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ExAC: Respiration and Ventilation: Separating Oxygenation from CO2 Clearance

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Respiration and Ventilation: Separating Oxygenation from CO2 Clearance

Items in this domain test whether you can separate oxygenation from ventilation, interpret a capnogram in real time, and recognize respiratory depression before desaturation occurs. Expect vignettes about sedation in obesity, obstructive sleep apnea, or procedural apnea.

Core physiology

  • Oxygenation is reflected by SpO2 and PaO2; ventilation by EtCO2 and PaCO2. A patient can hold a normal saturation while becoming dangerously hypercapnic.
  • Causes of hypoxemia: low inspired oxygen, hypoventilation, ventilation-perfusion mismatch, shunt, and diffusion limitation. Shunt responds poorly to added oxygen; hypoventilation responds well.
  • Capnogram phases: inspiratory baseline, expiratory upstroke, alveolar plateau with the end-tidal point, and inspiratory downstroke. A sloping plateau suggests expiratory obstruction such as asthma or COPD.
  • The normal EtCO2 to PaCO2 gradient is about 2-5 mmHg; it widens with ventilation-perfusion mismatch and narrows in healthy young patients.

Monitoring under sedation

Continuous capnography detects apnea and airway obstruction earlier than oximetry and is recommended for moderate sedation in at-risk patients, including obesity, OSA and ASA III or higher. Supplemental oxygen lengthens the time to desaturation and can hide hypoventilation until it is severe.

Worked example

A sedated 62-year-old with OSA on 4 L nasal cannula has SpO2 98 percent but a capnogram with a rising baseline and EtCO2 of 58 mmHg. Reasoning: hypoventilation is already present despite a reassuring saturation. Act by stimulating the patient, performing a jaw thrust and repositioning, reducing or stopping the sedative, and considering reversal if opioids or benzodiazepines are the cause. Do not wait for desaturation before intervening.

Traps that cost marks

  • Relying on SpO2 alone and treating a normal saturation as normal ventilation.
  • Calling an absent waveform a technical failure when it may represent apnea.
  • Ignoring dilution effects when sampling low-flow nasal cannula capnography.
  • Treating inspiratory stridor as bronchospasm; stridor with poor airflow suggests laryngospasm, which needs airway maneuvers and positive pressure, not just bronchodilators.
  • Forgetting that preoxygenation and supplemental oxygen delay, but do not prevent, respiratory failure.