clinical chemistry 1: blood gases, ph and buffer systems
TRANSCRIPT
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Clinical Chemistry 1: BLOOD GASES, pH
and BUFFER SYSTEMS
(LECTURE & LABORATORY)
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TOPIC OUTLINE:PART 1 - LECTUREI. BASIC CONCEPTII. BUFFER SYSTEM
III. ACID-BASE IMBALANCES
PART 2 - LABORATORYIV. LABORATORY APPLICATION
V. ABG INTERPRETATION
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I. BASIC CONCEPT
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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
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BUFFER
3.0 9.07.0pH
HCO3- H2CO3
pK= 6.1
Bicarbonate-Carbonic Acid System
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BUFFERBicarbonate-Carbonic Acid System
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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
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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
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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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PHOSPHATE 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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Respiratory Mechanism in the Regulation ofAcid-Base Balance
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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
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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
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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 Analyzer
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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
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V. ABG INTERPRETATION
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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
6
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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
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STEP 1 Evaluate the pH
If pH is not given in the problem, calculate the pH using Henderson-Hasselbalch Equation:
wherein: pk’ is 6.1 (constant)
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ABG INTERPRETATION
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STEP 1 Evaluate the pH
Example: In healthy individual.. HCO3 – 24mmol/L, pCO2 – 40 mm Hg
pH = 6.1 + log ( 24 / 0.0307 x 40)pH = 7.35
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ABG INTERPRETATION
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STEP 1 Evaluate the pH
If dC02 is given, calculate the pH using this formula:
wherein: pk’ is 6.1 (constant)
dC02
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ABG INTERPRETATION
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STEP 2 Evaluate the ventilation (lungs)
pCO2 = 35 – 45 mm Hg
< 35 = respiratory alkalosis> 45 = respiratory acidosis
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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
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ABG INTERPRETATION
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STEP 3 Determine which is the primary and compensating disorder
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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
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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
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STEP 6 FINAL INTERPRETATION
a. Degree of Compensation
b. Primary Disorderc. Degree of Oxygenation
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SAMPLE CASE
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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%
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STEP 1 Evaluate the pH
pH = 7.35 – 7.45
less than 7.35 = acidosismore than 7.45 = alkalosis
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STEP 2 Evaluate the ventilation (lungs)
pCO2 = 35 – 45 mm Hg
< 35 = respiratory alkalosis> 45 = respiratory acidosis
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STEP 2 Evaluate the metabolic process (kidneys)
[HCO3-] = 22 – 26 mmol/L (mEq/L)
< 22 = metabolic acidosis> 26 = metabolic alkalosis
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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
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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
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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
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STEP 6 FINAL INTERPRETATION
a. Degree of Compensationb. Primary Disorderc. Degree of Oxygenation
Partially Compensated Metabolic Acidosis with Mild Hypoxemia
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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
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