Blood GasesBlood GasesFor beginnersFor beginners

Lynsey wardLynsey ward44thth February 2008February 2008

AimsAims

To have a basic understanding of Blood To have a basic understanding of Blood Gas analysis.Gas analysis.

ObjectivesObjectives

To state what acid and alkaline in the To state what acid and alkaline in the value of PHvalue of PHWhen analysing a blood gas determine When analysing a blood gas determine which are:which are:--

AcidoticAcidoticAlkaloticAlkaloticRespiratory Respiratory MetabolicMetabolic

ObjectivesObjectives

Be able to give the correct values for Be able to give the correct values for Three of the Blood gas measurements. Three of the Blood gas measurements. Give two reasons for poor Blood Gas Give two reasons for poor Blood Gas resultsresults

Why do we take blood gases?Why do we take blood gases?

To assess the effectiveness of ventilation, To assess the effectiveness of ventilation, circulation and perfusioncirculation and perfusionBlood gases indicate if the baby is able to Blood gases indicate if the baby is able to move air in and out of the lungs well move air in and out of the lungs well enough to obtain the oxygen it needs, and enough to obtain the oxygen it needs, and dispose of the carbon dioxide it doesndispose of the carbon dioxide it doesn’’ttBlood gases can also tell us if the baby is Blood gases can also tell us if the baby is having to use its energy stores in having to use its energy stores in anaerobic metabolism anaerobic metabolism

What do blood gases tell us?What do blood gases tell us?

Changes in the levels of Changes in the levels of acidity within the bloodacidity within the bloodoxygen and carbon dioxide within the bloodoxygen and carbon dioxide within the blood

The pattern of these changes over time, The pattern of these changes over time, which helps nursing and medical staff to which helps nursing and medical staff to evaluate and plan careevaluate and plan care

What is acidity? What is acidity?

Acidity is a measurement of the number of Acidity is a measurement of the number of Hydrogen ions (H+) within a solution.Hydrogen ions (H+) within a solution.A solution is:A solution is:

Neutral if it has a pH of 7Neutral if it has a pH of 7Acid if it has a pH less than (<) 7Acid if it has a pH less than (<) 7Alkaline if it has a pH greater than (>) 7Alkaline if it has a pH greater than (>) 7

What is Acidity?What is Acidity?

Acid= gives up Hydrogen ions when in Acid= gives up Hydrogen ions when in solution.solution.Some give up H+ more readily= Strong Some give up H+ more readily= Strong and weak Acidsand weak AcidsBase= Alkaline which accepts Hydrogen Base= Alkaline which accepts Hydrogen

ions. When acid added to base a weaker ions. When acid added to base a weaker acid is formedacid is formed

Normal values for pHNormal values for pH

For a premature baby or a baby with a For a premature baby or a baby with a respiratory problem the pH of the blood should respiratory problem the pH of the blood should lie between 7.25 and 7.35 lie between 7.25 and 7.35 (Neonatal Guidelines 2007, SSBC (Neonatal Guidelines 2007, SSBC Newborn Network)Newborn Network)

Babies with a pH below 7.25 are acidoticBabies with a pH below 7.25 are acidoticThis can be a This can be a respiratoryrespiratory or a or a metabolicmetabolic acidosisacidosisLooking at the balance of carbon dioxide, Looking at the balance of carbon dioxide, oxygen, bicarbonate and base excess can tell us oxygen, bicarbonate and base excess can tell us which type of acidosis it iswhich type of acidosis it is

OxygenOxygen

When we take a blood gas we look at the When we take a blood gas we look at the partial pressure of oxygen (PaO2) partial pressure of oxygen (PaO2) dissolved within the blooddissolved within the bloodPartial pressure is the pressure exerted by Partial pressure is the pressure exerted by the oxygen gas molecules within the bloodthe oxygen gas molecules within the bloodThe partial pressure of oxygen is The partial pressure of oxygen is measured in Kilopascals (KPa)measured in Kilopascals (KPa)

Normal Values of OxygenNormal Values of Oxygen

For a premature baby or a baby with a For a premature baby or a baby with a respiratory problem:respiratory problem:The partial pressure of oxygen should lie The partial pressure of oxygen should lie between:between:--

