Refractory Hypercapnia

Hannah Kieffer

Background

  • Definition:
    • Inadequate clearance of CO2 leading to respiratory acidosis (pH ≤ 7.20) despite maximum RR&TV (i.e. minute ventilation) tolerated without causing barotrauma or autoPEEP
  • Common causes:
    • Obstructive lung disease (COPD, emphysema, asthma)
    • Hypoventilation syndromes (congenital central hypoventilation, brainstem injury, sleep apnea, obesity, sedative medications (i.e. opiates), neuromuscular weakness, chest wall trauma, ascites/pleural effusion)
    • Increased CO2 load (shock, sepsis, malignant hyperthermia)
  • Presentation:
    • Shortness of breath, AMS, somnolence, hypoxemia, tachycardia, HTN (in some cases)

Evaluation

  • Physical exam, mental status, recent medications
  • ABG or VBG
    • Respiratory acidosis: Expect high CO2; normal pH suggests chronic condition with compensation vs low pH suggests more acuity
    • If increased PCO2 and normal pH, always treat the pH and not the PCO2 (i.e., if compensated chronic hypercarbia, decreasing CO2 below what the patient has adapted to -> decreased respiratory drive, cerebral vasoconstriction, Bohr effect)

Management Algorithm

  • Maximize conventional ventilation strategies
    • Consider HFNC or BiPAP if safe
    • If refractory (PaCO2 >60 mmHg for >6 hours w pH <7.25), consider ECCO2R
    • Consider escalation to V-V ECMOS
  • Special considerations
    • If history of OSA, make sure they are on home CPAP/BiPAP
    • If opiate related, trial Narcan
    • If reactive airway disease contributing, bronchodilators
  • BiPAP
    • Contraindicated if pt unable to remove BiPAP mask independently
    • Increase MV by increasing Δ between IPAP/EPAP or increasing RR
  • Mechanical ventilation
    • Allows you to control rate and TV (in VC modes)
    • To increase MV and CO2 clearance:
      • Increase RR
        • Up to ~30-35 breaths/min
        • Need to keep in mind I/E time to avoid breath stacking/autoPEEP
          • Evidence of autoPEEP: Increased WOB/vent dyssynchrony, worsening hypotension, and failure of the expiratory limb on the flow waveform on the vent to return to zero
          • Possible corrective measures: Lower RR, decrease inspiratory time, increase expiratory phase time
      • Increase TV
        • Usually start at 4-6mL/kg PBW. Can consider increasing to 8mL/kg PBW as long as plateau pressures remain < 30 cm H2O
        • Goal peak pressures ≤ 35 cmH2O / plateau pressures ≤ 30 cmH2O
    • If ARDS, allow for permissive hypercapnia (goal pH ≥ 7.2)
  • V-V ECMO / Extracorporeal carbon dioxide removal (ECCO2R)
    • Indications for ECMO for hypercapnia:
      • Severe dynamic hyperinflation and/or severe respiratory acidosis
      • pH ≤ 7.20 with PaCO2 ≥ 60 for 6h with RR at 35/min and TV increased to target maximum MV while keeping plateau pressure ≤ 32 cmH2O
    • Similar considerations and contraindications as refractory hypoxemia (see above)
      • Benefits: Reduces work of breathing, promotes early ventilator weaning/extubating -> allows early mobilization and recovery
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