous access. Although commercially available since 1989 for application in newborns and small children, no alternative for adults exists. Recirculation, kinking and insufficient blood flow plague broader application.23,24 Our W-Z DLC is designed to almost eliminate recirculation and enhance performance in a larger patient. When combined with a compact pump-gas exchanger, this system may supply total respiratory support via minimally invasive percutaneous single venous cannulation to adults and large children.
Our current feasibility study demonstrates a very low recirculation rate at only 2 L/min blood flow with proper DLC placement. The asymmetric balloon below the SVC drainage opening also contributes to the minimal recirculation seen in our acute studies. Throughout our 15 day study, the O2 saturation gain across the gas exchanger is significant (up to 50%) at 2 L/min blood flow, but recirculation was relatively higher (20%) than in the acute studies. Positioning may have contributed to suboptimal DLC function evidenced by a more cephalad RA position (conjuction between SVC and RA) of the DLC infusion lumen opening found at autopsy in this animal.
We demonstrated the feasibility of percutaneous insertion and advancement of the W-Z DLC into the SVC-RA-IVC (25 Fr cannula) without fluoroscopic guidance in six consecutive cadaver sheep before the current study. Our 1 day in vivo studies further demonstrated feasibility of safe insertion without fluoroscopic guidance. However, our single chronic study emphasized the need for proper placement. In future studies and for clinical applications, we recommend fluoroscopic guidance during insertion. Similarly, transthoracic echocardiography may prove helpful allowing safe percutaneous insertion, advancement and optimal positioning in SVC-RA-IVC to avoid right heart injury, and minimize recirculation.
Benefiting from our unique cannula construction with an extremely thin membrane sleeve infusion lumen and thin wall stainless steel reinforced polyurethane outer wall, our 26 Fr DLC can achieve 2.0 L/min blood flow with <120 mm Hg ΔP. We have previously shown that for total CO2 removal, only 1 L/min arterial blood flow is needed.25 At 2 L/min venous blood flow, with no recirculation, a gas exchange device can remove the total CO2 production and transfer over 200 ml/min O2 with normal hemoglobin (12 g/l). Therefore, 2 L/min blood flow can meet the gas exchange requirements for most patients. If necessary, a larger DLC (up to 30 Fr) could be used for more blood flow (up to 5 L/min) and more gas exchange allowing total respiratory support under conditions of stress, hypermetabolism or large body surface area.
The W-Z DLC prototype is constructed with a high silicone content polyurethane copolymer with proven biostability characteristics.26 Under mild systemic heparinization (ACT 180–230 seconds), we found the DLC shaft and drainage openings completely thrombosis free at autopsy after the 15 day in vivo animal study. During our 15 day feasibility study, our W-Z DLC system did not require blood transfusion (blood hemoglobin was maintained above 9 g/dl).
In this first study of our W-Z DLC, only one long-term sheep was used; more studies are needed to prove consistent performance. We did not directly verify DLC position until autopsy, which can be very easily accomplished by fluoroscopy or echocardiography in a clinical setting. Our measures of recirculation are clinically sufficient but a computational fluid dynamics (CFD) analysis may better elucidate the in vivo flow characteristics of the catheter.
In conclusion, the W-Z DLC minimizes recirculation rate, maximizes the cross-sectional flow area at a given DLC size, to maximize flow and to enhance the PAL system’s gas exchange performance. The one site percutaneous venous cannulation may allow total gas exchange as an ambulatory PAL circuit. Our design refinements will be further tested in long-term large animal studies and in prospective randomized outcome studies in our sheep model of ARDS.
Supported in part by a National Institutes of Health STTR Grant No. 5 R42 HL067523 and Avalon Laboratories, Rancho Dominguez, California. The large animal experiments were conducted at UTMB, Galveston, Texas.
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