Clin Path 3 (Blood Gases)

What are 2 hard rules for sample collection for blood gases?

1. Sample has to be capped so no extra O2 mixes

2. Sample cannot be clotted

T/F

The same sample for chemistries and blood gases can be used and venous blood can be appropriate

True

What is the pH of blood?

7.35-7.45

What causes decreases in pH?**

Decreases indicates increase in free hydrogen ions (more hydrogen, more carbonic acid, pH goes down because of excess H, carbonic acid is indirect player that affects H concentrations)

What are the normal values for PCO2?

Partial pressure of CO2

• 35-40mmHg

What does a blood gas analysis entail? What are the normal values for each?

1. HCO3 = 15-23mmol/L

• Bicarbonate concentration, drives pH up**

• Ph and bicarbonate most useful for detection of acid-base problems

2. PO2 = 85-100mmHg

• Partial pressure of O2

• Indicates oxygenation - only useful in arterial blood samples

What can affect changes in pH?

PH affected by changes in PCO2

What are some sample requirements covered in lecture?**

1. Avoid manual heparinization of syringes

2. No gas bubbles

3. Capped immediately

4. Used to require icing sample immediately, but new containers have too much gas exchange when cold

What are the major buffers used to regulate pH? Minor buffers?

1. Major = hemoglobin, bicarbonate buffer system

2. Minor = inorganic phosphate, plasma proteins

Bicarbonate buffer system**

1. Important, fast acting, measurable

2. H2O + CO2 ←> H2CO3 (carbonic acid) ←> HCO3 (bicarbonate) + H**

What do hydrogen and CO2 control?

1. Hydrogen drives acidity

2. CO2 drives bicarbonate

What is the use of bicarbonate? PCO2? What causes the 4 main acid-base pathologies?**

1. Bicarbonate is the metabolic component of blood pH (driven by kidneys)

2. PCO2 is respiratory component (CO2 driven by lungs)

Acid-base pathology

1. Bicarbonate (HCO3) depletion or accumulation of fixed acid when in effect the amount of fixed acid that is produced exceeds the buffering capacity (kidney can’t compensate for acidosis) = metabolic acidosis

• Most common issue is kidney complications (bicarbonate makes pH more alkaline, depletion = acidic)

• Dehydration, poor tissue perfusion

2. pCO2 retention due to hypoventilation = respiratory acidosis (CO2 drives carbonic acid up, acidic pH)

• Acute respiratory failure

• Accumulation of CO2 - hypoventilation/impairment of gas exchange

3. Bicarbonate excess = metabolic alkalosis (fewer free H ions since bicarbonate and H come together to make carbonic acid, less H = more alkaline)

• Most common cause - upper GI obstruction

4. HCO2 loss due to hyperventilation - respiratory alkalosis (less CO2 means body compensates by using bicarbonate and H to make carbonic acid to make more CO2)

• Pneumonia - gas exchange impaired, oxygen diffuses slower than CO2 (CO2 goes down)

What does bicarbonate and hydrogen control concerning pH?

1. Bicarbonate controls alkalinity

2. Hydrogen controls acidity

What is the relationship of acid-base pathology to the anion gap?

1. Increased anion gap = accumulation of aid due to production/toxicity (most common)

2. Normal anion gap = bicarbonate loss - severe diarrhea or renal loss of bicarbonate

What are the 4 steps to evaluating blood gas results?

1. Evaluate pH

• Determine if bicarb (kidney) or pCO2 (respiratory) is causing pH imbalance

2. Evaluate bicarbonate

3. Evaluate pCO2

4. Combined abnormalities between hCO3 and pCO2 

• Defined by most abnormal value

• Lower value may be compensatory

What is a mixed acid-base disturbance?

2 or more pathologic processes that affect pH

• EX = dog w/ pancreatitis

• Dehydration, poor tissue perfusion = metabolic acidosis 

• Vomiting = metabolic alkalosis

What is used to calculate arterial blood oxygenation? What is the normal value?

1. Calculated w/ pO2 value to give saturation

2. Normal solution = 95%

What is the base excess calculation? What is normal? When is it used?

1. Calculation based on pH, pCO2, and hCO3

2. Normal = near 0

3. Useful in planning fluid therapy

What is cooximetry? What does it measure?

1. Measurement of hemoglobin spectophotometrically

2. Measures:

• Oxygenated hgb

• Unoxygenated hgb

• Carboxyhemoglobin - carbon monoxide poisoning

• Methemoglobin

Useful only on arterial blood

What are the effects on overall values (pH, pCO2, HCO3) with the 4 main abnormalities concerning blood gases?

1. Respiratory acidosis

• pH = decreased

• pCO2 = increased

• HCO3 = normal

2. Metabolic acidosis

• pH = decreased

• pCO2 = normal

• HCO3 = decreased

3. Metabolic alkalosis

• pH = increased

• pCO2 = normal

• HCO3 = increased

4. Respiratory alkalosis

• pH = increased

• pCO2 = decreased

• HCO3 = normal

What are 3 causes for the 4 main abnormalities concerning blood gases?

1. Respiratory acidosis

• Most common = alveolar destruction (cancer)

• Impaired respiratory drive (CNS disorders)

• Airway obstruction (foreign objects, tumors)

2. Respiratory alkalosis

• Drugs - pharmacologic stimulation (salicylate)

• Psychegenic factors - stress (hyperventilation)

• Hypoxia - low levels of pCO2 stimulate respiratory effort

3. Metabolic alkalosis

• Endogenous increase in alkali (K depletion, diuretic therapy)

• Exogenous increase in alkali (ingestion of Na bicarbonate, penicillin therapy)

• Increased loss of acid (vomiting)