Metabolic
This section of acid-base determinations (including blood gases) will cover topics associated with biochemical theory and pathways, including:
- How pH relates to acidity and alkalinity
- How the Henderson-Hasselbalch equation explains the interactions
- How to evaluate parameters associated with normal and abnormal states
- How to classify disease states
Biochemical Theory
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Acids, Bases, pH
- Acids donate \(H^+\), bases accept them
- \(pH = -log[H^+]\), reflecting acidity or alkalinity
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Buffer Systems
- Resist pH changes (bicarbonate, hemoglobin, phosphate, protein)
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Henderson-Hasselbalch Equation
- \(pH = pKa + \log \left( {\frac {[HCO_3^-]} {0.03 \times PaCO_2}} \right)\)
- Relates pH, bicarbonate, and Pa\(CO_2\), crucial for acid-base assessment
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pH and H+ Ion Concentration
- Inverse relationship: Increased [\(H^+\)] decreases pH
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CO2 and O2 Transport
- \(O_2\) transport relies on hemoglobin, affected by pH, \(CO_2\), temperature, 2,3-BPG
- \(CO_2\) transported as dissolved gas, carbaminohemoglobin, and bicarbonate
- Carbonic anhydrase catalyzes \(CO_2\) to bicarbonate, chloride shift maintains balance
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Regulation
- Chemical Buffers: Immediate action to maintain pH balance by binding or releasing \(H^+\) ions
- Lungs (respiratory compensation)
- Kidneys (metabolic compensation)
Metabolic Pathways
- Acid production is a normal bodily process, requiring regulation by chemical buffers, as well as the renal and respiratory systems
- Kidneys filter and selectively secrete or reabsorb ions to maintain balance
- Lungs adjust breathing patterns in response to pH imbalances
Normal States
- Blood pH = 7.35-7.45
- Arterial Blood Gas Values
- Pa\(O_2\): 80-100 mmHg
- Pa\(CO_2\): 35-45 mmHg
- \(HCO_3^-\): 22-26 mEq/L
- Base Excess: -2 to +2 mEq/L
- Sa\(O_2\): 95-100%
- Normal acid-base balance requires functional renal, pulmonary, and metabolic systems
Abnormal States
-
Acidosis (pH < 7.35)
- Respiratory: Increased Pa\(CO_2\)
- Hypoventilation (COPD, drug OD, muscular disorders)
- Metabolic: Decreased \(HCO_3^-\)
- Increased acid production (DKA, lactic acidosis, toxins)
- Bicarbonate loss (diarrhea, RTA)
- Impaired acid excretion (kidney failure)
- Respiratory: Increased Pa\(CO_2\)
-
Alkalosis (pH > 7.45)
- Respiratory: Decreased Pa\(CO_2\)
- Hyperventilation (anxiety, pain, hypoxia)
- Metabolic: Increased \(HCO_3^-\)
- Acid loss (vomiting, suctioning)
- Bicarbonate intake or retention
- Respiratory: Decreased Pa\(CO_2\)
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Other Abnormalities
- Hypoxemia (Pa\(O_2\) < 80 mmHg)
- Reduced inspired oxygen, hypoventilation, diffusion impairment, V/Q mismatch
- Hypercapnia (Pa\(CO_2\) > 45 mmHg)
- Hypoventilation, increased dead space ventilation, increased \(CO_2\) production
- Hypoxemia (Pa\(O_2\) < 80 mmHg)