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The unit for measurement of blood pressure is mostoften mm Hg (1 mm Hg = 1 torr), and the pressure gradient(ΔP) for flow is arterial pressure minus venous pressure.Resistance is the impedance to flow and can be measured inunits of mm Hg/mL/min. Resistance can be quantified as thepressure rise associated with an incremental rise in flow.
Thegreatest resistance in the circulation occurs in the smallestarteries and arterioles (see Fig. 8.1). Factors that determineresistance are considered later in the chapter (see “Biophysicsof Circulation”).BLOOD PRESSUREThe blood pressures in the venous systems of both pulmonaryand systemic circulations are considerably lower than pressures in the respective arterial systems. In addition, bloodpressures in the pulmonary circulation are lower than corresponding pressures in the systemic circulations.
Systemicarterial pressure at rest is normally about 120/80 mm Hg (systolic/diastolic), compared with 25/10 mm Hg for pulmonaryartery pressure. Some important definitions related to arterialpressure are as follows:■■Systolic arterial pressure: Peak arterial pressure reachedduring ejection of blood by the heart.Diastolic arterial pressure: Lowest arterial pressurereached during diastole, while the heart is relaxed andfilling (not ejecting blood).Flow, pressure, and resistance can be related in theformulaQ = ΔP/Rwhere Q is flow, ΔP is the pressure gradient, and R is resistanceto flow.Rearranging this formula,ΔP = QRThis equation is analogous to Ohm’s law, which states that V =IR, where V is electrical potential (electrical gradient), I iscurrent (electrical flow), and R is impedance (electricalresistance).■■Arterial pulse pressure: The difference between systolicand diastolic pressures; dependent on stroke volume(volume ejected by one ventricle during one contraction) and arterial compliance.Mean arterial pressure (MAP): The average pressureover a complete cardiac cycle of systole and diastole;dependent on peripheral resistance and cardiac output(volume ejected by one ventricle per unit time).Mean arterial pressure is not the simple arithmetic mean ofsystolic and diastolic pressures, because of the irregular shapeof the arterial pressure curve (Fig.
10.1). MAP can be approximated by adding one third of the pulse pressure to the diastolic pressure.Physiological pressures are usually given in units of mmHg or cm H2O. In other words, 1 mm Hg is the pressurethat would support a column of mercury (Hg) at a height of 1millimeter (mm), and 1 cm H2O is the pressure that wouldsupport a column of water 1 centimeter (cm) high. Another wayto conceptualize these units is that a column of water 1 cm highwould exert 1 cm H2O pressure at its base, and a column ofmercury 1 mm high would exert 1 mm Hg pressure at its base.Although these may appear to be unconventional units forquantifying pressure, they are useful because water or mercurymanometers are often used to measure pressure.
In some texts,the unit torr is used; it is equal to 1 mm Hg.108Cardiovascular PhysiologySystolic pressureDichrotic notchStroke volume120Cardiac outputPulse pressureMean arterialpressure (MAP)Arterialcompliance80PeripheralresistanceMAP = Pdiast. +PressureDiastolic pressure( Psyst. ⫺ Pdiast. )3TimeFigure 10.1 The Arterial Pressure Wave The arterial pressure wave represents the changes in pressure in the arterial system over periodsof systole, during which the stroke volume is ejected from the left ventricle, and diastole, during which the heart is refilling and blood in the arterialsystem continues to flow downstream.
Arterial pressure is affected by cardiac output, stroke volume, arterial compliance, and peripheral resistance.Mean arterial pressure can be approximated based on the formula shown. The first upward deflection in the curve marks the beginning of systole,the period of cardiac ejection. The transient irregularity in the downward slope of the wave is known as the dicrotic notch, which is produced byclosure of the aortic valve, marking the beginning of diastole. During diastole, the heart refills, while blood in the arterial system runs downstream,reducing arterial pressure.Pressure waves vary through the cardiovascular system(Fig. 10.2). Both high and low pressures are encountered inthe ventricles (approximately 120/0 and 25/0 mm Hg at restfor left and right ventricles, respectively).
High systolic pressure is necessary for pumping of blood through the circulation, whereas the low pressures are required for return ofblood to the heart during diastole. Note that while mean pressures in the large arteries are somewhat lower than in theaorta, pressure pulsations are greater in larger arteries.
Thisphenomenon is mainly attributable to two factors: changes inpressure travel more rapidly downstream than the actualblood flow, which accentuates the pulsatility downstream,and pressure changes are reflected back at branch points,again accentuating downstream pulsatility.BIOPHYSICS OF CIRCULATIONBlood flow through vessels is a complex phenomenon, involving a nonhomogenous fluid flowing in a pulsatile mannerthrough distensible, branching tubes of various dimensions.Under most conditions, this flow can be described byPoiseuille’s law:Q⫽⌬Pr48Lwhere Q is flow, ΔP is the pressure gradient from one end of atube to the other, r4 is the radius of the tube to the fourth power,η is the viscosity of the fluid, and L is the length of the tube.
