How to do it Article
Use of the EXCOR ventricular assist device (VAD; Berlin Heart Inc, The Woodlands, TX) has become a standard treatment in pediatric bridge to transplantation.1–3 Despite its increased use in the pediatric population, hematologic issues, such as postoperative hemorrhage (44%), and neurologic dysfunction (29%), represent the major complications with this device, as described in the EXCOR investigational device exemption trial.2 The majority of neurologic events occurred within the first 14 days of support and represented the most frequent cause of death.4 This certainly reflects the challenging nature of finding a balance between hemorrhagic and thrombotic risks in the acute postoperative period. In order to mitigate the complication risks during such a vulnerable period, a temporary use of an extracorporeal centrifugal pump has been suggested.5 In this report, we describe our approach using a temporary centrifugal pump connected to cannulas designed for the Berlin EXCOR VAD.
VAD implantation is conducted in a usual manner; the Berlin EXCOR cannulas (left ventricular apical and aortic) are inserted on cardiopulmonary bypass support. Instead of the Berlin EXCOR pump, an extracorporeal centrifugal pump is connected to the Berlin cannulas via a heparin-coated tubing (Figure 1). Depending on the size of the Berlin cannula, the sizes of the tubing and connectors are determined. A Berlin 6 mm cannula fits a 1/4-inch connector, and a 9 mm cannula works with a 3/8-inch connector. We use the Jostra Rotaflow (MAQUET Cardiovascular, Wayne, NJ) system for this purpose, which requires a 3/8-inch attachment, so an additional connector would be necessary to connect the smaller 6 mm cannula with the Rotaflow device. This centrifugal pump has a servo-regulation safety feature with which the machine console continuously monitors the pump inlet pressure. If the inlet pressure exceeds the alarm limit (set to −20 mm Hg below each patient’s baseline inlet pressure), the pump speed (i.e., rpm) automatically slows down to prevent a suction event. The pump speed is then automatically returned to the baseline once the inlet pressure returns within the normal range. In addition to the inlet pressure, we routinely monitor both right and left atrial pressures, and the pump flow is continuously monitored with a flow probe (Transonic Systems Inc., Ithaca, NY).
The flow measurement, as well as pressure monitoring (i.e., right and left atrium, and VAD inlet), helps to confirm the adequacy of VAD support and to determine the cause of VAD failure (Figure 2). The definition of “full-flow” at our heart center includes calculations based on each individual patient’s age and weight or body surface area (Table 1). By systematically analyzing these parameters, identification of VAD failure, if present, is greatly facilitated. Additionally, this information provides guidance regarding the necessity of RVAD support; specifically that an RVAD is indicated if flow is inappropriate due to RV failure. Once the patient is hemodynamically stabilized postoperatively and deemed ready to be converted to the EXCOR, flow information provides insight into size selection of the EXCOR pump. Since the cardiac output generated by the EXCOR depends on the pump size (i.e., stroke volume) and pump rate (beat per minutes), one can choose an optimal pump size and rate for each individual patient. For instance, if the patient’s ideal flow is measured at 900 ml/min, a 10 ml EXCOR pump should be set at 90 beats per minute, provided full-fill and full-eject of the EXCOR pump. When the measured flow falls into the “overlapping” areas, a decision needs to be made judiciously considering the patients’ clinical status. We prefer to choose a larger pump for these overlapping zones because the pump flow requirement typically increases over time as the patient progresses postoperatively.
The use of a temporary centrifugal pump connected to cannulas designed for the Berlin EXCOR VAD can be advantageous due to several factors, including the ability to bridge severely sick patients to a decision for transplantation listing, financial and logistic reasons, and reduced bleeding complications.6 In addition, at our institution, we use the Rotaflow device before transitioning to the EXCOR pump to provide hemodynamic data to determine the adequacy of VAD support or potential causes of VAD failure, as well as a patient’s clinical suitability to transition to the EXCOR pump, and to help choose the optimal Berlin pump size for each individual patient.
