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Pediatric Circulatory Support and Perfusion

Early Experience with Low-Prime (99 ml) Extracorporeal Membrane Oxygenation Support in Children

Yamasaki, Yasumori; Hayashi, Teruyuki; Nakatani, Takeshi; Yotsuida, Hideki; Nishigaki, Takayuki; Takahashi, Yuzo; Inamori, Shuji; Kagisaki, Kuji; Hagino, H; Ishizaka, T; Yagihara, Toshikatsu

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doi: 10.1097/01.mat.0000178960.76823.c7
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Abstract

Postcardiotomy patients and those with severe congenital heart disease cannot be managed by medical treatment alone and most often require some form of mechanical support. In adult patients, mechanical circulatory support after conventional medical therapy is established has become a standard procedure in the treatment of acute cardiopulmonary collapse.

However, mechanical circulatory support in children is much more challenging, and most device manufactures have focused their research on adult-sized assist devices.

Many groups have reported their experiences in the use of extracorporeal membrane oxygenation (ECMO) for cardiotomy circulatory support; survival rates of nearly 50% can be expected.1–9 However, ECMO use in postcardiotomy mechanical support is not necessarily established. Nevertheless, these devices are in fact used in cardiopulmonary resuscitation; it is therefore necessary to design separate devices for children and adults. Quick setup is mandatory for cardiopulmonary resuscitation using ECMO assist devices.

Our conventional ECMO circuit for children consists of a centrifugal pump and membrane oxygenator. Because of the large priming volume (260 ml), the circuit had to be primed with donor blood and required 30 minutes for setup. In 2000, we started using a low-prime (99 ml) ECMO with a small centrifugal pump and small membrane oxygenator for the induction of ECMO. Herein, we review our experiences with cardiopulmonary resuscitation for sudden cardiopulmonary collapse in children.

Materials and Methods

Patients

Data were obtained in a retrospective manner from 35 children who required ECMO support from July 1997 to December 2004 because of postcardiotomy or sudden cardiopulmonary collapse in the intensive care unit (ICU), catheterization room, or children’s unit.

Thirteen of these patients had two-ventricle (collection) physiology and 22 patients had single-ventricle (palliation) physiology (Table 1). From 1997 to 2000, 23 patients underwent ECMO support with conventional circuit: group A. From 2000 to 2004, we used low-prime circuit for induction of ECMO in 12 patients: group B. After the induction of ECMO with low-prime ECMO with low-prime circuit, ECMO was converted to conventional circuit for the longer support.

Table 1
Table 1:
Physiological Background of the Conventional Group (Group A; n = 23)

ECMO Circuit

EMASEVE CHILD CX-ESL-002: The Main ECMO Circuit Normally Used for Children.

Our conventional ECMO circuit, EMASEVE CHILD ESL-002 for children, consists of a centrifugal pump (CX-HP Terumo, Tokyo, Japan) and a membrane hollow fiber oxygenator with a heat exchanger (Capiox-10H, Terumo INC, Tokyo, Japan). The circuit’s volume capacity is 260 ml, with a maximum flow 4.0 l/min. Blood surface area is all heparin-bonded. In-and-out blood tubing is 6 mm in diameter, with a length of 50 cm (Figure 1).

Figure 1.
Figure 1.:
The EMASEVE CHILD CX-ESL-002, the ECMO circuit we normally use. Except for cases of emergency ECMO, this circuit is primed with donor blood and then used in the child. The picture was taken during an actual operation.

99kun: A Starter Circuit for ECMO for Low-Body-Weight Children

In 2000, we started to use a low-prime circuit, 99kun, with a volume capacity of only 99 ml (Figure 2). The circuit has a small centrifugal pump (HPM-15, Edwards, Tokyo, Japan) and a small oxygenator (Menox αCube 2000EL, DIC-Edwards Inc, Tokyo, Japan). These devices are placed in the chest area of a patient by an expansion and contraction arm. The circuit can be primed without donor blood even for the smallest patient and requires only 10 minutes to set up. Except for a convert from cardiopulmonary bypass for cardiac surgery, we use it for cardiopulmonary resuscitation. After the induction of ECMO with 99kun, ECMO was converted to prime on the donor blood EMASEVE CHILD ESL-002 for a longer support period. This was a result of our experiences with the 99kun, in which performance was not sustained for a long period. Additionally, no heat exchanger is provided in this device to reduce the priming volume, thus increasing the risk of hypothermia. However, the EMASEVE CHILD ESL-002 features a heat exchanger and performs sufficiently for long periods, but because its large volume, priming with donor blood and adjustment of electrolytes and albumin are required. The 99kun was developed to complement ECMO during the preparation period.