Theeffects of this relationship are illustrated in Figure 10.3.Based on Poiseuille’s law, flow (Q) through a tube will be:■■■■Directly proportional to the longitudinal pressure gradient (inflow pressure minus outflow pressure).Inversely proportional to the length of the tube.Inversely proportional to the viscosity of the fluid.Directly proportional to the fourth power of the radiusof the tube.For example, if the radius of a tube is doubled, flow will beincreased by a factor of 16, assuming the pressure gradient ismaintained. Physiological regulation of regional blood flow ona moment-to-moment basis mainly involves changes in radius(vasodilation and vasoconstriction) of the small arteries andarterioles, taking advantage of this powerful factor.
Undernormal circumstances, viscosity of blood is not an issue; however,changes in hematocrit are associated with large changes in bloodviscosity, as occur in anemia and polycythemia.Because Q = ΔP/R, resistance can be described as:R⫽8Lr4Of the factors affecting flow through a tube, the mostimportant is the radius of the tube. While flow is inverselyproportional to the length of the tube and the viscosity of thefluid, and directly proportional to the hydrostatic pressure gradient in the tube, it is directly proportional to the fourth powerof the radius of the tube.
Therefore, doubling the radius of thetube will cause a 16-fold increase in flow, if other factors areconstant.Flow, Pressure, and ResistanceCLINICAL CORRELATEMeasurements of Blood PressureArterial pressure is routinely measured by sphygmomanometry,in which a blood pressure cuff is inflated above the systolic pressure, compressing vessels and stopping blood flow. As pressure inthe cuff is gradually released, the practitioner listens for the soundsof Korotkoff through a stethoscope.
These sounds are producedby the pulsatile flow of blood in arteries under the cuff when thecuff pressure is between the systolic and arterial pressure. Thus,the sounds are first heard when cuff pressure falls below systolicpressure, and the sounds disappear when the cuff pressure fallsbelow diastolic pressure. Arterial pressure can also be measureddirectly through an arterial catheter.
Such a catheter can be passedin a retrograde direction (against the flow of blood) for measurement of pressures in the arteries, aorta, and left ventricle, as thecatheter advances toward the heart.Although arterial pressure monitoring is important for detectionof hypertension, monitoring of additional pressures is often highlydesirable in surgical and intensive care settings. Pulmonary capillary wedge pressure (“wedge pressure”) is one such measurement.A venous catheter can be passed antegrade (in the direction ofblood flow) for measurement of pressures in the veins, rightatrium, and right ventricle.
However, it is not possible to measurepulmonary venous and left atrial pressures by these direct methods.Instead, we must rely on measurement of pulmonary capillarywedge pressure.With the aid of a partially inflated balloon, a catheter is passedfrom a systemic vein, through the right heart, into the pulmonaryartery (PA), to a branch of the pulmonary artery. The lumenopens at the distal end of the catheter and is continuous; theballoon is on the outside of the catheter only.
When the cathetercan be advanced no further, the balloon is fully inflated (the catheter is actually a multilumen Swan–Ganz catheter—one lumen iscontinuous to the end of the catheter; another leads to the balloon,allowing inflation; other lumens may open along the length ofthe catheter). Inflating the balloon has no effect on patency of thecatheter lumen through which pressure is measured. When theballoon is fully inflated, pressure falls downstream beyond thatpoint. Vascular pressure beyond the point of occlusion equilibrates with downstream pressure, and wedge pressure (measuredat the catheter tip) is thus an indicator of pulmonary venous andleft atrial pressure.
It also approximates left ventricular end-diastolic pressure (LVEDP), when LV pressure has equilibrated withthe pressure in the left atrium and pulmonary veins with filling ofthe ventricle. Wedge pressure is useful in hemodynamic assessment (e.g., in acute heart failure).Catheter tip detailAscending aortaPulmonary trunkInflated balloonPulmonary artery branchDistal lumenSuperior vena cavaLeft atriumPulmonic valveMitral valveRight atriumDistal lumen hubBalloon inflation valveLeft ventricleInferiorvenacavaBalloon inflation hubThermistorconnector109Proximal lumen hubTricuspid valveRight ventriclePulmonary Artery Catheterization Pulmonary artery catheters (Swan–Ganz catheters) are multilumencatheters that can be advanced into a branch of the pulmonary artery. With the tip thus wedged with theballoon inflated, flow is occluded and pressure can be measured beyond the balloon occlusion through oneof the lumens.
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