The auscultatory method is a more sensitive and therefore more precise technique for measuring systolic pressure, and it also enables diastolic pressure to be estimated. The practitioner listens with a stethoscope applied to the skin of the antecubital space over the brachial artery. While the pressure in the cuff exceeds systolic pressure, the brachial artery is occluded, and no sounds are heard (see
Fig.
17.20B ). When the inflation pressure falls just below the systolic level (120 mm Hg in
Fig. 17.20A ), a small spurt of blood escapes the occluding pressure of the cuff, and slight tapping sounds (called Korotkoff sounds) are heard with each heartbeat. The pressure at which the first sound is detected •Fig. 17.19 Pulse Pressure Curves Recorded From Various Sites in the Arterial Trees of Humans at Different Ages. Inthe24-yearold,thearterialpulsedisplaysstrikingchangesinthepulsepressureamplitudeandcontourasitpassesdownthearterialtree.Thepulsepressurewaveinthe68-year-oldshowslittleamplificationandisrelativelyunchangedasthepulsetravelsbecausethereislesswavereflection.(ReproducedbypermissionofHodderEducationfromNicholsWW,O’RourkeM,eds.McDonald’s Blood Flow in Arteries: Theoretical, Experimental and Clinical Principles. 5thed.London:Arnold;2005.) represents systolic pressure. It usually corresponds closely to the directly measured systolic pressure. As the inflation pressure of the cuff continues to fall, more blood escapes under the cuff per beat and the sounds become louder. When the inflation pressure approaches the diastolic level, the Korotkoff sounds become muffled. When the inflation pressure falls just below the diastolic level (80 mm Hg in
Fig. 17.20A ), the sounds disappear; the pressure reading at this point indicates diastolic pressure. The origin of the Korotkoff sounds is related to the discontinuous spurts of blood that pass under the cuff and meet a static column of blood beyond the cuff; the impact and turbulence generate audible vibrations. Once the inflation pressure is less than diastolic pressure, flow is continuous in the brachial artery, and sounds are no longer heard (see
Fig. 17.20C
The Venous System
Veins are elements of the circulatory system that return blood to the heart from tissues. Moreover, veins constitute a very large reservoir that contains up to 70% of the blood in the circulation. The reservoir function of veins makes them able to adjust the volume of blood returning to the heart,
Theankle-brachialindex(ABI)istheratioofsystolicbloodpressuresattheankle(dorsalispedisartery)tothatinthebrachialartery.TheABI,whichisobtainedbysimplemeasurements,isanindicatorofpossibleperipheralarterydisease.TheABIhasalsobeenproposedasapredictorofriskforcardiovascularandcerebrovasculardisease.PeoplewithanormalABIratioof1.1to1.4havealowerincidenceofeithercoronaryorcerebrovasculareventsthandothosewitharatioof0.9orlower.Inaddition,astherateofABIincreaseswithtime,theincidencesofcardiovascularmorbidityandmortalityalsoincrease.
or preload, so that the needs of the body can be matched when cardiac output is altered (see
Chapter 19). This high capacitance is an important property of veins.