Blood Gases and Acid-Base Balance

The pH of plasma is function of two independent variables:

• Partial pressure of Carbon dioxide (pCO2) - regulated by lungs

• Concentration of Bicarbonate (HCO3-) - regulated by kidneys

Blood Gas Analysis - Specimens

• In most institutions, the respiratory department is responsible for collection and analysis, but some clinical laboratories still perform blood gas analysis

• Arterial puncture - evaluate pH, oxygen content, and CO2 content of blood

• Capillary specimens - infants

Order of Draw: BSEON

Arterial Blood Gas

• method of determination could be done through: (2)

• Gasometer (Van Slyke or Natelson)

• Electrodes

Arterial Blood Gas

• Electrode for:

→pH:

→pCO2

→pO2:

→pH: Silver- Silver Chloride Electrode or Calomel Electrode

→pCO2: Severinghaus Electrode

→pO2: Clarke Electrode

Sample Collection and Handling

• Anticoagulant:

• Must use _ collection for pH and blood gases

• Sodium heparin

• Anaerobic collection

Sample Collection and Handling

• If blood is exposed to air (bubbles in syringe; uncapped tube):

→CO2 and PCO2

→pH

→PO2

→CO2 and PCO2: DECREASED

→pH: INCREASED

→PO2: INCREASED

Sample Collection and Handling

• If testing is prolonged (>15 minutes), blood should be kept in cracked ice to prevent hemolysis

• Hemolysis leads to:

→pCO2 and CO2:

→pH:

→pO2:

→pCO2 and CO2: INCREASED

→pH: DECREASED

→pO2: DECREASED

ABG Values

• pH:

• pCO2:

• HCO3-:

• pO2:

• pH: 7.35 to 7.45

• pCO2: 35 to 45 mmHg

• HCO3-: 22 to 26 mmol/L

• pO2: 80 to 110 mmHg

3-Step Interpretation of ABG Results

1. Identify it is alakalosis or acidosis

2. Identify if it is respiratory or metabolic

3. Identify if it is compensated or uncomepensated

• Most CO2 combines with H2O to form H2CO3 (carbonic acid) which immediately dissociates into H+ and HCO3 (bicarbonate)

• This reaction is accelerated by _

• The dissociation of H2CO3 increases HCO3 in RBC causing it diffuse into plasma

• To maintain electroneutrality, _ diffuse into the cell as an exchange to moving out HCO3

• carbonic anhydrase

• chloride

Action of the Lungs

• The H+ that was carried on the reduced hemoglobin in venous blood is released to recombine with HCO3 - to form H2CO3 (carbonic acid) which dissociates into H2O and CO2

• The CO2 diffuses into the alveoli and is eliminated through ventilation

• Slow or non-removal of CO2 results to increased H+ concentration

• Rapid or fast elimination of CO2 results to decrease of H+ ion concentration

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Action of the Kidneys

• Excrete considerable amounts of acid-base for acid-base balance

• The main role is to reclaim _ from the glomerular filtrate and add it to blood

• The H+ combine with HCO3, forming carbonic acid, and later converted to H2O and CO2 by carbonic anhydrase

• CO2 diffuses into the tubule an reacts with H2O, reform carbonic acid and then go back to bicarbonate

bicarbonate

Metablic acidosis

• low pH with bicarbonate deficiency

• production of increased amount of acid (3)

• Lab findings:

→pH:

→HCO3:

→pCO2:

• low pH with bicarbonate deficiency

• production of increased amount of acid:

→Diabetic ketoacidosis

→Lactic acidosis

→Renal failure

• Lab findings:

→pH:DECREASED

→HCO3: DECREASED

→pCO2: NORMAl

Metaboli Acidosis - Compensation

Hyprventilation

Metablolic Alkalosis

• Bicarbonate excess

• May be due to excess ingestion of base, decreased elimination of base or loss of acididc fluis

• Caused by:_

• Lab Findings:

pH: INCREASED

pCO2: NORMAL

HCO3-: INCREASED

• Caused by:

→Intestinal obstruction

→Vomiting

→Glucocorticoid excess - Cishing's Syndrome

→Mineralocorticoid excess - Hyperaldosteronism

• Lab Findings:

pH: INCREASED

pCO2:

HCO3-:

Metabolic Alkalosis - Compensation

• Hypoventilation - CO2 is converted to H2CO3 (carbonic acid) which lowers blood pH

Respiratory Acidosis

• Excessive amount of pCO2

• May be due to inability to exhale CO2

• Caused by: _

• Lab Findings:

pH:

pCO2:

HCO3-:

• Caused by:

→Chronic bronchitis

→Emphysema

→Ingestion of narcotics and barbiturates

→Meningitis

• Lab Findings:

pH: DECREASED

pCO2: INCREASED

HCO3-: NORMAL

Respiratory Acidosis - Compensation

• Increased bicarbonate retention/reabsorption

Respiratory Alkalosis

• Decreased amount pCO2

• May be due to excessive exhalation pCO2

• Caused by: _

• Lab Findings:

pH:

pCO2:

HCO3-:

• Caused by:

→Hypoxia

→Anxiety, Nervousness, Excessive crying

→Pulmonary embolism

→Pneumonia

→Congestive heart failure

→Salicylate overdose

• Lab Findings:

pH: INCREASED

pCO2: DECREASED

HCO3-: NORMAL

Respiratory Alkalosis - Compensation

• Increased excretion of bicarbonate

• When the kidney and lungs are working properly, a _ ratio of bicarbonate to carbonic acid will be maintained

• The body's cellular and metabolic actibities are pH-dependent, thus the body tries to return pH toward normal whenever imbalance occur

• The lungs compensate immediately but the response is _ and incomplete

• The kidneys are slow but response is _ and complete

• 20:1

• short-term

• long-term