| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
a Department of Anaesthesia, Yong Loo Lin School of Medicine and National University Hospital, National University of Singapore, Singapore
b Department of Cardiac, Thoracic and Vascular Surgery, Yong Loo Lin School of Medicine and National University Hospital, National University of Singapore, Singapore
Accepted for publication September 4, 2007.
* Address correspondence to Dr Ti, Department of Anaesthesia, National University Hospital, 5 Lower Kent Ridge Rd, Singapore 119074, Singapore (Email: anatilk{at}nus.edu.sg).
 |
Abstract |
|---|
 Top Abstract IntroductionMaterial and MethodsResultsCommentAcknowledgmentsReferences |
|---|
Methods: Data of 60 patients who underwent cardiac surgery with a newer mini-cardiopulmonary bypass system incorporating an air purge from August 2005 to July 2006 (group A) were retrospectively collected and compared with that of 60 matched patients who underwent cardiac surgery with conventional cardiopulmonary bypass during the same period (group B). Matching criteria were prebypass hematocrit, body surface area, age, and surgical procedure.
Results: Demographic and background data were similar for both groups. There were no detectable episodes of air embolism. Patients in group A had higher initial and nadir hematocrits during cardiopulmonary bypass and received fewer transfusions. However, postoperative blood loss and transfusion requirements were similar in both groups. Episodes of low indexed flows during cardiopulmonary bypass commonly occurred in group A, and this was associated with a greater than 50% decrease in urine output and lower mixed venous oxygen saturations (58% ± 6% versus 68% ± 5%) as compared with group B. There were no differences in clinical outcomes.
Conclusions: The newer mini-cardiopulmonary bypass system addressed the problem of air embolization. It preserved hematocrit and reduced transfusion during cardiopulmonary bypass, but did not improve outcomes postoperatively. It is unclear whether these benefits outweigh the potential risk of hypoperfusion associated with its use.
|
Introduction |
|---|
|
TopAbstractIntroductionMaterial and MethodsResultsCommentAcknowledgmentsReferences |
|---|
More recently, the incorporation of an air purge into newer MCPB systems has largely eliminated this danger. The air purge has been shown to effectively trap and remove entrained air from the circuit [2, 11]. Despite the paucity of direct evidence of better patient outcomes, our institution has started using the newer MCPB system because the theoretical benefits were compelling. We also believed that the benefits of using the MCPB system may be more apparent in our patients, who are predominantly Asian and smaller in size than Western patients. Intuitively, smaller patients would suffer from greater hemodilution and be exposed to more blood transfusions if CCPB systems were used. Therefore, we decided to conduct a retrospective case-control study to compare the benefits and possible risks associated with the use of MCPB system with the CCPB system in our cohort of Asian patients undergoing cardiopulmonary bypass (CPB) for cardiac surgery.
|
Material and Methods |
|---|
|
TopAbstractIntroductionMaterial and MethodsResultsCommentAcknowledgmentsReferences |
|---|
Mini-Cardiopulmonary Bypass System
The MCPB system used was the ECCO (extracorporeal circulation, optimized) system (Dideco; Sorin Group, Milan, Italy). This is a new-generation MCPB system able to handle air emboli entering the closed circuit, a weakness and source of danger in early MCPB systems [2]. The system consisted of a centrifugal pump, a soft buffer bag, a bubble trap, an arterial filter, and an oxygenator. Venous return, through a 3/8-inch polyvinyl chloride tubing, was facilitated by the centrifugal pump acting as a kinetic-assisted venous drainage system to a maximum negative pressure of –30 mm Hg. The whole system was coated with phosphorylcholine, a substance that mimics the human cell membrane, making it biocompatible.
The ECCO system had two features aimed at addressing the potential problem of air entrainment and air embolization. The first is the air purge control, which is used in conjunction with a bubble sensor on the venous inlet line. The presence of venous air bubbles of a size of 0.065 cm3 or greater activates a dedicated roller pump that removes air from the bubble trap through a purge line. The second is an electric remote clamp. This works in conjunction with a bubble sensor situated on the tubing line between the bubble trap outlet and the centrifugal pump inlet. Air detected by this sensor causes the electric remote clamp on the arterial line to automatically close, stopping the pump, preventing retrograde flow, and preventing loss of prime of the MCPB circuit in the event of massive air entry.
