Chapter_14_Blood_Pressure_and_Hemodynamics__1_slide_per_page

Chapter_14_Blood_Pressure_and_Hemodynamics__1_slide_per_page...

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Chapter 14 Blood Pressure and Hemodynamics BME 501 T. K. Hsiai, MD, PhD, FACC
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Blood pressure Blood pressure is measured in terms of the column of mercury (in millimeters) that could be supported by the pressure inside the arteries at two times: maximum thrust by the heart, and when the heart is relaxed. These are normally about 120 mm and 80 mm, respectively. Recent medical guidelines suggest that the familiar old “normal” 120/80 values are too high, and that somewhat lower values are desired!
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Arterial BP
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Blood Pressure Measurement
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Schematic diagram of arterial blood pressure measurement by Korotkoff sounds After the cuff is placed snugly over the arm, the radial artery is palpated while the pressure is increased until the pulse can no longer be felt, then 30 mm Hg. more. As the pressure is released the artery is palpated until the pulse is felt again. This palpatory method will detect systolic pressure only. The auscultatory method detects diastolic as well as systolic pressure. The sound heard when a stethoscope bell (or diaphragm) is applied to the region below the cuff were described by Korotkoff in 1905 and are called Korotkoff’s sounds.
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Elastic Tissue Elastic recoil=PE + KE Æ conversion from potential energy into kinetic energy during diastole
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W = PdV t 1 t 2 Objective 1: How is the pulsatile blood flow in the larger arteries converted into a steady flow in the capillary? At steady flow, the hydraulic work, W, is defined as the change in volume times the pressure over time. The work done in pumping the fluid for 1 s would be 10,000mmHg . ml/s(V=100 ml) Given Q=100ml/s through a Resistance (R)=1 mmHg/ml/s Æ then a constant P=Q x R = 100mmHg A: Continuous flow pump Q=100 ml/s Q=100ml/s W = PxV = 100X100 = 10,000 mmHg . ml/s R=1mmHg/ml/s P=100mmHg dP Q time time Rigid tube
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B: Intermittent pump Pump flow: P= downstroke: RxQ =1x200 = 200mmHg upstroke=R x Q =1x 0 = 0 R=1 mmHg/ml/s Outflow: 200ml/s for 0.5 s 0 ml/s for 0.5 s W=PxV=200 x100=20,000 mmHg . ml/s dP Q time time 200mmHg 0 200ml/s 0 The pump is intermittent: steady for half cycle and ceases for the remainder of the cycle. The conduit is rigid, and therefore the flow produced by the pump during its downstroke must exit through the resistance during the same 0.5 second that elapses during the downstroke. The pump must do twice as much work as the pump in continuous flow (A).
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P=constant at 100mmHg Compressible tube R=1 mmHg/ml/s Outflow=100ml/s dP Q time time Pumped flow: downstroke: RxQ=1X200= 200ml/s for 0.5 s upstroke: RxQ=1x0= 0 for 0.5 s W=P x V=100X100= 10,000mmHg . ml/s The presence of compressible tube results in perfect filtering of the pressure; ie, the pressure is steady, and the outflow through the resistance is also steady. The work equals that in continuous pump (A).
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This note was uploaded on 02/27/2008 for the course BME 501 taught by Professor Yamashiro,hsiai during the Fall '07 term at USC.

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