pH

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pH Blood Gas Analysis

• Dr. Farhan Javed Dar

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Introduction

• Arterial blood gas measurement is a blood test
  that is performed to determine the concentration
  of oxygen, carbon dioxide and bicarbonate, as
  well as the pH, in the blood.
• Its main use is in pulmonology, as many lung
  diseases feature poor gas exchange, but it is
  also used in nephrology (kidney diseases) and
  electrolyte disturbances.

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

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• The analyzer is use in the quantitative
  determination of pH,PCO2,PO2,Oxygen saturation
  and hematocrit. It also displays hemoglobin.
• The 2-point calibration performed every two hours
  gap 1-point calibration after every half an
  hour gap by machine automatically.

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• The PO2 PCO2 electrode has a membrane with a
  three months life, change it after every three
  months.
• Two buffer solution having pH 6.840 and 7.382 are
  being used in the calibration of pH electrode
  are supplied with system
• Buffer solution, a solution which resists change
  of pH upon addition of small amounts of acid or
  base, or upon dilution

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THE BUFFER SYSTEMS OF THE BODY

• Proteins
• Phosphate
• HCO3-

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Instrument

• Nova (STAT. Profile ultra analyzer).

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Procedure

• To initiate a sample sequence the operator
  carefully watch the analyzer that it must be
  calibrated, reagent pack are perfectly installed
  no error code is blinking on the display screen
• Before introducing the sample, be sure that
  sample is not clotted mix the syringe for few
  seconds to remove any air bubble

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

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• Give sample identify number in the data screen of
  nova other related information so that the
  result can be transmitted automatically to AKUH
  computer system
• Results also appears on the paper inserted in the
  nova printer.

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pH

• The pH is a measure of hydrogen ion (H) in blood
  which indicates the acid or base (alkaline)
  nature of blood
• A pH of less than 7 is acidic, and a pH greater
  than 7 is called basic (alkaline).
• The normal blood pH range is 7.35 to 7.45

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Principle Of pH measurement

• pH is measured using a hydrogen ion selective
  glass membrane.
• One side of the glass is in contact with a
  solution of unknown pH.
• A change in potential develops which is
  proportional to the pH difference of these
  solutions.

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• This change in the potential is measured against
  a reference electrode of constant potential.
• The magnitude of the potential difference is then
  measure, the pH of unknown solution.

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Partial Pressure Of CO2

• Definition
• The amount of carbon dioxide dissolved in
  arterial blood.
• Its level indicates how well carbon dioxide is
  able to move out of the blood into the airspace
  of the lungs and out with exhaled air.
• The normal range is 35 to 45 mm Hg.

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Principle Of PCO2 Measurement

• PCO2 is measured with a modified pH electrode.
• CO2 in the unknown solution makes contact with a
  gas permeable membrane mounted on a combination
  measuring/reference electrode.
• CO2 diffuses across the membrane into a thin
  layer of electrolyte solution in response to
  partial pressure difference.

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• This solution then becomes equilibrated with the
  external gas pressure.
• CO2 in the solution becomes hydrated producing
  carbonic acid which results in a change in
  hydrogen ion activity
• CO2 H2O ? H2CO3 ? H HCO3

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• The electrolyte solution behind the membrane is
  in contact with a glass hydrogen ion selective
  electrode.
• The change in hydrogen ion activity in the
  electrolyte solution produce a potential, which
  is, measured against the internal filling
  solution.

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• This change in potential is measured against the
  constant potential of the reference electrode
  is related to the PCO2 of the unknown sample.

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Partial Pressure of Oxygen

• Definition
• the partial pressure of oxygen is the gas 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 from
  the airspace of the lungs into the blood.
• The normal range is 80 to 100 mm Hg.

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Principle of PO2 Measurement

• PO2 is measured amperometrically by the
  generation of a current at the electrode surface.
• As oxygen diffuses through a gas permeable
  membrane, the oxygen molecules are reduced at the
  cathode, consuming 4 electrons for every molecule
  of oxygen reduced.

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• This flow of electrons is then measured by the
  electrode is directly proportional to the PO2.

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Base Excess Of Blood

• Base excess of blood is defined as the
  concentration of titrable base needed to titrate
  blood to pH 7.40 at 370 C while the PCO2 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

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Oxygen Content

• Oxygen content is defined as the total amount of
  oxygen contained in a given volume of whole blood
  including dissolved oxygen bound to hemoglobin.
• It is expressed in milliliters of oxygen per 100
  ml of blood (vol. ) as calculated from the
  oxygen saturation and the hemoglobin
  concentration.
• The normal range is 95 to 100.

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Controls

• Level I,II III are available one control run
  in each 8 hours shift.
• These controls are formulated from a buffered
  bicarbonate solution of pH sodium
  concentration.
• The solution are equilibrated with known
  concentration of oxygen carbon dioxide.