Blood in the surgical field was not returned to the MCPB system to reduce air-blood interface. Instead, a cell-saving device was used to suction blood from the field. Left ventricular venting was used in all patients, with the aspirated blood channeled to the buffer bag. The priming volume was approximately 750 mL.
Conventional Cardiopulmonary Bypass System
The CCPB system (Stockert SIII; Sorin Group) was an open system incorporating a hard-shell reservoir, which acted as volume store and air trap. All components were bigger when compared with the MCPB system. A roller pump was used, and gravity-dependent venous return was facilitated through 1/2-inch polyvinyl chloride tubing. Blood from the surgical field and the left ventricle was returned to the hard-shell reservoir through the cardiotomy suction and left ventricular vent. respectively. The oxygenator was the only part of the CCPB system with biocompatible coating. The total priming volume was approximately 1,300 mL. Blood priming of the circuit was electively used when the calculated postdilution hematocrit was less than 22%. No cell-saving device was used. However, most cases had an additional hemofilter added to remove extra volume from the system.
Anesthesia and Surgical Management
All patients were anesthetized with a low-opioid technique, using 10 to 20 µg/kg of fentanyl. Aprotinin or antifibrinolytics were not routinely given. Anesthesiologists were asked to limit the amount of fluid infused before commencement of CPB to 1 L in the MCPB group, hemodynamics permitting. Liberal use of phenylephrine was also requested for back-priming of the MCPB circuit with patients’ blood. Surgical technique did not differ between groups. Arterial access was achieved through ascending aortic cannulation, and venous access was achieved through a two-stage venous cannula inserted through the right atrial appendage, or bicaval cannulation for mitral valve surgery. In particular, extra pursestring ties for the venous cannulas, as advocated by some authors to reduce air entrainment [10], were not routinely used. Standard air removal techniques were used for both groups, including use of Trendelenburg positioning, positive pressure to the lungs during chamber closure, and aortic root venting.
Perfusion Management
Moderate hypothermia to 32° to 33°C, alpha-stat pH management, and target flow rates of 2.0 to 2.2 L · min–1
· m–2 were used for both groups. Mean arterial pressure of at least 50 mm Hg was maintained with the use of phenylephrine. Anticoagulation was achieved with 300 IU/kg of heparin to a target activated clotting time of greater than 400 seconds. Cardioplegia regimens were similar for both groups. In the MCPB group, limited autologous blood back-priming of the circuit was performed before the start of CPB. First, blood from the venous cannula would be allowed to fill the circuit anterograde until the venous bubble trap. Then, blood from the aortic cannula was allowed to fill the circuit retrograde until the oxygenator. The displaced crystalloid prime was collected into the buffer bag. Autologous blood back-priming enabled reduction of crystalloid prime volume by up to 300 mL, reducing the total volume of crystalloid prime in the MCPB circuit to 450 mL. The amount of autologous blood back-priming was limited by the hemodynamics of the patient, as hypotension typically occurred despite boluses of phenylephrine. Mannitol was added to the circuits of both groups, with the MCPB group receiving 250 mL and the CCPB group receiving 500 mL of 20% mannitol. Targeted hematocrit during CPB for both groups was 22% or greater, with red blood cell transfusions given as necessary.
Intensive Care Unit
In the intensive care unit (ICU), arterial blood gases, together with electrolytes and hemoglobin levels, were monitored every 6 hours while patients remained intubated and every 12 hours after extubation. The threshold for red blood cell transfusion was a hemoglobin level of 9 mg/dL. Blood loss and urine output were recorded hourly. The coagulation profile was routinely checked on admission to the ICU. Excessive bleeding (>100 mL/h for 2 continuous hours) or an abnormal coagulation profile would be corrected with the use of platelets and fresh-frozen plasma. Patients who were hemodynamically stable, extubated, and not receiving significant inotropic agents (dopamine <5 µg · kg–1
· min–1) were transferred to a high-dependency unit before discharge to the surgical ward when close nursing care was no longer required.
