372 SECTION IV Pediatric Critical Care Cardiovascular microemboli, and cause retrograde flow in the venous line if the vacuum apparatus malfunctions or becomes occluded 44,45 Con siderations for safe[.]
Trang 1microemboli, and cause retrograde flow in the venous line if the
vacuum apparatus malfunctions or becomes occluded.44 , 45
Con-siderations for safe VAVD operation include using a pressure
monitor to maintain a venous line pressure between 0 to 240
mm Hg (gravity siphon will typically provide 25 to 215 mm
Hg), adding both positive and negative pressure relief valves to
the sealed venous reservoir, being vigilant to recognize and ask the
surgeon to correct sources of venous line air, including a moisture
trap in the vacuum kit, and only using the minimal amount of
vacuum to achieve full arterial flow.46–49 When on total CPB, the
mean arterial pressure (MAP) and CVP should confirm that the
heart is empty by showing a flat tracing However, the many
ana-tomic variations of the congenital patient can lead to blood
re-turning to the heart despite adequate drainage Variations such as
a patent ductus arteriosus, major aortopulmonary collateral
arter-ies, an unrecognized left SVC draining to the coronary sinus, and
aortic insufficiency can return blood to the heart and should be
considered when evaluating venous drainage Assessment of
ade-quate venous drainage and perfusion flow rate is critical before
proceeding to the surgical repair A poorly positioned venous or
arterial cannula can restrict optimal perfusion flow; addressing
this issue during the aortic cross-clamp period would waste
un-necessary myocardial ischemic time Once cannula placement is
deemed acceptable, full perfusion flow is achieved, and
oxygen-ation from the oxygenator is confirmed, the anesthesiologist can
turn off the ventilator
Determining and Monitoring Effective Perfusion
Flow Rate
The fundamental goals of bypass are to meet the metabolic
de-mands of all tissues and to attenuate the deleterious
pathophysi-ologic effects of artificially supporting a patient Once the patient
is transitioned to CPB, several management techniques are used
to safely optimize the level of support Perfusion flow rate, which
represents the cardiac output during CPB, is altered to meet the
O2 consumption needs of the patient Global adequacy of flow is
estimated in real time by the display of O2 saturation by a sensor
in the venous return line Assessing regional O2 consumption to
the brain, kidneys, or bowel, for instance, is a challenge
Addi-tionally, owing to age-related differences of body surface area
(BSA)-to-blood volume ratios, flow rate indexes are higher in
neonates than adults The optimal effective flow rate or cardiac
index for any size patient remains unclear Considering that the
perfusion flow rate is not fixed during the different phases of
CPB, several variables are helpful in determining a safe minimal
rate Initial normothermic target rates are calculated for CPB
initiation by weight and BSA At Children’s Health Dallas, in
ad-dition to the CPB initiation flow rates calculated by weight, the
perfusionist calculates several cardiac indexes ranging from 2.4 to
3.0 L/min per m2 Factors such as the degree of hypothermia,
acid-base balance, depth of anesthesia and neuromuscular
block-ade, hematocrit, venous saturation, lactate level, urine output,
and near infrared saturation trends are used to guide perfusion
flow rates Patient temperature is the greatest factor affecting
per-fusion flow; rates as low as 40 to 50 mL/kg are routinely used at
core temperatures in the 20°C range
A growing area of concern is the avoidance and early detection
of acute kidney injury (AKI) in congenital heart disease patients
While traditional diagnostic approaches have not been
thor-oughly validated in children, it has been widely reported that
there is a 40% to 50% incidence of AKI in congenital heart
disease patients, and 64% incidence in neonatal patients.50–52 The kidneys perceive nonpulsatile flow or a decrease in arterial flow as hypovolemia, and the resultant neurohormonal cascade is thought
to trigger the AKI complex Since most perioperative risk factors, such as younger age and the incidence of higher surgical com-plexity, are nonmodifiable, therapeutic strategies have focused
on optimally managing perfusion flow rate, arterial pressure, and hematocrit.53
Arterial Pressure
The MAP will slowly lose its pulsatile trace and flatten out as the heart empties on CPB Though the MAP is calculated by factor-ing the systolic and diastolic pressures, the value of the flat tracfactor-ing
