Circulation. report good agreement between blood pressures. determined by the direct intra-arterial and the

Transcription

1 Crculaton A Journal of the Amercan -Heart Assocaton OCTOBER 1954 VOL. X NO. 4 Downloaded from by on November 27, 2018 Comparson of Indrect and Drect Methods of Measurng Arteral Blood Pressure D. STUART WEATIIERIJEAD, M.D., ALAN E. TRELOAR, PH.D., ALLAN B. DOBKIN, M.D., AND JosEPhI J. BUCKLEY, MN.D. Drect and ndrect determnatons of blood pressure have been recorded n 70 human subjects. Statstcal analyss of the results s presented. frequent dscrepancy between drect and ndrect readngs s evdent wth the drft of the A latter fallng ncreasngly below the drect measurement as blood pressure rses. The greatest dscrepancy s found n the young hypertensve subject and the possble clncal mplcatons of ths fndng are dscussed. An attempt s made to explan some of the factors contrbutng to the varable error by whch auscultatory readngs underestmate the true ntraarteral pressure. Ru FREDERICK H. VAN BERGEN. M.D.. IN the course of recordng arteral pressures of patents subjected to hypotensve anesthesa, t was observed that blood pressures oftell were unobtanable by any of the ndrect methods, whle at the same tme ntra-arteral manometry demonstrated systolc levels of 60 to 70 mm. Hg. One frequently s mpressed by the uncertanty, f not nadequacy, of blood pressure determnatons upon patents n surgcal shock when ndrect methods alone are employed. Clncans, usng the three ndrect methods under customary condtons, have observed dsturbng dscrepances among the results obtaned. The usual practce of employng one technc only does not permt such revelatons. False confdence ll absolute values thus arses all too easly. Conflctng opnons exst concernng the valdty of auscultatory pressures.' Several observers2-4 n studes of the naccuracy of From the Dvson of Anesthesology, and the Dvson of Bostatstcs, Unversty of Mnnesota, Mnneapols, Mnn. These nvestgatons were supported by grants from the Research Dvson of the El Llly Laboratores and the Commttee on Fellowshps and Grants of the Squbb Insttute. 481 ndrect determnatons have enumerated factors contrbutng to the errors. Others5-8 report good agreement between blood pressures determned by the drect ntra-arteral and the auscultatory methods. Cognzant of the confuson exstng nl ths basc physologc measurement, we undertook the followng nvestgaton to comple data seekng to defne the relatve accuracy of the three ndrect methods over a wde blood pressure range, and to dentfy some of the factors contrbutng to the current engma. METHODS Indrect Measurement of Blood Pressure. Two to four traned observers partcpated sequentally n determnng auscultatory systolc and dastolc levels, and oscllometrc and palpatory systolc levels, on each patent, usng a mercury manometer. Care was taken to nsure that these observers dd not see the ntra-arteral pressure recordngs. Another assstant entered each observer's ndrect blood pressure fndngs on the tracngs at the approprate ponts. In the dletermnaton of auscultatory systolc and dastolc pressure, the standards recommended by Bordlev and hs co-workers8 were followed. Oscllometrc systolc pressure was recorded as the pont on the manometer scale at whch the menscus of the pulsatng mercury column became convex. Palpatory systolc pressures were determned by the percepton of a pulse n the radal artery. I)astolc Crculaton, I'olume -, October, 1954