Outcome Measures
The main outcomes in this study were starting and nadir hematocrits during CPB, detection of air embolization, postoperative blood loss, and transfusion requirements during CPB and postoperatively in the ICU. Secondary outcomes were intraoperative variables and clinical outcomes including lengths of stay. Postoperative myocardial infarction was defined as electrocardiographic and biochemical evidence of a fresh myocardial infarction. Cardiac failure was defined as clinical evidence of a low cardiac output state, presence of pulmonary edema, and echocardiographic evidence of a fall in ejection fraction. New arrhythmias were defined as onset of a new arrhythmia sustained for at least an hour, and compromising hemodynamic stability or requiring pharmacologic treatment. Inotropic agent use was defined as use of dopamine infusion of greater than 5 µg · kg–1
· min–1 or any other inotropic agent infusion for more than 1 continuous hour. Postoperative infection was defined as fever of greater than 38°C or white blood cell count of greater than 11,000 cells/µL, with or without a positive culture result. Delayed extubation was defined as the patient remaining intubated for 24 hours after arrival in the ICU. Adverse cerebral outcome was taken as a new type 1 or type 2 adverse cerebral outcome [12].
Statistical Analysis
All data were tabulated and analyzed with the SPSS statistical software package (SPSS 14.0; SPSS Inc, Chicago, IL). Continuous data was presented as mean ± standard deviation, and categorical data was presented as percentages for each group. Comparison of data between the two groups was performed using unpaired Student’s t test for continuous data and 
2 test for categorical data. Level of significance was taken as a probability value less than 0.05.
|
Results |
|---|
|
TopAbstractIntroductionMaterial and MethodsResultsCommentAcknowledgmentsReferences |
|---|
|
|
Intraoperative Variables
Episodes of low indexed flows commonly occurred in group A, typically brought about by poor venous return. Nadir indexed flow during CPB, defined as the lowest recorded indexed flow with duration of at least 2 minutes, for group A was 1.75 L · min–1
· m–2 compared with 2.2 L · min–1
· m–2 in group B (Table 2). The duration and severity of low-flow periods, defined as periods during which cardiac index was less than 2.0 L · min–1
· m–2, was significantly greater in group A. This resulted in a slightly lower nadir mean arterial pressure during CPB for group A. Indexed flow and arterial pressure were typically supported with supplementary fluids, surgical adjustment of the venous line, and boluses of phenylephrine. Urine output during CPB was markedly poorer in group A, at less than 50% of that of group B. The mixed venous saturation was also lower in the MCPB group. However, lactate levels, base excess levels, and pH during CPB were similar in both groups. There were no differences in postoperative urine output, creatinine change, diuretic use, incidence of new-onset dialysis, or postoperative renal dysfunction (Tables 3, 4).
|
|
|
Comment |
|---|
|
TopAbstractIntroductionMaterial and MethodsResultsCommentAcknowledgmentsReferences |
|---|
Older MCPB systems were inherently prone to air entrainment and embolization as they were closed systems incorporating kinetically assisted venous drainage. This danger was highlighted in the study by Nollert and associates [10]. They described visible air entering the MCPB system in 2 of their 15 patients; in 1 patient, CPB had to be immediately stopped to allow for complete removal of air. Occult air embolism possibly occurred in every patient when the older MCPB systems were used, as a bench study showed that the number of air emboli entering the cerebral circulation increased eightfold to tenfold when older MCPB systems were used as compared with CCPB [13]. This danger would not be acceptable to most clinicians and may be the primary reason why MCPB remains underutilized despite its attractive theoretical benefits. However, this study and the work of others suggest that the air purge systems integrated into the newer MCPB systems have largely eliminated this danger [2, 11].
