แปลบทความวิจัยCase 15
Hypovolemic Shock: Regulation of Blood Pressure
Mavis Byrne is a 78-year-old widow who was brought to the emergency room one evening by
her sister. Early in the day, Mrs. Byrne had seen bright red blood in her stool, which she attributed
to hemorrhoids. She continued with her daily activities: she cleaned her house in the
morning, had lunch with friends, and volunteered in the afternoon as a "hugger" in the newborn
intensive care unit. However, the bleeding continued all day, and by dinnertime, she could
no longer ignore it. Mrs. Byrne does not smoke or drink alcoholic beverages. She takes aspirin,
as needed, for arthritis, sometimes up to 10 tablets daily.
In the emergency room, Mrs. Byrne was light-headed, pale, cold, and very anxious. Her
hematocrit was 29% (normal for women, 36%-46%). Table 2-3 shows her blood pressure and
heart rate in the lying (supine) and upright (standing) positions.
TABLE 2-3 Mrs. Byrne's Blood Pressure and Heart Rate
Parameter Lying Down (Supine) Upright (Standing)
Blood pressure 90/60 75/45
Heart rate 105 beats/min 135 beats/min
An infusion of normal saline was started, and a blood sample was drawn to be typed and crossmatched
to prepare for a blood transfusion. A colonoscopy showed that the bleeding came from
herniations in the colonic wall, called diverticula. (When arteries in the colon wall rupture,
bleeding can be quite vigorous.) By the time of the colonoscopy, the bleeding had stopped spontaneously.
Because of the quantity of blood lost, Mrs. Byrne received two units of whole blood
and was admitted for observation. The physicians were prepared to insert a bladder catheter to
allow continuous monitoring of urine output. However, by the next morning, her normal color
had returned, she was no longer light-headed, and her blood pressure, both lying and standing,
had returned to normal. No additional treatment or monitoring was needed. Mrs. Byrne was
discharged to the care of her sister and advised to "take it easy."
QUESTIONS
1. What is the definition of circulatory shock? What are the major causes?
2. After the gastrointestinal blood loss, what sequence of events led to Mrs. Byrne's decreased arterial
pressure?
3. Why was Mrs. Byrne's arterial pressure lower in the upright position than in the lying (supine)
position?
4. Mrs. Byrne's heart rate was elevated (105 beats/min) when she was supine. Why? Why was her
heart rate even more elevated (135 beats/min) when she was upright?
5. If central venous pressure and pulmonary capillary wedge pressure had been measured, would
you expect their values to have been increased, decreased, or the same as in a healthy person?
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6. What is hematocrit? Why was Mrs. Byrne's hematocrit decreased, and why was this decrease
potentially dangerous?
7. Why was her skin pale and cold?
8. If Mrs. Byrne's urinary Na" excretion had been measured, would you expect it to be higher,
lower, or the same as that of a healthy person? Why?
9. How was the saline infusion expected to help her condition?
10. Why did the physicians consider monitoring her urine output? How do prostaglandins
"protect" renal blood flow after a hemorrhage? In this regard, why was it dangerous that
Mrs. Byrne had been taking aspirin?
11. Had her blood loss been more severe, Mrs. Byrne might have received a low dose of dopamine,
which has selective actions in various vascular beds. In cerebral, cardiac, renal, and mesenteric
vascular beds, dopamine is a vasodilator; in muscle and cutaneous vascular beds, dopamine is
a vasoconstrictor. Why is low-dose dopamine helpful in the treatment of hypovolemic shock?
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FA 1 ANSWERS AND EXPLANATIONS
1. Shock (or circulatory shock) is a condition in which decreased blood flow causes decreased tissue
perfusion and 02 delivery. Untreated, shock can lead to impaired tissue and cellular metabolism
and, ultimately, death.
In categorizing the causes of shock, it is helpful to consider the components of the cardiovascular
system that determine blood flow to the tissues: the heart (the pump), the blood
vessels, and the volume of blood in the system. Shock can be caused by a failure of, or deficit
in, any of these components. Hypovolemic shock occurs when circulating blood volume is
decreased because of loss of whole blood (hemorrhagic shock), loss of plasma volume (e.g.,
burn), or loss of fluid and electrolytes (e.g., vomiting, diarrhea). Cardiogenic shock is caused
by myocardial impairment (e.g., myocardial infarction, congestive heart failure). Mechanical
obstruction to blood flow can occur anywhere in the circulatory system and cause a local
decrease in blood flow. Neurogenic shock (e.g., deep general anesthesia, spinal anesthesia,
spinal cord injury) involves loss of vasomotor tone, which leads to venous pooling of blood.
Septic or anaphylactic shock involves increased filtration across capillary walls, which leads
to decreased circulating blood volume.
2. Mrs. Byrne h