Extracorporeal membrane oxygenation (ECMO) has become a standard treatment option for neonates with severe respiratory failure.1 The two basic modes of vascular access in neonatal ECMO are venoarterial (VA) and venovenous (VV) approaches. Although VA ECMO has been the dominant mode used, concerns over risks of systemic embolization and carotid ligation have led to the development and use of the dual-lumen VV ECMO mode.2–4 This method allows for single venous cannulation and thereby eliminates the concerns of VA ECMO. Challenges to the VV ECMO approach include the dependence on good ventricular function and controlling and minimizing a phenomenon called recirculation.5 Recirculation is a dynamic event that results in a fraction of the oxygenated blood exiting the arterial lumen shunting back into the venous lumen. This may result in suboptimal oxygen delivery to the patient and is influenced by a variety of factors. These include cannula and patient position, volume status, pump flow rate, and cardiac output.6 Most commonly; the monitoring of recirculation during VV ECMO has been performed with the use of indirect methods. Recently, it has been demonstrated that a technique called dilutional ultrasound could be used to quantify the recirculation fraction.7
There are several reasons why monitoring and quantifying recirculation during VV ECMO may be important, as the percent of neonatal respiratory ECMO patients using the VV ECMO mode has steadily increased and is approaching 50% of all respiratory ECMO cases annually.8 Accounting for this increase has been the evidence of better outcomes of VV compared with VA modes and the extension of VV ECMO into a wider patient population.9 Although once reserved for lower-risk patient groups, VV ECMO is now being used more aggressively in more challenging ECMO patients. These include congenital diaphragmatic hernia patients and patients requiring inotropic support.10–12
To conduct a successful VV ECMO in the spectrum of respiratory patient populations, data that could aid in the optimization of oxygen delivery, such as accurate recirculation monitoring, could be important to trend and guide interventions in minimization of recirculation.
The purpose of this study was to investigate the utility of dilutional ultrasound monitoring during changing recirculation events in an animal model of VV ECMO. We hypothesize that dilutional ultrasound will provide accurate and rapid bedside determination of recirculation.
Materials and Methods
This study was performed on one healthy 16-kg swine in compliance with the Guide for the Care and Use of Laboratory Animals (NIH Publication 85-23, revised 1985). The Committee for the Humane Use of Animals at SUNY–Upstate Medical University, approved the protocol. One 16-kg pig was used. Anesthesia was induced with 50 mg/kg of sodium pentobarbital. The swine was then intubated and ventilated with a Galileo ventilator (Hamilton Medical, Reno, NV). Anesthesia was maintained with sodium pentobarbital (6 mg/kg per minute), using a continuous intravenous infusion. Heart rate and ECG were monitored with the use of a pacemaker/defibrillator system (Zoll Medical, Burlington, MA). Femoral arterial and central venous pressure lines were placed, and pressures were monitored with the use of Argon transducers (Model 049-992-00a, CB Sciences Inc., Dover NH) by a 16-channel Powerlab/16s (AD Instruments Pty Ltd., Milford, MA) interfaced with a Dell Dimensions XPS R400 computer (Dell Inc., Dallas, TX).
Placement of the 15F Origen dual-lumen (DL) VV cannula into the right atrium was accomplished through access of the right internal jugular vein. The animal was heparinized with a loading dose of 5000 units of porcine heparin.
The ECMO circuit consisted of quarter-inch polyvinyl chloride tubing, a silicon bladder (Gish), an Rx05 oxygenator (Terumo), heater/cooler (Seabrook ECMO Temp), a Cobe roller pump, and a stopcock AV bridge. The circuit was primed with 200 ml of lactated ringers, 5000 units of heparin, and 20 mEq of sodium bicarbonate. Stopcocks were placed in the circuit so that pre- and post-membrane blood saturations could be measured. A CDI 500 was also used to measure arterial and venous line blood parameters continuously.
