|
Pediatric x Adult Cardiopulmonary Bypass - Basic Comparison
|
|
|
" 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.
|