pH & Blood Gas pH & Blood Gas AnalysisAnalysis

Dr. Farhan Javed DarDr. Farhan Javed Dar

IntroductionIntroduction

Arterial blood gas measurement is a blood Arterial blood gas measurement is a blood test that is performed to determine the test that is performed to determine the concentration of oxygen, carbon dioxide concentration of oxygen, carbon dioxide and bicarbonate, as well as the pH, in the and bicarbonate, as well as the pH, in the blood.blood.

Its main use is in pulmonology, as many Its main use is in pulmonology, as many lung diseases feature poor gas exchange, lung diseases feature poor gas exchange, but it is also used in nephrology (kidney but it is also used in nephrology (kidney diseases) and electrolyte disturbances.diseases) and electrolyte disturbances.

As its name implies, the sample is taken As its name implies, the sample is taken from an artery, which is more from an artery, which is more uncomfortable and difficult than uncomfortable and difficult than venepuncturevenepuncture..

The analyzer is use in the quantitative The analyzer is use in the quantitative determination of pH,PCOdetermination of pH,PCO

22,PO,PO

22,Oxygen ,Oxygen

saturation and hematocrit. It also displays saturation and hematocrit. It also displays hemoglobin.hemoglobin.

The 2-point calibration performed every The 2-point calibration performed every two hours gap & 1-point calibration after two hours gap & 1-point calibration after every half an hour gap by machine every half an hour gap by machine automatically.automatically.

The POThe PO22 & PCO & PCO

22 electrode has a electrode has a

membrane with a three months life, membrane with a three months life, change it after every three months.change it after every three months.

Two buffer solution having pH 6.840 and Two buffer solution having pH 6.840 and 7.382 are being used in the calibration of 7.382 are being used in the calibration of pH electrode & are supplied with systempH electrode & are supplied with system

Buffer solution, a solution which resists Buffer solution, a solution which resists change of pH upon addition of small change of pH upon addition of small amounts of acid or base, or upon dilution amounts of acid or base, or upon dilution

InstrumentInstrument

NovaNova (STAT. Profile ultra analyzer). (STAT. Profile ultra analyzer).

ProcedureProcedure

To initiate a sample sequence the operator To initiate a sample sequence the operator carefully watch the analyzer that it must be carefully watch the analyzer that it must be calibrated, reagent pack are perfectly calibrated, reagent pack are perfectly installed & no error code is blinking on the installed & no error code is blinking on the display screendisplay screen

Before introducing the sample, be sure Before introducing the sample, be sure that sample is not clotted & mix the that sample is not clotted & mix the syringe for few seconds to remove any air syringe for few seconds to remove any air bubblebubble

Press the analyze button after few Press the analyze button after few seconds probe comes out to pick the seconds probe comes out to pick the sample, remove the needle then apply the sample, remove the needle then apply the syringe into the probe & press the analyze syringe into the probe & press the analyze button so that probe pick the samplebutton so that probe pick the sample

Give sample identify number in the data Give sample identify number in the data screen of nova & other related information screen of nova & other related information so that the result can be transmitted so that the result can be transmitted automatically to AKUH computer systemautomatically to AKUH computer system

Results also appears on the paper Results also appears on the paper inserted in the nova printer.inserted in the nova printer.

pHpH

The pH is a measure of hydrogen ion (H+) The pH is a measure of hydrogen ion (H+) in blood which indicates the acid or base in blood which indicates the acid or base (alkaline) nature of blood (alkaline) nature of blood

A pH of less than 7 is acidic, and a pH A pH of less than 7 is acidic, and a pH greater than 7 is called basic (alkaline). greater than 7 is called basic (alkaline).

The normal blood pH range is 7.35 to 7.45

Principle Of pH measurementPrinciple Of pH measurement

pH is measured using a hydrogen ion pH is measured using a hydrogen ion selective glass membrane.selective glass membrane.

One side of the glass is in contact with a One side of the glass is in contact with a solution of unknown pH.solution of unknown pH.

A change in potential develops which is A change in potential develops which is proportional to the pH difference of these proportional to the pH difference of these solutions.solutions.

This change in the potential is measured This change in the potential is measured against a reference electrode of constant against a reference electrode of constant potential.potential.