The major concern we had with the newer MCPB system was that many of our patients experienced episodes of poor venous return, and this resulted in periods of low indexed flow rates (cardiac index) during CPB. Surprisingly, this concern had not been highlighted in previous publications, although our experience is not unique. In their large retrospective series, Wiesenack and coworkers [14] reported significantly lower indexed flow rates during MCPB in their patients as compared with CCPB. Others have reported lower urine output during CPB with the MCPB system [2]. Inherently, the design of the MCPB system greatly increases the risk of periods of low indexed flow rates. First, the venous reservoir (buffer bag) is very limited, and therefore the system is highly susceptible to variations in venous return. Second, blood loss into the surgical field was returned to the cell-saving device and not to the MCPB circuit. This contributed to the depletion of the volume reserve of the patient. Similar to our own experience, others have also experienced substantial loss of volume from blood loss during MCPB [9]. Third, the use of active suction to enhance venous return runs the risk of causing collapse of the right atrium or inferior vena cava, thereby reducing venous return. This risk was compounded by our practice, as we limited the amount of volume infused before CPB in an attempt to defend the hematocrit. This may have left the patient with little volume reserve during CPB.
The significance of these episodes of low indexed flow rates is difficult to ascertain. An indexed flow rate of at least 2.0 L · min–1 · m–2 during CPB is typically adopted in most practices, which affords a comfortable safety margin to the patient. Kirklin and Barratt-Boyes [15] recommended indexed flow rates of at least 2.2 L · min–1 · m–2 during CPB in adults at temperatures above 28°C, with the exception of large adults with a body surface area greater than 2 m2. Similarly, a nomogram that used plateauing of oxygen consumption to indicate adequacy of perfusion at a given temperature would suggest a flow rate of at least 1.8 L · min–1 · m–2 at a temperature of 30°C [15]. However, in clinical practice, patients often tolerate periods of low indexed flow without any apparent detrimental effects. Furthermore, markers of global perfusion such as lactate levels and pH in the MCPB group were similar to that of the CCPB group.
Two factors caused us to be concerned that some degree of hypoperfusion may have occurred. First, the lower indexed flow rates were accompanied by a 50% fall in urine output during MCPB in our patients. Urine output during CPB is influenced by a variety of factors, and we were cognizant that the fall in urine output in the MCPB group may be explained by the prebypass fluid restriction, smaller prime volume, and smaller mannitol dose. Patients in the MCPB group did not suffer any obvious adverse effects either globally or specific to the renal system, and urine output in the ICU were similar for both groups. Nevertheless, we were concerned that subclinical hypoperfusion may have occurred as urine output is also a marker of general perfusion. Second, the mixed venous saturations were significantly lower during MCPB as compared with CCPB. Low mixed venous oxygen saturations indicate that systemic oxygen delivery was inadequate to meet demand. In the MCPB group, the mean mixed venous oxygen saturation was only 58%, just below the normal range of 60% to 70%. We believe that the lower indexed flow rates necessitated greater cellular oxygen extraction, therefore compromising the margin of safety for adequate perfusion.
Another source of disappointment with the MCPB system was its inability to reduce red blood cell transfusion exposure to our patients. It is known that smaller patients have poorer outcomes, including higher rates of morbidity, organ dysfunction, and mortality [16]. It is believed that this is partly a result of lower hematocrits during CPB and higher rates of blood transfusions. Our patients, being smaller in size than Western populations, are vulnerable to significant hemodilution and consequent red blood cell transfusion during CCPB. We had hoped that with MCPB, we could reduce the hemodilution, increase the hematocrit during CPB, and reduce red blood cell transfusion exposure.
However, this was not the case. Although the higher initial and nadir hematocrits in the MCPB group ensured that the number of patients receiving blood transfusions decreased during CPB, the degree of hemodilution was still significant enough to result in 40% of patients requiring blood to be added to the MCPB circuit and 70% of patients requiring postoperative transfusion. As a result, despite the lower percentage of patients receiving blood during CPB, the overall percentages of patients exposed to transfusions were similar in both groups. One reason may have been our difficulty in maintaining adequate volume in the system. This meant that patients frequently required additional fluid to be given into the MCPB circuit. This resulted in an increase of the final prime volume to 779 mL in the MCPB group. We speculate that smaller patients such as ours were unable to tolerate even modest amounts of hemodilution without requiring blood transfusions. As such, we believe that the use of aggressive blood back-priming of the entire circuit and minimization of intraoperative blood loss are crucial factors to reduce the volume of prime needed and prevent excessive hemodilution, and we have recently changed our practice accordingly.
