Perfusion Parameters

Ns. Ida Simanjuntak, S.Kep

Perfusionist Staff

National Health Cardiovascular Centre Harapan Kita, Jakarta

Monitored and controlled parameters

1. Oxygen supply2. Blood Flow3. Blood pressure4. Blood gases and electrolytes and acid base

status5. Haemodilution6. Coagulation status7. Temperatures

Oxygen Supply

The most important function of the perfusionist is to provide an OXYGEN SUPPLY which is adequate to cope with the patients OXYGEN DEMAND

Average adult has a basilar oxygen consumption of about 250 ml/min Affected by temp, depth of anaesthesia + + +

Oxygen Supply

One gram of haemoglobin can carry 1.34 ml of oxygen

100 mls blood can carry 0.003 x pO2 mls of oxygen THEREFORE;100mls of fully saturated blood can carry

1.34 x Hb + 0.003 x pO2 mls of oxygen100mls of partially saturated blood can carry

1.34 x Hb x Sat + 0.003 x pO2 mls of oxygen

Oxygen Supply

Oxygen used by patient =

Flow (L/min) x 10 x (Arterial O2 – Venous O2 content)= 10 x Flow x [1.34 x Hb x (Art sat – ven sat) +0.003

x ( paO2 - pvO2)]Flow = L/min Saturation is a decimal

Hb = grams/100ml pO2 = mm Hg

Therefore Oxygen Supply is directly related to;Pump Flow HaemoglobinArterial and Venous Saturations

Arterial and Venous pO2 ‘s

Oxygen Availability

Although pump flow may be adequate, oxygen availability may be reduced by;

A shift to the left in the oxygen dissociation curve

Arterial venous shunting Often diagnosed by low arterial PRESSURE and

high venous pO2

Carbon Dioxide and Oxygen Availability

Oxygen Dissociation Curve; Any factor shifting the curve to the left reduces oxygen availability – i.e. increases haemoglobins’ affinity for oxygen.

Factors include;

reducing temperature, rise in pH, fall in CO2 content or a decrease in 2,3-DPG

Pump Flow

Adequate flow on bypass is usually estimated at a cardiac index of 2.4 l/min/m2

This is dependant upon Adequacy of venous return Selection of a suitable prime and suitable policy of adding

fluids during bypass State of Venodilitation Fluid or blood lossHowever , when cold the flow can be reduced

Advantages are lower venous pressure, less oedema, less blood damage but may get unnecessarily low pressure

Blood Pressure

At the onset of bypass there is usually a drop in blood pressure caused by; Reduction in viscosity (prime) Patient going into shock Arterio – venous shunts opening Mannitol is a vasodilator Adverse reactions to prime constituents

1 in 1000 react to gelatins

Blood PressureAcceptable Limits

Lower limit – usually 40 – 50 mm Hg Higher as the patient gets older

Cerebral autoregulation keeps brain blood flow constant despite pressure or temperature

•Upper limit about 75mm Hg to stop non-collateral coronary circulation and reduce oozing

Control of Blood Pressure

Arterial Pressure Often (and wrongly)

controlled by pump flow

Can use Metaraminol, Methoxamine or Levophed to raise pressure

Phentolamine or Isoflurane used to lower pressure

Venous Pressure Used when possible to

measure adequacy of venous return

The higher the venous pressure the greater the chance of oedema

Blood Gases

pO2

Normal 10 – 13 KPa On bypass 20 – 30 or 40 KPa

pCO2

Normal 4.5 – 6 KPa

pH and Acid-Base status Venous Saturation

Note; 1 Kpa = 7.5 mmHg

Carbon Dioxide

Normal range 4.6 – 6.0 Very commonly 4.0 – 6.0

Reasons for maintaining control;1. Ph2. Brain blood flow3. Oxygen availability

Carbon Dioxide and pH control

Henderson-Hasselbalch equation;

pH = pK + Log [ Bicarbonate]

[ CO2 ]

This provides a major buffering system in the body and is totally linked with ph control and respiratory acidosis/alkalosis

Carbon Dioxide and pH control

Important to determine whether pH changes are due to metabolic or respiratory imbalance

Metabolic acidosis; Get low pH, normal pCO2 Increase flow mls 8.4% bicarbonate = Pts weight x base deficit

3

Respiratory acidosis; Get low ph and a high pCO2 Increase gas sweep speed

Carbon Dioxide and Cerebral Perfusion

Increased pCO2 causes cerebral vasodilitation

Changes in pO2 only have a small effect

However……..