Figure 2.
Figure 2.:
Use of the 99kun low-prime (99 mL) ECMO circuit during an actual operation. As shown in the picture, the circuit is shortened through placement near the chest of the patient. A surgeon sets up the circuit on the other side of a sterile sheet.

ECMO Management

In all patients treated with ECMO, access was by the venoarterial route through the operative or nonoperative median sternotomy. Venous drainage was achieved via the atrium using an 8 to 14 Fr Bio-Medicus pediatric cannula (Medtronic). The outflow was returned into the ascending aorta through an 8 to 14 Fr Bio-Medicus pediatric cannula. All patients were treated by a venoarterial bypass procedure.

Anticoagulation was initiated with 100 U/kg body weight of heparin and maintained to an activated clotting time of 200 seconds. Flow rates of about 100 ml·kg –1·min –1 were maintained depending upon the physiology and arterial blood gas, not metabolic acidosis. When an increase in the fall of oxygenation, fall of the carbon dioxide removal rate, and a hemolytic reaction were seen, the circuit exchange was done. Mechanical support was performed until the heart recovered, but even if recovery could not be expected, support was continued as long as the family requested it. (In Japan, organ transplantation and artificial heart support in children are prohibited. Thus no transient ECMO is performed before transplantation.) Weaning from ECMO was performed by lowering the flow volume to reduce the level of support and lowering the oxygen concentration of an artificial lung in the absence of hemodynamic changes (i.e., provided there was no exacerbation), elevated lactic acid, or metabolic acidosis.

Statistical Analysis

Patient data are presented as a range of the mean or as a percentage of patients in a group. The statistical package Statmate (Atoms, Tokyo, Japan) was used for all examinations. Differences between mean values were analyzed using a t test or Fisher exact test where appropriate. All other dichotomous variables were analyzed using a chi-square test. Differences were considered significant at the p < 0.05 level.

Results

Extracorporeal membrane oxygenation was used to support 35 children undergoing cardiology treatment. The median and mean age of these patients was 0.25 and 0.98 ± 1.93 years, respectively, and the median and mean body weight of these patients was 3.23 and 4.75 ± 3.45 kg, respectively. The median and mean days for weaning were 4 and 9.3 ± 16.3 days, respectively. ECMO was initiated in an ICU for 24 patients, a percutaneous coronary intervention room for four patients, a pediatric ward for two patients, and an operating room for three patients on an emergency basis during surgery without an artificial heart and lung. Tables 1 and 2 show the background factors and support period for both groups.

Table 2
Table 2:
Physiological Background of Group B (n = 12)

In group A (Table 1) (conventional methods; n = 23), the EMASEVE CHILD CX-ESL-002 was primed with adjusted donor blood. The mean age and body weight for this group was 1.34 ± 2.3 years and 5.37 ± 4.0 kg, respectively. The mean support time was 6.6 ± 7.7 days. In 16 patients, ECMO was initiated in an ICU because of low postoperative cardiac output and sudden cardiopulmonary collapse after cardiac resuscitation. ECMO was initiated for sudden arrhythmia and cardiac arrest in a percutaneous cardiac intervention room in four patients and in a pediatric ward in three patients. Fourteen patients received single-ventricular repair, whereas nine patients received biventricular repair. Weaning was successful in three patients (13%), and one patient was discharged (4.3%).

In group B (Table 2), after the low-prime 99kun was used, ECMO was performed using the EMASEVE CHILD CX-ESL-002 primed with adjusted donor blood. The mean age and body weight for this group was 0.30 ± 0.21 years and 3.55 ± 1.52 kg, respectively. The mean support time was 14.5 ± 25.6 days. In 17 patients, ECMO was initiated in an ICU because of low postoperative cardiac output and sudden cardiopulmonary collapse after cardiac resuscitation. In three patients, ECMO was performed unexpectedly because of intraoperative acute cardiopulmonary collapse without an artificial heart and lung in an operating room. Seven patients received single-ventricular repair, whereas five patients received biventricular repair. Weaning was successful in six patients (50%), and five patients were discharged (41.7%) (Table 3).