Postoperative management after the EXCOR placement is challenging due to the need for relatively heavy anticoagulation in the acute postoperative period.7 The myriad of abnormalities within the clotting cascade after cardiopulmonary bypass has been well documented,8 and concomitant renal and hepatic dysfunction lead to further coagulopathy. The ease of anticoagulation management is an additional benefit with the use of a temporary bridge with the Rotaflow centrifugal pump. Although there are no head-to-head comparison data, it is our experience that the Rotaflow support can be maintained with lower anticoagulation levels than the EXCOR support. Thus, the risk for surgical and neurologic hemorrhage during the most vulnerable period may be lessened by the use of the Rotaflow bridge without increased risk for thrombotic events. At our institution, we will generally use intravenous heparin as the standard anticoagulant of choice for the Rotaflow device. Heparin is typically started 2–3 hours postoperatively at an initial dose of 10U/kg/h without a bolus, granted there is no evidence of on-going surgical bleeding. The timing of heparin initiation and initial dose are the same for when we previously performed Berlin VAD implantation without initial use of a centrifugal pump. Bivalirudin will be used only if the patient develops heparin-induced thrombocytopenia, or if heparin treatment fails. We define failure of heparin therapy as thrombus formation within the circuit or thromboembolism in the patient while within a therapeutic range, or the inability to achieve a therapeutic range. In these instances, we would change to a second agent, specifically Bivalirudin. Our therapeutic range while using the rotaflow device is determined by an Anti-Xa level of 0.1–0.2 IU/ml, whereas when placing a patient on a full anticoagulation regimen, we will target an Anti-Xa level of 0.3–0.5 IU/ml.
Transition to the EXCOR device can be performed at bedside after the bleeding risk is lower and the perioperative coagulopathic state has been corrected. Given the significantly lower device cost of the Rotaflow pump compared with other devices, such as the Centrimag (Thoratec Corp., Pleasanton, CA) or the EXCOR, exchanging the Rotaflow pump head would pose a significantly less financial burden to the hospital. Although economy should not be the primary determinant of the patient care, such financial considerations are becoming more important in the current era. As pump exchanges are a simple bedside procedure without significant financial burden, clinicians may have a lower threshold for pump exchange and, in turn, accept a mild anticoagulation strategy, which eventually can result in less hemorrhagic complications. In our series of seven patients with the Rotaflow bridge to the EXCOR, we have not experienced any thromboembolic events despite a mild anticoagulation regimen, and have required two bedside tubing exchanges due to thrombus formation either within the circuit tubing or tubing connectors, which did not lead to any additional clinical sequelae (Table 2). Additionally, there were no cases of postoperative surgical bleeding which delayed initiation of heparin therapy in this group.
Potential downsides of this strategy include the larger size and complexity of the entire system compared with the simplified and mobile design of the EXCOR system. Temperature control may be a challenge particularly in small children with an open chest due to the longer VAD circuit that would cause heat loss from the exposed tubing. Therefore, we use as short of tubing as possible when connecting the patient to the Rotaflow device in an effort to minimize heat loss and exposure to foreign material, which leads to platelet activation. In our experience, the majority of thrombus formation occurs between the tubing connectors, as this is a site of turbulent flow, especially when there are differences in the tubing sizes being connected. Therefore, we attempt to limit the number of connections, and the length of tubing in our circuit to decrease thrombus formation. Covering the circuit with a warming blanket is an effective measure to prevent hypothermia. The inability to mobilize the patient is not a clinically significant issue because patients typically are not ambulatory yet in the first couple of days postoperatively.
In conclusion, interim use of an extracorporeal centrifugal pump allows time for hemodynamic stability, limits the hemorrhagic risks, and provides relevant hemodynamic information before conversion to the EXCOR. We hope this report will elicit more discussion on the EXCOR management for further outcome improvement.
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Keywords:Copyright © 2019 by the American Society for Artificial Internal Organs
pediatric ventricular assist device; Berlin EXCOR; pediatric centrifugal pump