The magnitude of the potential difference The magnitude of the potential difference is then measure, the pH of unknown is then measure, the pH of unknown solution.solution.

Partial Pressure Of COPartial Pressure Of CO22

DefinitionDefinition The amount of carbon dioxide dissolved in

arterial blood. It’s level indicates how well carbon dioxide is It’s level indicates how well carbon dioxide is

able to move out of the blood into the able to move out of the blood into the airspace of the lungs and out with exhaled air. airspace of the lungs and out with exhaled air.

The normal range is 35 to 45 mm Hg.

Principle Of PCOPrinciple Of PCO22 Measurement Measurement

PCOPCO22 is measured with a modified pH is measured with a modified pH

electrode.electrode. COCO

22 in the unknown solution makes in the unknown solution makes

contact with a gas permeable membrane contact with a gas permeable membrane mounted on a combination mounted on a combination measuring/reference electrode.measuring/reference electrode.

COCO22 diffuses across the membrane into a diffuses across the membrane into a

thin layer of electrolyte solution in thin layer of electrolyte solution in response to partial pressure difference.response to partial pressure difference.

This solution then becomes equilibrated This solution then becomes equilibrated with the external gas pressure.with the external gas pressure.

COCO22 in the solution becomes hydrated in the solution becomes hydrated

producing carbonic acid which results in a producing carbonic acid which results in a change in hydrogen ion activitychange in hydrogen ion activity

COCO22 + H + H

22O O ↔ H↔ H

22COCO

33 ↔ H ↔ H++ + [ HCO + [ HCO

33]]

The electrolyte solution behind the The electrolyte solution behind the membrane is in contact with a glass membrane is in contact with a glass hydrogen ion selective electrode.hydrogen ion selective electrode.

The change in hydrogen ion activity in the The change in hydrogen ion activity in the electrolyte solution produce a potential, electrolyte solution produce a potential, which is, measured against the internal which is, measured against the internal filling solution.filling solution.

This change in potential is measured This change in potential is measured against the constant potential of the against the constant potential of the reference electrode & is related to the reference electrode & is related to the PCOPCO

22 of the unknown sample. of the unknown sample.

Partial Pressure of OxygenPartial Pressure of Oxygen

DefinitionDefinition the partial pressure of oxygen is the gas the partial pressure of oxygen is the gas

phase in equilibrium with the blood.phase in equilibrium with the blood. The partial pressure of oxygen that is

dissolved in arterial blood It indicates how well oxygen is able to move It indicates how well oxygen is able to move

from the airspace of the lungs into the blood.from the airspace of the lungs into the blood. The normal range is 80 to 100 mm Hg.

Principle of POPrinciple of PO22 Measurement Measurement

POPO22 is measured amperometrically by the is measured amperometrically by the

generation of a current at the electrode generation of a current at the electrode surface.surface.

As oxygen diffuses through a gas As oxygen diffuses through a gas permeable membrane, the oxygen permeable membrane, the oxygen molecules are reduced at the cathode, molecules are reduced at the cathode, consuming 4 electrons for every molecule consuming 4 electrons for every molecule of oxygen reduced.of oxygen reduced.

This flow of electrons is then measured by This flow of electrons is then measured by the electrode & is directly proportional to the electrode & is directly proportional to the POthe PO

22..

Base Excess Of BloodBase Excess Of Blood

Base excess of blood is defined as the Base excess of blood is defined as the concentration of titrable base needed to concentration of titrable base needed to titrate blood to pH 7.40 at 37titrate blood to pH 7.40 at 3700 C while the C while the PCOPCO

22 is held constant at 40 mm Hg. is held constant at 40 mm Hg.

The base excess indicates the amount of excess or insufficient level of bicarbonate in the system.

The normal range is –2 to +2 mEq/liter

Oxygen ContentOxygen Content

Oxygen content is defined as the total Oxygen content is defined as the total amount of oxygen contained in a given amount of oxygen contained in a given volume of whole blood including dissolved volume of whole blood including dissolved oxygen bound to hemoglobin.oxygen bound to hemoglobin.

It is expressed in milliliters of oxygen per It is expressed in milliliters of oxygen per 100 ml of blood (vol. %) as calculated from 100 ml of blood (vol. %) as calculated from the oxygen saturation and the hemoglobin the oxygen saturation and the hemoglobin concentration.concentration.

