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Review Article

Pediatric Cardiopulmonary Bypass Devices: Trends in Device Use for Cardiopulmonary Bypass and Postcardiotomy Support

Groom, Robert C.

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doi: 10.1097/01.mat.0000180399.24938.a5
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Abstract

The devices used for pediatric cardiopulmonary bypass (CPB) undergo design improvements, and new innovative devices emerge periodically. Clinical evaluations of devices are frequently reported in the literature; however, information about the extent to which theses devices are actually used in clinical practice at centers is not well described. Historically, decisions related to device use are based on a number of factors including but not limited to perceived patient benefit, cost, safety, ease of use, and tradition.

Maine Medical Center began its open heart surgery program in 1958.1 The Clowes pump oxygenator was selected for use based on the perceived advantages of a membrane oxygenator over those systems with direct blood-gas interface.2 The Clowes membrane system required a large priming volume and was prone to leaking (Figure 1). It was not long before this system was replaced with the more reliable bubble type oxygenator that had a much lower surface area, require less homologous blood for priming, was easier to set up and prime, and was not prone to leaking. This article reviews the actual extent to which various devices are used for intraoperative CPB and for postcardiotomy support using extracorporeal membrane oxygenation (ECMO) for pediatric cardiac surgery patients. The reported findings are based on surveys of pediatric open heart centers that we conducted in 1989,3 1994,4 1999,5 and 2004.6 These surveys provide the most comprehensive information available related to pediatric CPB devices use and practices in North America. The most recent survey, completed in January of this year, included information from 51 centers with an average case volume of 195 procedures per year comprising a total of 9,358 CPB procedures during 2004. The series of surveys documents the historical trends in devices used for intraoperative CPB and postcardiotomy support for pediatric patients over the past 16 years. An understanding of the trends in practice provides important lessons that may shape our thinking related to future changes in practice and device use. Device selection requires careful consideration of a number of factors including, cost, utility, safety, and measurable benefit to the patient. The quality of care depends not only on the devices selected, but also on the fine details of the processes related to device use.

Figure 1.
Figure 1.:
Clowes Membrane Oxygenator in use at Maine Medical Center in 1958.

Intraoperative CPB Device Use

Pumps.

Two dominant pump design types are used for CPB: the occlusive dual roller pump first patented in 1855 by Porter and Bradley7 and then modified by DeBakey in 1934,8 and the centrifugal constrained vortex or kinetic pump introduced in 1976.9 The positive displacement roller pump offers the advantages of simplicity, low cost, and low prime volume. Advantages of the centrifugal pump include: 1) reduced potential for rupture or disconnection of a circuit component; 2) cessation of flow, if a large amount of air is introduced into the pump; 3) elimination of tubing wear or spallation; and 4) reduced blood trauma. Although the use of centrifugal pumps has grown to the point where they are used more than roller pumps for adult cardiac surgery,1,10 centrifugal pump use has remained extremely limited in pediatric cardiac surgery. The primary disadvantages of centrifugal pump use are the risks of retrograde flow and possibility of air embolism, if the pump were to be inadvertently stopped causing the fluid column from the aortic cannula to the venous reservoir to produce a siphon effect. This leading concern may possibly be prevented by placing a mechanical one-way valve11 in the CPB circuit or using a pump system with an automatic line clamp device.12 Another concern is the potential risk of drawing air into the circuit across the oxygenator fiber bundle when vacuum-assisted venous drainage (VAVD) is used.13 Centrifugal pumps require the use of a separate flow probe that must be carefully calibrated and thus subject to measurement error. The centrifugal pump disposable component adds a significant cost to the CPB circuit. A novel method for using a centrifugal pump for blood propulsion and augmentation of venous drainage for pediatric CPB was described in 2001 by Ojita and colleagues.14 This innovative adaptation is a departure from conventional circuit configurations and has not been widely adopted to date. A large series by Parolari and colleagues15 found no difference in mortality and a small but significant difference in the occurrence of permanent neurologic injury related to pump type.