is referred to as the MAP and perfusion pressure during CPB The transition to CPB often leads to hypotension; in contrast to adult cases, vasopressors (e.g., phenylephrine) are not typically admin-istered in the early phase of CPB in young patients The goal in the early phase of CPB is to cool the patient and reduce the metabolic demands Low perfusion pressure, 20 to 30 mm Hg, is accepted during the cooling phase, and vasodilators (e.g., phen-tolamine) are used to reduce arterial tone and increase uniformity
of perfusion and improve cooling Vasodilation has been shown to improve temperature distribution and reduce lactate production
in pediatric deep hypothermic CPB.54 Hemodilution, hypocalce-mia, and the inflammatory response are also factors that cause hypotension at the onset of CPB Hemodilution will lower the perfusion pressure because of the viscosity reduction, and hemoconcentration performed by the perfusionist with the hemo-concentrator can easily increase perfusion pressure by raising the hematocrit The systemic inflammatory response is triggered by the foreign surface contact of blood and bypass circuitry This response releases many vasoactive mediators, which can quickly drop the perfusion pressure, and highlights the importance of minimizing circuit surface area for the pediatric patient The de-crease in pressure in an adult patient with coronary or carotid stenosis would likely be treated with a vasopressor, while increas-ing perfusion flow is the preferred method in young patients
Arterial and Venous Oxygen Saturation
Most O2 saturation monitoring techniques are noninvasive,
inex-pensive, and can be used in real time Changing perfusion flow rate and O2 delivery will have immediate and direct effects on O2
saturation levels Pulse oximetry is a clinical mainstay used to monitor O2delivery to the extremities during the preoperative and postoperative periods However, due to the nonpulsatile flow pattern generated during CPB, this technology is ineffective
As mentioned earlier, a mainstay monitoring technique during CPB is to track the O2 saturation of the venous line blood
drain-ing into the venous reservoir As a general guideline, perfusion flow rate is adjusted to maintain this mixed venous O2 saturation
(Svo2) greater than 70% While this guideline is helpful during
“normal” physiologic conditions, the many nonphysiologic vari-ables of CPB cause shifts in the oxyhemoglobin dissociation curve (Fig 35.7); these venous O2 saturations may fail to represent satisfactory O2 delivery to the tissues Leftward shifts in the curve
prevent O2from being released from hemoglobin, which could deceivingly demonstrate an acceptable Svo2 in the presence of tissue hypoxia As the patient is cooled during CPB, regional de-oxygenation has been shown to occur despite a normal or rising Svo2 without increasing perfusion flow rate.55 Hypothermia not only strengthens the hemoglobin O2 affinity but also creates an
alkaline blood pH and causes a further leftward oxyhemoglobin
Trang 2shift In this situation, it is important to cool and warm patients
methodically to minimize the temperature gradient between the
blood and tissues and maintain perfusion flow so that the Svo2
does not trend downward As tissue temperature decreases,
ve-nous line Svo2 can be used to help guide the reduction of
perfu-sion flow rate In addition to hypothermia, other factors—such as
third spacing, myocardial dysfunction, and hemodilution—may
reduce O2 delivery and lead to cellular hypoxia Therefore, it is
common practice to augment O2 delivery using higher fraction of
inspired O2 settings and maintaining a partial arterial oxygen
pressure (Pao2) above normal physiologic values, with the goal of
increasing the O2 gradient between capillaries and tissue beds
However, the O2 tension strategy must be carefully managed to
avoid the generation of systemic reactive O2 species, systemic
in-flammation, O2radicals, and end-organ injury This paradox is
especially pronounced in patients with cyanotic defects owing to
their physiologic sensitivity to oxidative stress.56 While the
litera-ture does not clearly delineate what Pao2 value is considered high
or hyperoxic, it is suggested that CPB should be initiated by
con-trolling oxygenation in a slow, graded manner and to not exceed
Pao2 values of 350 mm Hg.57 , 58
It is especially critical to meet the metabolic needs of brain
tissue, and global Svo2 values may misrepresent regional O2
con-sumption It has been shown that considerable regional
differ-ences exist and Svo2 can overestimate regional saturations from
the brain.59 The majority of venous blood analyzed by the venous
line Svo2 comes from IVC cannula, and IVC saturation is