Dilutional Ultrasound Monitoring
Dilution ultrasound technology relies on the detection of changes in ultrasound velocity through a medium. A saline bolus into the blood path will change the velocity properties of ultrasound and produce a dilution curve. Two clamp-on ultrasonic flow sensors were used, one at the DL VV cannula inlet and one on the outlet. By injecting saline into the arterial line, an inlet dilution curve will be produced. Any recirculated saline will produce a second dilution curve on the cannula outlet probe. The ratio of the areas of two dilution curves determines the recirculation.
R = Svenous Line /Sarterial Line
Calibrated transonic dilution ultrasound flow probes were placed on the arterial and venous lines. The flow probe monitor was interfaced to a computer that was using dilution ultrasound curve analysis software to automatically calculate the recirculation rate (Figure 1).
For technique comparison, pre-oxygenator, post-oxygenator, and patient venous blood samples were taken to calculate recirculation according to the saturation method.6 Simultaneously, a dilutional ultrasound measurement was made by injection of a 5-ml bolus of saline into the circuit post-oxygenator. Each measurement was made in triplicate at the following flow rates; 200 ml/min, 400 ml/min, 600 ml/min, and 760 ml/min. Time to results were also notes between the techniques.
To evaluate the ability of dilutional ultrasound to rapidly detect changes in recirculation technical and physiologic variability was introduced. At a fixed pump flow rate of 500 ml/min, cannula position was intentionally changed as follows: (a) position 1 (P1) is the recommended position with the arterial side holes approximated to direct inflow toward the tricuspid valve, (b) position 2 was a 90° posterior rotation. A physiologic change was induced by using a 50-mg bolus of amiodarone hydrochloride with the cannula in P1. Three dilutional ultrasound measurements were made by using a 3-ml bolus of saline for each.
Data are expressed as mean ± standard deviation. Significance is defined as a level of p < 0.05.
The dilutional ultrasound method produced results in approximately 1 minute while the obtaining recirculation using the saturation method took between 3 and 5 minutes. The comparison of recirculation calculations between dilutional ultrasound and the saturation equation method is illustrated in Figure 2. At each flow rate, there was no significant difference between techniques. At a constant pump flow rate (500 ml/min), changes in recirculation induced by catheter repositioning and physiologic alterations are shown in Figure 3. Changes in catheter position and changes in animal hemodynamics were significantly different than baseline.
The use of the VV mode of ECMO is increasing and expected to continue to be used more often in neonatal respiratory failure.13 The significance of recirculation in adversely affecting VV ECMO has resulted in newer cannula designs that can reduce recirculation and increase oxygen delivery.14,15 In addition to product improvements, accurate bedside monitoring strategies also need to be used in managing adverse levels of recirculation.
Previous efforts to quantify recirculation during VV ECMO have significant limitations that have precluded widespread use.16,17 In hemodialysis, the dilutional ultrasound technique has been adopted to measure AV fistula recirculation, and these data have become an important adjunct in these procedures.18,19 Recently, the technique was effectively demonstrated by van Heijst et al.7 in DL VV ECMO applications.7 We corroborate this work in our trials, where the dilutional ultrasound calculations of recirculation agree closely with the accepted but clinically limited and time-consuming saturation method (Figure 2).
Because recirculation is a dynamic event that is influenced by a variety of factors, we were particularly interested in the capacity of dilutional ultrasound to respond to changes. By manipulating the catheter, we demonstrated that dilutional ultrasound rapidly and effectively revealed changes in recirculation rates (Figure 3).
From the results of this preliminary animal study, we conclude that the use of dilutional ultrasound to assess and monitor recirculation during VV ECMO can provide valuable data to the clinician that could allow early detection of recirculation changes. This could guide the clinician in interventions to reduce recirculation and optimize oxygen delivery to the patient. In the face of poor VV ECMO, quantification of recirculation could provide invaluable data in troubleshooting VV ECMO situations where conversion to VA ECMO is being considered. It is conceivable that having this quantified data could help the clinician with troubleshooting and possibly even prevent needless conversion to VA ECMO.
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