Ph Blood Gas Analysis

pH & Blood Gas Analysis Dr. Farhan Javed Dar

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.

 As its name

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

 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.

 The PO

& 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 2

THE BUFFER SYSTEMS OF THE BODY  Proteins  Phosphate  HCO3-

Instrument

Nova (STAT. Profile ultra analyzer).

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

 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

 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.

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

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.

 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.

Partial Pressure Of CO2  Definition 





The amount of carbon dioxide dissolved in arterial blood. It’s 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.

Principle Of PCO2 Measurement  PCO

is measured with a modified pH electrode.  CO in the unknown solution makes 2 contact with a gas permeable membrane mounted on a combination measuring/reference electrode.  CO diffuses across the membrane into a 2 thin layer of electrolyte solution in response to partial pressure difference. 2

 This solution then becomes equilibrated

with the external gas pressure.  CO in the solution becomes hydrated 2 producing carbonic acid which results in a change in hydrogen ion activity CO2 + H2O ↔ H2CO3 ↔ H+ + [ HCO3]

 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.

 This change in potential is measured

against the constant potential of the reference electrode & is related to the PCO2 of the unknown sample.

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.

Principle of PO2 Measurement  PO

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. 2

 This flow of electrons is then measured by

the electrode & is directly proportional to the PO2.

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

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%.

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.

Acid Base Disorders

 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

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

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

 Loss of HCO3 

Extrarenal losses • Acute Diarrhea • Drianage from pancreatic fistulae • Diversion of urine to gut



Renal Losses • Renal Tubular Acidosis

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

METABOLIC ALKALOSIS  Characterized by : 

a high pH-greater than 7.45 pH-



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



If compensation has occurred a High Pco2

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

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 )

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

Causes of Respiratory Acidosis  Thoracic Disease 

Restrictive defects • Hydrothorax • Pneumothorax • Flail Chest



Obstructive disease • • • • • • •

Bronchitis Emphysema Pneumonia Infiltrations Edema Pneumonia Foreign body obstruction

 Neuromuscular disease    

Poliomyletiis GB syndrome Multiple sclerosis Myopathies

 Central Depression     

Trauma Cerebrovascular accidents CNS infections CNS tumors Drug overdose

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

RESPIRATORY ALKALOSIS  Characterized by : 



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

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

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

pH

PCO2

HCO-3

Respiratory Acidosis

↓

↑

N

Respiratory Alkalosis

↑

↓

N

Metabolic Acidosis

↓

N

↓

Metabolic Alkalosis

↑

N

↑

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