notori-ously misleading as an estimate of O2 consumption owing to
“contamination” by highly saturated renal venous blood Though
the Svo2 does have its place in guiding perfusion flow rate, more
specific, regional oxygenation assessment is currently
recom-mended to ensure adequate perfusion flow distribution,
particu-larly for the brain
Near-Infrared Spectroscopy
Near-infrared spectroscopy (NIRS) is a noninvasive optical sensor
that can measure cerebral and somatic tissue oxygenation NIRS
monitoring is gaining considerable popularity because sensor pads may be placed over various regional tissue beds, particularly both cerebral hemispheres, and display real-time results Somatic mon-itoring sites—such as flank, abdominal, and muscle—are sug-gested to help broaden the assessment of systemic hypoperfusion The technology works by bouncing various wavelength arcs of near-infrared light from a sensor emitter and detector These pho-todetectors allow for selective measurement of tissue oxygenation This technology is widely used in the operating room and inten-sive care unit, although interpreting the results has been a topic of debate A validation study performed at Children’s Health Dallas demonstrated that cerebral NIRS values accurately predicted the
O2 saturation in the SVC on CPB and that flank NIRS values were significantly associated with IVC saturation.60 As increasing evidence validates tissue oximetry against invasive measurements, NIRS monitoring has shown its value in quickly detecting re-gional low flow.61–64 At Children’s Health Dallas, the perfusionists use NIRS trends and values to guide perfusion flow rate, hematocrit, blood gas strategy, temperature, and vasomotor tone (Box 35.1)
It is important to note that NIRS helps to guide rather than dic-tate perfusion management The upper limits and critical lower values reported by NIRS are poorly defined
Considering the regional O2 oxygen saturation variations of the neonatal and infant cardiac surgical patients, NIRS provides valuable information and early detection of poor perfusion in critical organs NIRS has been particularly useful as a real-time monitor on patients with hypoplastic left heart syndrome, for example Cerebral O2 saturation measured after stage I palliation has been shown to strongly correlate with hemodynamic param-eters and help to identify early postoperative complications.62 , 65 Hoffman et al.66 found that avoiding cerebral hypoxia with the use of NIRS monitoring was the most significant factor in im-proving childhood neurodevelopmental outcomes Additionally, Sood et al.67 demonstrated that perioperative NIRS monitoring was useful in predicting neurodevelopmental outcomes, especially when evaluating the percent decrease of cerebral O2 saturation from baseline values during the intraoperative period
Methods to Optimize Physiologic Management
Target Hematocrit and Ultrafiltration
Despite the progress of reducing the pediatric circuit prime volume, hemodilution during CPB remains difficult to avoid.68 Hemodilution can cause edema, coagulopathy, blood and colloid osmotic pressure reduction, and the need to transfuse blood products Blood product transfusion is the most straightforward solution to address these complications; however, the risk-benefit assessment of blood transfusion must be considered Transfusion-related complications include increased postoperative morbidity and mortality, prolonged mechanical ventilation and hospital stay,
100
20
50
30
70
90
40
60
80
10
Left shift:
Decreased 2,3-DPG
Fetal hemoglobin
Hypothermia
Hypocarbia
Alkalosis
Right shift:
Increased 2,3-DPG Hyperthermia Hypercarbia Acidosis
•Fig 35.7
Near-Infrared Spectroscopy Values
Increase perfusion flow rate Increase hematocrit pH-stat blood gas strategy Decrease temperature Increase mean arterial pressure; vasopressor Verify adequate superior vena cava drainage
Trang 3exacerbation of the inflammatory response, and infection.69–72
Modern blood bank testing and donor screening have
signifi-cantly reduced infectious complications, but noninfectious risk
remains a major concern In addition, smaller patients are exposed
to a higher transfusion risk because the transfusion effects may be
more pronounced than in adult patients With a goal of
minimiz-ing hemodilution and donor blood exposure, the cardiac team
must implement a transfusion algorithm and define a target
he-matocrit during CPB Perioperative and developmental outcomes
data reported from clinical trials at the Boston Children’s Hospital
demonstrate that, when compared with target CPB hematocrit
values of 30%, hematocrit values at or below 20% are associated
with adverse outcomes and that the benefits of hematocrit values
