 
Pediatric x Adult Cardiopulmonary Bypass - Basic Comparison Kelly Calvert, BSN, CCP Chief of Perfusion Services
(Prepared for the Course: Basis and Techniques of Neonatal Perfusion - Brazilian Page of Perfusion Line)
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" The Child is Not a Small Adult "
Probably the most overused cliché in pediatric perfusion, it is by far the most accurate. The perfusionist can not assume a passive transition from adult clinical practice to pediatric management.
I. HEMODILUTION
Although there has been a recent effort and consensus by manufacturers and practitioners to minimize circuit volumes there continues to be a gross degree of hemodilution realized in the neonatal patient. This can be as much as 3-15 times the amount of hemodilution seen in an adult. For example in a 3 kg child, given 85 cc/kg, an EBV(estimated blood volume) of 255cc contrasts to an average circuit prime volume of 400-800 cc. Thus a prime: EBV ratio can exceed > 2-3: 1, or >60-200% of a neonate's blood volume. An average adult circuit prime: EBV is usually < 1:2, given an 80 kg pt, 6400 EBV and prime volume of 1800 cc. Thus only a 25-33% dilution rate is realized.
Several significant aspects of this gross hemodilution are quickly noted. Most obviously is the increased propensity for capillary leak and volume shifting, but most concerning is the necessity of the addition of donor blood to maintain an adequate hematocrit for optimal oxygen delivery. We practice a routine dilute hematocrit of 18% and prime the circuit with one unit of washed, leukoreduced packed red cells. Not only is exposure to donor blood an issue, but the cost of processing and washing the blood can be of significance. The washing of cells, whether by the blood bank or the perfusionist, is of utmost importance for glucose and potassium management. It is highly recommended. Whole blood can be used in the prime but is probably most effective when given post bypass for factor depletion. Of interesting note is the recent push to run even higher hematocrits on pump. Will this require even more donor blood?
II. PERFUSION PRESSURES
Most universally accepted perfusion pressures in adult management during cardiac procedures range from 50-80 mmHg. This acceptable range offers an adequate driving pressure to maintain urine output, cerebral autoregulation and critical closing pressures. Vasoconstricting agents such as Phenylephrine are often utilized and well tolerated for management of 'ideal' pressures. These agents are especially helpful in the patient with significant atherosclerosis or carotid disease.
On the contrary, perfusion pressures in the neonate can be quite low, < 20-30mmHg. More accurately, flows are more reflective of adequacy of perfusion. This is due to the elasticity of the neonatal vasculature. Of significant consideration is the use of vasoconstricting agents, particularly in the presence of aortopulmonary collaterals. With meticulous management of venous blood gases, global temperature regulation and relative flow rates, there should be no indication for use of vasoconstricting agents to 'create' an ideal blood pressure. It is never a consideration in our management protocol for neonates or pediatric patients less than 4-5 years of age. In the presence of aortopulmonary collaterals it is actually a contraindication.
III. FLOW RATES
For the most part there is a narrow flow range in the adult patient, usually 50-65 cc/kg/min. This calculation based on cardiac index rarely varies and in conjunction with a fixed perfusion pressure creates a mentality of 'perfusion by numbers'.
The flow ranges for neonates on the other hand are quite variable and wide. Flows range from 0-200 cc/kg/min. DHCA( Deep Hypothermic Circulatory Arrest) is one end of the spectrum, contrasted with high metabolic demands, vent return, circuit shunts or patient collaterals, all of which contribute to the necessity of high flow rates on the other end. This can often exceed a cardiac index of 3 LPM/m2. The flow rates are calculated based on cc/kg but the perfusionist must adapt flows according to individual case and demands. In-line blood gas monitoring and/or frequent blood sampling is really a must in the management of these patients.
IV. BLOOD GAS MANAGEMENT
Although a common debate both historically and presently, I will discuss our current protocol, based on the adaptation from Boston Children's practice and current studies on circulatory arrest and developmental outcomes.
After a retrospective study and review of the incidence of choreoathetosis from 1983-1987 Boston Children's Hospital has provided insight to the management of these patients at cooler temperatures. It was concluded that during that time blood gas management had shifted to alpha-stat protocol and based on their findings it is current practice to employ pH-stat management on all cases in which temperatures are taken to 28oC or less. In this temperature-corrected strategy, the pCO2 remains unchanged from 37oC ( 40 torr). This strategy allows for cerebral vasodilation despite low perfusion pressures. Again, this is even of more significance in the presence of aortopulmonary collaterals. The reflective pCO2 ranges at 37oC can be > 80-100 torr. In the adult population alpha-stat is the strategy of choice as atherosclerotic changes inhibit positive outcomes for ph-stat management.
