MEDICAL PHYSIOLOGY Cardiovascular Conference #2 Quiz 4A October 19, 2001 1. 1. Provide a diagram or flow chart showing why heart rate changes during inspiration in a normal individual. Inspiration ==> Decreased intrathoracic pressure ==> Increased pooling of blood in pulmonary circulation, decreased pulmonary venous return ==> Decreased left ventricular stroke volume/cardiac output ==> Decreased Mean Arterial Pressure/Pulse Pressure ==> Decreased Arterial Baroreceptor nerve activity ==> Decreased Nucleus Tractus Solitarius Impulse activity ==> This leads to 2 branches as follows: Branch 1: Increased C1 activity ==> Increased cardiac sympathetic tone ==> Increased Heart Rate Branch 2: Decreased DMV/Nucleus Ambiguus activity ==> Decreased cardiac parasympathetic tone ==> Increased Heart Rate 2. What effect will atropine have on resting heart rate of a normal individual and why? Assuming there is normal sympathetic tone, atropine will increase heart rate. This is because atropine blocks cholinergic muscarinic receptors in the heart. Normally, the parasympathetic vagus nerve releases acetylcholine that activates these receptors and slows the heart. Blocking the effect of cholinergic muscarinic receptor activation removes the parasympathetic inhibition, causing heart rate to increase under the influence of the excitatory sympathetic nerves. 3. Provide a diagram showing why a phenylephrine injection that raises arterial pressure by 20 mm Hg will change heart rate in a normal individual? View diagram Phenylephrine ==> Alpha-1 mediated vascular smooth muscle contraction ==> IThis leads to 2 branches as follows: Branch 1: Arteriolar vasoconstriction ==> Increased TPR ==> Increased blood pressure ==> Increased baroreceptor nerve firing ==> Increased NTS activity Branch 2: Venoconstriction ==> Increased venous return, cardiac output ==> Increased blood pressure ==> Increased baroreceptor nerve firing ==> Increased NTS activity 4. How does ADH help maintain blood pressure in the absence of a functional sympathetic nervous system? ADH helps to maintain blood pressure in the absence of a functional sympathetic nervous system in two ways. In the long-term, by decreasing urine production, it causes volume expansion, which leads to increased right atrial/central venous pressure, which increases ventricular filling and increases stroke volume, and therefore cardiac output. However, in the short-term, decreased cardiopulmonary baroreceptor firing leads to diminished NTS inhibition of the A1 neurons, and therefore more ADH release, which increases TPR by eliciting arteriolar smooth muscle contraction, and venous return by venous smooth muscle contraction. 5. A patient with autonomic failure shows a smaller decrease in blood pressure when actively getting out of bed than when being passively tilted. Why? When an individual actively assumes an upright position, they rely on contraction of muscles in the trunk and extremities. When skeletal muscles contract, the pressure inside the veins within the muscles and between muscle and bone increases as the veins are compressed. This facilitates venous return to the heart and therefore increases cardiac output. MEDICAL PHYSIOLOGY Cardiovascular Conference #2 Quiz 4B October 22, 2001 1. Provide a diagram or flow chart showing why heart rate changes during expiration in a normal individual. Expiration ==> Increased intrathoracic pressure ==> Decreased pooling of blood in pulmonary circulation, increased pulmonary venous return ==> Increased left ventricular stroke volume/cardiac output ==> Increased Mean Arterial Pressure/Pulse Pressure ==> Increased Arterial Baroreceptor nerve activity ==> Increased Nucleus Tractus Solitarius Impulse activity ==> This leads to 2 branches as follows: Branch 1: Decreased C1 activity ==> Decreased cardiac sympathetic tone ==> Decreased Heart Rate Branch 2: Increased DMV/Nucleus Ambiguus activity ==> Increased cardiac parasympathetic tone ==> Decreased Heart Rate 2. What effect will propranolol have on resting heart rate in a normal individual and why? Proporanolol is a beta (ß) adrenergic receptor blocker, and prevents norepinephrine and epinephrine from acting on cardiac beta receptors to accelerate heart rate. Assuming the individual has normal parasympathetic cardiac tone, propranolol will eliminate the excitatory effects of sympathetic activation, thereby slowing heart rate. 3. Why will the increase in systemic blood pressure following injection of 1.0 mg phenylephrine be greater in the conference patient who has multiple systems atrophy with autonomic failure than in a normal person, and why? The response will be greater for 2 reasons. The patient is likely to show denervation supersensitivity, which means that the vascular smooth muscle alpha receptors will be more strongly activated by a given dose of an alpha adrenoceptor agonist. Further, the patient lacks a functional parasympathetic nervous system so the baroreflex is unable to slow the heart rate (thereby prolonging run-off time), thus decreasing blood pressure. 4. How does ADH help maintain blood pressure in the absence of a functional sympathetic nervous system? ADH helps to maintain blood pressure in the absence of a functional sympathetic nervous system in two ways. In the long-term, by decreasing urine production, it causes volume expansion, which leads to increased right atrial/central venous pressure, which increases ventricular filling and increases stroke volume, and therefore cardiac output. However, in the short-term, decreased cardiopulmonary baroreceptor firing leads to diminished NTS inhibition of the A1 neurons, and therefore more ADH release, which increases TPR by eliciting arteriolar smooth muscle contraction, and venous return by venous smooth muscle contraction. 5. Diagram the steps involved in maintaining blood pressure during rapid tilting in a normal person. View diagram Tilt to upright position ==> Venous pooling, decreased cardiac output ==> Decreased baroreceptor nerve activity ==> Decreased Nucleus Tractus Solitarius activity ==> This leads to 2 branches as follows: Branch 1: Less inhibition of C1 ==> Increased sympathetic tone Branch 2: Less stimulation of DMV ==> Decreased cardiac parasympathetic tone