higher than 25% should be further investigated.73 , 74 The protocol
at Children’s Health Dallas is to maintain a hematocrit of at least
30% during CPB
Reducing circuit prime volume and incorporating an
ultrafil-tration device can significantly reduce donor blood exposure
Originally, a hemoconcentrator added to the CPB circuit was
used primarily to remove plasma water and raise the hematocrit
This process is referred to as conventional ultrafiltration (CUF) and
its initial use was met with a few limitations Perfusionists were
accustomed to using diuretics to help increase the hematocrit;
however, this strategy offered little control and required adequate
kidney perfusion during CPB When ultrafiltration emerged as a
CPB technique in the mid-1980s, hemoconcentrators were
viewed as an expensive option for removing excess circuit volume
Also, while fluid is removed from the circuit during CUF, the
volume in the venous reservoir level diminishes and CUF must be
stopped before the reservoir is emptied, which would have
cata-strophic consequences Thus, the amount of fluid removed during
CUF is dependent on available volume in the venous reservoir,
which limits the ability to effectively raise the hematocrit In
1991, a report described a modified ultrafiltration (MUF)
tech-nique performed after weaning the patient from CPB that
en-abled the perfusionist to concentrate the entire circuit and return
most of that volume back to the patient.75 During this era,
pedi-atric circuit prime volume was still rather high; various MUF
techniques proved to be a valuable resource for reducing total
body water post-CPB As the prime volumes of CPB circuits have
become increasingly smaller relative to the blood volume of the
patient, perfusionists have begun to abandon the cumbersome
MUF technique in favor of preventing hemodilution rather than
reversing hemodilution.76–77 The early popularity of MUF led
investigators to explore the potential reduction of
proinflamma-tory mediators during ultrafiltration An additional ultrafiltration
technique, zero balance ultrafiltration (ZBUF), emerged as an
al-ternate method to attenuate the inflammatory response ZBUF is
performed by removing the ultrafiltration effluent from the
cir-cuit during CPB while administering a replacement solution (e.g.,
Plasma-Lyte A) to the venous reservoir in a 1:1 ratio The
high-volume filtration of fluid that is able to be exchanged allows the
perfusionist to control electrolyte and glucose levels (e.g., high
potassium levels after delivering cardioplegia) and more effectively
remove inflammatory mediators The increasing acceptance of
ultrafiltration during CPB led to developing many technique
variations during the preoperative, perioperative, and
postopera-tive phases, which can be categorized into two groups: blood
concentration and blood filtration
In preparation for neonatal CPB at Children’s Health Dallas,
for example, the perfusionist adds approximately 300 mL of
PRBCs to the venous reservoir after priming the circuit with
Plasma-Lyte A The circuit volume is recirculated through the hemoconcentrator, and volume is removed until the reservoir is almost empty The circuit prime is then “washed” by adding ap-proximately 500 mL of Plasma-Lyte A and then removing that
volume This process is known as prebypass ultrafiltration
(Pre-BUF) In addition to concentrating the circuit, Pre-BUF has been shown to reduce high potassium, glucose, lactate, citrate, and bradykinin levels found in PRBCs CUF is then performed dur-ing the early phase of CPB to remove any excess circuit volume and maintain a hematocrit of 30% During the warming phase of CPB, the perfusionist performs CUF and ZBUF This combined ultrafiltration strategy, coupled with a miniaturized circuit, allows the perfusionist to filter the blood and exceed or meet baseline hematocrit values before weaning from CPB
Hypothermia
Deep Hypothermic Circulatory Arrest vs Antegrade Cerebral Perfusion
The therapeutic potential of hypothermia has been known for centuries and has been routinely used in cardiac surgery since its inception This concept relies on the fundamental physiologic relationship between O2 consumption and temperature In 1950, Bigelow and colleagues78 compared the use of normothermia and topical cooling on dogs and reported superior ischemic tolerance
by surface cooling after 15 minutes of circulatory arrest The first clinical application in cardiac surgery was reported in 1953
by Lewis and Taufic, who described the successful repair of an ASD in a 5-year-old girl using topical cooling and total body hypothermia.2 To achieve this, patients were submerged in an ice bath to reduce their temperature to approximately 28°C Then, the defect was closed with the aid of inflow occlusion In 1958, Sealy and colleagues79 successfully reported the use of hypothermia
