Hypercapnia | MD Nexus

Determinants of pCO2

Terms
- pCO2 = Arterial Partial Pressure of CO2
- VCO2 = CO2 Production (normal = 90-130 L/min/m2)
  - Measured using a metabolic cart (measures expired CO2)
- VE = Minute Ventilation (Respiratory Rate x VT)
- VD/VT Ratio = Dead Space/Tidal Volume Ratio

Terms
- PAO2: alveolar PO2 (alveolar oxygen tension)
Assumptions
- FIO2: room air
- Altitude: sea level
- Note: arterial PaCO2 (pCO2) is assumed to be nearly the same as alveolar PACO2 in this equation
- Respiratory Exchange Ratio = 0.8

Assumptions
- A-a gradient remains the same (in this case, A-a gradient = 10)
- Respiratory Exchange Ratio = 0.8

At a constant alveolar (minute) ventilation, increased CO2 production would thoretically increase the pCO2
- Normal Patient: increased CO2 production leads to an increase in alveolar (minute) ventilation -> therefore, the patient is able to maintain normal pCO2
- Patient with Moderate-Severe Lung Disease: patient may be unable to generate an increased alveolar (minute) ventilation to compensate for the increased CO2 production -> pCO2 may rise, possibly resulting in respiratory failure

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
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)

Key Point: VD/VT Ratio determines how efficiently the lungs excrete CO2 per breath
Assumptions
- Graph Assumes a Constant CO2 Production (VCO2) of 200 ml/min
- VCO2 = VA x (PaCO2/PB)
- VE = VA x 1.21/(1-VD/VT)
At Low VD/VT Ratio: a relatively low minute ventilation (VE) must be maintained to keep pCO2 constant at 40
At High VD/VT Ratio: a high minute ventilation (VE) must be maintained to keep pCO2 constant at 40
- If this cannot be maintained, pt will develop hypercapnic respiratory failure

Impact of Shunt Fraction on Arterial pO2: pO2 declines linearly with increasing shunt fraction -> the more shunt that exists, the more hypoxemia occurs
Impact of Shunt Fraction on Arterial pCO2: pCO2 remains relatively constant over a range of shunt fractions
- Note: pCO2 only rises after shunt fractions exceeds 50% -> therefore shunt does not typically cause hypercapnia

Respiratory Compensation for Metabolic Alkalosis (see Metabolic Alkalosis, [[Metabolic Alkalosis]])
- Mechanism: elevated pH results in hypoventilation with a compensatory increase in pCO2
  - However, the degree of hypoventilation is limited by the hypoxic drive to breathe
  - The predicted compensatory increase in pCO2 in response to a primary metabolic alkalosis obeys the acid-base rules: expect 7 increase in pCO2 for each 10 increase in HCO3 or Expected pCO2 = (bicarb x 0.7) + 21 + 1.5
  - Clinical Pearl: ALL cases of subacute or chronic hypercapnia are accompanied by elevated serum bicarbonate (on serum chemistry or ABG)
  - Presence of elevated serum CO2 should raise the suspicion for presence of either a primary metabolic alkalosis OR a primary respiratory acidosis with compensatory metabolic alkalosis
  - Order an ABG to differentiate these conditions
Increased CO2 Production: occurs with overfeeding (with tube feedings or TPN)
- Only causes hypercapnia when alveolar ventilation (VE) is inadequate (ie: in the presence of significant lung disease)
Acute Hypoventilation (see Acute Hypoventilation, [[Acute Hypoventilation]]): with acutely decreased VE
Chronic Hypoventilation (see Chronic Hypoventilation, [[Chronic Hypoventilation]]): with chronically decreased VE
Increased Dead Space Ventilation: with increased VD/VT Ratio
- Note: hypercapnia only occurs when VD/VT ratio exceeds 50%

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%
  - COPD: 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
Normal/Unchanged A-a Gradient (see Acute Hypoventilation, [[Acute Hypoventilation]] and Chronic Hypoventilation, [[Chronic Hypoventilation]])
- Obstructive PFT’s
  - COPD: hypercapnia in COPD is multifactorial (due to hypoventilation, V/Q mismatch, etc)
- Restrictive PFT’s + Normal MIP
  - Obesity Hypoventilation Syndrome
  - Obstructive Sleep Apnea
  - Primary Idiopathic Alveolar Hypoventilation Syndrome/Ondine’s Curse
  - Brainstem Disease
  - Pharmacologic Central Respiratory Depressants: opiates, sedatives, etc
- Restrictive PFT’s + Decreased MIP
  - Motor Neuron Disease
  - Neuropathy
  - Neuromuscular Junction Disease
  - Myopathy
  - Chest Wall Disease

Shifts Oxyhemoglobin Dissociation Curve to the Right
- Decreases Hb affinity for oxygen in the lungs (decreasing loading) and increasing oxygen unloading at the tissues.
Decreases Alveolar pO2
Both Stimulation and Depression of Cardiovascular System
- Via competing effects on SVR, CO, etc
- Usually causes pulmonary hypertension and and increase in CO though
Central Nervous System Depression
Stimulation of Ventilation
Dilation of Vascular Bed
Increased Intracranial Pressure (ICP)
Anesthesia: seen with pCO2 >200 mm Hg
Decreased Renal Blood Flow: seen with pCO2 >150 mmHg
Leakage of Intracellular Potassium: seen with pCO2 >150 mmHg
Alteration of the Action of Pharmacologic Agents
- Due to intracellular acidosis, not due to the pCO2 itself