Hypercapnia

Etiology of Hypercapnia

Background

Determinants of pCO2

Terms and Assumptions

  • pCO2: arterial partial pressure of carbon dioxide
  • k: constant
  • VCO2: carbon dioxide production (normal = 90-130 L/min/m2)
    • Measured Using a Metabolic Cart (Which is Capable of Measuring Expired Carbon Dioxide)
  • VE: minute ventilation (respiratory rate x tidal volume)
  • VD/VT Ratio: dead space/tidal volume ratio

Specific Etiologies of Hypercapnia

  • Respiratory Compensation for Metabolic Alkalosis (see Metabolic Alkalosis)
    • Mechanism
      • Elevated pH Results in Hypoventilation with a Compensatory Increase in pCO2
        • However, the Degree of Hypoventilation is Limited by the Hypoxic Respiratory Drive to Breathe
      • The Predicted Compensatory Increase in pCO2 in Response to a Primary Metabolic Alkalosis Obeys the Acid-Base Rules (see Acid-Base)
        • Expect an Increase of 7 in pCO2 for Each Increase of 10 in the HCO3
        • Expected pCO2 = (bicarb x 0.7) + 21 + 1.5
      • Clinical Pearls
        • ALL Cases of Subacute or Chronic Hypercapnia are Accompanied by Elevated Serum Bicarbonate (on Serum Chemistry or Arterial Blood Gas)
        • Presence of Elevated Serum Carbon Dioxide Should Raise the Suspicion for Presence of Either a Primary Metabolic Alkalosis OR a Primary Respiratory Acidosis with Compensatory Metabolic Alkalosis
        • One Would Order an Arterial Blood Gas to Differentiate These Conditions
  • Increased Carbon Dioxide Production (VCO2)
    • Occurs with Overfeeding (Generally with Tube Feedings or Total Parenteral Nutrition)
    • Increased Carbon Dioxide Production Only Results in Hypercapnia When Alveolar Ventilation (VE) is Inadequate (ie: in the Presence of Significant Lung Disease, Such as Chronic Obstructive Pulmonary Disease, Acute Respiratory Distress Syndrome, etc)
  • Acute Hypoventilation with Acutely Decreased Minute Ventilation (VE) (see Respiratory Failure)
  • Chronic Hypoventilation with Chronically Decreased Minute Ventilation (VE) (see Respiratory Failure)
  • Increased Dead Space Ventilation with Increased VD/VT Ratio
    • Hypercapnia Only Occurs When the VD/VT Ratio Exceeds 50%

Simplified Alveolar Gas Equation (see also Hypoxemia)

Terms and Assumptions

  • PAO2: alveolar partial pressure of oxygen (PO2 or alveolar oxygen tension)
  • Respiratory Exchange Ratio: 0.8
  • Arterial PaCO2 (pCO2) is Assumed to Be Nearly the Same as Alveolar PACO2 in This Equation
  • FIO2 is Assumed to Be Room Air (21% FIO2)
  • Altitude is Assumed to Be Sea Level

Inverse Relationship Between Arterial pCO2 and pO2

Terms and Assumptions

  • A-a Gradient Remains the Same (in this Case, A-a Gradient = 10)
  • Respiratory Exchange Ratio: 0.8

Increased Carbon Dioxide (CO2) Production Normally Results in a Compensatory Increase in Alveolar Ventilation

At a Constant Alveolar (Minute) Ventilation, Increased Carbon Dioxide (CO2) Production Should Theoretically Increase the pCO2

  • In a Normal Patient
    • Increased Carbon Dioxide (CO2) Production Results in an Increase in Alveolar (Minute) Ventilation, Decreasing the pCO2 Back to a Normal Level (i.e. Approximately 40 mm Hg)
  • In a Patient with Moderate-Severe Lung Disease
    • Patient May Be Unable to Increase Their Alveolar (Minute) Ventilation to Compensate for the Increased Carbon Dioxide (CO2) Production
    • Therefor, pCO2 May Increase, Possibly Resulting in Respiratory Failure

Alveolar Ventilation is Inversely (But Not Linearly) Related to pCO2 (at Varying Levels of CO2 Production)

