Initiation of Cardiopulmonary Bypass
Arterial and venous cannula in the heart prior to initiating CPB
may result in significant problems in the peri-bypass period. A malpositioned venous
cannula has the potential
for vena caval obstruction. The problems of venous obstruction are magnified during
CPB in the neonate because arterial pressures are normally low (20 to 40 mm Hg),
and large, relatively stiff cannulas easily distort these very pliable venous vessels.
[94]
[95]
[96]
A cannula in the inferior vena cava may obstruct venous return from the splanchnic
bed, resulting in ascites from increased hydrostatic pressure or directly reduced
perfusion pressure across the mesenteric, renal, and hepatic vascular beds. Significant
renal, hepatic, and gastrointestinal dysfunction may ensue and should be anticipated
in the young infant with unexplained ascites. Similar cannulation problems may result
in superior vena cava obstruction. This condition may be more ominous during bypass.
Under these circumstances, three problems may ensue: (1) cerebral edema, (2) a
reduction in regional or global CBF, and (3) reduced proportion of pump flow reaching
the cerebral circulation, causing inefficient brain cooling.
In the operating room, it is advisable to monitor superior vena
cava pressures via an internal jugular catheter or by examining the patient's head
for signs of suffusion after initiating bypass. Discussions with the perfusionist
regarding adequacy of venous return and large cooling gradients between the upper
and lower body should alert the anesthesiologist and the surgeon to potential venous
cannula problems. Patients with anomalies of the large systemic veins (persistent
left superior vena cava or azygous continuation of an interrupted inferior vena cava)
are at particular risk for problems with venous cannulation and drainage.
Problems with aortic cannula placement can also occur. The aortic
cannula may slip beyond the takeoff of the innominate artery and therefore selectively
flow to the right side of the cerebral circulation. Also, the position of the tip
of the cannula may promote preferential flow down the aorta or induce a Venturi effect
to steal flow from the cerebral circulation. This problem has been confirmed during
CBF monitoring by the appearance of large discrepancies in flow between the right
and left hemispheres after initiating CPB. The presence of large aortic to pulmonary
collaterals, such as a large PDA, may also divert blood to the pulmonary circulation
from the systemic circulation, thereby reducing CBF and the efficiency of brain cooling
during CPB.[102]
The surgeon should gain control
of the ductus either prior to or immediately after instituting CPB to eliminate this
problem and, if possible, large aortopulmonary collaterals should be embolized in
the cardiac catheterization laboratory prior to the operative procedure. Neonates
with significant aortic arch abnormalities (e.g., aortic atresia, interrupted aortic
arch) may require radical modifications of cannulation techniques, such as placing
the arterial cannula in the main pulmonary artery and temporarily occluding the branch
pulmonary arteries to perfuse the body via the PDA, or even dual arterial cannulation
of both the ascending aorta and main pulmonary artery. Such adaptations require
careful vigilance to ensure effective, thorough perfusion and cooling of vital organs.
Once the aortic and venous cannulas are positioned and connected
to the arterial and venous limb of the extracorporeal circuit, bypass is initiated.
The arterial pump is slowly started, and, once forward flow is ensured, venous blood
is drained into the oxygenator. Pump flow rate is gradually increased until full
circulatory support is achieved. If venous return is diminished, arterial line pressure
high, or mean arterial pressure excessive, pump flow rates must be reduced. High
line pressure and inadequate venous return are usually caused by malposition or kinking
of the arterial and venous cannulas, respectively. The rate at which venous blood
is drained from the patient is determined by the height difference between the patient
and the oxygenator inlet and the diameter of the venous cannula and line tubing.
Venous drainage can be increased by using vacuum-assisted drainage under certain
circumstances.
In neonates and infants, deep hypothermia is commonly used. For
this reason, the pump prime is kept cold (18 to 22°C). When the cold perfusate
contacts the myocardium during the institution of CPB, heart rate slows immediately
and contraction is impaired. The contribution of total blood flow pumped by the
infant's heart rapidly diminishes. Therefore, to sustain adequate systemic perfusion
at or near normothermic temperatures, the arterial pump must reach full flows quickly.
CPB is initiated in neonates and infants by beginning the arterial
pump flow first. Once aortic flow is ensured, the venous line is unclamped and blood
is siphoned out of the right atrium into the inlet of the oxygenator. Flowing before
unclamping the venous line prevents the potential problem of exsanguination if aortic
dissection or misplacement of the aortic cannula occurs. Neonates and infants have
a low blood volume/priming volume ratio, and intravascular volume falls precipitously
if the venous drainage precedes aortic inflow. Once the aortic cannula position
is verified, pump flow rates are rapidly increased to maintain effective systemic
perfusion. As coronary artery disease is rarely a consideration, the myocardium
should cool evenly unless distortion caused by the cannulas compromises the coronary
arteries. When a cold prime is used, caution must be exercised in using the pump
to infuse volume prior to initiating CPB. Infusion of cold perfusate may result
in bradycardia and impaired cardiac contractility before the surgeon is prepared
to initiate CPB.
Once CPB is begun, careful observation should be focused to ensure
appropriate circuit connections, myocardial perfusion, and optimal cardiac decompression.
Ineffective venous drainage can rapidly result in ventricular distention. This
is especially true in infants and neonates, in whom ventricular compliance is low
and the heart is relatively intolerant of excessive preload augmentation. If ventricular
distention occurs, pump flow must be reduced and the venous cannula repositioned.
Alternatively, the heart may be decompressed by placing a cardiotomy suction catheter
or small vent in the appropriate chamber.
Pump Flow Rates
Recommendations for optimal pump flow rates for children have
historically been based both on the patient's body mass and on evidence of efficient
organ perfusion as determined by arterial blood gases, acid-base balance, and whole
body oxygen consumption during CPB.[49]
[103]
At hypothermic temperatures, metabolism is reduced,
and CPB flow rates can therefore be reduced and still meet or exceed the tissue's
metabolic needs (see the discussion of low-flow CPB in the following section).