in conjunction with CPB The use of CPB with various degrees of hypothermia or deep hypothermic circulatory arrest (DHCA) dramatically increased the “safe” period of support, which enabled surgeons to repair increasingly complex anomalies and allowed cardiac surgery to flourish
Hypothermia suppresses metabolic activity, preserves high-energy phosphate stores, and reduces the reaction rate of bio-chemical reactions Several factors are used in determining the type and degree of hypothermia during CPB The most significant factor is the degree of surgical difficulty and anticipated CPB sup-port time Complex surgical repairs requiring lengthy supsup-port times would benefit from more pronounced hypothermia The degree of hypothermia varies greatly and is typically classified as mild, moderate, deep, and profound (Table 35.5) Deep hypo-thermia might be seen as desirable when low flow (#50 mL/kg per minute) or DHCA is desired, as is the case in operations involving complete aortic arch reconstruction Circulatory arrest
is a process in which the perfusion flow is turned off and the
TABLE
35.5
Category Core Temperature
Hypothermia Classifications in Cardiac Surgery
Trang 4patient’s blood volume is allowed to drain into the venous
reser-voir This dramatic application provides an asanguineous and
completely motionless surgical field, facilitating complex repairs
In very small patients, the venous cannula may be obstructive and
DHCA is required in order to remove the cannula and access the
surgical site Hypothermia also facilitates exposure of the surgical
field by allowing decreased perfusion flow rates, which reduces the
amount of collateral blood returning to the heart via the
pulmo-nary veins (Fig 35.8) Patients with pulmonary blood flow
re-strictions (e.g., tetralogy of Fallot, pulmonary atresia) can develop
major aortopulmonary collateral arteries; these collaterals can
flood the heart during the aortic cross-clamp period if CPB flow
is maintained This excessive blood return not only obscures the
surgical site but may also warm the cold arrested myocardium or
wash out cardioplegia from the coronary arteries Hypothermia
and perfusion flow rate reduction can attenuate this collateral
flow while maintaining adequate oxygenation to the patient
The rate of cooling varies greatly between different tissue beds;
thus, multiple measurement sites are recommended to ensure
uniform cooling distribution (Fig 35.9) The optimal
tempera-ture measurement site is controversial, but choosing sites that
closely reflect tissue temperatures of vital organs, particularly the
brain, is widely accepted At Children’s Health Dallas, the
pa-tient’s nasopharyngeal, rectal, and bladder temperatures are
mon-itored during surgery Nasopharyngeal temperature closely
corre-lates with brain temperature; however, it may underestimate the
global core temperature considering the slower cooling rates of
other tissue beds For this reason, rectal and bladder temperature
monitoring sites with slower rates of cooling are typically used to
guide cooling end points
The concept of a “safe” circulatory arrest time is controversial,
and most guidelines are met with a degree of uncertainty A
no-mogram focused on neurologic protection has been devised that
estimates safe circulatory arrest times, but values should not be
used as absolutes (Fig 35.10) The historic incidence of
periop-erative cerebral injury during DHCA has led investigators to
ex-plore alternative techniques to protect the brain during complex
repairs Antegrade cerebral perfusion (ACP, also known as
selec-tive cerebral perfusion) uses a cannulation technique that directs
perfusion flow to only the brain with the theoretic advantage of
protecting it from hypoxic ischemic injury Though this tech-nique has been adopted among many surgical centers, investiga-tions comparing DHCA and ACP have failed to definitively demonstrate superiority of either technique,80–82 and not all of this work is focused only on neurologic issues Recent literature has suggested that during ACP, the resultant partial perfusion from collateral vessels provides better protection to abdominal organs than DHCA.83 , 84 In addition, the ideal temperature dur-ing ACP remains unknown However, recent reports have demon-strated superior results using moderate to mild hypothermia, in the 25°C to 30°C range, rather than deep levels of hypother-mia.85 , 86 Considering the coagulopathy, inflammatory response, and the vascular and organ dysfunction associated with deep hy-pothermia, investigators have been prompted to explore warmer
