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Ann Thorac Surg 1997;64:521-525
© 1997 The Society of Thoracic Surgeons

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Original Articles: Cardiovascular

Modified Ultrafiltration After Cardiopulmonary Bypass in Pediatric Cardiac Surgery

Department of Cardiothoracic Surgery, University Hospital Leiden, Leiden, the Netherlands

Accepted for publication February 25, 1997.

	Abstract

Top Footnotes Abstract Introduction Material and Methods Results Comment References

 
Background. Cardiopulmonary bypass in children results in considerable water retention, especially in neonates and small infants. Dilution of plasma proteins increases water loss into the extravascular compartments. Excessive total body water may prolong ventilatory support and may contribute to a prolongation of intensive care convalescence. After discontinuation of cardiopulmonary bypass, modified ultrafiltration can be used to withdraw plasma water from the total circulating volume.

Methods. This retrospective study included 198 pediatric patients who underwent cardiac operations in the period from September 1991 to November 1994. Two groups were compared: 99 patients without ultrafiltration and 99 patients receiving modified ultrafiltration. The following indices were analyzed: cardiopulmonary bypass prime volume, transfused blood volume during and after the operation, postoperative chest drain loss, and hemoglobin and hematocrit levels before, during, and after the procedure.

Results. Modified ultrafiltration resulted in a significant increase in hemoglobin and hematocrit levels and a significantly lower amount of transfused blood. Mean postoperative chest drain loss was significantly less in the patients who underwent modified ultrafiltration.

Conclusions. Modified ultrafiltration decreases blood transfusion requirements and chest drain loss after pediatric cardiac surgical procedures.

	Introduction

Top Footnotes Abstract Introduction Material and Methods Results Comment References

 
Cardiopulmonary bypass (CPB) with hypothermia and hemodilution results in an increase of total body water. Water retention is especially important in neonates and infants. The ratio of prime volume to patient blood volume is greater in smaller patients. Hemodilution increases tissue perfusion during CPB and allows the use of hypothermia, which is needed to protect against ischemic organ damage during periods of low flow and circulatory arrest [1]. Hemodilution also reduces donor blood requirements during CPB. Dilution of plasma proteins increases water loss into the extravascular compartment and postoperative blood loss as a result of clotting disturbances [2]. Cardiopulmonary bypass itself produces an important inflammatory response, and this "whole body inflammatory response" is especially large in children [3]. One of the consequences of this inflammatory reaction is increased capillary leakage. Higher capillary permeability accounts for an increase in total body water, especially in the extracellular interstitial compartment. All of these factors may have negative consequences in the postoperative course. Renal immaturity together with decreased cardiac output further delay the return to normal body water content.

Intravenous diuretics and inotropic agents frequently are necessary in the younger age group to reduce the increase in total body water. If diuresis is inadequate, peritoneal dialysis may be needed temporarily. To avoid an excessive increase in body water while aiming to reduce the use of blood products as much as possible, synthetic large molecular substances are added to the CPB prime to increase the colloid osmotic pressure.

Conventional ultrafiltration during CPB to reduce excess water is limited by the need to maintain a minimum level in the venous CPB reservoir. This is especially true in pediatric CPB because lower prime volumes are used. Naik and Elliott [4] could not demonstrate a significant effect of conventional ultrafiltration on reduction of the total body water increase and donor blood requirements. Using ultrafiltration immediately after the cessation of CPB (modified ultrafiltration, MUF) they were able to diminish significantly the total body water increase and donor blood requirements [4]. Another advantage of MUF over conventional ultrafiltration is its ability to return the contents of the CPB circuit to the patient in a concentrated form.

In this study, we compared two groups of 99 patients each who underwent pediatric cardiac operations in our institution. In one group we used MUF; in the other group ultrafiltration was not used. The technique of MUF is described and its effects are evaluated on total donor blood use, postoperative chest drain blood loss, and preoperative, perioperative, and postoperative hemoglobin and hematocrit levels.

	Material and Methods

Top Footnotes Abstract Introduction Material and Methods Results Comment References

 
The study included 198 patients who underwent cardiac operations with CPB from September 1991 to November 1994. Two longitudinal cohorts were studied. The first cohort had procedures between September 1991 and July 1993, at which point MUF was introduced and used in all pediatric patients operated on thereafter. The second cohort consisted of patients operated on from July 1993 to November 1994. In the first 99 patients, no ultrafiltration was used; MUF was used in the second group of 99 patients. The two groups were comparable in sex, age, and body weight, as well as the duration of CPB. Preoperative values of hemoglobin and hematocrit were comparable (Table 1). Diagnoses did not differ between the groups (Table 2). Methods of surgery and anesthetic techniques essentially did not change during the study period. No changes in perfusion techniques concerning prime solutions or blood transfusion policy occurred in our unit from September 1991 to November 1994. Aprotinin (Bayer AG, Germany) is not used in our institution for pediatric cardiac operations. Conventional ultrafiltration also was not used in our unit for these procedures.

