INTRODUCTION

This case study is designed for upper division physiology students, most of whom are interested in pursuing a career in the health sciences. The students are expected to have a basic knowledge of physiology of the respiratory system, and some idea of how homeostatic mechanisms operate in a normal human. The case is particularly good at illustrating how organ systems interact with each other, and is a useful introduction to acid-base balance in the body.

CASE OBJECTIVES

By working through this case, the student will be able to:

  1. Explain the physiological changes (respiratory, cardiovascular, and otherwise) that humans encounter at high altitudes
  2. Understand the pathophysiology of pulmonary edema
  3. Evaluate physiological data in a "clinical" setting to make a "diagnosis"
  4. Understand the integration of the cardiovascular, respiratory, and nervous systems in regulating homeostatic variables such as blood gas levels, pH, etc.
  5. Recognize the multiple influences on blood pH

CLASSROOM MANAGEMENT

Numbers of students vary depending on the institution's enrollment. Typically, this case is best managed with small groups of four to six students working together within a larger classroom (i.e., in a laboratory setting). The case is divided into four distinct sections, each of which has its own set of questions, and is intended to be given to the students in a progressive disclosure format. There is also a fifth, final section that serves as a sort of coda for the case. Allowing approximately 10-15 minutes of discussion for each section permits the case to be managed in a one-hour class. If more depth is desired, the case can be extended to a two-hour period by simply splitting the case across two periods. This will allow for more lengthy interpretation of the data sets.

Student Preparation

Prior to the use of this case, students need to review the basics of respiratory physiology: basic anatomy, normal values for oxygen and carbon dioxide in ambient air as well as in the blood, control of respiration, effects of hypo- and hyperventilation, etc.

ANALYSIS OF MAJOR ISSUES

This case is divided into four parts. The story opens, in Part I, with a description of the physical situation: exhausted climbers on a steep mountain. Because of the much-chronicled Mount Everest disaster of 1996, this is a familiar and interesting topic for most students. In the opening paragraphs, one climber in particular is having severe difficulty breathing, and he quickly becomes the focus of the story as his condition deteriorates. The opening questions ask students to assess the physiological changes that occur at altitude, and has them determine the characteristics of the ambient air at the climbers' altitude. This provides them with the basic facts necessary to address the later issues that will arise for the ill climber. Possible answers to the questions are listed below:

  1. What types of physiological problems do humans encounter at high altitudes?
  2. What symptoms did the climbers exhibit that might be related to altitude? Explain.

These questions can be addressed together, and should not be exclusive to Mark, the ill climber. Emily, for example, has a headache (a common symptom at altitude), and all of the climbers are short of breath. If time and class level permit, this could lead to discussions of why these symptoms occur (cerebral vasodilation; decreased oxygen content, etc.)

  1. Compare the air at 18,000 feet (atmospheric pressure 280 mm Hg) to the air at sea level (760 mm Hg). What specific changes in the primary atmospheric gases (nitrogen, oxygen, carbon dioxide) might occur? Are they significant?

This will require students to recall the "makeup" of the ambient air - 21% oxygen, 79% nitrogen, <1% carbon dioxide - and to perform calculations to determine the partial pressures at altitude. The significance of the changes will likely lead back to a discussion of the climbers' symptoms.

QUESTIONS 4, 5, AND 6 ARE MORE APPROPRIATE FOR UPPER DIVISION OR ADVANCED STUDENTS

  1. What is the specific pulmonary response to this altitude? [Assume you are considering a subject at rest.]

This question is necessary to show that the altitude has an effect on breathing that is independent of any exercise. Students should correctly answer with "hyperventilation," which can lead to a discussion distinguishing hyperventilation from hyperpnea, and the causes for each.

  1. How will this response affect overall blood gases? What about oxygen loading and unloading from hemoglobin? Explain how you arrived at your conclusions.

These questions are designed to lead the students to the conclusion that hyperventilating actually makes oxygen unloading from hemoglobin more difficult, as the oxygen-hemoglobin dissociation curve is shifted to the left with decreasing carbon dioxide.

  1. After breathing at altitude for a few days, the body normally begins producing more 2,3-DPG. What is the significance of this change? How will it affect the pulmonary changes observed?

This question ties in the cardiovascular system to a larger extent, with increased 2,3-DPG production shifting the curve to the right. I have found that for many students this type of manipulation is necessary for them to finally understand the oxygen-hemoglobin dissociation curve.

Part II, the very short second section of the case, describes how the climbers lived at altitude (acclimated) for a month before attempting their climb. Most students are familiar with this idea, but have never questioned why (in a physiological sense) this might work. In describing the production of erythropoietin and the subsequent increase in hematocrit, the renal and endocrine systems interact with the cardiovascular and respiratory systems in a homeostatic loop - just the type of integration we like to show in physiology!

Part III (also short) describes a dramatic loss of consciousness by our stricken climber, as he falls unconscious and appears to be in serious trouble. The questions at the end of this section ask the students to play "doctor" and decide what to test to determine the details of his condition. It is critical in this portion that students do not get caught up in the details of the tests - if they know the types of things they want to measure, that is best. Ask them not to think of the feasibility of a test (as many of them will), but rather on what information would be most useful to them. List all suggested tests on the board and have the class decide why or why not each of the proposed tests should be run. You may want to tell the class they must justify all tests to their superiors or an insurance company. Further, they must decide which tests are most critical and must be run first - after all, someone's life is at stake!

After answering the above questions, hand students the results of the medical findings in the fourth section (Part IV). The questions are pretty straight forward and should lead to the conclusion that Mark is suffering from pulmonary edema. If students are having difficulty making sense of the data, try "guiding" them in the following way:

  1. In comparing Alveolar Oxygen Tension and Arterial Oxygen, what do you see? (Alveoar is high; Arterial is low)
  2. What does that tell you about oxygen levels in the lungs vs. oxygen levels in the blood? Apparently, he's breathing in plenty of oxygen, but he's not getting it into his blood for some reason. What could be preventing this from happening?

More advanced students could also be challenged to examine the changes in blood pH and decide how he could be acidic. Remind them that carbon dioxide levels are low, so it's not a respiratory acidosis. [Answer: lactic acidosis resulting from low oxygen levels...but you knew that]

Finally, the last short section, Part V, can be used to begin a discussion of HAPE and its sudden onset. Students could also discuss High Altitude Cerebral Edema, or HACE, a similarly insidious disease that has taken the lives of many experienced climbers. The pathophysiology of these disorders are not entirely understood, although there is quite a bit of anecdotal information on the Internet (see the sites listed below).

REFERENCES

  1. Adamson, T.P., and Ishida, A. Systemic Physiology Laboratory Manual, Second Edition, University of California, Davis. 1994. McGraw-Hill, Inc. College Custom Series.
  2. High Altitude Medicine Guide. http://www.gorge.net/hamg/AMS.html
  3. Jerome, E.H., and Severinghaus, J.W. The New England Journal of Medicine Editorial, March 7, 1996, Vol. 334, No. 10 "High-Altitude Pulmonary Edema" http://www.nejm.org/content/1996/0334/0010/0662.asp
  4. Additional pertinent websites:

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