pH Management during CPB

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pH Management during

Hypothermic CPB

Dr. Rashad SiddiqiDepartment of Cardiac Anaesthesia,

AFIC NIHD Rawalpindi

Asst Prof of Anaesthesiology,

Army Medical College, Rawalpindi



INTRODUCTION



37ºC pH = 7.40

PaCO2

40mmHg

40ºC pH = 7.35

PaCO2

Higher

20ºC pH = 7.65

PaCO2

lower

Normal values for pH 7.4 and PCO2 40mmHg are

appropriate only at 37ºC

There is a temp-dependent spectrum of normal

values in the body



Temperature PCO2 Relation

� CO2 solubility in blood changes with temperature

Temperature

CO2 Solubility

PaCO2

while the overall gas content remains constant



Temperature pH Relation

� Relationship of pH and temperature is complex

PaCO2

H+

pH



Temperature pH Relation

EquilibriumTemp

Kinetic energydissociationun-dissociated

= ionized

All acids and bases, including water



Neutrality of Water (Neutrality of Water (pHpHNN))

Neutral water: [H+] equals [OH-]

pH at which water is neutral increases 0.017 units foreach °C decrease in temperature

At 37°C, pH of neutral water equals 6.8

At 25°C, pH of neutral water equals 7.0



Body Fluids & Hypothermia

Biological fluids behave much like water in

terms of intrinsic temperature related changes

in the dissociation constants of the weak-acids

and weak-bases they are composed of



Body Fluids & Hypothermia

� At 37ºC blood and tissue fluids are alkaline (lower

[H+] and higher pH) relative to water at same

temperature

� Buffer systems keep [OH-] : [H+] ratio of 16 : 1

�Major buffering system is imidazole of the amino

acid histidine



-Stat Management



-Stat Management

ci - ase management strategy in ic

� pH is maintained constant at varying

temperatures

�

the net charge (dissociation) of proteinsremains constant as temperature

changes



-Stat Management

Intracellular

pH Extracellular

pH

Temperature

PaCO2

Temperature

trans-membrane pH gradient

(o.6 units) remains

unchanged



Advantages

preservesauto-

regulation

optimizescellularenzymeactivity

lessmetabolic

suppression



pH Stat Management



pH Stat Management

Temperature Corrected Values

Respiratory Alkalosis in Hypothermia

CO2 is added to correct the pH

eeping pH at 7.4 and PCO2 at 40mmHg



Effects

Shifts Hb-O2 curve to right (reverses effect of

hypothermia on Hb-O2 curve)

Results in vasodilatation (reverses effect of

hypothermia reduced tissue perfusion secondary to

increased viscosity)



improves cerebral blood flow

cerebral oxygenation

brain cooling efficiency during CPB

greater risk of microembolism and freeradical-mediated damage



pH Stat vs -Stat

� For example:

� If the PCO2 at 28ºC is 35 and it is not temperature

corrected (alpha-stat) then it will be reported as

35. If it is temperature corrected (pH stat), it

would be around 20mmHg and it will be

necessary to add CO2 or decrease the FGF at heart

lung machine



pH Stat vs -Stat



THE DEBATE



� constant intracellular electrochemical neutrality appears

to be essential for normal cellular function

� most enzymes depend on an optimal pH for their

function

Which is the better technique?

� the response of CBF to changes in PCO2 remains intact,

therefore -stat with a decreasing PaCO2 results in areduced cerebral blood flow

� at the same time hypothermia reduces cerebral

metabolic rate

� cerebral blood flow auto-regulation is lost with pH-stat

and CBF is perfusion pressure dependant

� excessive blood flow un-necessarily exposes brain to

high ICP and microemboli



REVIEW OF LITERATURE



CHOICE OFLPHA STAT OR PH STAT

MANAGEMENT AND NEUROLOGICAL

OUTCOME AFTER CARDIAC SURGERY

Hindman BJ. Anaesthesiology 1998;89:5-7



� Jonas

et al (1993) & du Plessis

et al (1997)

� Better neurological outcome in children with pH stat

management after DHCA

� Kurth et al (1998)

± pH stat management increases the rate of brain cooling

± Rate of brain oxygen depletion during DHCA is

considerably slower



�Skaryak et al (1995), Stephan et al (1992) & Patel et

al (1996)

± in adults undergoing routine cardiac surgery with

moderately hypothermic CPB, post-op neurological or

neuropsychological outcome is slightly, but persistently,

better with stat management



�Barbut

et al (1994), Pugsley

et al (1998) & Clark

et al

(1995)

± Lesser CBF of stat management results in fewer cerebral

emboli during CPB

± In adults cardiac surgery patients post-op neurologic

outcome is related to the number of cerebral emboli that

occur during CPB



± With use of membrane oxygenators and arterial filters, the

number of emboli originating from CPB circuit is pretty low

± so with short CPB runs, the acid-base management may

not significantly affect the neurological outcome

� Murkin et al (1995)

± stat management is not associated with better outcome

until CPB time is > 90 minutes



IS PH-STAT OR ALPHA-STAT THE BEST TECHNIQUETO FOLLOW IN PATIENTS

UNDERGOING DEEP HYPOTHERMIC CIRCULATORY

ARREST?