Clinical Examples

  • In a Patient with Acute/Chronic Hypocapnia: assuming a constant CO2 production (VCO2), a significant increase in minute ventilation (VE) must be present to maintain the low pCO2
    • Example: DKA patient with pH 7.40 and pCO2 30 must maintain a significanty increased VE to maintain the pCO2 at that level -> despite a normal pH, rapid respiratory failure can occur if, for any reason, patient cannot maintain that high VE
  • In a Patient with Acute/Chronic Hypercapnia: assuming a constant CO2 production (VCO2), a relatively small decrease in VE can produce a significant increase in pCO2
    • Example: chronically hypercapnic COPD with pCO2 60 can experience a significant increase in pCO2 with even a small decrease in VE (due to minimal sedation, etc)

Minute Ventilation (VE) is Inversely (But Not Linearly) Related to pCO2 (at Varying VD/VT Ratios)

Terms and Assumptions

  • Graph Assumes a Constant Carbon Dioxide Production (VCO2) of 200 ml/min
  • VCO2 = VA x (PaCO2/PB)
  • VE = VA x 1.21/(1-VD/VT)

Key Points

  • VD/VT Ratio Determines How Efficiently the Lungs Excrete Carbon Dioxide (CO2) Per Breath (i.e How “Sick” the Lungs Are)
    • Low VD/VT Ratio = More Efficient Carbon Dioxide (CO2) Excretion Per Breath
    • High VD/VT Ratio = Less Efficient Carbon Dioxide (CO2) Excretion Per Breath
  • At a Low VD/VT Ratio (Healthy Lungs), a Relatively Low Minute Ventilation (VE) is Required to Maintain pCO2 Constant at 40 mm Hg
  • At a High VD/VT Ratio (“Sick Lungs”), a High Minute Ventilation (VE) is Required to Maintain pCO2 Constant at 40 mm Hg
    • If This Level of Increased Minute Ventilation Cannot Be Maintained, the Patient Will Develop Hypercapnic Respiratory Failure

Impact on Shunt Fraction on Arterial pO2 and pCO2

Key Points

  • pO2 Decreases Linearly and Inversely with Increasing Shunt Fraction
    • The Higher the Degree of Shunt, the Lower the pO2
  • In Contrast, pCO2 Remains Relatively Constant Over a Wide Range of Shunt Fractions
    • pCO2 Only Increases After the Shunt Fraction Exceeds 50%
      • For This Reason, Shunt Does Not Typically Result in Hypercapnia

Clinical Evaluation of Hypercapnia (see also Respiratory Failure)

Normal/Unchanged A-a Gradient

Increased A-a Gradient

  • Normal VCO2 (Normal CO2 Production)
    • V/Q Mismatch (V/Q Ratio >1, Dead Space Ventilation): hypercapnia only occurs when VD/VT ratio >50%
    • Chronic Obstructive Pulmonary Disease (COPD) (see Chronic Obstructive Pulmonary Disease): hypercapnia in COPD is multifactorial (due to hypoventilation, V/Q mismatch, etc)
  • Increased VCO2 (Increased CO2 Production): these conditions usually cause hypercapnia only in the setting of underlying lung disease (with inability to excrete CO2)
    • Hypermetabolism
    • Overfeeding: especially wtih excessive carbohydrate, whch generates more CO2 per calorie than do fats
    • Organic Acidosis

Physiologic and Clinical Effects of Hypercapnia

  • Shifts the Oxyhemoglobin Dissociation Curve to the Right (see Hypoxemia)
    • Decreases Hemoglobin Affinity for Oxygen in the Lungs (Decreasing Loading) and Increases Oxygen Unloading at the Tissues
  • Decreases the Alveolar pO2
  • Both Stimulates and Depresses the Cardiovascular System
    • Via Competing Effects on Systemic Vascular Resistance (SVR), Cardiac Output (CO), etc
    • Hypercapnia Usually Results in Pulmonary Hypertension and an Increase in Cardiac Output (CO) Though
  • Depresses the Central Nervous System
  • Increases Intracranial Pressure (ICP) (see Increased Intracranial Pressure)
    • Physiology
      • Hypercapnia Causes Cerebral Vasodilation
    • Clinical
      • May Undesirably Potentiate Neurologic Injury in Traumatic Brain injury (TBI), etc (see Traumatic Brain injury)
  • Stimulates Ventilation
  • Dilates Vascular Beds
  • Anesthesia
    • Observed with pCO2 >200 mm Hg
  • Decreases Renal Blood Flow
    • Observed with pCO2 >150 mm Hg
  • Results in Leakage of Intracellular Potassium
    • Observed with pCO2 >150 mm Hg
  • Alters the Action of Pharmacologic Agents
    • Due to Intracellular Acidosis, Not Due to the pCO2 Itself

References

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