Mini cardiopulmonary bypass: Anesthetic considerations.

This review article is going to elaborate on the description, components, and advantages of mini-cardiopulmonary bypass (mini-CPB), with special reference to the anesthetic management and fast track anesthesia with mini-CPB. There are several clinical advantages of mini-CPB like, reduced inflammatory reaction to the pump, reduced need for allogenic blood transfusion and lower incidence of postoperative neurological complications. There are certainly important points that have to be considered by anesthesiologists to avoid sever perturbation in the cardiac output and blood pressure during mini-CPB. Fast-track anesthesia provides advantages regarding fast postoperative recovery from anesthesia, and reduction of postoperative ventilation time. Mini bypass offers a sound alternative to conventional CPB, and has definite advantages. It has its limitations, but even with that it has a definite place in the current practice of cardiac surgery.

INTRODUCTION

Mini-cardiopulmonary bypass (mini-CPB) or (minimally invasive CPB) is gaining a considerable popularity in recent years. It combines the benefits of shed blood separation, biocompatible coating of the circuit, and reduced priming.

Cardiopulmonary bypass is an integral part of cardiac surgery; it is associated with some complications like generalized inflammatory reaction due to the release of interleukins that can lead to acute lung injury and at times even multiorgan dysfunction. Additionally it has an adverse effect on coagulation that is due to hemodilution caused by priming solution of the pump and trauma to the blood components in the pump circuit, which lead to homologous blood transfusion, and blood products administration.[1]

Cardiac anesthetists are concerned about the perioperative blood loss and transfusion in general, but they are aware of the negative impact of systemic inflammatory reaction on the patient, and the incidence of neurological events as well.

Mini-CPB has a clear benefit in reducing homologous blood and blood products transfusion, and maintaining coagulation status after CPB, consequently reducing the transfusion-related complications.[2]

Also, the active bubble removal system include gaseous microemboli GME-sensing system, which removes air from the venous line automatically via the venous bubble trap.

This mechanism is an important tool to reduce the postoperative neurological dysfunction related to GME.[3]

COMPONENTS OF MINI CPB

Currently, different brands of mini-CPB are available in the market. Although, there are some technical differences between them, they are similar in their principle components and function.

Main components are as follows.

Centrifugal head, which substitutes the main roller pump in the CCPB.

A closed circuit, with or without a collapsible venous reservoir.

Oxygenator: There is biocompatible coating of the circuit and oxygenator; it can be heparin coated, phosphorylcholine coated, or multimolecular coated.

Air removal device.

Arterial filter.

Left ventricular blood venting is collected in a flexible reservoir, then given via a roller pump into the venous side.

Cardioplegia will be given through a line branching from the circuit.

Other related components:

heat exchanger;

suctioned blood from the surgical field will not go directly to the circuit, but it will be first processed in the cell saver then it will be given to the patient if needed [Figure 1].

Figure 1

Components of mini cardiopulmonary bypass

BENEFITS OF MINI-CPB

Mini-CP bypass has the following main characteristics: shed blood separation, biocaompatible coating, effective detection and removal of gaseous microemboli from the venous and arterial sides of the circuit, and reduced prime volume, 700–800 mL, compared to 2000 mL in the conventional cardiopulmonary bypass (CCPB) and a significant reduction in blood contact surface area, 1.4–2 m2 in mini bypass, while it is two- to threefolds in the conventional CPB. These features can lead to improved homeostasis, reduced intra- and postoperative blood and blood products transfusion requirement and better outcome. The biocompatible coating in mini-CPB can reduce the reaction between plasma proteins and surface of the bypass circuit, which will reduce platelets activation and postoperative bleeding. Using biocompatible (phosphorylcholine) coating and shed blood separation showed improvement in patient's homeostasis and maintained physiological coagulation similar to that achieved with off pump coronary artery bypass grafting (CABG).[4]

Retransfusion of shed blood may obscure the benefit of biocompatible coating of the circuit,[56] also the material independent blood activation due to blood–air interface, can obscure the beneficial effect of biocompatible surfaces.[7]

In the study of Formes et al., the systemic inflammatory response was investigated on 60 patients undergoing CABG, using either mini-CPB or CCPB. Interleukine-6, TNF-a, and neutrophil elastase levels were significantly lower in the mini-CPB group. Platelet count remained at higher values with mini-CPB compared to CCPB, also Thromboglobulin levels showed slightly lower platelet activation in the mini-CPB group.[8]