Current Practice

Ninety percent of North American pediatric cardiac centers use roller pumps for arterial return, whereas 2% use centrifugal pumps exclusively and 8% use some of each type (Tables 1 and 2)

Table 1
Table 1:
Safety Devices and Circuit Components
Table 2
Table 2:
Online Monitoring in the CPB Circuit

Oxygenators

Hollow fiber membrane oxygenators have been the predominantly used type since the late 1980s, when 5% of centers reported exclusive use of bubble oxygenators and 13% reported occasional use of bubble oxygenators for selected simple procedures that would require a short period of CPB. Membrane oxygenators have been used exclusively over the past 10 years. Great improvements have been made in hollow fiber oxygenators during the past two and one half decades. The first hollow fiber devices used designs with blood flowing through the fiber with the gas compartment surrounding the fibers. These devices required a larger surface area to transfer sufficient oxygen. With the passage of blood through the fibers, the resistance to flow was higher, producing higher shear forces on the blood formed elements. All of the more recent designs are configured with gas flowing through the fiber and blood flowing around the outside of the fiber, thereby reducing blood flow resistance, increasing the boundary layer, and improving gas exchange.

Although once there was equal use of oxygenators with collapsible venous reservoirs and those with hard shell reservoirs, there has been a steady trend away from the use of oxygenators with collapsible venous reservoirs (closed systems) to oxygenators with hard shell (open systems).6 The use of an open system offers several distinct advantages. Unlike collapsible reservoirs, it is not necessary to actively aspirate air that may be entrained in the venous line during CPB. The buoyant air migrates to the top and escapes. However, a number of studies have reported a higher incidence of gaseous microembolism (GME) caused by air entrained in the venous line and, furthermore, that VAVD further increases GME counts.16–20 Wilcox19 has raised concern that VAVD has been used clinically without any significant redesign of the components of the CPB circuit to improve the gas handling performance in negative pressure conditions. Currently, VAVD is used by 47% of centers. There are some cost and ease-of-use issues as well, because hard-shell venous reservoirs also include an integrated cardiotomy reservoir. Furthermore, the prime volume may be slightly reduced, because the integration of the venous reservoir with the cardiotomy eliminates connecting circuitry, and one may opt for a smaller-bore venous line and use VAVD. There are a number of disadvantages to the open systems. The circulating blood is exposed to a larger and more complex surface that contains defoaming sponges and antifoam agents. With these systems, air entrained in the venous line is likely to be ignored because is not necessary to actively purged the air as in the case of the closed system. As previously mentioned, air entrained in the venous line is not completely removed from the circuit and be infused into the arterial outflow of the CPB circuit and into the patient’s arterial circulation, including cerebral circulation, producing contact activation of the vascular endothelium or obstruction at the microcapillary level. Most hard-shell open systems do not provide a means of sequestering cardiotomy blood that may contain fat, bone, lipids, and other debris from the surgical field, which may exacerbate the systemic inflammatory response or traverse the CPB circuit or likewise obstruct the microcapillary circulation.21 When arterial outflow rate exceeds the venous return rate in a closed system, the amount of air introduced into the circuit is minimal because the reservoir collapses before a large amount of air is introduced into the arterial line or centrifugal pump.

In summary, most of the advantages of the open system are related to ease of use. Many of the disadvantages of open systems may be attenuated by systematically adopting good techniques; for example, eliminating the entrainment of air in the venous line should it occur, careful use of the cardiotomy suction system, maintaining a safe operating level in the venous reservoir, and using a level detector on the venous reservoir. However, two recent randomized clinical trials have found superior clinical outcomes with systems equipped with a closed reservoir and a centrifugal arterial pump.22,23

Current Practice

Membrane oxygenators are now used exclusively. The use of open systems (hard shell) has increased to 88% of all centers, whereas closed systems (collapsible reservoirs) are used by 10% of centers and 2% use some of each type. Forty-six percent of the centers use VAVD (Table 1).

