Oxygenation versus ventilation
An alveolus and capillary with a thickening membrane on the left, a hinged bellows with diaphragm and airway column on the right, under one strip asking whether oxygen is getting in and whether air is moving.
Oxygenation is a membrane problem. Ventilation is a bellows problem. Pulse oximetry measures only the first, which is why the exam loves the sedated patient with a comfortable saturation.
| OXYGENATION | VENTILATION | |
|---|---|---|
| What moves | Oxygen crossing the alveolar membrane into blood | Carbon dioxide leaving the body, driven by rate and depth |
| What fails | Pneumonia, pulmonary edema, ARDS, atelectasis, pulmonary embolism | Opioid or sedative depression, neuromuscular weakness, chest wall injury, severe COPD, exhaustion |
| What you see | Hypoxemia, restlessness then confusion, tachypnea, accessory muscle use, late cyanosis | Falling respiratory rate or shallow effort, somnolence, morning headache, a saturation that stays acceptable while the patient deteriorates |
| What fixes it | Increase inspired oxygen, position for the best lung, recruit alveoli with positive end-expiratory pressure | Stimulate, reverse the sedative, support the breath mechanically |
| The trap | Cyanosis appears far too late to be an early cue | The oximeter cannot detect it at all |
The trap row
A patient on supplemental oxygen can hold a saturation in the mid-nineties while carbon dioxide climbs and the level of consciousness falls. Oxygen keeps the hemoglobin loaded; nothing about that number tells you whether the bellows is still working. The ventilation assessment is rate, depth, and mentation, watched over time. Count the respirations yourself for a full minute and look at whether the chest is actually moving air rather than merely moving.
Why the exam builds items here
- The post-operative patient on patient-controlled analgesia who becomes hard to rouse.
- The patient with a neuromuscular disorder whose vital capacity is falling while saturation holds.
- The patient with COPD who is drowsy and comfortable rather than distressed.
In every one of these the correct answer assesses the level of consciousness and the respiratory effort, not the pulse oximeter.
Margin notethe pulse oximeter cannot see carbon dioxide.
Watch outa sedated patient with a falling respiratory rate is failing even with an acceptable saturation. Never withhold oxygen from a hypoxic patient, including a patient with COPD.
TakeawaySaturation answers one of the two questions, so ask the other one out loud.
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