The Effects of Cardiopulmonary Bypass Following Pediatric Cardiac Surgery
11.7 Components of CPBP .1 Pumps
During cerebral ischemia, hyperglycemia may increase the release of excitatory neurotransmitters. By contrast, in the adult population, hypergly cemia is not associated with neurologic impairment; instead, hypo- glycemia is deleterious and should be avoided [21].
Experimental pharmacological neuroprotection has not been proven as beneficial but in clinical practice some of the medications used have included thiopen- tal, steroids, and aprotinin.
regions modify fibrinogen adsorption, thus changing its ability to interact with circulating platelets. Some studies of the use of these circuits are associated with a drop in tissue plasminogen activator release and preser- vation of platelet numbers with less platelet activation.
11.7.3 Oxygenators
Oxygenators are able to perform gas exchange, and are the place where volatile anesthetics can be delivered.
New oxygenators are being developed for use in pedi- atric cardiac surgery that require less priming volume.
The smallest oxygenator available is 0.3 m2 in surface area developed to provide less than 0.8 lpm of flow.
Historically, rotating disks and bubbles were used dur- ing the first CPBP and were associated with massive air embolism. Currently, the oxygenators that are used are membrane oxygenators, there are two types:
LP 0.1
Fig. 11.3 Roller pump
Fig. 11.2 Schematic diagram of cardiopulmonary bypass circuit
Heat Exchanger Oxygenator
Reservoir Arterial
Filter
Roller
“ True membrane” and micro-porous membrane. Hollow fiber oxygenators are a type of membrane oxygenator made up of polypropylene fibers, which are porous.
The new oxygenators are smaller and allow high flow rates with higher gas transfer rates. Hollow fiber oxy- genators cannot be used for a prolonged period because of the leak of proteins and serum through the small pores. These proteins can occlude the membrane and decrease its efficiency.
The heat exchanger is integrated into the oxygenator and allows cooling and rewarming of the blood.
11.7.4 Venous Reservoir
The venous reservoir is the component of the CPBP where blood is collected from the venous line at the initiation of the CPBP. Transfusion products, crystalloids solutions, and blood obtained from suction systems (which aspirate blood and air from the field and the heart chambers) drain into the reservoir as well.
11.7.5 Cardioplegia Delivery System
This system is connected to an independent roller pump to drive blood from the cardioplegia solution
into the aortic root. The system also has an independent heat exchanger, and the pressure and temperature are also independently monitored. Blood from the oxygenator is mixed with the crystalloid cardioplegia solution before transfer to the aortic root (Fig. 11.4).
11.7.5.1 Venting of the Left Heart
The left ventricle normally receives venous blood flow from the bronchial and Tebesian veins during CPBP, this flow is collected in the right superior pulmonary vein. Bronchial blood flow is increased in cyanotic patients and abnormal blood flow to the left ventricle is present in patients with left superior venous cava, PDA or aortic regurgitation. Adequate drainage of the left ventricle prevents distension, decreases wall tension, improves subendocardial perfusion, and also improves surgical exposure. The decreased wall tension and improved subendocardial perfusion are essential to the pediatric population where the compliance of the small chambers is decreased.
11.7.5.2 Filters and Bubble traps
Micropore filters trap the air decreasing the risk for embolization. They are located at various sites through
Fig. 11.4 Cardioplegia delivery system Cardioplegia
Solution
Blood cardioplegia Heat exchanger
Monitor
Temperature/Pressure
Manifold
Oxygenator
the system. Leukocyte-depleting filters prevent the activated leukocytes from reaching the systemic circu- lation although their clinical benefit has not been proven.
Residual blood from the CPBP after finish support can be procesed by the cell saver and is usable during the first 4 h after surgery, it is an usual practice from our surgical group.
Small improvements in early postoperative lung function and attenuation of the reperfusion injury at a cellular level in patients receiving systemic leukode- pletion was reported. However, it is not reflected on hospital stay or survival.
11.7.5.3 Circuit Miniaturization
Current advances in the design of the CPBP circuit help to reduce the use of peri-operative blood products.
Recent studies in neonates have shown clinical benefit with its use, which includes reduction of the postop- erative edema, improvement of the systolic blood pressure, and reduced mechanical ventilation time.
The prime volume used is 115 ml in patients with less than 6 kg of weight [23].
11.7.6 Hemoconcentrators and Ultrafiltration
Hemofiltration and ultrafiltration are techniques used to remove water (with inflammatory molecules) from the circulatory blood flow. This effect is achieved through the filtration of water across a semi-permeable membrane as the result of a hydrostatic pressure gradient. The blood flows through a hemoconcentrator (which is composed by a bundle of hollow fibers) creating a positive pressure that drives water across the membrane through an ultrafiltration reservoir system (Figs. 11.5 and 11.6).
There are three approaches to ultrafiltration in pediatric cardiac surgery:
1. Conventional ultrafiltration (CUF) (performed during the CPBP)
2. Modified ultrafiltration (MUF) (after termination of the CPBP)
Fig. 11.5 Continuous hemofiltration system
Hemofilter
Manifold Arterial filter
3. Ultrafiltration of the prime (PUF) (before the onset of the CPBP, it is performed occasionally when the CPBP circuit is primed with blood products).
In 1991, Naik et al. reported the use of MUF [24]. It is initiated after separation from CPBP; the blood from the aortic cannula is pumped through the hemofilter and then warmed by a heat exchanger and returned through the cardioplegia circuit to the patient’s venous cannula(s) [25]. There is not a global consensus about the amount to be removed, but in general, the fluid is removed depending on arterial pressure, CVP, and left atrium pressure. This technique is generally used for patient less than 10 kg of weight [26].
The major advantage of ultrafiltration is to remove excess fluid from patients, which leads to an increase in the hematocrit level and coagulation factors. MUF decreases the level of low-molecular weight inflamma- tory mediators and other deleterious substances.
Several clinical trials have demonstrated the clinical benefits of CUF and MUF after pediatric cardiac surgery, however, controversy remains regarding the optimal ultrafiltration strategy [27].
MUF and CUF reduce blood loss, blood transfusion, and mechanical ventilation time.
Other demonstrated effects of the use of MUF include:
1. Improvement in the postoperative hemodynamics 2. Improvement in the alveolar–arterial oxygen
difference.
3. Decrease pulmonary vascular resistance.
4. Decrease the incidence of pleural effusions (after superior cavopulmonary connection and Fontan procedure)
5. Decrease myocardial edema.
6. Improvement in the left ventricular function.
There is lack of consensus in the type of MUF (arterio- venous, venovenous), duration of ultrafiltration during CPBP, volume of ultrafiltrate, and the type of hemofil- ter to be used, leading to difficulty in the interpretation of the published studies and the definition of the best method of filtration [28].
Hemofiltration carries the potential for human and equipment error and increases plasma heparin
Filter Fig. 11.6 Modified ultrafiltration
system
concentration. The removal of blood from the systemic circulation may result in hemodynamic instability or impaired aortopulmonary shunt flow. High flow rates through the ultrafilter decrease cerebral blood flow velocities and cerebral mixed venous oxygen saturation [29].