Cardiopulmonary Bypass Circuit Surface Modification

The surface area of the circuit relative to the patient’s surface size is substantially higher for pediatric patients, especially neonates and infants. The work of Gourlay and colleagues24,25 in an animal model has shown that the ratio of circuit surface area to subject body size is related to the expression of the adhesion molecule CD11b(mac-1) on neutrophils, suggesting that minimizing circuit surface area or surface material would be beneficial in that it would reduce the systemic inflammatory response. Heparin-bonded CPB circuits have been available since the late 1980s; more recently, surface modifications that inhibit the inflammatory response and the activation of platelets have become available. The advantages of surface-coated circuits have been well documented for adults and pediatric patients; however, it has taken 20 years for the use of modified circuits to become routine.26–29

Current Practice.

Surface modifications, or coatings of the CPB circuits, are routinely used by 74% of all centers (Table 1).

Ultrafiltration

Ultrafiltration has become an important strategy to reduce fluid overload, increase hematocrit, and remove inflammatory response mediators. These devices have been used before CPB to condition the prime (PreBUF), during CPB intermittently and continuously (UF), and have also been used after CPB to remove fluid from the patient and concentrate and return the blood from the CPB circuit (MUF).30 The extent to which these techniques are used is shown in Figure 2. Given the reported clinical benefits of MUF, it is not surprising that it is now used at 75% of centers in North America. However, it cannot be overstated that MUF is a highly technical process that requires the use of safety devices and vigilant personnel. More than 80% of the centers that use this technique have reported technical problems related primarily to human error.31

Figure 2.
Figure 2.:
Use of ultrafiltration. (From Groom RC, Froebe S, Martin J, Quinn R, et al: An update on pediatric perfusion practice in North America: 2004 survey. J Extra Corpor Technol, 2005.)

Current Practice.

Ninety-eight percent of the centers reported routine use of ultrafiltration. There were several methods of ultrafiltration reported, as shown in Figure 2. The total number of centers using modified ultrafiltration was 43% in 1994, 64% in 1999, and 75% in 2004. Of these centers, arteriovenous modified ultrafiltration is used by 65% of the centers and venovenous modified ultrafiltration is used by 10% of the centers (Figure 2).

Cardiac Support Systems

The availability of systems for cardiac support at centers has increased from 67% to 96% of centers having some system for ventricular assist for cardiac surgery patients. Thirty-one percent of centers report the availability of pneumatic assist devices. The pneumatic devices are used primarily on adolescent patients because pneumatic devices for use in infants and neonates have not to date been approved by the US Food and Drug Administration, and are only available on a compassionate-use basis. Respondents to our recent survey reported that they used assist devices (including ECMO) on 2.7% of their pediatric open heart surgery cases in 2004 (or approximately 268 support procedures in 2004).

The most common assist device available at centers was ECMO. The first use of long-term extracorporeal support was with the Kolobow spiral flow membrane oxygenator reported in 1971 in this journal.32 It is remarkable that the spiral flow oxygenator and servoregulated roller pump system have been the principle oxygenator and system used for long-term support for respiratory failure and for postcardiotomy heart failure for more than 30 years. However, during the past 15 years, there have been a growing number of reports regarding the use of hollow fiber membranes and centrifugal pumps for ECMO support, particularly for postcardiotomy heart failure.33–35 Two reports from the ELSO registry from 199036 and 200437 indicate that the Kolobow oxygenator and roller pump are universally used for support. However, Gunst et al.38 recently conducted a survey of 79 open heart centers in the United States and found that although 60% of the centers use the Kolobow oxygenator exclusively, 19% of centers exclusively use hollow fiber oxygenators, and 40% reported use of at least one hollow fiber oxygenator in 2004. For blood propulsion, 65% reported exclusive use of roller pumps, 12% exclusive use of centrifugal pumps, and 35% reported at least use of one centrifugal pump in 2004. These findings were identical to our recent survey.6 The reports from the ELSO registry do not accurately reflect actual device use for postcardiotomy support of cardiac surgery patients. According to Gunst and colleagues,38 40% of the centers that they surveyed were not ELSO Registry participants.

