Extra Corporeal Circulation in Paediatrics

Cardiopulmonary Bypass circulation [CPB]
When all the systemic venous drainage to the heart is drained and returned to the CPB circuit and again to the patient through arterial cannulas is described as cardiopulmonary bypass circulation.

Cannulation for CPB
Routinely systemic venous return can be drained through two cannulae-SVC and IVC. Alternative or extra cannulae can be required to drain venous blood through LSVC, Interrupted IVC with hepatic veins or azygous continuation of IVC to Rt. /Lt. side of heart.

Both cavae are cannulated surgically, tapes are passed around the vessels externally.these tapes after tightening isolate the Rt. Heart completely.So that after opening the chamber entrainement of air and venous blood obscuring the surgical field is avoided.At the time of antegrade cardioplegia one tape should untightened to avoid distenstion og RA and egress of solution from the coronary sinus.

Patient wieght in kg	SVC /IVC SIZE of cannulae
<3.5	12/12
3.5-6	12/14
6-8	12/16
8-12	14/16
12-16	14/18
16-23	16/18
23-28	16/20
28-36	18/20
36-48	18/22

Sizes of aortic cannulas.
Arterial limb is pressurized side of CPB. Straight tip, curved tip cannulas available.

Pumps used

1.Rollerpump Commonly used in paediatric cardiac surgery.
➤ Provide continuous nonpulsatile flow.
➤ Capable of generating both positive and negative pressure to propel blood through pump and to pull blood in venous reservoir and for cardiotomy suction.

2.Centrifugal pump Newer devices.
➤ Less priming volume and less haemolysis of blood.

Filters and Bubble Traps
➤ Minimises risk of embolisation.Leukocyte filters for avoiding leukocytes in systemic circulation and reduce generation free radical generation which improves myocardial protection and post op lung function recovery.

Priming the Pump
➤ Priming volume dilutes the patients lood content 2 to 3 times.maintainance of Haematocrit is important and according to indivisual protocols.PCV transfusion is necessary to maintain desired Hct.[haematocrit].
➤ Blood products used should be fresh[within one week] to avoid higher potassium, lactate and pyruvate levels.
➤ Citrate in blood bag causes Calcium depletion, paediatric patients are susceptible for hypocalcemia.
➤ Colloid osmotic pressure should be maintained to avoid capillary leak.[target 16-18 mmof Hg]
➤ Mannitol- diuretic.
➤ Prevents adhesion of platelets to circuit surface.
➤ Free radical scavenger.

Steroids- Anti-inflammatory agent.
➤ Magnessium, calcium and sodium bicarbonate are also added to the prime solution.
Anaesthetic drugs –IV.Fentanyl, iv midazolam and muscle relaxant should be given on CPB.

Ultrafiltration
1.conventional
2.dilutional.
3.Modified.

Diagrammatic presentation of CPB

Differences between Adult and Paediatric CPB.

Glucose management

PARAMETERS	ADULT	PAEDIATRICS
Temp.	Minimum CPB temperature Rarely o251-321C	321C Frequently 151-251C
DHCA	Use of total circulatory arrest Rare	common
Pump prime	DILUTION Dilution of blood volume 25%-33%	150%-300%
Whole blood or RBC added	Rare	Frequent
Perfusion pressure	50-80 mmHg	20-50 mmHg
Acid-base management strategy	Alpha-stat	pH-stat at temperature o281-301C
	Hyperglycemia: Frequent requires insulin.Hypoglycemia-rare	Less common, Hypoglycemia-common.

Physiologic explaination for above differences
High flow rates
Procedures per formed on infant and children may require the extreme measures that are not necessary for adults, including deep hypothermia, hemodilution, different acid-base strategies, low perfusion pressures, and wide variation of perfusion flow rates. These measures notably vary from those of normal physiology and affect protection of normal organ function duringCPB. In addition, their smaller circulating bloodvolume, higher oxygen consumption rate, reactive pulmonary vascular bed, immature organ systems, and altered thermoregulation may cause vulner- ability to deleterious effects of CPB in infants.

The cardiac pathology with large intra-and extracardiac shunts may result in a greater redistribution of flow away from the vital organs during CPB.
Development of collateral vessels secondary to cyanosis and vascular obstruc- tion may result in significant bloodloss and impair surgical field exposure.
Infants are characterized by a high metabolic rate and, thus oxygen demand requiring higher flow rates per body surface area.