The normal range is 95% to 100%.

ControlsControls

Level I,II & III are available & one control Level I,II & III are available & one control run in each 8 hours shift.run in each 8 hours shift.

These controls are formulated from a These controls are formulated from a buffered bicarbonate solution of pH & buffered bicarbonate solution of pH & sodium concentration.sodium concentration.

The solution are equilibrated with known The solution are equilibrated with known concentration of oxygen & carbon dioxide.concentration of oxygen & carbon dioxide.

Acid Base DisordersAcid Base Disorders

There are four simple acid-base disordersThere are four simple acid-base disorders Metabolic AcidosisMetabolic Acidosis Metabolic AlkalosisMetabolic Alkalosis Respiratory AcidosisRespiratory Acidosis Respiratory AlkalosisRespiratory Alkalosis

A pt. can also suffer from two simple A pt. can also suffer from two simple disorders simultaneously which is termed disorders simultaneously which is termed a mixed acid base disturbance a mixed acid base disturbance

METABOLIC ACIDOSISMETABOLIC ACIDOSIS

Characterized by :Characterized by :

A low HCOA low HCO33 - - less than 22 mEq/L

A low pH - A low pH - less than 7.35

And if compensation has occurred ,a low PcoAnd if compensation has occurred ,a low Pco2 2

Causes of Metabolic AcidosisCauses of Metabolic Acidosis Addition of HAddition of H+ +

Increased productionIncreased production• Ketoacidosis Ketoacidosis • Lactic acidosisLactic acidosis• ToxinsToxins

• Ingestion/infusion( HCl,NHIngestion/infusion( HCl,NH44Cl )Cl )

Decreased Renal ExcretionDecreased Renal Excretion• Renal failureRenal failure• Obstructive uropathyObstructive uropathy• Renal tubular acidosis Type 1Renal tubular acidosis Type 1• Mineralcorticoid deficiencyMineralcorticoid deficiency

Loss of HCO3Loss of HCO3 Extrarenal lossesExtrarenal losses

• Acute DiarrheaAcute Diarrhea• Drianage from pancreatic fistulaeDrianage from pancreatic fistulae• Diversion of urine to gutDiversion of urine to gut

Renal LossesRenal Losses• Renal Tubular AcidosisRenal Tubular Acidosis

Consequences of Metabolic Consequences of Metabolic AcidosisAcidosis

Cardiac FailureCardiac Failure Hyperkalemia e.g diabetic ketoacidosisHyperkalemia e.g diabetic ketoacidosis Hypokalemia in in renal tubular acidosisHypokalemia in in renal tubular acidosis Mobilization of calcium from bone. Renal Mobilization of calcium from bone. Renal

reabsorption of calcium producing reabsorption of calcium producing hypercalciuria, leads to nephrocalcinosis & hypercalciuria, leads to nephrocalcinosis & urolithiasis urolithiasis

METABOLIC ALKALOSISMETABOLIC ALKALOSIS

Characterized by :Characterized by :

a high pH-a high pH-greater than 7.45

A high bicarbonate-A high bicarbonate-greater than 26 mEq/liter

If compensation has occurred a High PcoIf compensation has occurred a High Pco2 2

Causes of Metabolic AlkalosisCauses of Metabolic Alkalosis Increased exogenous bicarbonateIncreased exogenous bicarbonate

Oral/IV. BicarbonateOral/IV. Bicarbonate Antacid therapy, e.g, magnesium carbonateAntacid therapy, e.g, magnesium carbonate Organic acid salts e.g , lactate ,citrateOrganic acid salts e.g , lactate ,citrate

Loss of hydrogen ionsLoss of hydrogen ions Gastrointestinal tract lossesGastrointestinal tract losses

• Stomach : vomitting, gastric suctionStomach : vomitting, gastric suction• Bowel : diarrheaBowel : diarrhea

Kidney lossesKidney losses• Diuretic therapyDiuretic therapy• Mineralcorticoid excessMineralcorticoid excess