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• Acid Base Disorders

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• There are four simple acid-base disorders
• Metabolic Acidosis
• Metabolic Alkalosis
• Respiratory Acidosis
• Respiratory Alkalosis
• A pt. can also suffer from two simple disorders
  simultaneously which is termed a mixed acid base
  disturbance

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METABOLIC ACIDOSIS

• Characterized by
• A low HCO3 - less than 22 mEq/L
• A low pH - less than 7.35
• And if compensation has occurred ,a low Pco2

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Causes of Metabolic Acidosis

• Addition of H
• Increased production
• Ketoacidosis
• Lactic acidosis
• Toxins
• Ingestion/infusion( HCl,NH4Cl )
• Decreased Renal Excretion
• Renal failure
• Obstructive uropathy
• Renal tubular acidosis Type 1
• Mineralcorticoid deficiency

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• Loss of HCO3
• Extrarenal losses
• Acute Diarrhea
• Drianage from pancreatic fistulae
• Diversion of urine to gut
• Renal Losses
• Renal Tubular Acidosis

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Consequences of Metabolic Acidosis

• Cardiac Failure
• Hyperkalemia e.g diabetic ketoacidosis
• Hypokalemia in in renal tubular acidosis
• Mobilization of calcium from bone. Renal
  reabsorption of calcium producing hypercalciuria,
  leads to nephrocalcinosis urolithiasis

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METABOLIC ALKALOSIS

• Characterized by
• a high pH-greater than 7.45
• A high bicarbonate-greater than 26 mEq/liter
• If compensation has occurred a High Pco2

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Causes of Metabolic Alkalosis

• Increased exogenous bicarbonate
• Oral/IV. Bicarbonate
• Antacid therapy, e.g, magnesium carbonate
• Organic acid salts e.g , lactate ,citrate
• Loss of hydrogen ions
• Gastrointestinal tract losses
• Stomach vomitting, gastric suction
• Bowel diarrhea
• Kidney losses
• Diuretic therapy
• Mineralcorticoid excess

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Consequences Of Metabolic Alkalosis

• Alkalemia enhances binding of calcium ions to
  protein which results in increased neuromuscular
  activity Characteristic Chvostek Trousseaue
  signs may occur
• Hypokalemia
• Increased calcium reabsorption
• Enhanced glycolysis ( stimulation of
  phosphpfructokinase by a high intracellular pH )

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RESPIRATORY ACIDOSIS

• Characterized by
• Increased Pco2
• pH less than 7.35 with a PCO2 greater than 45 mm
  Hg.
• It is always due to decreased excretion of CO2 by
  the lungs

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Causes of Respiratory Acidosis

• Thoracic Disease
• Restrictive defects
• Hydrothorax
• Pneumothorax
• Flail Chest
• Obstructive disease
• Bronchitis
• Emphysema
• Pneumonia
• Infiltrations
• Edema
• Pneumonia
• Foreign body obstruction

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• Neuromuscular disease
• Poliomyletiis
• GB syndrome
• Multiple sclerosis
• Myopathies
• Central Depression
• Trauma
• Cerebrovascular accidents
• CNS infections
• CNS tumors
• Drug overdose

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Consequences Of Respiratory Acidosis

• On Brain
• Hypercapnia induces cerebral vasodilation
  increased cerebral blood flow which in turn
  increses intracerebral pressure producing
• Drowsiness
• Headaches
• Stupor
• Coma
• On Potassium
• Release of potassium from cells (exchange for H
  
• But not a constant feature

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RESPIRATORY ALKALOSIS

• Characterized by
• Hypocapnia ( low Pco2 ) due to increased
  ventilation.
• pH greater than 7.45 with a PCO2 less than 35 mm
  Hg

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Causes Of Respiratory Alkalosis

• Central Stimulation
• Anxiety
• Pregnancy
• Hypoxemia
• Hepatic Encephalopathy
• Gram ve septicaemia
• Salicylate overdose
• Infection, trauma
• Tumour
• Pulmonary Pathology
• Embolism
• Congestive cardiac failure
• Asthma,Pneumonia

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Consequences Of Respiratory Alkalosis

• On Calcium Metabolism
• Tetany as alkalemia causes increased binding of
  calcium ions to protein.
• On Potassium
• Initially mild hypokalemia but generally plasma
  potassium remains normal
• On Phosphate
• Transient severe hypophosphataemia
• On Glucose Metabolism
• Increased lactate production
• On Brain
• Cerebral vasoconstriction, which may results in
  light headedness

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pH PCO2 HCO-3
Respiratory Acidosis ? ? N
Respiratory Alkalosis ? ? N
Metabolic Acidosis ? N ?
Metabolic Alkalosis ? N ?
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• THANK YOU