The calculated volume of red blood cells needed for CPB circuit priming was deduced from the following formula:

(1)

where C_v = circulating volume (weight [kg] x 80 mL); CPB_v = prime volume in the CPB circuit (mL); Ht₁ = preoperative hematocrit; Ht₂ = target hematocrit during CPB; Ht_TBV = hematocrit of transfused blood; and TBV = transfused blood volume (mL).

The pH was corrected with 10 mL NaHCO₃ 8.4% for every 250 mL of red blood cell volume. One hundred milliliters of human albumin 20% solution (CLB, Amsterdam, the Netherlands) was added, using Ringer's solution to complete the CPB priming volume. Mannitol was substituted in a dose of 0.5 g per kilogram of body weight.

A Minntech Hemocor HPH hemoconcentrator (Minntech Corporation, Minneapolis, MN) was used for MUF. In the patients with a body weight up to 14 kg, we used the Minntech HPH 400 ultrafilter with a prime volume of 27 mL. In the group of patients with a body weight from 14 to 35 kg, the Minntech HPH 600 ultrafilter with a prime volume of 43 mL was used, as the higher circulating volume in these patients necessitated a filter with a higher filtration rate. The molecular cutoff weight of both ultrafilters is 65,000 D.

The ultrafilter was primed together with the rest of the CPB circuit. The arterial cannula was connected to the inlet of the ultrafilter while the venous cannula was in connection with the outlet of the ultrafilter. During CPB, the inlet of the ultrafilter is partially clamped to allow limited flow through the filter (Fig 1). In this setup, conventional ultrafiltration is possible. Immediately after discontinuation of CPB, both cannulas remain in situ, the inlet is completely opened, and blood flows from the patient through the arterial cannula with the aid of a roller pump through the ultrafilter and back to the patient through the venous cannula (Fig 2). Our method of MUF leaves the arterial and one venous cannula in situ, whereas other groups replace the venous cannula for the purpose of MUF [4]. During MUF, the remainder of the volume in the CPB circuit is gradually remitted to the patient after having been concentrated by passage through the ultrafilter. The outlet of the ultrafilter may be partially clamped to allow a higher transmembrane pressure and a higher ultrafiltration rate. No vacuum suction is used. The volume loss in the venous reservoir is replaced first by the blood remaining in the venous tubing and later by added Ringer's solution. Modified ultrafiltration is finished when the residual fluid in the CPB circuit is almost completely replaced by clear Ringer's solution. Because the CPB system always remains fully primed, CPB can be restarted at any moment.

View larger version (16K): [in this window] [in a new window]   Fig 1. . Placement and blood flow of the ultrafilter in the circuit during cardiopulmonary bypass. (Ao = aorta; R.A. = right atrium; ven. res. = venous reservoir.)

View larger version (18K): [in this window] [in a new window]   Fig 2. . Placement and blood flow of the ultrafilter during modified ultrafiltration. (Ao = aorta; R.A. = right atrium; ven.res. = venous reservoir.)

Statistical analysis of the compared variables was with the Student's t test for all variables with the exception of the sex differences between the two cohorts, for which the ² test was used. A p value of 0.05 or less was regarded as statistically significant (A. H. Zwinderman, PhD, Department of Medical Statistics, Leiden University).

	Results

Top Footnotes Abstract Introduction Material and Methods Results Comment References

 
Mean CPB prime volumes were larger in the MUF group, with a mean (± standard error of the mean [SEM]) volume of 707 ± 7.89 mL in the group without ultrafiltration and 809 ± 8.64 mL (p < 0.01) in the group with MUF. This difference is attributed exclusively to the volume needed to fill the ultrafilter and the extra tubing required for MUF.

No difference was found between the groups for the red blood cell volume supplied during the operation. However, postoperative red blood cell transfusion was significantly less frequent in the group receiving MUF. The total of perioperative red blood cell transfusions was also lower in the group with MUF (Table 3; Fig 3).

View larger version (13K): [in this window] [in a new window]   Fig 3. . (A) Red blood cell volume transfused during cardiopulmonary bypass. (B) Red blood cell volume transfused after cardiopulmonary bypass. (C) Total transfused red blood cell volume. (MUF = modified ultrafiltration; UF = ultrafiltration.)

Preoperative hemoglobin and hematocrit levels did not differ between the groups (see Table 1). During CPB, the mean values of hemoglobin and hematocrit were lower for the group having MUF (p < 0.05). After discontinuation of CPB, the hemoglobin and hematocrit levels did not change in the group in which MUF was not used. Red blood cell transfusions were used in the early postoperative period to increase the hemoglobin concentration to a target level of 7.0 mmol/L in all patients. Modified ultrafiltration resulted in a significant rise of hemoglobin and hematocrit levels (p < 0.001). Early red blood cell transfusion could therefore be avoided in most patients in the MUF group. Hemoglobin and hematocrit values 4 hours after arrival to the intensive care unit were not different for both groups, as a result of blood transfusion in the group of patients without MUF (Table 4; Fig 4).