2 482 MEASUREMENT OF ARTERIAL BLOOD PRESSURE FIG. 1. Stran gage-needle system employed n recordng brachal artery pressures. Stopcock connects system to pressure bottle for square wave producton and to a syrnge contanng 2.5 per cent sodum ctrate for "flushng out" the system. levels were not recorded by ether the oscllometrc or the palpatory methods. Drect Measurement of Blood Pressure. The ntraarteral pressure was measured drectly wth hgh accuracy by means of a standard resstance wre pressure transducer (model P23A Statham stran gage) and regstered by a drect-wrtng electronc recorder (Sanborn Polyvso). A 15 gage needle, connected drectly to the face of the transducer by a double male adapter, was nserted nto the brachal artery drectly under the lower edge of the blood pressure cuff (fg. 1). Ths entre system was demonstrated to have a 100 per cent frequency response, flat to 17.5 cycles per second. There was a 105 per cent response at 23 cycles per second (fg. 2). The system was calbrated statcally aganst a mercury manometer before and after each recordng. Square waves of 200 mm. Hg were passed before, durng, and after each recordng to detect dampng due to clottng or malposton of the needle. Readng the Tracngs. The Sanborn Polyvso has a recordng accuracy of ±0.5 mm. deflecton wthn the nner 4 cm. of the recordng paper. Ths ncreases an addtonal -0.5 mm. deflecton error n the remanng 0.5 cm. at each margn of the paper. Wth the attenuaton used to cover the pressure ranges nvolved, the over-all readng accuracy was judged to be wthn ranges of ±2.5 mm. Hg n the center 4 cm. of the paper and wthn ±5 mm. Hg n the margnal 0.5 cm. In quantfyng the tracngs an average of the ndvdual systolc and dastolc levels for a perod of 15 seconds mmedately before begnnng nflaton of the cuff was used. Ths was deemed desrable so that effects of varatons n blood pressure produced by respraton rregular cardac rhythm and such other mnor factors mght be mnmzed. Tracngs mmedately followng deflaton of the cuff were not used. SUBJECTS The subjects n the present study conssted of 70 surgery patents rangng n age from 2.5 to 82 years (average of 52 years). In every nstance the blood pressure comparsons were made n the mmedate postoperatve perod whle the subjects stll were partally under the effects of general anesthesa. All measurements were made wth the patents n a horzontal supne poston. The subjects used for these observatons and the condtons prevalng were not normal. It may be nferred safely from the average age (52 years) that many of the patents were ether mld or severe hypertensves. Many of the patents were afflcted wth some form of cardovascular dsease other than hypertenson. The postoperatve and postanesthetc states are often accompaned by marked alteratons n stroke volume output, blood volume, total perpheral resstance, pulse pressure and heart rate. Nevertheless, these subjects and these condtons represent a cross secton of the surgcal patents who requre careful and accurate montorng of ther blood l)ressures durng the operatve and I:ostoperatve perods. RESULTS Crcumstances delmted both the perod of observaton and number of avalable observers for these blood pressure determnatons on each patent. Wth 5 of the 70 subjects, *4I 130 ISO 110 C00 9C s8 7( SC SC 4C t ta, B 4 " FIG. 2. Graph llustratng the frequency response curves of the two recordng systems employed n ths study. S ld lne represents the stran gage-15 gage needle system; broken lne represents the stran per cent re- gage-catheter system. Ordnate lne n sponse. Abscssae lne n cycles per second. T...~~~~~_

3 VAN BERGEN, ET AL ! ^o ^ '/ Downloaded from by on November 27, 2018 o DIRECT READING (MM O FIG. 3. Relatonshp of auscultatory readng to drect determnaton of systolc blood pressure. only one drect readng was secured, followed mmedately by one each of the ndrect measurements n four cases and only the auscultatory measurements n the other. For the remanng 65 subjects, 2 to 14 drect measurements were made, wth varyng numbers of ndrect determnatons. In studyng the relatonshp between the drect measurement and each of the ndrect determnatons of blool pressure, we have avalable 180 values for auscultatory systolc pressure, 133 auscultatory dastolc, 166 oscllometrc systolc, and 132 palpatory systolc. Each ndrect readng has ts assocated drect readng taken closely f not mmedately n advance of t. Thus there are from 132 to 180 pars of values defnng the relatonshps, each of the 70 subjects contrbutng from 1 to 7 ponts for the 3 correlaton surfaces. The possblty of dsturbng effects arsng from the unequal representaton of the 70 subjects led us to select one par of values at random from each subject for each of these three assocatons, usng a table of random numbers. Thus we establshed a sample of pared values for each assocaton n whch there was no possblty of dsturbance arsng from consderng patents unequally. The basc data need not be reproduced here n tabular form. Graphc methods wll be adequate. Fgures 3, 4 and 5 present the correlaton scatters for the three relatonshps of ndrect determnaton of blood pressure to OIRECT READING tum HG) FIG. 4. Relatonshp of oscllometrc readng to drect determnaton of systolc blood pressure. the drect method of measurement. Each sold dot or open crcle defnes a par of assocated values for some patent; the correspondng drect and ndrect blood pressure values may be ascertaned by tracng these ponts to the horzontal and vertcal scales, respectvely. The nscrbed thcker straght lnes defne the average relatonshp of the ndrect readng of blood pressure to the drect measure. Snce the latter has neglgble error n comparson wth the ndrect determnaton, t s approprate to consder only ths lne for the average relatonshp when errors alone reduce the correlaton below perfecton. The full lne s ftted to all avalable pars of values (dots plus crcles). The broken lne s ftted only to the I!! l ALL OBSERVATIONS»»0«0 1o RANDOM SAMPLE LINE OF EDUAL VALUES DL **6- -- f 8OC 8 - -,uu DIRECT REAODNG <Mt. H«.) / -^ ~~~~~~~~~~~~~~~~~rr FIG. 5. Relatonshp of palpatory readng to drect determnaton of systolc blood pressure. f.d '~ 4gv

