Explain why muscle weakness occurs in myasthenia gravis.

Derek Zoolander is a 32-year-old fashion model for a world famous modeling agency. Over the last 8 months, he experienced “strange” symptoms. He had severe eyestrain when he modeled for longer than 15 minutes. He became tired when he did common tasks, such as eating, and he had extreme fatigue on the job. Despite his strong work ethic, Derek had to excuse himself from several photo shoots because he simply could not pose. Derek is not a complainer, but he began to worry about these vague symptoms. His physician suspected myasthenia gravis. While awaiting the results of a serum antibody test, the physician initiated a trial of an acetylcholinesterase inhibitor. Derek immediately felt better while taking the drug; his strength returned and he could work normally. Meanwhile, the results of the antibody test were

positive, confirming the diagnosis of myasthenia gravis.

1. What are the steps of neuromuscular transmission?

2. What step of neuromuscular transmission is blocked by myasthenia gravis (review pages 86-87 in Guyton and Hall)?

3. Explain why muscle weakness occurs in myasthenia gravis.

4. How does curare compare to myasthenia gravis in action?

Study 2

Your patient is a 56-year-old office manager who is significantly overweight. His diet consists of fatty foods that included red meats and high-calorie desserts. He had occasional chest pains that were relieved by nitroglycerin.

One night, he went to bed early because he wasn’t feeling well. He awakened at 4:00 A.M. with crushing pressure in his chest and pain radiating down his left arm that was not relieved by nitroglycerin. He was nauseated and sweating profusely. He also had dyspnea, especially when he was in the recumbent position. His wife stated his breathing was “noisy.” He called 911. The paramedics arrived and transported him to the nearest hospital.

In the emergency department, his blood pressure was 105/80. Inspiratory rales were present, consistent with pulmonary edema, and his skin was cold and clammy. Successive electrocardiograms and lab results of cardiac enzymes (creatine phosphokinase and lactate dehydrogenase) suggested a left ventricular wall myocardial infarction. His heart rate was variable during the sequential ECGs. His heart rate was 116 bpm during the first recording and 80 during the last recording. During cardiac catheterization, pulmonary capillary wedge pressure was 32 mm Hg (normal, 5 mm Hg). His ejection fraction, measured was 0.35 (normal, 0.55). His stroke volume was measured at 38 mL (normal range is 50-80 mL).

5. Why did he suffer orthopnea (difficulty breathing while recumbent)? Why would pulmonary edema be worse while lying supine compared to sitting or standing?

6. Why did pulmonary edema develop? Elaborate on the major Starling forces involved after reviewing pages 181-186 of Guyton and Hall.

7. Which information in the case tells you that his stroke volume was decreased?

8. Assuming that the patients decreased stroke volume was consistent from the first ECG measurement until the cardiac catheterization, what were his cardiac output measurements during the ECG readings?

9. The normal range for cardiac output is between 4.0 – 8.0 L/min. Were his measurements within normal range? What is the result of a low cardiac output on circulation?

10. Using the blood pressure measured in the emergency department, what is his mean arterial pressure and pulse pressure.

11. Why is mean arterial pressure not the simple average of systolic and diastolic pressures?

Study 3

Blanche Devereaux is a 78-year-old widow who was brought to the emergency room one evening by her sister. Earlier in the day, she had seen bright red blood in her stool. She continued with her daily activities. However, the bleeding continued all day. By dinnertime, she could no longer ignore it. Blanche takes aspirin, as needed, for arthritis, sometimes up to 12 tablets daily. In the emergency room, she was light-headed and her skin was pale and cool to the touch. Her hematocrit was 26% (normal for women, 35%-45%). The ER doctor ordered orthostatic vital signs. Below are the results of the test:

Measurement

Supine

Standing

Blood Pressure

88/60

74/46

Heart Rate

106 bpm

130 bpm

12. What is the definition of circulatory shock? What are some causes of circulatory shock (Review pages 273 through the top of 276 in Guyton and Hall)?

13. After blood loss, what is the fundamental problem in shock? Why does shock occur due to loss of blood?

14. Calculate Mrs. Devereaux’s pulse pressure and mean arterial pressure while supine and standing.

15. What was the compensatory mechanism that leads to her heart rate being fast in both positions?

Study 4

Mr. Ortega returned to the hospital after a recent myocardial infarction. He has experienced a number of fainting spells. The cardiac rehabilitation nurse reports that his PR intervals were longer than normal. Although his QRS complexes had a normal configuration, there were occasional nonconductive P waves, P waves that are not followed by QRS complexes. His physicians believed that a recent myocardial infarction caused a block in his atrioventricular conducting system (Review pages 115 through 118 in Guyton and Hall).

16. What does the PR interval on the ECG represent? What is the normal value?

17. What does the QRS complex on the ECG represent? What is implicit in the information that the QRS complexes on Mr. Ortega’s ECG had a normal configuration?

18. How is it possible to have P waves that are not followed by QRS complexes?

19. Why did Mr. Ortega faint?

Case 5

Tim celebrated his dental school acceptance by completing the seven days trek to Machu Pichu (Day 7: Aguas Calientes elevation 2,430 m). Tim is in good physical condition. He runs 3-5 miles per day, weight trains, and plays midfield in the local soccer club. (Review pages 527-the top of 531 in Guyton and Hall).

20. Describe Tim’s normal O2 saturation curve and the curve at 2,430 m.

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21. Predict whether each of the following parameters would be increased, decreased, or unchanged on Aguas Calientes. Explain why each of the predicted changes would occur.

a. Breathing rate

b. Percent saturation of hemoglobin

c. PO2 at which hemoglobin is 50% saturated (PO50%)

d. Pulmonary artery pressure

Case 6

Sally is a 60-year-old retiree in northern Alberta. One bitter cold March morning (-50 °C), she decided to warm her car in the garage. Thirty minutes later, Sally’s husband found her slumped in the front seat of the car, confused and breathing rapidly. She was taken to a nearby emergency department, where she was diagnosed with acute carbon monoxide poisoning and given 100% O2 to breathe. An arterial blood sample had an unusual cherry-red color. ((Review pages 501 through 502 in Guyton and Hall)

22. The normal percent O2 saturation of hemoglobin in arterial blood is 95%-100%. Why was Sally’s O2 saturation reduced to 50%?

23. What percentage of the heme groups on her hemoglobin were bound to carbon monoxide? Describe normal O2-hemoglobin dissociation curve, and compare the O2-hemoglobin dissociation curve that would have been obtained on Sally in the emergency department.

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24. What effect did CO poisoning have on her O2-binding capacity? What effect did CO poisoning have on the affinity of hemoglobin for O2?

25. What was the rationale for giving Sally 100% O2 to breathe?