Many authors reported about the correlation between GME and impairment of neuro-cognitive functions after CCPB, which plays an important role in the morbidity, mortality, and quality of life after cardiac surgery.[9-11]

In the study of Perthel and associates on GME that looked at 60 patients divided in two equal groups, mini-CPB, and CCPB, a Doppler sonography device was used as a bubble counter to detect and count GME (detecting GME 10–120 μm) on the inlet of the venous bubble trap and at the outlet of the arterial filter in the mini-CPB, also at the outlet of the hard shell venous reservoir and outlet of arterial filter in the CCPB during CABG surgery. No statistical difference has been detected between the GME sensed at the outlet of venous reservoir in the CCPB, compared to the venous line of the mini-CPB, but there was a statistically significant difference in terms of GME volume postarterial filter in mini-CPB as compared to the arterial filter outlet in CCPB (in favor of mini-CPB). Transcranial Doppler (TCD) revealed less microemboli signals sensed bilaterally in the middle cerebral arteries in the mini-CPB, compared to CCPB, but that was not statistically significant.[12]

Anastasiadis et al. studied 64 patients undergoing coronary artery bypass grafting with mini-CPB or CCPB; patients were monitored with cerebral oximeter. A battery of neurocognitive tests as complex scanning, visual tracking, focused attention, short- and long-term memory were performed before surgery, at discharge, and 3 months later. Results showed that patients on mini CPB had significantly less intraoperative episodes of brain desaturation, and better neurocognitive function at discharge and 3 months later.[13]

In the study of Harostock et al., it was found that mini-CPB has the benefit of reducing the transfusion rate, less perioperative blood loss, and less hemodilution; the latter is related to the lower priming volume.[14]

Ranucci and associates compared a bigger group of patients, 1663 patients undergoing CABG surgery; they were divided in two equal groups, either with mini-CPB or CCPB. This study concluded that mini-CPB group demonstrated a shorter ventilation time, reduced ICU, and hospital stay, better renal function, lower rate of intra-aortic balloon pump use, atrial fibrillation, ventricular arrhythmias, and peripheral thromboembolism.[15]

In a meta analysis of nine randomized controlled studies, some of them reported significant differences in favor of mini-CPB regarding ventilation time, ICU and hospital stay, renal function, and low cardiac output; three studies showed no differences in the clinical outcome. The only homogenous outcome in seven of these studies was the less transfusion rate in mini-CPB patients.[16]

Also in a wider meta-analysis study including the best 14 evidenced paper on 1000 patients undergoing mini- CPB: 10 studies demonstrated reduced hemodilution, seven showed reduced packed red blood cell transfusion, two showed less fresh frozen plasma administration, two showed less platelets usage, three showed less postoperative blood loss, one study documented more blood loss, three showed better postoperative renal function, four showed better cardiac index, and two studies showed shorter postoperative ICU stay. Inflammatory markers were found less in six studies, and in five studies there was better myocardial protection with mini-CPB.[17] Huybregts and associates studied the inflammatory reaction, renal and intestinal injury, on 49 patients undergoing CABG either with mini-CPB or CCPB. Urinary interleukine-6, urinary N-acetyl-saminidase were three times lower, and urinary intestinal fatty acid binding protein was 40% decreased in the patients on mini CPB, as compared to CCPB. It has been concluded that mini-CPB, in addition to reduced transfusion requirement, it reduced the systemic inflammatory reaction and the immediate postoperative renal and intestinal tissue injury.[18]

Regarding the learning curve for the best performance of mini-CPB, Abdel–Rahman and associates reported a reduction in the intraoperative blood loss after 50 cases with mini-CPB explained by the surgeon's “learning curve.”