Alpha-stat and pH-Stat control of pCO2 confuses the whole issue;

Alpha-stat maintains normal Carbon dioxide content but reduced pCO2 and raised pH

pH-Stat maintains normal pCO2 and normal pH with a resultant raised carbon dioxide content

Brain blood flow is normalised with alpha stat but raised to a maximum with pH-stat

However……..continued

Typical blood gas result

pH 7.37 pCO2 4.24

pO2 33.8

Real Values if the patient was at 30oC

7.47 3.12 29.6

pO2 ControlThe three groups of thought

Group 1; “Normal Values” Range 10 – 14 Kpa

The body is used to it Minimal gaseous microemboli No risk of damaging the lungs with hyperbaric

oxygen toxicity However

Difficult to achieve and MUST have on line monitoring (Pearson 1984)

pO2 ControlThe three groups of thought

Group 2 “The Practical Ones” Range 20 – 30 KPa but accept up to 40

KPaEasily achievable with modern day oxygenatorsGives reasonable room for error – on-line monitoring

not essentialPossible increase in gaseous microemboli not proven

pO2 ControlThe three groups of thought

Group three “Does it matter” Range ; 20 – 100 KPa

Scientific proof that there is any need to control pO2 is relatively scarce

Any bubbles pumped into patient would be mainly composed of oxygen rather than nitrogen

May be advantageous prior to deep hypothermic arrestPossibly the main domain of older perfusionists

trained on bubble oxygenators which were difficult to control (Newland 1982)

pO2 ControlThe three groups of thought

Group three “Does it matter” However;

Increased gaseous microemboli out of oxygenator (Pearson 1988)

Increased gaseous microemboli out of cannula (Kuntz 1982

High pO2 causes cerebral vasoconstriction (Henriksen 1986)

Higher the pO2 the greater the risk of reperfusion injury

Advantages of On-Line Blood Gas Monitoring

1. Carbon Dioxide Changes in pCO2 are slow as CO2 is distributed throughout the body.

Therefore value of monitoring depends upon frequency of off-line sampling

2. pH As carbon dioxide

3. Oxygen Allows one to maintain lower values, therefore less microemboli. No

known advantage of high oxygen levels on bypass Changes are fast so gives better patient protection On-line monitoring probably more accurate than B.G. machine due to

temperature effects (Reeder 1983)

Disdvantages of On-Line Blood Gas Monitoring

Cost Training Only pO2 is quick changing. Everything else

is very slow changing and can be monitored using off-line blood gases

Venous Saturation

O2 used = 10 x Flow x [1.34 x Hb x (Art sat – ven sat) +0.003 x ( paO2 - pvO2)]

A low venous saturation (less than 70%) means that there is insufficient oxygen supply as a result of either; Low pump flow Low paO2

Low Hb

Haematocrit

Haematocrit (Hct) is the percentage of whole blood taken up by erythrocytes Normal value in adult male is about 45%

Haemoglobin Concentration (Hb) is measured in grams of haemoglobin per 100 mls blood Normal value in adult male is about 14.5 g/100ml

“Rule of thumb” Hb x 3.3 = Hct Higher the haematocrit the higher the oxygen

carrying capacity but also the higher the viscosity Lowest acceptable Hct on bypass usually about 19%

Haematocrit

Excessive haemodilution occurs when blood flow cannot increase enough to compensate for reduced O2 carrying capacity

Reduced temperature helps by reducing O2 demand

Need also to consider O2 supply Anatomy of arteries Left shifts of dissociation curve Coming of bypass

Reduced L.V. function ? Lung disease

Anticoagulation

Anticoagulation must be achieved before going on bypass INCIDENCE of problems

Stoney 1979 1 in 787 cases (374,000)Wheeldon 1981 3,667 ( 33,000)Kurusz 1986 1,479 (573,000)Svenmarker 1991 400 ( 8,800)Jenkins 1997 3,005 (27,000)Mejak 2000 2,283 (671,000)

Heparin Monitoring

Activated clotting time usually maintained above 400 or 480 seconds

Several types of equipment available from Hemochron, Hemocue + +

Heparin Monitoring

Can measure heparin concentrations with such equipment as the Hepcon

Can then determine exact heparin and protamine requirements

Heparin Monitoring

Celite is a diatomaceous earth which increases the speed of clotting by about 12 times. The presence of Aprotinin slows down the process and so Kaolin is used instead

Kaolin mining in Bulgaria

Temperature For every one degree drop in temperature there is a 7%

reduction in oxygen demand If the temperature drops below 32o C there is an increased

chance of fibrillation Temperatures below 15o C do not give any additional

protection due to Bohr shift Mild Hypothermia – minimum temp 32o C helps to

preserve the functions of a beating heart Moderate Hypothermia – 25 – 30oC preserves the

functions of a non-beating heart Profound hypothermia – 15 – 20o C Allows for total

circulatory arrest

Uses of Profound Hypothermia

Neonatal and Paediatric surgery Allows for simple cannulation in a complex

problem Allows for a clear surgical area

Aortic Arch Surgery Helps preserve the brain Retrograde cerebral perfusion may be used Can use continuous antegrade warm cerebral

perfusion with a cold body

Uses of Profound Hypothermia

Giant Cerebral Aneurysms Deflates the aneurysm allowing it to be clipped

Descending Aortic Aneurysms Preserves the spinal chord against ischaemia

which may result in paraplegia