Constituents of priming fluid relative to patient size and priming volume of the oxygenator 1

Limitations and validity of haemodilution. 2

Formulae for calculation of haemodilution. 3

CO₂ flushing prior to priming the oxygenator 4

PreCPB filter placement and pore size relative to priming. 4

Constituents of priming fluid relative to patient size and priming volume of the oxygenator

· Due to benefits of haemodilution and concerns about blood borne diseases, is standard practice to use a nonblood CPB prime

· Patients < 35 kg may need some blood in prime to avoid excessive haemodilution

· Haemodilution [Hct: 21-24%] reduces blood viscosity resulting in:

· increased tissue perfusion

· reduced blood cell trauma

· improved renal function

· reduced need for homologous blood

1. Osmolality

a) Isosmotic or slightly hypertonic to minimise tissue oedema

b) Use of mannitol or albumin

2. Electrolytes

a) Normal electrolyte balance must be maintained to prevent post CPB electrolyte abnormalities

3. Volume

a) Enough volume to fill & prime:

i) Circuitry, oxygenator, filters, venous reservoir

b) Enough volume to enable safe conduct of perfusion:

i) venous level, flow rates required

c) Not too much volume to cause excessive haemodilution

4. Haemodilution

a) Avoid < 18 % Hct

b) To determine the initial Hct on CPB:

Hct_int = initial Hct on CPB

EBV = estimated patient blood volume

Hct = preoperative Hct

c) To determine the volume of red blood cells needed to prime the circuit:

PBV = patient’s blood volume

ECCV = extracorporeal circuit volume

CPBHct = desired Hct on CPB

PtHct = patient’s pre CPB Hct

*Estimate of total blood volume by age*
Age	cc/kg
Adult	65
3 years	70
1 year	75
6 months	80

*Acceptable values for prime*
pH	7.35-7.45
pO₂	80-300 mmHg
pCO₂	35-45 mmHg
Na⁺	< 140 mEq
K⁺	3.5-5 mEq
Ca²⁺ (ionised)	approx 1 mmol
Glucose	< 200 mg/dl
Hct	Depends on anticipated degree of hypothermia
Oncotic press	13-16 mmHg
Osmotic press	300 mOsm

Avoid SPPS in vacuum primed oxygenators (SciMed)

Limitations and validity of haemodilution

1. Benefits of haemodilution

a) Lowers blood viscosity

i) Counteracting sludging effect of hypothermic blood

ii) Improves tissue perfusion during hypothermia

b) Reduced risks associated with blood transfusions

c) See appropriate modules

2. Risks of haemodilution

a) Reduced MAP upon commencement of CPB

i) Due to reduced viscosity

b) Lowered colloid osmotic pressure

i) Dilution of plasma proteins

ii) Require increased I.V. fluid due to 3^rd space shifts

c) SaO₂ must be kept at 100%

i) To prevent any decline in O₂ transport

ii) O₂ transport = O₂ content ´ blood flow

iii) [O₂ content = 1.34 ´ Hb ´ SaO₂ + dissolved O_2]

d) Excessive haemodilution

i) Blood flow cannot increase to compensate for reduced O2 content

ii) Ischaemia of critical organs

iii) anaerobic metabolism

e) Increased flow

i) At similar VO₂ (O₂ consumption rates), a halving of the Hct requires a doubling of flow

ii) Importance of hypothermia to reduce VO₂

iii) If normothermic, SvO₂ may fall on CPB

f) Factors reducing ability of body to tolerate significant reductions in Hct:

i) Stenosis

a) Limits maximum flow of blood

b) Reduced viscosity does help however

ii) Cardiac failure after CPB

a) Unable to maintain sufficient C.O. necessary to maintain adequate O₂ delivery

iii) Left shifted O₂-Hb curve

a) Reduces O₂ release in tissues

b) Hypothermia, alkalosis, reduced 2,3 DPG

iv) Lung disease

a) Unable to maintain sufficient gas exchange necessary to maintain adequate O₂ delivery after CPB

g) ‘Healthy’ patient

i) Able to tolerate down to Hct of 20

Formulae for calculation of haemodilution

Hct_int = initial Hct on CPB

EBV = estimated patient blood volume

Hct = preoperative Hct

Blood volume = Patient weight ´ 8% [infants]

7.5% [child, adult male]

7.0 % [adult female]

Red cell volume = Blood volume ´ Hct

CO₂ flushing prior to priming the oxygenator

· CO₂ purging of the assembled circuitry prior to priming

· Used to displace atmospheric N₂ which is difficult to debubble

· Occurs for several minutes

· CO₂ is used as has a high water solubility; any CO₂ bubbles not washed from the circuit during priming are quickly adsorbed

· Recirculation of the prime with a O₂ gas flow allows for excessive CO₂ to be removed from the circuit before commencing CPB

· The slightly unoccluded pump head will allow the CO₂ to permeate the pump boot

PreCPB filter placement and pore size relative to priming

· 0.2 microns

· Upstream of arterial line

· Removes tubular and oxygenator dust and other debris, microbes and spores, early onset spallation (silastic), particulate matter of what ever source

· Positioned where all the pump prime will flow, yet is removable prior to commencement of CPB

Inline monitoring of haemodilution

Method 1

By using the electrical conductivity of blood, the Hct can be measured with an electrolyte in line analyser

Method 2

Hct is measured by using reflected infrared light from formed particles in the blood
The reflected infrared light will be proportional to the percent of red cells in the blood

Priming Solutions

*Fluid*	*grms/l*	*(ml)*	*Glu (g)*	kJ	Na	K	Cl	Ca	*Lactate*	pH	*Osmolality*
0.9% NS	9	1000	—	—	154	—	154	—	—
5% Dextrose	5	1000	50	850	—	—	—	—	—	4	278
Hartman’s		1000	—	—	131	5	112	2	29	5 -7	278
Ringers		1000	—	—	147	4	156	2.2	—		309?

5% DEXTROSE

• Readily available & cheap

• Slightly hypotonic and becomes steadily more so as the dextrose is metabolised leaving the patient with a sizeable water load to excrete

• The marked dilutional effect on plasma bicarbonate produces a marked systemic metabolic acidosis with hyponatraemia & hypochloraemia

• Not recommended for diabetics as may lead to very high glucose levels during CPB

• Due to its effect in raising osmotic pressure in prime, may be associated with reduced postoperative fluid retention & perioperative fluid requirements

• However, CNS damage may occur with hyperglycaemia when associated with global or focal injury is followed by immediate reperfusion of the ischaemic region [therefore should avoid hyperglycaemia particularly in thoracic aortic repairs when are subjecting global CNS ischaemia]

HARTMAN’S

• Similar ion concentration to plasma

• Lactate renders Hartman’s slightly acid until liver converts lactate into bicarbonate, eventually producing a metabolic alkalosis

• Diabetic patients are less able to handle the lactate peripherally so that it is more readily converted to glucose thereby exacerbating hyperglycaemia [use of NS or Ringer’s]

• Due to exchange of Na for K in kidneys, may exacerbate hypokalaemia

• Danger of exacerbating any metabolic acidosis by producing lactic acidosis, particularly in seriously ill patients with poor tissue perfusion or impaired hepatic function