6 and 12 KPa6 and 12 KPa

Carbon DioxideCarbon Dioxide

When we take a blood gas we look at the When we take a blood gas we look at the partial pressure of carbon dioxide partial pressure of carbon dioxide (PaCO2) dissolved within the blood(PaCO2) dissolved within the bloodPartial pressure is the pressure exerted by Partial pressure is the pressure exerted by the carbon dioxide gas molecules within the carbon dioxide gas molecules within the bloodthe bloodThe partial pressure of carbon dioxide is The partial pressure of carbon dioxide is measured in Kilopascals (KPa)measured in Kilopascals (KPa)

Normal Values of Carbon dioxideNormal Values of Carbon dioxide

For a premature baby or a baby with a For a premature baby or a baby with a respiratory problem:respiratory problem:The partial pressure of carbon dioxide The partial pressure of carbon dioxide should lie between:should lie between:--

5.5 and 8 KPa5.5 and 8 KPa(Neonatal Guidelines 2007, SSBC Newborn Network)(Neonatal Guidelines 2007, SSBC Newborn Network)

BuffersBuffers

Minimises changes in ph.Minimises changes in ph.Addition of strong acid or base become Addition of strong acid or base become weaker solutions.weaker solutions.Normally sufficient buffers to keep pH Normally sufficient buffers to keep pH narrow range.narrow range.Three buffers, which are?Three buffers, which are?

BuffersBuffers

BicarbonateBicarbonate

HaemoglobinHaemoglobin

PhosphatePhosphate

BicarbonateBicarbonate

Bicarbonate (HCO3) is produced by a buffer Bicarbonate (HCO3) is produced by a buffer within haemoglobinwithin haemoglobinThe conversion of carbonic acid to bicarbonate The conversion of carbonic acid to bicarbonate reduces the acidity (lowers the pH) of the bloodreduces the acidity (lowers the pH) of the blood

BicarbonateBicarbonate

Bicarbonate is measured in millimoles/litreBicarbonate is measured in millimoles/litreThe normal range for bicarbonate is 18The normal range for bicarbonate is 18--25 25 mm/Lmm/L

Base ExcessBase ExcessThe base excess is the amount of acid which would have The base excess is the amount of acid which would have to be added to blood to correct the pH to 7.4 to be added to blood to correct the pH to 7.4 Base excess is expressed as a + or Base excess is expressed as a + or –– valuevalueThe normal range for base excess is from The normal range for base excess is from --4 to +44 to +4A baby with a base excess below A baby with a base excess below --4 is 4 is acidoticacidotic, as H+ , as H+ ions need to be taken away to return the pH to 7.4ions need to be taken away to return the pH to 7.4A baby with a base excess above +4 is A baby with a base excess above +4 is alkaloticalkalotic, as H+ , as H+ ions need to be added to return the pH to 7.4ions need to be added to return the pH to 7.4

BloodBloodBlood has a pH of 7.4Blood has a pH of 7.4Blood maintains this pH by using buffersBlood maintains this pH by using buffersIf the pH of the blood rises ( becomes more If the pH of the blood rises ( becomes more alkaline) buffers release H+ ions to lower the pH alkaline) buffers release H+ ions to lower the pH to 7.4to 7.4If the pH of the blood falls (becomes more If the pH of the blood falls (becomes more acidic) buffers absorb H+ ions to raise the pH to acidic) buffers absorb H+ ions to raise the pH to 7.47.4Changes in pH should be avoided, as they affect Changes in pH should be avoided, as they affect the way in which the body absorbs oxygen and the way in which the body absorbs oxygen and excretes carbon dioxideexcretes carbon dioxide

Using blood gas resultsUsing blood gas results

Look at resultsLook at resultsCheck previous resultsCheck previous resultsInform Medical Staff/Senior NurseInform Medical Staff/Senior Nurse

Looking at blood gasesLooking at blood gases

So looking at blood gases means looking So looking at blood gases means looking at absolute values at absolute values andand patternspatternsWe need to consider the numerical results We need to consider the numerical results of the blood gas in relation to previous of the blood gas in relation to previous blood gases and take in to account the blood gases and take in to account the patientspatients’’ history history Which is why blood gases can be so Which is why blood gases can be so confusing; results which are fine for some confusing; results which are fine for some patients are unacceptable for others!patients are unacceptable for others!

Normal Gas valuesNormal Gas values

PhPhP02P02Co2Co2Standard bicarbonate.Standard bicarbonate.Base Excess.Base Excess.