4 484 44MEASUREMENT OF ARTERIAL BLOOD PRESSURE random sample of 1 pont per patent (defned by the crcles). These heavy lnes are the statstcal regresson lnes. The lght dagonal lne s that of equal values on the two scales; the dstrbuton of the pont scatter n relaton to ths lne defnes the nature of the errors of the ndrect method relatve to the drect method over the range of values covered. Certan features common to all three relatonshps are outstandng. Frst, over the entre range of observatons, ndrect readngs of blood pressure fall below the drect measurement made as nearly as possble at the same tme. There was only one case of equalty n 611 pared observatons, and relatvely few approaches to equalty. Second, the drft of ndrect readngs s to fall ncreasngly below the drect measurement as blood pressure ncreases. Thrd, no serous dsturbance s ntroduced by consderng all values (as opposed to one per patent for the random sample) n arrvng at these average relatonshps. We therefore have chosen to use the total sample lnes n further dscusson. The three regressons of ndrect on drect readngs (usng all observatons) are supermposed n fgure 6. It wll be observed that the oscllometrc method makes the nearest approach on the average to equalty wth the drect readng. The palpatory method falls furthest from the equalty lne n these data, wth auscultatory results holdng the ntermedate poston. It would be unwse to conclude that these rather small dvergences of j le,i. srrscucmrorr svstoce -1 1I /I / V OSCILLOHEIRIC SYSTOLIC P.LPIPTORI SYSTOLlC I I I_X / I IEbO 40!!, t 00 nuscultnronu MISTOLIC I I I / LINE OC EOUIL V.LUtS / FIG. 6. determnaton of blood pressure. r r Regressons of ndrect readngs on drect r ZZO z.o! j / ^~~~ /. f/ 7o SO ^-'^^ ^ ----' :--' ANDooM sample o~lv) ' ' ^^ ~~~~~~~~~~LINE 0f EOUAL VALUES 0--- A OBSERVATIONS (000) DIRECT READING (MM HG) FIG. 7. Relatonshp of auscultatory readng to drect determnaton of dastolc blood pressure. the three regressons among themselves would persst f more extensve data were avalable. However, there would be no queston about ther departure n general (collectvely) from the equalty lne. Fgure 6 also ncludes the regresson of auscultatory dastolc readng. The full scatter of ponts, together wth the regresson and equvalence lnes for the auscultatory dastolc data, s gven n fgure 7. It s nterestng to note that the agreement of ndrect and drect readngs s even less satsfactory for dastolc pressures than for systolc pressures. The absolute errors of the ndrect readngs on the average are of the same general order of magntude as for the auscultatory systolc pressure at each end of the lne. Snce the general level of dastolc pressures s much less than for systolc pressures, the errors become much more mportant n the relatve sense. It s of some nterest to consder at ths pont the nter-relatonshps of the ndrect readngs, one to the other, for systolc pressure. The charts are reproduced as fgures 8, 9 and 10. Snce both varables are now subject to error, and one may presume more or less equally so, there s less justfcaton for consderng only one of the two regresson lnes whch may be drawn for each surface. However, the amount of data avalable to us for ths study s lmted to not more than 47 ponts. We shall omt, therefore, regresson analyss and be content to study the scatters alone.