Aziz KAA et al Interact CardioVasc Thorac Surg 2010;10:271-282



�Medline search from 1950 to 2009 resulted in 206papers out of which 16 represented best evidence

1 paper provided inconclusive results

7 studies found better cerebrovascular metabolism with

alpha-stat

4 studies found better cerebrovascular metabolism with

pH-stat

4 other studies showed no significant



� Piccioni et al. gave inconclusive results.

- no significant difference between the two groups with

regard to tissue perfusion as demonstrated by

the similar tissue oxygenation parameters

lactate concentrations.

- after cardiopulmonary bypass (CPB), the acidosis

caused by this procedure was less significant in

patients managed by alpha-stat strategy than the pH- stat

strategy

Piccioni MA, et al. Artif Organs 2004;28:347352.



Hoover et al . , Sakamoto et al . , Hickey and du Plessis

et al . demonstrated a significant benefit in favour of

pH-stat

Favouring pH Stat



In a prospective double-blind study

PjVO2 and SjVO2 measured

adults with high risk of impaired cerebral blood flow

� aged 70 years old� diabetic

� prior stroke

� uncontrolled hypertension

pH-stat group have higher oxygen tensions andsaturation during CPB

Favouring pH Stat

Hoover LR, et al. Anesth Analg 2009;108:138993



Randomized controlled trial

paediatric patients with cyanotic anomalies in the pH-

stat group have

- decreased pulmonary collateral circulation

- lower lactate levels

Favouring pH Stat

Sakamoto T, et al. J Thorac Cardiovasc Surg 2004;127:129



prospective randomized study

92 infants (<9 months) for TGA surgeries

- less post-op seizures with pH-stat group

- shorter recovery time to first EEG reading

Favouring pH Stat

Hickey PR. Ann Thorac Surg 1998;65:S65S69.



prospective randomized study

182 infants with congenital heart disease

- alpha stat group required more inotropes

- alpha stat group had more hypocalcemia

- alpha stat group had more coagulopathy

- alpha stat group had more seizures

Favouring pH Stat

du Plessis AJ. J Thorac Cardiovasc Surg 1997; 114:9911000.



Randomize study

52 CABG patients

the pH-stat acid-base strategy is associated with

- increased jugular venous oxygen saturation

- a decrease in arterio-venous oxygen content

- glucose differences in adult patients

Favouring alpha-Stat

Kiziltan HT, et al. Anesth Analg 2003;96:644650.



Randomized study

20 Adults undergoing CABG

- less systemic vascular resistance in the

alpha-stat group


Buhre W et al. Acta Anaesth Scand 1998;42:167171.



Randomized studies in adults

- decreased blood flow velocity in the middle cerebral

artery at 28ºC in alpha stat group

- decreased CERO2 in pH stat group

- less disruption of cerebral auto-regulation in stat group

- higher incidence of postoperative cerebral dysfunction in

pH stat group


Patel RL, et al. J Thorac Cardiovasc Surg 1996;111:12671279.

Venn GE, et al. Ann Thorac Surg 1995;59:13311335.

Patel RL, et al. . Eur J Cardiothorac Surg 1993;7:457463.



Kancir and Madsen

- studied plasma phosphate concentration during and

after hypothermic CPB as reflection of metabolic

disturbance- showed a lower level in the alpha- stat group

Stephan et al.

- better postoperative clinical neurological assessment

results in alpha stat group


Kancir CB, et al. Scand J Thorac Cardiovasc Surg 1992;26:151155.

Stephan H etal. . Br J Anaesth 1992;69:5157.



Bellinger et al.

no differences in neuro-developmental

outcome between the two groups in paediatric

population .

Engelhardt et al., Badner et al., and Baraka et al.

no significant difference in early cerebral function

outcome, renal function, and oxygen delivery,

respectively, between the two groups.

No Significant Difference



CONCLUSION



After numerous studies, we find that the

answer to which technique is better is

simple..... It depends

In children undergoing DHCA pH stat strategyhas been found to be betterin adults undergoing routine cardiac surgery

with moderate hypothermia, stat strategy

is beneficial

pH Management during CPB

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