clinical chemistry 1: blood gases, ph and buffer systems

Clinical Chemistry 1: BLOOD GASES, pH

and BUFFER SYSTEMS

(LECTURE & LABORATORY)

TOPIC OUTLINE:PART 1 - LECTUREI. BASIC CONCEPTII. BUFFER SYSTEM

III. ACID-BASE IMBALANCES

PART 2 - LABORATORYIV. LABORATORY APPLICATION

V. ABG INTERPRETATION

I. BASIC CONCEPT

ACIDS AND BASES⬢ Acid

Is a substance that can yield a hydrogen ion (H+) when dissolved in water⬢ Base

Is a substance that can yield hydroxyl ions (OH-)

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ACIDS AND BASES

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ACIDS AND BASES⬢ Dissociation Constant (K value)-aka ionization constant-relative strengths of acids and bases – their ability to dissociate in water

⬢ pKa-negative log of the ionization constant-the pH in which the protonated and unprotonated forms are present in equal concentration

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ACIDS AND BASES

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ACIDS AND BASES⬢ Strong Acids -have pK values of less than 3.0-raising the pH above the pK will cause it to dissociate and yield a H+

⬢ Strong Bases-have pK values of greater than 9.0-lowering the pH below the pK will cause it to release OH-

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ACIDS AND BASES

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ACIDS AND BASES

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ACIDS AND BASES

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BUFFER

3.0 9.07.0pH

BUFFER

3.0 9.07.0pH

HCO3- H2CO3

pK= 6.1

Bicarbonate-Carbonic Acid System

BUFFERBicarbonate-Carbonic Acid System

pCO2⬢ Partial Pressure of CO2 (pCO2)⬢ Pressure or tension exerted by CO2 gas dissolved

in blood (dCO2)⬢ An index of efficiency of gas exchange in the

lungs⬢ Not a measure of CO2 concentration in the blood⬢ 35 to 45 mm Hg

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Total CO2 Concentration⬢ Total CO2 content in the blood⬢ Consisting of ionized (HCO3-, CO3-, carbamino

compound) and unionized fraction (H2CO3) and physically dissolved CO2

⬢ 23 to 27 mmol/L

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Bicarbonate Ion Concentration⬢ The bicarbonate ion concentration in the blood

that has been equilibrated with CO2 at 40 mm Hg at 37 degrees Celsius

⬢ 22 to 26 mmol/L

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pO2⬢ Partial Pressure of O2 (pO2)⬢ The pressure or tension exerted by oxygen gas

dissolved in arterial blood which reflects the availability of the gas in blood but not its content

⬢ 80 to 110 mmol/L

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pH⬢ the pH of a solution is defined as the negative

logarithm of the hydrogen ion activity (pH = −log aH+).

⬢ a measure of the concentration of hydrogen ions in an aqueous solution

⬢ average pH of blood (7.40)

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pH

⬢ Henderson-Hasselbalch Equation[A-] = proton acceptor (HCO3-)[HA] = proton donor, or weak acid (H2CO3)[pK’] = pH at which there is an equal concentration of protonated and unprotonated species

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pH

function of kidneys (metabolic)pH = --------------------------------------

function of lungs (respiratory)

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⬢ Henderson-Hasselbalch Equation

pH

function of kidneys (metabolic)pH = --------------------------------------

function of lungs (respiratory)

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⬢ Henderson-Hasselbalch EquationA fall in HCO3- or rise in pCO2 will cause a fall in pHA rise in HCO3- or fall in pCO2 will cause a rise in pH

Normal Values

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II. BUFFER SYSTEM

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BUFFER SYSTEM

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PHOSPHATE BUFFER SYSTEM

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PROTEIN BUFFER SYSTEM

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HEMOGLOBIN BUFFER SYSTEM

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BICARBONATE-CARBONIC ACID BUFFER SYSTEM

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BICARBONATE-CARBONIC ACID BUFFER SYSTEM⬢ H2CO3 dissociates into CO2 and water (CO2

eliminated by the lungs thru expiration)⬢ Changes in CO2 modify the ventilation

(respiration) rate⬢ HCO3- concentration can be altered by the

kidneys

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BUFFER SYSTEM

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Respiratory Mechanism in the Regulation ofAcid-Base Balance⬢ Respiration⬢ Exchange of Gases in the Lungs and Peripheral

Tissues⬢ Respiratory Response to Acid-Base

Perturbations

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Respiratory Mechanism in the Regulation ofAcid-Base Balance⬢ Exchange of Gases in the Lungs and Peripheral

Tissues

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Respiratory Mechanism in the Regulation ofAcid-Base Balance⬢ Respiration⬢ Exchange of Gases in the Lungs and Peripheral

Tissues⬢ Respiratory Response to Acid-Base

Perturbations

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Respiratory Mechanism in the Regulation ofAcid-Base Balance