or normothermic high-flow CPB Recent studies have shown that high-flow normothermic CPB is as safe as hypothermic CPB during low-risk procedures, and may reduce postoperative inotro-pic and respiratory support, shorten length of stay, and reduce intraoperative blood transfusion.87 , 88
pH and Partial Pressure of Arterial Carbon Dioxide Strategy
CO2 concentration and pH are primary determinants of cerebral blood flow As body temperature decreases, the solubility of CO2
increases, resulting in a decreased partial pressure of arterial CO2
(Paco2) and increased pH Manipulating the acid-base manage-ment during hypothermic bypass can be classified by two mecha-nisms of control: a-stat or pH-stat Both mechamecha-nisms have been studied intensely in reptiles (ectotherms) and hibernating mam-mals (endotherms), looking at their adaptive blood pH alterations that allow them to withstand extreme temperature fluctuations pH-stat acid-base management is practiced by hibernating mam-mals, accomplished by decreasing ventilatory rate and raising Paco2 while maintaining a constant pH during hypothermic con-ditions Maintaining pH while varying temperature during hiber-nation is thought to preserve O2 stores by decreasing metabolic activity Alternatively, reptiles use a-stat and allow their pH to enter an alkaline state by reducing Paco2
The perfusionist can maintain a pH-stat or “temperature-cor-rected” acid-base strategy during CPB by allowing CO2 to pas-sively rise or actually adding CO2 to the CPB circuit Cerebral blood flow has been shown to decrease during hypothermia using the a-stat strategy and demonstrates a linear relationship to the increased Paco2 when a pH-stat strategy is used.89 In addition to preserving cerebral blood flow during hypothermia, a pH-stat strategy induces a rightward shift of the oxyhemoglobin dissocia-tion curve (see Fig 35.7) potentially allowing for increased O2
off-loading from hemoglobin at the capillary level
Whether pH-stat or a-stat acid-base management during hy-pothermic CPB demonstrates clear benefits on clinical outcomes has been difficult to demonstrate However, it is suggested that a pH-stat strategy is optimal for pediatric patients, and a-stat is the optimal strategy for adult patients.90
Myocardial Protection
Myocardial protection refers to the strategies and techniques
em-ployed to allow the surgeon to work on the heart in a bloodless and motionless field yet recover the best possible postischemic myocardial function and cardiac output for the patient Strategies for both adults and children attempt to reduce cardiac workload and minimize the metabolic demands and consequences of O2
• Fig 35.8 Nomogramrelatingoxygenconsumptiontoperfusionflowrate
andtemperature.(FromKirklinJW,Barratt-BoyesBG.Hypothermia,cir-culatoryarrest,andcardiopulmonarybypass.In:KirklinJW,Barratt-Boyes
BG, eds. Cardiac Surgery. 2nd ed. New York: Churchill-Livingstone;
1993:91.)
50
150
100
0
2.5 2.0
1.5 1.0
0.5 0.0
–1 · m
–2 )
37 °C
30 °C
25 °C
20 °C
15 °C
Trang 55
90 80 70 60 50 40 30 20 10 40 30 20 10 0 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40
Time (minutes)
Arterial cannula Myocardial Brain Nasopharyngeal Rectal
P < 05
Temperature (± SEM)
• Fig 35.9 Relationshipsoftemperaturesmeasuredatvarioussitesovertimeduringcoolingandwarming
fromcardiopulmonarybypass.(FromStefaniszynHJ,NovickRJ,KeithFM,etal.Isthebrainadequately
cooledduringdeephypothermiccardiopulmonarybypass?Curr Surg.1983;40:294–297.)
deprivation during ischemia Reducing afterload and emptying
the heart by initiating CPB is a myocardial protection technique
during beating heart procedures (e.g., palliative shunts,
bidirec-tional Glenn procedure) When the heart needs to be stopped and
opened for intracardiac repairs, the aorta is cross-clamped and
cardioplegia is delivered to the coronary circulation to cause
pro-longed asystole Cardioplegia is a myocardial arrest-producing
solution that is formulated to prolong myocardial tolerance to
ischemia There are many techniques to protect the myocardium,
but cardioplegia strategies are often the focus when discussing protection techniques In North America, high-potassium depo-larizing solutions are the most common type of cardioplegia used.34 Universal agreement on an optimal myocardial protective technique is widely debated; most strategies are guided by surgeon
or institutional preference
After the first successful cardiac surgical correction using CPB
in 1953, surgeons explored a variety of techniques to support the patient during the repair It did not take long for them to realize