Table 3
Table 3:
Group A versus Group B

Discussion

Over the last 10 years, the number of adult patients undergoing ECLS has markedly increased, and different ECLS devices have been developed and proven useful. Although the number of pediatric patients with congenital heart disease increased beginning in 1995, ECMO has mostly been performed using devices designed for adults. Because the number of pediatric patients is small relative to the number of adult patients, it makes less business sense for manufacturers to design devices for children; this is compounded by the low rate of weaning.12 Even when transient ECMO is performed before artificial heart support in heart transplantation, the rate of weaning is low. Finally, ECMO is not necessarily an established technique.3

In adults, extracorporeal life support is positioned as effective mechanical support during cardiopulmonary resuscitation when the subject is unresponsive to other treatments. The design of extracorporeal life support equipment is simple, and setup is quick and convenient. In emergency cardiopulmonary resuscitation, quick extracorporeal life support and circulation setup can prevent the onset of irreversible organ injury. As is the case with adults, quick setup is also necessary in children during acute cardiopulmonary collapse, as in conventional cardiopulmonary resuscitation. Device setup and blood access establishment hinder quick ECMO setup in children.5 When making a switch from cardiopulmonary bypass to ECMO, promptness is not an issue because this is merely a switch from one type of mechanical support to another. Therefore, device factors (heat exchanger, heparin coating, durability, and adjustability) are more important, and circuit volume is not a major issue. In other reports,10 switches from cardiopulmonary bypass are mixed with other cases, and different devices were used; as a result, it is difficult to compare data among studies.

We developed a low-prime circuit for rapid ECMO setup except for cases in which weaning from artificial lung heart support is difficult. Although most past pediatric reports used adult devices, our circuit is designed especially for children. If an existing circuit is used without any donor blood, hemodilution could easily have exceeded 30% for the patients in the present study. Severe hemodilution during cardiopulmonary resuscitation can lower oxygen transport and can further impair the peripheral circulatory system. To resolve these problems, donor blood is needed, but it has been shown that the level of potassium in donor blood increases with the duration of storage due to erythrocyte damage. Therefore, to adjust donor blood components, another circuit in addition to ECMO is prepared to adjust these components; the resultant preparation requires 30 to 40 minutes. The low volume of the 99kun low-prime circuit, which we have been using since 2000, allows for minimization of hemodilution. Also, because no donor blood is used, preparation can be markedly shortened, to less than 5 minutes.

As is the case with ECMO in adults,8 the study results also suggest that quick setup can affect vital prognosis during acute cardiopulmonary collapse in children (Table 3). However, due to the significant variation in patient and disease factors, the study findings do not indicate definitively that the low-prime circuit is useful in all patients; further investigation is therefore necessary. Additionally, there are no established criteria for pediatric ECMO. Standards for ECMO methods, management, and initiation must therefore be established.

Another problem arises in that even with rapid ECMO setup, the blood access line presents an issue. In all of our patients, a blood access line was secured by performing a thoracotomy. Securing a sufficiently thick blood access line is essential for the adjustment of flow volume. With a cervical or inguinal blood access line, the degree of freedom in flow volume adjustment is low. However, the low invasiveness of a cervical or inguinal blood access line should be taken into consideration except in cases of heart surgery. Additionally, thoracotomy requires a proper surgical environment and skilled surgeons; these points also need to be taken into account. There have been few reports on unprimed ECMO circuits. We agree on this point with Morris et al.6 that even if rapid preparation were possible without using donor blood, the time required for thoracotomy remains an issue. It will thus be necessary to develop a technique to secure a blood access line.

The duration of mechanical support in the present study was longer relative to durations reported elsewhere.7–9 Longer mechanical support clearly compromises prognosis. This is because heart transplantation and artificial heart support in children are not approved in Japan. At our institution, even if a clinician determines that there is no hope for cardiac recovery, there is no other alternative (i.e., heart transplantation or artificial heart support), and our only option is to continue ECMO until the patient dies while on mechanical support. Furthermore, it is the family’s decision whether to continue ECMO. It is our hope that pediatric heart transplantation and artificial heart support will be approved in the near future. We expect that this will further improve weaning and discharge rates.

Conclusion

In the present study, we initiated ECMO in 36 patients due to acute cardiopulmonary collapse in an ICU, PCI room, or ward. Of these, the rate of weaning and discharge improved for 12 patients who were first placed on a low-prime circuit before ECMO after 2000. These findings suggest the usefulness of a low-prime circuit before ECMO. However, it cannot be concluded based solely on the study findings that the use of a low-prime circuit before ECMO is effective in all cases, and further investigation is thus required.

References

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