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Renal Mechanisms in the Regulation ofAcid-Base Balance⬢ Na+-H+ Exchange⬢ Renal Production of Ammonia and Excretion of

Ammonium Ions⬢ Excretion of Hydrogen as Dihydrogen

Phosphate⬢ Reclamation of Filtered Bicarbonate

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Renal Mechanisms in the Regulation ofAcid-Base Balance⬢ Na+-H+ Exchange

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Renal Mechanisms in the Regulation ofAcid-Base Balance⬢ Renal Production of Ammonia and Excretion of

Ammonium Ions

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Renal Mechanisms in the Regulation ofAcid-Base Balance⬢ Excretion of Hydrogen as Dihydrogen

Phosphate

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Renal Mechanisms in the Regulation ofAcid-Base Balance⬢ Reclamation of Filtered Bicarbonate

III. ACID-BASE IMBALANCES

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Acid-Base Imbalances⬢ Respiratory Acidosis⬢ Respiratory Alkalosis⬢ Metabolic Acidosis⬢ Metabolic Alkalosis

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Respiratory Acidosis⬢ Due to excessive carbon dioxide accumulation⬢ COPD, myasthenia gravis, CNS disease, drug

overdose (barbiturates, morphine and opiates) and pneumonia

⬢ COMPENSATION: Kidney retain HCO3 because of increased pCO₂

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Respiratory Alkalosis⬢ Due to excessive CO2 loss (because of rapid

breathing)⬢ Anxiety, severe pain, aspirin over dosage, hepatic

cirrhosis and gram negative sepsis

⬢ COMPENSATION: Decreased reabsorption of HCO₃

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Metabolic Acidosis⬢ Renal Tubular Acidosis: Impaired H+ secretion⬢ Diarrhea: Increased HCO3 excretion⬢ Vomiting: Increased HCO3 excretion⬢ Diabetic Ketoacidosis ⬢ Tissue hypoxia/ increased anaerobic respiration

⬢ COMPENSATION: Hyperventilation

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Metabolic Alkalosis⬢ Too much chloride is lost without replacement ⬢ Sweating, Vomiting, Nasogastric suction⬢ Ingestion of alkaline drugs⬢ Too much amount of lactate, acetate and

bicarbonate is intravenously infused⬢ Aldosteronism: increased hydrogen excretion

⬢ COMPENSATION: Hypoventilation

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Acid-base Imbalances⬢ Respiratory Acidosis: excess CO2, caused by

hypoventilation⬢ Respiratory Alkalosis: deficit CO2, caused by

hyperventilation⬢ Metabolic Acidosis: deficit HCO3, common in

cases of kidney disease and diabetes⬢ Metabolic Alkalosis: excess HCO3, caused by

diarrhea, steroid or diuretic therapy.

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Acid-base ImbalancespH pCO2 HCO3-

Respiratory Acidosis ↓ ↑ Normal or ↑

Respiratory Alkalosis ↑ ↓ Normal or ↓

Metabolic Acidosis ↓ Normal or ↓ ↓

MetabolicAlkalosis ↑ Normal or ↑ ↑

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Acid-base Imbalances

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Acid-base Imbalances

IV. LABORATORY APPLICATION

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Specimen Collection⬢ 1) Heparinized plastic syringe

Disadvantages:-Excess heparin causes downward shifting of blood pH-Leaking of gas through plastic

⬢ 2) Glass syringe pretreated with heparin⬢ 3) Heparinized evacuated tubes

With oxygen contamination = ↑ pO2

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Specimen Handling And Storage⬢ Blood samples should be chilled with the use of ice chips

-prevent O2 consumption by the RBCs and release of acidic metabolites⬢ Sources of Error- specimen exposed to air:=decrease pCO2, increase pH, increase pO2-specimen at RT more than 30 mins:=decrease pO2, decrease pH, increase pCO2

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Instrumentation: Arterial Blood Gas Analyzer⬢ Uses 3 electrodes as sensing devices to measure pO2, pCO2, and pH

⬢ pO2 measurement is amperometric-amount of current flow is an indication of the oxygen present-Clark electrode

⬢ pCO2 and pH measurements are potentiometric-a change in voltage indicates the activity of each analyte-Severinghaus electrode

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Instrumentation: Arterial Blood Gas Analyzer

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Instrumentation: Arterial Blood Gas AnalyzerReference electrodes:⬢ Calomel electrode-mercury/mercurous chloride⬢ Silver/silver chloride-overall better and faster⬢ Normal hydrogen electrode

V. ABG INTERPRETATION

Normal Values

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ABG INTERPRETATION

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Check pHCheck HCO3

& pCO2

Determine the primary & compensatin

g disorder

Degree of compensation (check pH)

Check oxygenation

Final Interpretion

STEP

1

STEP

2

STEP

3

STEP

4

STEP

5STEP

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ABG INTERPRETATION

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STEP 1 Evaluate the pH

pH = 7.35 – 7.45

less than 7.35 = acidosismore than 7.45 = alkalosis

ABG INTERPRETATION

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If pH is not given in the problem, calculate the pH using Henderson-Hasselbalch Equation:

wherein: pk’ is 6.1 (constant)

