SECTION III THE CARDIOVASCULAR SYSTEM Hypertension In the course of a routine physical exanination a 40-year-old man was found to have an arterial blood pressure of 175/95 mm Hg and a beart rate of 70 beats/min. The history.physical examination,and laboratory findings disclosed nothing significant other than the suggestion of slight left ventricular hypertrophy(slight increase in left ventricular mass). 1.If the patient's stroke rolune.cardiac output,and central venous pressure were all normal,what hemodynamic factor must account for the subsequent development of an elevated mean arterial pressure (Pa)?Pa is estimated to be 122 m Hg.as determined by the equation Pa Pd (Ps -Pd)/3,where Ps and Pd are the systolic and diastolic pressures.respectively. 2.If the patient's arterial compliance is mormal for his age,how do you explain that his arterial pulse pressure is abnormally large (80 mm Hg),in contrast to the average normal value of about 40 m Hg? ANSVER 1.The equation that defines total peripheral resistance (R)is R,"(P.-P.)/Q. Under steady state conditions.the peripheral runoff (Q)and cardiac output (Q) are equal.If we substitute for Q in the above equation.and solve that equation for P..we find: P■9，·RtP Thus mean arterial pressure depends on cardiac output,peripheral resistance, and central venous pressure.In this patient,however.cardiac output and central venous pressure were norml (and the latter is ordinarily very small compared to P.).Therefore by exclusion,the significant hemodynamic factor responsible for the
SECTION III THE CARDIOVASCULAR SYSTEM Hypertension In the course of a routine physical examination a 40-year-old man was found to have an arterial blood pressure of 175/95 mm Hg and a heart rate of 70 beats/min. The history, physical examination, and laboratory findings disclosed nothing significant other than the suggestion of slight left ventricular hypertrophy (slight increase in left ventricular mass). 1. If the patient's stroke volume, cardiac output, and central venous pressure were all normal, what hemodynamic factor must account for the subsequent development of an elevated mean arterial pressure (Pa)? Pa is estimated to be 122 mm Hg, as determined by the equation Pa = Pd + (Ps - Pd)/3, where Ps and Pd are the systolic and diastolic pressures, respectively. 2. If the patient's arterial compliance is normal for his age, how do you explain that his arterial pulse pressure is abnormally large (80 mm Hg), in contrast to the average normal value of about 40 mm Hg? ANSWER 1. The equation that defines total peripheral resistance (Rt) is Rt = (Pa – Pv)/Qr. Under steady state conditions, the peripheral runoff (Qr) and cardiac output (Qh) are equal. If we substitute Qh for Qr in the above equation, and solve that equation for Pa, we find: Pa = Qh *·Rt + Pv Thus mean arterial pressure depends on cardiac output, peripheral resistance, and central venous pressure. In this patient, however, cardiac output and central venous pressure were normal (and the latter is ordinarily very small compared to Pa). Therefore by exclusion, the significant hemodynamic factor responsible for the
patient's elevated mean arterial pressure is the high total peripheral resistance. Clinically most patients with essential hypertension,which is the most comon variety of hypertension,do have an elevated peripheral resistance and a norml or slightly reduced cardiac output. 2.The arterial pulse pressure reflects the relationship between the arterial distensibility and the disparity between the volune of blood ejected into the systenic arteries during the rapid ejection phase of systole and the volume that runs out of the arteries through the microcirculation during the same phase of the cardiac cycle.Ve can refer to this disparity between the input and output of blood during this brief time interval as the arterial "volume increment,"AV..The pressure increment"during a cardfac cycle is the arterfal pulse pressure,by definition.From the definition of arterial compliance (C.).the arterial pulse pressure (P.P)is related to the arterial compliance and the volume increment as follows: P.=Pa=△/C Thus P.P is directly proportional to the volume increment and inversely proportional to the arterial compliance.In general,AV.varies directly as the stroke volue.This patient had a normal stroke volune,and therefore we nay presune that AV.was also normal.Similarly,the distensibility of the patient's arterial systen was normal for her age.Presumably,a normal AV.imposed on a normal C.would result in a normal arterial pulse pressure.The key to understanding why the patient had a pulse pressure of 80 an Hg (twice the normal value of about 40 m Hg)is that the arterial compliance varfes as a function of the transmural pressure vithin the arteries.When we increase the pressure by adding fluid (gas or liquid)to a partially filled balloon.we find that the distensibility (compliance)of the balloon tends to decrease as we overdistend it.It takes a greater and greater increment of pressure to force a given additional volume into the balloon A similar pressure-volume relationship exists for the arteries.As the arteries are progressively distended
patient's elevated mean arterial pressure is the high total peripheral resistance. Clinically most patients with essential hypertension, which is the most common variety of hypertension, do have an elevated peripheral resistance and a normal or slightly reduced cardiac output. 2. The arterial pulse pressure reflects the relationship between the arterial distensibility and the disparity between the volume of blood ejected into the systemic arteries during the rapid ejection phase of systole and the volume that runs out of the arteries through the microcirculation during the same phase of the cardiac cycle. We can refer to this disparity between the input and output of blood during this brief time interval as the arterial "volume increment," ∆Va. The "pressure increment" during a cardiac cycle is the arterial pulse pressure, by definition. From the definition of arterial compliance (Ca), the arterial pulse pressure (Ps – Pd) is related to the arterial compliance and the volume increment as follows: Ps – Pd = ∆Va/Ca Thus Ps – Pd is directly proportional to the volume increment and inversely proportional to the arterial compliance. In general, ∆Va varies directly as the stroke volume. This patient had a normal stroke volume, and therefore we may presume that ∆Va was also normal. Similarly, the distensibility of the patient's arterial system was normal for her age. Presumably, a normal ∆Va imposed on a normal Ca would result in a normal arterial pulse pressure. The key to understanding why the patient had a pulse pressure of 80 mm Hg (twice the normal value of about 40 mm Hg) is that the arterial compliance varies as a function of the transmural pressure within the arteries. When we increase the pressure by adding fluid (gas or liquid) to a partially filled balloon, we find that the distensibility (compliance) of the balloon tends to decrease as we overdistend it. It takes a greater and greater increment of pressure to force a given additional volume into the balloon. A similar pressure-volume relationship exists for the arteries. As the arteries are progressively distended
with blood,they become progressively less compliant as the pressure rises abowe normal.This tendency is exaggerated with advancing age. Therefore in hypertensive patients,if the arterial compliance did not change with the arterial distending pressure and if the stroke volume (and presumably the volune increment)were normal,the pulse pressure would not be appreciably different regardless of whether the patient's arterial pressure was normal or elevated. However,if the arterial compliance dininishes substantially with distending pressure,then a given stroke volune (and volume increaent)would be associated with a much greater pulse pressure when the arterial blood pressure was elevated than when the arterial blood pressure was norral
with blood, they become progressively less compliant as the pressure rises above normal. This tendency is exaggerated with advancing age. Therefore in hypertensive patients, if the arterial compliance did not change with the arterial distending pressure and if the stroke volume (and presumably the volume increment) were normal, the pulse pressure would not be appreciably different regardless of whether the patient's arterial pressure was normal or elevated. However, if the arterial compliance diminishes substantially with distending pressure, then a given stroke volume (and volume increment) would be associated with a much greater pulse pressure when the arterial blood pressure was elevated than when the arterial blood pressure was normal