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Key Clinical Questions

  • image What key clinical entities must be considered in the initial assessment of a hospitalized patient with hypoxia?

  • image What are initial diagnostic tests and assessments that should be obtained in the hypoxic patient?

  • image What are the potential pitfalls of reliance on pulse oximetry to define hypoxia, and how are these avoided?

  • image How should supplemental oxygen be delivered in the hypoxic hospitalized patient?

CASE 95-1

A 63-year-old white female with a history of chronic obstructive pulmonary disease (COPD), requiring 2 L/min (LPM) home oxygen supplementation, coronary artery disease, hypertension, obesity, diabetes mellitus (DM), and lung cancer is admitted to the hospital from clinic with worsening shortness of breath. In the past few months, she has experienced worsening shortness of breath and dyspnea on exertion, and is now unable to walk to the bathroom without getting short of breath. She admits to orthopnea, lower-extremity edema, fatigue, and chest pain. She now requires 4 LPM of oxygen in order to maintain an oxygen saturation >89%.

Her lung cancer was diagnosed in 1996 and was treated with lobectomy and radiation therapy. She has a 50 pack-year tobacco history but quit smoking 12 years ago. Her physical exam is notable for an oxygen saturation of 94% on 4 L, but she desaturates to 81% when semirecumbent. Crackles are heard in the left lung base, and she becomes quite dyspneic and tachypneic upon minimal effort. A one-sixth systolic ejection murmur is heard, along with a loud P2 and paradoxical splitting of S2. Lower extremities have +1 pitting edema, and she has mild digital clubbing

What are the next steps needed to appropriately evaluate and treat this patient’s hypoxia?

Hypoxemia is defined as an abnormally low arterial oxygen tension (PaO2). Hypoxia refers to insufficient oxygen in the tissues, and can be generalized or local. The two terms are often used interchangeably, but in actuality hypoxia can be caused by hypoxemia as well as other entities such as anemia or ischemia. This chapter discusses hypoxemic hypoxia.

A PaO2 of 60 mm Hg approximately 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 95-1). 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 may be needed.

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