Normal blood gas valuesNormal blood gas values7.25<pH<7.357.25<pH<7.35 pH/ aciditypH/ acidity

(Premature baby or (Premature baby or baby with respiratory baby with respiratory problem)problem)

6<PaO2<126<PaO2<12 Partial pressure OxygenPartial pressure Oxygen(O2)(O2)

5<PaCO2<75<PaCO2<7 Partial pressure Carbon Partial pressure Carbon Dioxide (Co2)Dioxide (Co2)

--4 to +44 to +4 Base Excess (Be)Base Excess (Be)

18 18 –– 25 mm/L25 mm/L Bicarbonate (HC03)Bicarbonate (HC03)

Disturbances of Acid/Base.Disturbances of Acid/Base.Respiratory Acidosis Respiratory Acidosis

Poor gases exchangePoor gases exchangeLow pHLow pHRaise Co2Raise Co2Other values normal.Other values normal.what do we need to do?what do we need to do?

Respiratory AlkalosisRespiratory Alkalosis

Over ventilation.Over ventilation.Low co2 = reduce cerebral blood flowLow co2 = reduce cerebral blood flowTherapeutic in cerebral oedema.Therapeutic in cerebral oedema.pH is highpH is highpCo2 is lowpCo2 is lowManagement Management

Metabolic AcidosisMetabolic Acidosis

Accumulation of acidsAccumulation of acidsAnaerobic respiration Anaerobic respiration Metabolic disordersMetabolic disordersTPN.TPN.Low pHLow pHNegative base ExcessNegative base ExcessNormal Co2, O2 normal or low.Normal Co2, O2 normal or low.Treat acidosis, correction, O2, stop TPN.Treat acidosis, correction, O2, stop TPN.

Metabolic AlkalosisMetabolic Alkalosis

Too much BicarbonateToo much BicarbonatePersistent VomitingPersistent VomitingHigh pHHigh pHHigh bicarbonateHigh bicarbonateNormal values for pco2 and o2Normal values for pco2 and o2Correct underlying cause.Correct underlying cause.

Mixed AcidosisMixed Acidosis

Poor gaseous exchange results in anaerobic Poor gaseous exchange results in anaerobic metabolism and excessive hydrogen ions metabolism and excessive hydrogen ions pH LowpH LowO2 LowO2 LowC02 HighC02 HighBicarbonate is lowBicarbonate is lowNegative base excessNegative base excessTreat by improving gaseous exchange and Treat by improving gaseous exchange and bicarbonatebicarbonate

Respiratory acidosisRespiratory acidosis Respiratory alkalosisRespiratory alkalosis

CauseCause Unable to move CO2 Unable to move CO2 out of lungs (poor out of lungs (poor ventilation)ventilation)

Losing too much CO2 Losing too much CO2 (over ventilation)(over ventilation)

SignsSigns Low pHLow pHHigh pCO2High pCO2pO2, HCO3 and BE pO2, HCO3 and BE may be normalmay be normal

High pHHigh pHLow pCO2Low pCO2

ActionAction CPAP, Ventilation, CPAP, Ventilation, change ventilationchange ventilation

Change CPAP, Change CPAP, change ventilationchange ventilation

Metabolic AcidosisMetabolic Acidosis Metabolic Metabolic AlkalosisAlkalosis

CauseCause Large loss HCO3Large loss HCO3Anaerobic metabolismAnaerobic metabolism

Too much Sodium Too much Sodium BicarbonateBicarbonateLoss of H+ Loss of H+ (vomiting)(vomiting)

SignsSigns Low pHLow pHBase excess is negativeBase excess is negativepO2 is normalpO2 is normal

High pHHigh pHHigh HCO3High HCO3Normal pCO2Normal pCO2

ActionAction Minimise the risk of Minimise the risk of anaerobic metabolismanaerobic metabolism

Correct CauseCorrect Cause

SummarySummarypHpH pO2pO2 pCO2pCO2 HCO3HCO3 BEBE

RespiratoryRespiratory LowLow LowLow HighHigh Normal Normal to Highto High

NormalNormal

MetabolicMetabolic LowLow Low or Low or NormalNormal

NormalNormal Normal Normal to Lowto Low

LowLow

MixedMixed LowLow LowLow HighHigh LowLow LowLow

Practical Implications.Practical Implications.Quality of the specimenQuality of the specimen--flowing clots.flowing clots.Poorly perfuse babyPoorly perfuse babyBeware bubblesBeware bubblesBeware unexpected resultsBeware unexpected results--? Comparable with last ? Comparable with last gas,? repeatgas,? repeat