A 20-year-old, previously healthy woman is brought to the emergency department after a motor vehicle collision. The patient was a restrained, rear-seat passenger when another car struck the side door. There was extensive damage to the vehicle, resulting in a prolonged extrication time. On arrival at the emergency department, blood pressure is 90/60 mm Hg and pulse is 120/min. On physical examination, the patient is alert but appears anxious, pale, and diaphoretic. There is no obvious head or neck injury. The lungs are clear bilaterally and cardiac auscultation reveals tachycardia. The abdomen is soft. Several actively bleeding lacerations are present on the lower extremities. X-rays show pelvic and left femoral shaft fractures. Which of the following findings are most likely present in this patient?
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This patient's hypotension and tachycardia in the setting of multiple lacerations and pelvic fracture (a common cause of severe internal bleeding) are consistent with hypovolemic shock due to massive hemorrhage. Following significant intravascular volume depletion, baroreceptors in the aorta and carotid arteries rapidly adjust autonomic output to help maintain blood pressure.
Baroreceptors are stretch-sensitive mechanoreceptors that increase their firing rate in response to arterial distension. These impulses are transmitted to the solitary nucleus, which then modulates sympathetic and parasympathetic output in response to the firing rate.
With an abrupt decrease in cardiac output and blood pressure (eg, acute blood loss, standing from a seated position), reduced stretch of the arterial walls leads to reduced baroreceptor firing and decreased activation of the solitary nucleus. This causes an increase in sympathetic outflow with a corresponding reduction in parasympathetic tone, leading to vasoconstriction (increases peripheral resistance) and increased heart rate and contractility (improves cardiac output), minimizing the drop in blood pressure.
Via a similar mechanism, stretch receptors in the atrial walls of the heart control secretion of atrial natriuretic peptide (ANP). With hypovolemia there is reduced atrial stretch that leads to decreased ANP secretion.
(Choices A, F, G, and H) Because baroreceptor firing reduces sympathetic output, the degree of baroreceptor firing is usually inversely related to cardiac contractility.
(Choice B) In hypervolemic states, blood pressure is increased, leading to increased baroreceptor firing with a decrease in sympathetic outflow and corresponding increase in parasympathetic outflow. A resulting decrease in vascular tone, heart rate, and cardiac contractility reduces blood pressure. Increased stretching of the atrial walls leads to increased ANP release, which promotes vasodilation and diuresis.
(Choices C and D) Because conditions that increase (eg, hypervolemia) or decrease (eg, hypovolemia) baroreceptor stretch typically have the same effect on atrial stretch, the degree of baroreceptor firing is usually directly related to the level of ANP release.
Educational objective:
Low blood pressure results in decreased arterial distension and reduced firing of aortic and carotid baroreceptors. This leads to an increase in sympathetic outflow and decrease in parasympathetic outflow, stimulating vasoconstriction and increased heart rate and contractility in order to help maintain blood pressure. Hypovolemia reduces atrial stretch, causing decreased atrial natriuretic peptide secretion.