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In the event of end-stage heart failure, a left ventricular assist device (LVAD) can be used as a heart transplant bridge to keep a patient alive. The first iterations of LVADs were pulsatile and mimicked the physiological pumping action of the heart (Figure 1) .
Figure 1 A pulsatile-flow LVAD schematicA pulsatile-flow LVAD assists ventricular systole by mechanically pumping blood from a weakened left ventricle into the aorta through a pair of one-way valves. The pressure differential ΔP generated by the pump is related to cardiac output (CO) and vascular resistance (VR) :ΔP = CO × VREquation 1The efficiency and performance of the heart (or LVAD) can be determined by the patient's cardiac pressure-volume (PV) loop. A cardiac PV loop plots the pressure and volume of the blood in the left ventricle throughout a single cardiac cycle (Figure 2) .
Figure 2 Cardiac PV Loop of a pulsatile-flow LVADBlood pressure is often represented by only two numbers: arterial systolic and diastolic pressures. A more detailed representation of blood pressures is shown in a blood pressure profile, which graphs the blood pressure throughout the length of the different vessel groups (Figure 3) . Multiple pressure fluctuations are shown within a vessel because the pressure is traced across multiple cardiac cycles.
Figure 3 Blood pressure profile of the different vessels
-If the radius of the outflow tube is 1 cm and the LVAD pump generates a pressure of 45 mm Hg, what is the velocity of blood through the outflow tube if peripheral resistance is 1 mm Hg⋅s⋅mL⁻¹?

Question

Passage
In the event of end-stage heart failure, a left ventricular assist device (LVAD) can be used as a heart transplant bridge to keep a patient alive. The first iterations of LVADs were pulsatile and mimicked the physiological pumping action of the heart (Figure 1) .

Figure 1 A pulsatile-flow LVAD schematicA pulsatile-flow LVAD assists ventricular systole by mechanically pumping blood from a weakened left ventricle into the aorta through a pair of one-way valves. The pressure differential ΔP generated by the pump is related to cardiac output (CO) and vascular resistance (VR) :ΔP = CO × VREquation 1The efficiency and performance of the heart (or LVAD) can be determined by the patient's cardiac pressure-volume (PV) loop. A cardiac PV loop plots the pressure and volume of the blood in the left ventricle throughout a single cardiac cycle (Figure 2) .

Figure 2 Cardiac PV Loop of a pulsatile-flow LVADBlood pressure is often represented by only two numbers: arterial systolic and diastolic pressures. A more detailed representation of blood pressures is shown in a blood pressure profile, which graphs the blood pressure throughout the length of the different vessel groups (Figure 3) . Multiple pressure fluctuations are shown within a vessel because the pressure is traced across multiple cardiac cycles.

Figure 3 Blood pressure profile of the different vessels
-If the radius of the outflow tube is 1 cm and the LVAD pump generates a pressure of 45 mm Hg, what is the velocity of blood through the outflow tube if peripheral resistance is 1 mm Hg⋅s⋅mL⁻¹?

Quizplus · Accepted Answer

11ecba2d_12e8_6fb5_a3bf_c960731e4724_MD0008_00 The volumetric flow rate of blood through the outflow tube is determined by the heart (or LVAD) and is equal to cardiac output. From Equation 1, the pressure generated by the heart (ΔP) is the product of cardiac output (CO) and vascular resistance (VR): 11ecba2d_12e8_6fb6_a3bf_714b2f867c1e_MD0008_00 The CO of the LVAD is calculated using the given ΔP = 45 mm Hg and VR = 1 mm Hg⋅s⋅mL⁻¹, 11ecba2d_12e8_6fb7_a3bf_41ac88d18a8d_MD0008_00 11ecba2d_12e8_96c8_a3bf_5fe4121b7304_MD0008_00 The volumetric flow rate Q = CO = 45 mL/s. This value can be rewritten in base SI units (1 mL = 1 cm³) as Q = 45 cm³/s. Volumetric flow rate is the product of cross-sectional area A and flow velocity v: 11ecba2d_12e8_96c9_a3bf_cb7b8f847446_MD0008_00 Because the cross-section of a tube is a circle (A = πr²) with a radius r = 1 cm, the outflow tube's cross-sectional area A = π(1 cm)² = π cm². Flow velocity v is calculated using Q = 45 cm³/s and A = π cm²: 11ecba2d_12e8_96ca_a3bf_87c15033ee0a_MD0008_00 11ecba2d_12e8_96cb_a3bf_55abbe98e071_MD0008_00 The velocity of blood through the outflow tube is 45/π cm/s. (Choice B) Flow velocity is given in units of [length]/[time] whereas volumetric flow rate is given in units of [length]³/[time]. (Choice D) 45 cm³/s is the volumetric flow rate (not flow velocity) in the outflow tube. Educational objective: The volumetric flow rate of blood in the outflow tube is equal to cardiac output (CO), which is the pressure (ΔP) generated by the heart divided by peripheral resistance (VR): Q = CO = ΔP/VR. Flow velocity is determined from volumetric flow rate, which is the product of flow velocity and cross-sectional area: Q = Av.

Passage In the Event of End-Stage Heart Failure, a Left Ventricular