V. CANNULATION
Whether CABG or Valve, standard or femoral, adult cannulation techniques are quite predictable. The larger vessels accommodate the necessary cannulae, causing rare anatomic distortion with cannula placement. Usually one or two cannulae are used, with femoral cannulation available as an alternative choice if need be.
In neonatal and pediatric cannulation consideration, the choices in decision-making are variable. Anatomic limitations may be not only by patient weight but by maldevelopment, such as in aortic atresia or hypoplasia of the pulmonary vasculature. Gross distortion can occur or the size may be prohibitive to flow requirements. One, two or three cannulae may be needed, such as in the presence of PLSVC. Unlike the adult patient, femoral cannulation as an alternative is not an option. The perfusionist and surgeon must have good communication for cannulation techniques to match flow requirements.
VI. AORTOPULMONARY COLLATERALS
Uncommon in adults, but rather common in patients with chronic cyanosis with decreased pulmonary blood flow, aortopulmonary collaterals can be a challenge. Flow rates are frequently affected and temperatures may need to be altered(lowered) substantially to acccomodate the field. Flows may need to be decreased in conjunction with the use of vasodilating agents. Phentolamine .1-.2mg/kg is the drug of choice during cooling and Nitroglycerine is used during rewarming, titrating to effect of global rewarming. Phenylephrine is contraindicated and would only further complicate matters and enhance collateral flow.
VII. TEMPERATURE RANGES
As a very basic comparison, suffice it to say that there is a trend to conducting adult cases at tepid or normothermic temperatures while many neonatal cases are utilizing very cool temperatures with or without DHCA. The perfusion considerations are multiple and this topic will be discussed in a future lecture on Hypothermia.
VIII. GLUCOSE MANAGEMENT
It is of utmost importance to maintain euglycemia in the neonate. Although there are increased glycogen stores in neonatal myocardium, there are low hepatic glycogen stores. There is decreased hepatic function in the newborn and compounded with a defect such as Coarctation of the Aorta or HLHS there can be decreased systemic perfusion during the neonatal transition period, only to make matters more complex. Exogenous glucose may be necessary in the early neonatal period to maintain normal glucose levels.
Perfusion considerations on CPB are directed at efforts to maintain normal glucose levels such as washing packed cells or minimizing glucose content in cardioplegia or IV fluids. When glucose levels are greater than 300mg/dl a saline hemodilutional washout with the hemoconcentrator is utilized. Frequent monitoring is recommended. Levels should be maintained approximately 150 mg/dl just prior to DHCA.
Hyperglycemia worsens neurologic injury as elevated glucose levels result in increased anaerobic metabolism of glucose and increased lactic acidosis. This leads to further depletion of ATP. Hypoglycemia alone can be treated, but coupled with hypothermia, cerebral blood flow may be compromised by altering autoregulation. This can be further exacerbated with hyperventilation as which may occur in weaning a patient with pulmonary hypertension from cardiopulmonary bypass.
The non-diabetic adult patient can handle the variable ranges in glucose levels. Large hepatic glycogen stores are present to provide easier regulation.
IX. INOTROPIC RESPONSE
Adults will have a fairly predictable response to a given dose of inotropic or vasoactive agent. The response of the neonate can be variable, leading to quite astounding doses of inotropes to achieve a desired response.
X. EQUIPMENT
One circuit for the adult patients at a given institiution can adequately provide flows for patients 50kg to 150kg. Most pediatric centers employ two or three circuits based on patient weight, procedure and flow requirements. Given that a particular institution may only perform 100 total pediatric cases a year, the stocking levels and inventory issues can be an additional consideration.
XI. ULTRAFILTRATION
Utilization of ultrafiltration in some form whether it be conventional or modified is seen on > 90 % of our neonatal and pediatric cases. Several system modifications may be necessary to allow for these options.
Adult CPB cases only employ ultrafiltration approximately <20% of the time and only in a conventional fashion.
Lake, Carol L. Pediatric Cardiac Anesthesia. Appleton and Lange: Norwalk. 1993.
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