ABG INTERPRETATION

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Example: In healthy individual.. HCO3 – 24mmol/L, pCO2 – 40 mm Hg

pH = 6.1 + log ( 24 / 0.0307 x 40)pH = 7.35

ABG INTERPRETATION

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If dC02 is given, calculate the pH using this formula:

wherein: pk’ is 6.1 (constant)

dC02

ABG INTERPRETATION

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STEP 2 Evaluate the ventilation (lungs)

pCO2 = 35 – 45 mm Hg

< 35 = respiratory alkalosis> 45 = respiratory acidosis

ABG INTERPRETATION

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STEP 2 Evaluate the metabolic process (kidneys)

[HCO3-] = 22 – 26 mmol/L (mEq/L)

< 22 = metabolic acidosis> 26 = metabolic alkalosis

ABG INTERPRETATION

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STEP 3 Determine which is the primary and compensating disorder

ABG INTERPRETATION

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STEP 4 Determine the degree of compensation:a. non-compensatoryb. partial compensation

7.31 – 7.34 acidosis7.46 – 7.49 alkalosis

c. complete compensation

Partial = Implies that the pH is approaching normalComplete = Implies that the pH has returned to the normal range

ABG INTERPRETATION

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STEP 5 Evaluate the degree of oxygenation

pO2 = 80 – 110 mm Hg(adequate oxygenation)

Hypoxemia:Mild = 60 – 79 mm HgModerate = 40 – 59 mm HgSevere = 39 mm Hg or less

ABG INTERPRETATION

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STEP 6 FINAL INTERPRETATION

a. Degree of Compensation

b. Primary Disorderc. Degree of Oxygenation

SAMPLE CASE

ABG INTERPRETATION

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20/M with diarrhea of greater than 5x/day.Intrepret the ABG result:

pH: 7.32pC02 = 28 mmHgHCO3 = 14 meq/L02 – 78%

ABG INTERPRETATION

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pH = 7.35 – 7.45

less than 7.35 = acidosismore than 7.45 = alkalosis

ABG INTERPRETATION

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STEP 2 Evaluate the ventilation (lungs)

pCO2 = 35 – 45 mm Hg

< 35 = respiratory alkalosis> 45 = respiratory acidosis

ABG INTERPRETATION

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STEP 2 Evaluate the metabolic process (kidneys)

[HCO3-] = 22 – 26 mmol/L (mEq/L)

< 22 = metabolic acidosis> 26 = metabolic alkalosis

ABG INTERPRETATION

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STEP 3 Determine which is the primary and compensating disorder

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In this case, change in HC03 is greater than pC02, therefore the acidosis is METABOLIC in origin.Basis: Change in HC03 = (24-14)/ 24 = 0.42 Note: (24 is the normal HC03 level, 14 is the given HC03 value in the case)Change in pC02 = (40-28)/ 40 = 0.30 Note: (40 is the normal pC02 level, 28 is the given value in the case)

Therefore 0.42 is greater than 0.30, change in HC03 is greater than pCo2.

Primary Disturbance: Metabolic AcidosisCompensating Disturbance: Respiratory Alkalosis

ABG INTERPRETATION

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STEP 4 Determine the degree of compensation:a. non-compensatoryb. partial compensation: pH - 7.32

7.31 – 7.34 acidosis7.46 – 7.49 alkalosis

c. complete compensation

Partial = Implies that the pH is approaching normalComplete = Implies that the pH has returned to the normal range

ABG INTERPRETATION

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STEP 5 Evaluate the degree of oxygenation

pO2 = 80 – 110 mm Hg(adequate oxygenation)

Hypoxemia:Mild = 60 – 79 mm HgModerate = 40 – 59 mm HgSevere = 39 mm Hg or less

ABG INTERPRETATION

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STEP 6 FINAL INTERPRETATION

a. Degree of Compensationb. Primary Disorderc. Degree of Oxygenation

Partially Compensated Metabolic Acidosis with Mild Hypoxemia

ABG INTERPRETATION: SUMMARY

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Case Step 1 Evaluate pH (and calculate if applicable) Acidosis

Step 2 Evaluate the Ventilation Respiratory Alkalosis

Step 2 Evaluate the Metabolic Process Metabolic Acidosis

Step 3 Determine the Primary Disturbance Metabolic Acidosis

Step 3 Determine the Compensating Disturbance Respiratory Alkalosis

Step 4 Degree of Compensation Partially Compensated

Step 5 Check Oxygenation Moderate Hypoxemia

Step 6 Final Interpretation Partially Compensated Metabolic Acidosis with Mild Hypoxemia

REFERENCES:

clinical chemistry 1: blood gases, ph and buffer systems

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