Nevertheless, several thousand cases have been safely performed with mini-bypass that proved to be an efficient technique for reducing blood transfusion.[19]

CONTROVERSIAL OPINIONS

On the other side, Nollert and associates have reported in a randomized controlled study, that mini-CPB has only a minimal effect on inflammation and coagulation variables, and did not produce a clear clinical difference regarding blood loss and blood products use. Furthermore because of the Closed circuit nature, safety margins for volume loss, air emboli, and weaning from CPB are reduced (there were two dangerous incidents of air leaks occurred in this study that indicated low safety margins).[20]

In the study of Svitec et al. on 54 patients undergoing CABG with mini-CPB or CCPB, though the patients on mini-CPB had a lower concentration of serum immune markers as interleukin-6, monocyte chemo-attractant protein-1 and neutrophil elastase, but there was no difference regarding the clinical outcome between both groups.[21]

ANESTHETIC MANAGEMENT OF MINI-CPB

Fast tracking anesthesia for cardiac surgery with the use of mini-CPB helps in achieving early extubation and reduction of the postoperative ventilation time, which can reduce the length of ICU and hospital stay, consequently reducing morbidity, mortality, and cost of cardiac surgery.

Fast-track anesthesia induction include:[2223] Either midazolam 0.05–0.1 mg·kg−1 (or etomidate 0.2–0.3 mg.kg−1), sufentanil 0.25–0.5 μg·kg−1, rocuronium 1 mg·kg−1.

For maintenance of anesthesia: Midazolam 14 μ·kg−1·h−1 (or propofol 4–6 mg·kg -1·h-1), sufentanyl 0.28 μ·Kg−1·h−1 (or remifentanil 0.1 mcg·Kg−1·min−1), rocuroniom 280 μ·Kg−1·h−1 and sevoflurane 0–2% according to the level of anesthesia, can be used throughout the surgery. Anesthesia level is monitored via the bisectral index monitor (BIS), targeting a range of BIS values between 40 and 60 is advocated to prevent anesthesia awareness while allowing a reduction in the administration of anesthetic agents.[24] The volatile anesthetic dose is also adjusted according to the hemodynamic parameters to maintain blood pressure (20%) of the preinduction value and stage of surgery, as during sternotomy, chest retraction, the anesthetic requirement increases.

Before transfer to the ICU, sevoflurane is discontinued, and propofol infusion 2–4 mg·kg−1·h−1 will be started. After admission in the ICU Paracetamol bolus of 1000 mg is given intravenously for the sake of analgesia, for acute pain or before tracheal extubation, an incremental dose of IV morphine 0.5-2 mg is given.

Patient's temperature is kept within normal range 36.5°C–37.5°C by using forced air warming.

In preparation before tracheal extubation, propofol infusion is stopped when the patient has acceptable ventilatory parameters (Pao2 >90 mmHg with Fio2 <60%, peak end-expiratory pressure <7.5 cm H2O) during synchronized intermittent mandatory ventilation (SIMV). Patient is normothermic, hemodynamically stable, blood drained from chest is less than 100 mL·h−1, urine output is >0.5 mL·kg−1·h−1, no arrhythmia, and no high dose of inotropic support is needed.

Tracheal extubation is done when the patient meets the following criteria: Awake, oriented, obeying commands (cooperative), absence of any residual muscle relaxation, and has adequate ventilator parameters: Vital capacity >12 mL·kg−1, respiratory rate 10-25 bpm, minute ventilation >90 mL·kg−1·min−1, Fio2 <0.6, peak end-expiratory pressure <7.5 cm H2O), arterial blood gases are acceptable: With Fio2 0.6: Pao2 is >90 mmHg, Paco2 is 35–45 mmHg, Sao2 >95%, and no acidosis. Arterial blood gases are repeated 30 and 60 min after extubation, then patient controlled analgesia with hydromorphone 150 μg IV boluses, on demand, with 10 min intervals will be started.

Patient is discharged from ICU, when he meets the following criteria: Patient is fully oriented, not sedated, not on intropic support, Spo2 >90% with Fio2 <0.6, absent CO2 retention, urine output >0.5 mL·kg−1·h−1, and chest tube output <50 mL·h-1. Postoperative nausea and vomiting and pruritis were treated with ondansetron, 4 mg IV, and nalbuphine, 5 mg IV, respectively.

CONCLUSION

This article illustrates different aspects of mini-cardiopulmonary bypass. According to the best evidenced studies, mini-CPB reduces the hemodilution during extracorporeal circulation, and lowers the transfusion requirements. Positive effects on renal function, cardiac output, ICU stay, and hospital stay were not clear in all studies. Postoperative morbidity can be positively affected but there is no evidence that it improved the postoperative mortality. Gaseous microemboli can be adequately and safely removed from the circuit. Anesthetic management in mini-CPB has been described in details.