View larger version (15K): [in this window] [in a new window]   Fig 4. . (A) Course of hemoglobin (Mb) levels. (B) Course of hematocrit (Ht) levels. (CPB = cardiopulmonary bypass; ICU = intensive care unit; MUF = modified ultrafiltration; UF = ultrafiltration.)

	Comment

Top Footnotes Abstract Introduction Material and Methods Results Comment References

Modified ultrafiltration removes water from the circulating volume, leading to an immediate increase in hemoglobin and hematocrit levels. Removing water from the circulation gives us the opportunity to return almost entirely the volume of the CPB circuit to the patient. The need for blood transfusion decreases significantly, as does postoperative blood loss.

In many patients, mean blood pressures increased during MUF. This has also been observed by others [4, 5]. The rise in blood viscosity due to water loss may be responsible for this blood pressure increase. Another explanation may be found in the removal of vasoreactive substances by MUF. Inflammatory mediators such as interleukins, tumor necrosis factor, and activated complement components—many of them having cardiodepressive characteristics—are reported to be removed by ultrafiltration [6, 7].

We did not measure total body water content, but others have reported significant decreases in total body water using MUF [5]. The problem of excess water is especially important after neonatal cardiac procedures. In the pre-MUF period (before July 1993), we observed this problem much more often than in the patients in whom MUF was used (after July 1993).

Modified ultrafiltration is a useful tool to combat water retention after pediatric cardiac operations. It diminishes the need for blood transfusion by both removing excess water and returning all CPB blood to the patient in a concentrated form. It also decreases postoperative chest drain loss. Of course, the use of MUF does not reduce the need for further efforts to limit the CPB prime volume to diminish water overload while at the same time trying to restrict the use of blood and blood products as much as possible. Our CPB prime volumes for pediatric cardiac operations have been reduced substantially in recent years. Up to 6 kg body weight, a Dideco Liliput oxygenator is now used with a total prime volume of 350 mL, including the ultrafilter and extra tubing needed for MUF. In the group of patients with a body weight of 6 to 14 kg, a Dideco 701 oxygenator is used with a total CPB prime volume of 650 mL, and in the group of patients with a body weight of 14 to 29 kg, a Dideco 702 is used with a total CPB prime volume of 750 mL.

We studied retrospectively two comparable cohorts of patients and found significantly lower blood transfusion requirements and chest drain blood loss after MUF. Our perioperative protocols did not change during the period of the study. We believe that despite the shortcoming of not being prospective and randomized, this study clearly demonstrates the beneficial effects of MUF in a pediatric cardiac operative population.

	Footnotes

Top Footnotes Abstract Introduction Material and Methods Results Comment References

 
Address reprint requests to Dr Hazekamp, Department of Cardiothoracic Surgery, D6-S, University Hospital Leiden, PO Box 9600, 2300 RC Leiden, the Netherlands.

	References

Top Footnotes Abstract Introduction Material and Methods Results Comment References

 

1. Utley JR, Wachtel C, Cain RB, Spaw EA, Collins JC, Stephens DB. Effects of hypothermia, hemodilution, and pump oxygenation on organ water content, blood flow and oxygen delivery, and renal function. Ann Thorac Surg 1981;31:121–33.[Abstract]
2. Kern FH, Morana NJ, Sears JJ, Hicky PR. Coagulation defects in neonates during cardiopulmonary bypass. Ann Thorac Surg 1992;54:541–6.[Abstract]
3. Kirklin JK, Blackstone EH, Kirklin JW. Cardiopulmonary bypass: studies on its damaging effects. Blood Purif 1987;5:168–78.[Medline]
4. Naik SK, Elliott MJ. A successful modification of ultrafiltration for cardiopulmonary bypass in children. Perfusion 1991;6:41–50.[Abstract/Free Full Text]
5. Naik SK, Knight A, Elliott MJ. A prospective randomized study of a modified technique of ultrafiltration during pediatric open-heart surgery. Circulation 1991;84(Suppl 3):422–31.
6. Journois D, Pouard P, Greeley WJ, Mauriat P, Vouhé P, Safran D. Hemofiltration during cardiopulmonary bypass in pediatric cardiac surgery. Anesthesiology 1994;81:1181–9.[Medline]
7. Wang MJ, Chiu IS, Hsu CM, et al. Efficacy of ultrafiltration in removing inflammatory mediators during pediatric cardiac operations. Ann Thorac Surg 1996;61:651–6.[Abstract/Free Full Text]

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This Article Abstract Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Add to Personal Folders Download to citation manager Author home page(s): Mark G. Hazekamp Michael Frank Hans A. Huysmans Permission Requests Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Draaisma, A. M. Articles by Huysmans, H. A. Search for Related Content PubMed PubMed Citation Articles by Draaisma, A. M. Articles by Huysmans, H. A.