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Chapter 93. Hypoxia

Natalie E. West, MD, MHS; Noah Lechtzin, MD, MHS

  1. What key clinical entities must be considered in the initial assessment of a hospitalized patient with hypoxemia?

  2. What are initial diagnostic tests and assessments that should be obtained in the hypoxemic patient?

  3. What are the potential pitfalls of reliance on pulse oximetry to define hypoxemia, and how are these avoided?

  4. How should supplemental oxygen be delivered in the hypoxemic hospitalized patient?

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Case 93-1

A 63-year-old white female with a history of chronic obstructive pulmonary disease (COPD), coronary artery disease with three stents, hypertension, obesity, diabetes mellitus (DM), and lung cancer was admitted to the hospital from clinic with worsening shortness of breath over the last 2 months, requiring oxygen supplementation. She had been on 2 liters per minute (LPM) of oxygen via nasal cannula for the past 3 years for COPD, but in the past few months has experienced worsening shortness of breath and dyspnea on exertion. At this point she cannot walk to the bathroom in her house without getting short of breath. She admited to orthopnea, lower extremity edema, fatigue, and chest pain as well. She now required 4–8 LPM of oxygen in order to maintain an oxygen saturation > 89%.

Her COPD was diagnosed in 2005, and she has been on supplemental oxygen since then. She had lung cancer in 1996, which was treated with lobectomy and radiation therapy. She had a 50 pack-year tobacco history but quit smoking 12 years ago. Pertinent medications included hydrochlorothiazide, simvastatin, pioglitazone, fl uticasone/salmeterol inhaler, and tiotropium inhaler.

Her physical exam was notable for an oxygen saturation of 94% on 4 liters, but she desaturated to 81% when semirecumbent. Crackles were heard in the left lung base, and she became quite dyspneic and tachypneic upon minimal effort. A 1/6 systolic ejection murmur was heard, along with a loud P2 and paradoxical splitting of S2. Lower extremities had +1 pitting edema, and she had mild digital clubbing

What were the next steps needed to appropriately evaluate and treat this patient's hypoxia?

Hypoxia is defined by an abnormally low arterial oxygen tension. A PaO2 of 60 mm Hg generally corresponds with the point on the oxygen–hemoglobin dissociation curve in which hemoglobin is 90% saturated. The curve is steep at this point, and further decreases in oxygen tension correspond with dramatic falls in hemoglobin saturation and resultant inadequate oxygen delivery to tissues (Figure 93-1). Oxygen affinity can be affected by pH, carbon dioxide (CO2), 2,3-diphosphoglycerate (2,3-DPG), and temperature. As pH decreases and CO2 increases, oxygen is more readily released, shifting the oxyhemoglobin curve to the right, increasing delivery of oxygen to the tissues. Red blood cells contain 2,3-DPG, which helps modulate oxygen affinity. Increasing levels of 2,3-DPG decrease the oxygen affinity, also shifting the dissociation curve to the right. Elevated body temperature shifts the dissociation curve to the right, helping to unload oxygen at a time when additional oxygen to tissues ...

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