Horton and colleagues39 reported superior performance of the poly-4-methyl-1-pentene diffusion membrane in patients weighing 2.2 to 51.0 kg (n = 23), with the longest support lasting 1,119 hours. Unlike the current microporous hollow fiber membranes, this device uses hollow fiber technology with a true (nonmicroporous) membrane, and it is awaiting approval by the US Food and Drug Administration at the time of this writing. Other nonporous hollow fiber oxygenators are currently in early stages of development as well.40,41

Current Practice.

Ninety-six percent of the responding centers reported the availability of assist devices at their centers (Figure 3). Of the pediatric ventricular assist devices currently available, ECMO was the most common, available at 90% of responding centers. ECMO system components were described as follows: roller pump systems 65%, centrifugal pump systems 22%, and both types 12%. Oxygenators used in the ECMO systems were as follows: silicone rubber membranes 55%, hollow fiber membranes 16%, and both types 29%.

Figure 3.
Figure 3.:
Percentage of centers with ventricular assist devices. (From Groom RC, Froebe S, Martin J, Quinn R, et al: An update on pediatric perfusion practice in North America: 2004 survey. J Extra Corpor Technol, 2005.)

Decisions and the Diffusion of Innovations

How are decisions made about device use, and how do innovative new devices find their way into clinical use? Stammers and Mejak42 documented that cost pressures may have great influence over decisions to use various devices, even when these devices are known to have superior safety and effectiveness. Everett Rodgers43 developed a theoretical framework that describes the diffusion of innovations. In his model, he described five characteristics that affect the rate at which innovations are adopted: relative advantage, compatibility, complexity, trial-ability, and observe-ability. Clearly, all of these factors are at play in the diffusion of innovation in CPB. Rodgers went on to describe five categories of adopters of new ideas: innovators, early adopters, the early majority, the late majority, and laggards. Each of these categories of practitioners provides value to the change. Those in the latter categories add historical context and, although the rate of change is diminished by this group, they add value in that they require more complete evidence before making a change. They are less deliberate but more cautious. The rate of diffusion may be related to a learning curve, to the lack of sufficient evidence that demonstrates the benefit of the proposed change, or to social factors related to those proposing the change. Understanding the patterns of past decision making provides instructive insight into the diffusion of change in clinical practice. Uncertainty and diversity will continue to emerge along with new techniques and devices. Those that practiced cardiac surgery in the 1950s during the era of the Mayo-Gibbon heart-lung machine and the Clowes pump oxygenator marvel at the quality and sophistication of the devices available to cardiac teams today. As in the past, we grapple with difficult decisions. For them, it was the choice between the Clowes Membrane and the Rotating Disc. For us, it may be the use of open versus closed reservoirs, or roller versus centrifugal pumps. It is important to be careful and thorough when making decisions about which devices we select to support cardiac surgery patients; it is of equal importance to have an understanding of the context in which they are used and to develop appropriate guidelines locally that allow for their use in a way that is safe and affords the best possible outcome for pediatric heart surgery patients. Device selection requires careful consideration of a number of factors, including cost, utility, safety, and measurable benefit to the patient. The quality of care depends not only on the devices selected, but also on the fine details of the processes of care related to device use. For today’s teams, careful attention to performance characteristics and the limitations of devices should be paramount. Some device design characteristics (e.g., hard-shell venous reservoirs) may make devices easier to use; however, if they are not used carefully, the patient may be unwittingly subjected to increased risk of injury from GME. Further understanding of the variation that occurs during the use of CPB devices and of the underlying precursors to patient injury are important areas for future study that will lead to substantial improvements in the care of patients requiring CPB support.44

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