Full Cardiopulmonary Bypass Flow Rates in Infants

Patient Weight(kg )	FullCPBRates(mL/kg/min)
3	150-200
3-10	125-175
10-20	80-150

The relatively large bypass prime volumes. compared with the
circulating blood volumes, in infants and children lead to significant haemodilution.

Even with the newest technology, the minimum prime volume for circuits allowing full support in neonates at normothermia is 220 ml, 180 ml if the arterial filter is excluded.

The advantage of haemodilution is improved viscosity.

The disadvantages of haemodilution
1.Anaemia with decreased oxygen-carrying capacity,
2.Reduced levels of plasma proteins
3.Reduced clotting factors leading to tissue oedema and coagulopathy,
4.Electrolyte imbalances
5.Exaggerated release of stress hormones and complement activation.

Maintanance of haematocrit on CPB
Haemodilution becoz of large priming volume improves viscosity and microcirculation but at the same time reduced Hb and Hct[20% or <25%] impairs oxygen carrying capacity and oxygenation. Now a days maintainance of Hct -25-30% is the protocol as many studies shown improved outcome with Hct nearly 30% on CPB.

DHCA
Deep hypothermic circulatory arrest [DHCA]-commonly used in paediatric CPB, based on the principle of lowering metabolic demands of the brain.

Deep hypothermia temp.<18 degree celcius either with low flow CPB or circulatory arrest has been used for surgical procedures and organ protection.

Complex anatomy and development of collaterals in cyanotic congenital heart diseases requires complete cessation of circulation for blood less surgical field.

Neuroprotection and other organ protection is very important in CPB, hypothermia helps to reduce metabolic rate and oxygen consumption and increases the tolerable limits of ishaemia.

Glycemic control
Immaturity of liver function till age of one year making infants susceptible for poor control of glucose and so tendancy to hypo or hyperglycemia is high in children than adults.causes of hypoglycemia are
1.Reduced synthetic function.
2.Decreased glycogen stores.
3.Systemic hypoperfusion impairing hepatic function and glucose production.
Hyperglycemia
1.glucose containing Iv Maintainance fluids.
2.Steroids given on CPB always causes hyperglycemia.

Adverse effects of hyperglycemia- impairement of neurologic outcome.
pH Management

Two types of pH management.

1. ALPHA stat pH management
a] No CO2 is added.
b] Temperature not corrected.3.pH is maintained at 7.4

How it works?
ALPHA stat management maintains normal cerebral blood flow and autoregulation.It improves metabolic recovery during rewarming.
By maintaining cerebral blood flow it reduces the embolic load to brain and adverse nuerooutcome.

2. Ph stat management
A] CO2 is added to circulation
B] Temperature is to be corrected with pts. Actual temp.
C] Ph is maintained at 7.4

How it works?

Ph stat strategy results in greater cerebral blood flow.Autoregulation is lost.Overperfusion exceeding need of tissues metabolic need improves brain cooling homogeneity and oxygenation[before, during and after deep hypothermia]. It carries higher load of embolus and risk of embolic cerebral injury.

In children the prominent cause of cerebral injuries are ishaemia and hypoperfusion while in adults its embolism becoz of atherosclerosis.
There are lot of standard studies carried out to compare both strategies in congenital cardiac procedures, but exact differences in outcome not established.

Many authers suggested combined strategy such as using Ph stat during the cooling phase of deep hypothermic CPB and alpha stat during rewarming period.

Systemic inflammatory response

these responses on CPB are becoz of
➤ Injury to blood elements
➤ Activation of complex inflammatory responses
➤ Stress response system.

Immature immune system leads to more morbidity and mortality in infants.
It manifest as post bypass ARDS, Pulmonary hypertension crisis, myocardial dysfunction and haemodyanamic instability.

Measures to reduce
➤ Use of corticosteroids [controversial but used in many OTs as protocol ]
➤ Use of serine protease inhibitors,
➤ Use of phosphodiasterase inhibitors, nitric oxide, antioxidants and complement inhibitors.
➤ Circuit miniaturization
➤ Heparin bonded circuits
➤ Use of haemofilters.

Monitoring on CPB
1.Direct measures like-cardiac index and mean arterial pressure onpump.
2.Urine output
3.temperature
4.TCD, NIRS, JUGULAR BULB SATURATION
5.Systemic venous oxygenation (sVo2)
6.ABG.