Pediatric bypass circuit.
Prime fluid calculations
First, calculate the patient’s circulating blood volume (PV), which varies with weight as shown below:
| Patient weight range | Calculated circulating volume (PV) |
|---|---|
| 0–10 kg | weight (kg) × 85 ml |
| 10–30 kg | weight (kg) × 80 ml |
| > 30 kg | weight (kg) × 70 ml |
The patient’s calculated circulating blood volume (PV) is added to the prime volume, making up the total circulating volume (TCV). If the calculated hematocrit (HCT) of the TCV is less than 24%, red cells need to be added to the pump prime volume.
Example 1. A 20-kg child has a calculated circulating blood volume of 20 × 80 = 1600 ml. If their hematocrit is 40% and the prime volume is 1000 ml then the total circulating volume is 2600 ml and the calculated hematocrit is 1600 divided by 2600 multiplied by 40 = 25%. Therefore bypass can be performed without the addition of red cells.
Example 2. A 5-kg child has a calculated circulating blood volume of 5 × 85 = 425 ml. If their hematocrit is 40% and the prime volume is 500 ml then the total circulating volume is 925 ml and the calculated hematocrit is 425 divided by 925 multiplied by 40 = 18.4%. Red cells must therefore be added to the prime volume to increase the hematocrit to 24% using the following formula:
Transfused red cells have a hematocrit of 70%, so in this example the amount of red cells needed is:
Typical prime fluid constituents
Fresh red blood cells (to ensure HCT of 24% on bypass)
8.4% sodium bicarbonate (20 ml/l of prime volume)
FFP/Octaplas 50 ml (if patient is 7 kg or less in weight)
Heparin 225 IU per kg body weight
Ringer’s solution to make up the remainder volume
Most circuits require the addition of red cells in order to maintain hemoglobin (hematocrit) levels at 80 g/l (24%), which is thought to be the optimal level for enhanced rheological properties during bypass. Red cells added to the prime circuit should ideally be fresh (less than 6 days old) and in babies less than a month old they should be irradiated and cytomegalovirus (CMV) negative. Fresh frozen plasma/Octaplas is often added to maintain circulating levels of clotting factors. Octaplas is treated pooled human plasma (derived from more than 1000 donors) that is rich in plasma proteins such as albumin, immunoglobulins, and clotting factors. It is solvent/detergent treated which reduces the levels of Protein S and alpha-2 anti-plasmin. It must be ABO group compatible and can cause adverse effects such as urticaria (in 1% of patients), thrombosis (due to reduced Protein S), excessive bleeding (due to low alpha-2 antiplasmin), and potentially transmits infection such as variant Creutzfeldt-Jakob disease (vCJD) and other, as yet unknown, viruses. The addition of any blood products will cause a much higher glucose load in the prime.
The addition of mannitol to the pump prime is carried out in some units to increase the osmolarity of the prime, and so reduce tissue edema. While mannitol certainly causes a diuresis due to its action as an osmotic diuretic, the volume of urine output per se is not a reliable indicator of renal function, although some anesthetists aim to maintain an hourly urine output of greater than 0.5 ml/kg/hour. As well as being an osmotic diuretic, mannitol is an oxygen free radical scavenger.
A sample from the pump prime, after addition of the red cells and Octaplas, should be run through a blood gas analyzer to ensure biochemical values, especially potassium, are acceptable before initiation of CPB.
Cardiotomy suction
Following full heparinization and confirming an ACT above the level considered safe for initiation of CPB, generally more than 400 to 480 seconds, shed blood is suctioned from the operative field and collected in the reservoir for re-circulation. The perfusionist needs to be vigilant in order to alter the pump flow rate and alert the surgeon when changes in suction occur because the degree of negative pressure that develops at the sucker tip may lead to hemolysis of red cells or occlusion. Occasionally a second reservoir is incorporated into the bypass circuit specifically to collect pump sucker blood in order to reduce the amount of foam formation within the circuit.
Cannulation
In pediatric patients blood vessels are small in caliber and the correct choice of both arterial and venous cannulae together with accurate placement are crucial in order to avoid obstructing vascular branches, misdirecting flow or impairing venous drainage.
Bi-caval cannulation is routinely performed in all but the simplest of operations as it allows more complete emptying of the heart and improves the surgeon’s view of intra-cardiac anatomy.
Ineffective venous drainage results in ventricular distension, which is poorly tolerated by neonates due to their low ventricular compliance. If drainage is inadequate, the pump flow must be reduced and the venous cannulae repositioned.
Presence of large aorto-pulmonary collaterals or a patent ductus arteriosus (PDA) may divert blood to the pulmonary circulation from the systemic circulation thereby reducing cerebral blood flow. This can be mitigated by the surgeon gaining early control of the ductus after chest opening or the embolization of large collaterals in the cardiac catheterization laboratory prior to surgery. Patients with significant aortic arch abnormalities may require radical modifications of cannulation techniques, including formation of a temporary Gore-Tex shunt.
CPB flow rate
Full flow is calculated to be 2.6 l/minute/m2 of body surface area (BSA). The BSA is calculated as the square root of [height (cm) × weight (kg)/3600]. For example, a 12-month-old, 10 kg in weight and length 60 cm gives a BSA of 0.41, so full flow for this child would be approximately 1 l per minute.
The infant vascular tree offers low resistance to flow so that low arterial pressures (mean = 20–40 mmHg) may be seen despite “full flow,” but these are usually well tolerated.
Cardioplegia solutions
Cold crystalloid, cold blood, and tepid blood cardioplegia are all used and depend on the individual cardiac unit and surgeon preference.
Antegrade cerebral perfusion
This facilitates cerebral perfusion during long periods of deep hypothermic circulatory arrest, for example during a Norwood Stage I procedure or an extended aortic arch repair. The aortic cannula is removed from the aortic root and inserted into the right innominate artery. Perfusion is carried out at a third of the normal flow rate with cerebral oximetry and careful arterial blood pressure monitoring.
Fluid balance
There is evidence that fluid overload is an independent marker for an increase in mortality. Children are susceptible to renal impairment and many methods are employed to monitor renal perfusion and optimize fluid balance, e.g. near infrared spectroscopy using electrodes placed over the loin area is a simple non-invasive means of monitoring kidney oxygenation. Methods of renal replacement therapy include conventional hemofiltration, modified ultrafiltration, and zero balance ultrafiltration. In a recent survey of North American units 70% of respondents utilized some form of modified ultrafiltration in a parallel circuit in order to hemoconcentrate and filter CPB fluid.
Conduct of CPB
Drug administration by the perfusionist on bypass
Heparin to maintain an ACT of at least 400 seconds (depending on the local protocol)
Fluids
Potassium chloride
8.4% sodium bicarbonate – patient weight × base excess × 0.15
Cardioplegia solution
Vasodilators and vasopressors in conjunction with the anesthetist
Isoflurane
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