Postoperatively, there were no differences in blood loss. MCPB has been reported to reduce postoperative blood loss, resulting in less transfusions [5, 6]. However, this has not been consistent in the literature with others showing no difference in postoperative blood loss [9, 10]. Similarly, we could not show a reduction in postoperative blood loss with the use of MCPB. This may have also contributed to the number of patients receiving blood transfusions in the ICU being similar in both groups.
The other major impetus to adopt MCPB systems is its potential to improve clinical outcome. Certainly, the evidence based on markers and indirect indicators of organ function is very compelling. Mini-cardiopulmonary bypass systems reduce inflammation, reduce blood loss, and preserve coagulation [2–5]. It has been suggested to preserve renal function [4], an important benefit as renal dysfunction after cardiac surgery is a serious morbidity. It may improve lung function by reducing inflammation [8]. It preserves cardiac function better by reducing myocardial edema and preventing arrhythmias [7]. However, like several studies before us [9, 10], these theoretical benefits are not readily apparent in clinical practice, and we could not demonstrate any clinical benefit as well.
There are several limitations in this study. First, this was a retrospective study and the patients who received MCPB were not randomly selected. However, the use of a matched cohort mitigates some of this concern. Second, there is a learning curve to the implementation of the MCPB for perfusionists, surgeons, and anesthesiologists. As these patients represented our initial use of the MCPB system, this may have limited the efficacy of the system in reducing hemodilution and preserving flow. Third, pharmacologic agents to reduce blood loss such as aprotinin or antifibrinolytics were not used routinely in our practice. In contrast, these agents were used in a study that successfully demonstrated a reduction in postoperative blood loss [5]. It may be that the complementary effects of these agents to preserve coagulation and reduce blood loss were required for the benefits of MCPB to be apparent. Lastly, our small sample size was likely insufficient to demonstrate a clinical benefit.
In conclusion, we found that the new MCPB system incorporating the air purge has adequately addressed concerns with air embolization. We showed that its use resulted in better preservation of hematocrit and reduction of transfusions during CPB. However, postoperative blood loss and exposure to blood transfusion were no different than CCPB. There were also no differences in other clinical outcomes. This lack of significant apparent benefit was compounded by extended periods of low indexed flow rates during CPB in the MCPB group, although no adverse effects were detected. It remains unclear whether the potential and theoretical benefits of MCPB outweigh the potential risk of hypoperfusion associated with its use.
|
Acknowledgments |
|---|
|
TopAbstractIntroductionMaterial and MethodsResultsCommentAcknowledgmentsReferences |
|---|
Funding for this study was provided by the Department of Anaesthesia, National University Hospital, Singapore.
|
References |
|---|
|
TopAbstractIntroductionMaterial and MethodsResultsCommentAcknowledgmentsReferences |
|---|
Related Article
This article has been cited by other articles:
|
|
 |
|
  F Biancari and R Rimpilainen Meta-analysis of randomised trials comparing the effectiveness of miniaturised versus conventional cardiopulmonary bypass in adult cardiac surgery Heart, June 15, 2009; 95(12): 964 - 969. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 M. P. Sakwa, R. W. Emery, F. L. Shannon, J. M. Altshuler, D. Mitchell, D. Zwada, and A. R. Holter Coronary artery bypass grafting with a minimized cardiopulmonary bypass circuit: a prospective, randomized trial. J. Thorac. Cardiovasc. Surg., February 1, 2009; 137(2): 481 - 485. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 J.-P. Remadi Invited Commentary Ann. Thorac. Surg., March 1, 2008; 85(3): 1000 - 1001. [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| ANN THORAC SURG | ASIAN CARDIOVASC THORAC ANN | EUR J CARDIOTHORAC SURG |
| J THORAC CARDIOVASC SURG | ICVTS | ALL CTSNet JOURNALS |