Consequences Of Metabolic Consequences Of Metabolic AlkalosisAlkalosis

Alkalemia enhances binding of calcium ions Alkalemia enhances binding of calcium ions to protein which results in increased to protein which results in increased neuromuscular activity & Characteristic neuromuscular activity & Characteristic Chvostek & Trousseaue signs may occurChvostek & Trousseaue signs may occur

HypokalemiaHypokalemia Increased calcium reabsorptionIncreased calcium reabsorption Enhanced glycolysis ( stimulation of Enhanced glycolysis ( stimulation of

phosphpfructokinase by a high intracellular phosphpfructokinase by a high intracellular pH )pH )

RESPIRATORY ACIDOSISRESPIRATORY ACIDOSIS

Characterized by :Characterized by : Increased PcoIncreased Pco2 2

pH less than 7.35 with a PCO2 greater than 45 mm Hg.

It is always due to decreased excretion of It is always due to decreased excretion of COCO2 2 by the lungsby the lungs

Causes of Respiratory AcidosisCauses of Respiratory Acidosis Thoracic DiseaseThoracic Disease

Restrictive defectsRestrictive defects• HydrothoraxHydrothorax• PneumothoraxPneumothorax• Flail ChestFlail Chest

Obstructive diseaseObstructive disease• BronchitisBronchitis• EmphysemaEmphysema• PneumoniaPneumonia• InfiltrationsInfiltrations• EdemaEdema• PneumoniaPneumonia• Foreign body obstructionForeign body obstruction

Neuromuscular diseaseNeuromuscular disease PoliomyletiisPoliomyletiis GB syndromeGB syndrome Multiple sclerosisMultiple sclerosis MyopathiesMyopathies

Central DepressionCentral Depression TraumaTrauma Cerebrovascular accidentsCerebrovascular accidents CNS infectionsCNS infections CNS tumorsCNS tumors Drug overdoseDrug overdose

Consequences Of Respiratory AcidosisConsequences Of Respiratory Acidosis

On BrainOn Brain Hypercapnia induces cerebral vasodilation & Hypercapnia induces cerebral vasodilation &

increased cerebral blood flow which in turn increased cerebral blood flow which in turn increses intracerebral pressure producing :increses intracerebral pressure producing :• DrowsinessDrowsiness• HeadachesHeadaches• StuporStupor• ComaComa

On PotassiumOn Potassium Release of potassium from cells (exchange for Release of potassium from cells (exchange for

HH+ + But not a constant featureBut not a constant feature

RESPIRATORY ALKALOSISRESPIRATORY ALKALOSIS

Characterized by : Characterized by : Hypocapnia ( low PcoHypocapnia ( low Pco2 2 ) due to increased ) due to increased

ventilation.ventilation. pH greater than 7.45 with a PCO2 less than 35

mm Hg

Causes Of Respiratory AlkalosisCauses Of Respiratory Alkalosis Central StimulationCentral Stimulation

AnxietyAnxiety PregnancyPregnancy HypoxemiaHypoxemia Hepatic EncephalopathyHepatic Encephalopathy Gram –ve septicaemiaGram –ve septicaemia Salicylate overdoseSalicylate overdose Infection, traumaInfection, trauma TumourTumour

Pulmonary PathologyPulmonary Pathology EmbolismEmbolism Congestive cardiac failureCongestive cardiac failure Asthma,PneumoniaAsthma,Pneumonia

Consequences Of Respiratory AlkalosisConsequences Of Respiratory Alkalosis On Calcium MetabolismOn Calcium Metabolism

Tetany as alkalemia causes increased binding Tetany as alkalemia causes increased binding of calcium ions to protein.of calcium ions to protein.

On PotassiumOn Potassium Initially mild hypokalemia but generally plasma Initially mild hypokalemia but generally plasma

potassium remains normalpotassium remains normal On PhosphateOn Phosphate

Transient severe hypophosphataemiaTransient severe hypophosphataemia On Glucose MetabolismOn Glucose Metabolism

Increased lactate productionIncreased lactate production On BrainOn Brain

Cerebral vasoconstriction, which may results in Cerebral vasoconstriction, which may results in light headednesslight headedness

pHpH PCOPCO22 HCOHCO--33

Respiratory Acidosis

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Respiratory Alkalosis

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Metabolic Acidosis

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Metabolic Alkalosis

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