5 VAN BERGEN, ET AL. 485 Downloaded from by on November 27, AUSCUILTARY READING tl.w4.) FIG. 8. Correlaton between auscultatory and oscllometrc readngs of systolc blood pressure. // ~ 90 PAI.PATORY READING (MH~ FIG. 9. Correlaton between auscultatory and palpatory readngs of systolc blood pressure. comparatvely It wll be noted that there s good agreement between these ndrect readngs. As before, the thn dagonal lne traces equal values by both methods. It wll also be noted that the scatters of ponts are n closer agreement wth ths lne than n fgures 3, 4 and 5. Ths would be antcpated from the proxmty of the regresson lnes n fgure 6. Because of the restrcted number of pared values we would not care to suggest that ndcatons of somewhat closer agreement n one par than another are sutable bases for clams that such better agreement does exst n general. However, t s notceable that these readngs agree wth one another as ndvdual pars more I' / I1 50 7o PALPATORY READING 1CMM H FIG. 10. Correlaton between oscllometrc and palpatory readngs of systolc blood pressure. closely, n general, than each ndrect readng agrees wth ts drect assocate. That s, the ntensty of correlaton n fgures 8, 9 and 10 s greater than n fgures 3, 4, and 5. Ths may arse from crcumstances; the drect readng s unknown to an observer, whereas hs ndrect readngs cannot very well be dsmssed from mnd mmedately as each s made. The Relatonshp of Body Buld, Age and Pulse Pressure to A uscultatory-drect Blood Pressure Measurement 1. Body Buld. The patents were dvded nto three weght groups (48 slm, 88 medum and 39 heavy) to study whether there s any -.-- o«t~~~~~~~~~~~~ 2 oo, ^"Z~~~~~~~~~~I. -- -^. UN OF EQUAL VALUES /^* / > '200 _\0 SO _I O_ ISO fo DIRECT READING (mnm Ml) FIG. 11. Weght as a factor n the correlaton of ndrect and drect systolc blood pressure readngs..-

6 486 46MEASUREMENT nfluence of body buld on the accuracy of systolc auscultatory determnatons. Fgure 11 presents the full scatters and regressons of auscultatory pressure on drect pressure for the three weght groups. It s nterestng to note that the three regressons form a systematc array of lnes ntersectng at essentally the same value of approxmately 150 mm. Hg drect systolc pressure. Varaton n blood pressure s very wde for both the heavy and medum buld patents, and relatvely restrcted only for the thn persons. But the average dvergence of auscultatory readng from drect determnaton s more varable for the thn than for the heavy, rangng from 0 to 45 mm. Hg for the former and 20 to 40 mm. Hg for the latter. 2. Age. The results of regroupng to study ths relatonshp are gven ll fgure 12. Snce weght tends to ncrease wth age, t s not surprsng that there s some smlarty between fgures 11 and 12. Unfortunately our data are too scant to allow of age analyss wthn weght groups. It s evdent that the two older age groups could well be combned to present a sharper contrast wth the younger group. Snce the regresson lne of the young age group slopes rapdly away from the lne of equvalence, one s challenged by the mplcaton that serous errors exst n determnng auscultatory pressures of young hypertensve patents. 3. Pulse Pressure. Auscultatory systolc TO 45 YEARS Ig 45 TO 65 YEA\RS \ 65 TO 82 YtARS / -,,! : LINE OF EQUAL \«LUES '.^Yr.'. ' J/~~~~~ _-.--- I" L ; T T _ 60?^ -7 ^" ^^ [* :" f 60I BO 160 DIRECT READING (mm Hg) 2~) 22~ 24 FIG. 12. Age as a factor n the correlaton of ndrect and drect systolc blood pressure readngs. OF ARTERIAL BLOOD PRESSURE f I [ EE 160 r e6o, I a '00 d 8ol 60 P11lse poess11e soo, Hg oso Hg Us- Ln of eqoosleoce Drect (Intro-orterol) Systolc Pressure (mm Hg) FIG. 13. Effect of pulse pressure on the auscultatory regresson lnes (all samples). pressure regresson lnes were computed to determne the nfluence of pulse pressure upon ther slope (fg. 13). For ths purpose the data were dvded nto two groups on the bass of the drect ntra-arteral pulse pressures; the frst group conssted of 76 observatons n whch the pulse pressures ranged from 14 to 49 mm. Hg, whle the second conssted of 104 observatons where the pulse pressure ranged from 50 to 151 mm. Hg. The regresson lne of the low pulse pressure group slopes away from the lne of equvalence more rapdly than the lne of the hgh pulse pressure group. The suggeston s that, on the average, errors n auscultatory systolc readngs are dfferent for persons of low pulse pressure than for those wth hgh pulse pressure. DIscussIoN Accuracy of the Drect Recordng System. Attenton should be drawn to the mportance of the flud flled system used to conduct dynamc pressure changes to the daphragm of the I transducer. The frequency response characterstcs of both the transducer and the recorder have been shown to be more than adequate for recordng pressure pulse waves.9 The lmtng factor of all such systems, then, s the needle or catheter and ts connectons to the pressure transducer. Ths s partcularly true where accurate reproducton of the detaled components of a pressure pulse wave

7 are desred. Even n ths nstance expert observers do not agree on the over-all frequency response requred. Wood9 reports no sgnfcant dfferences n recorded pressure levels when measured by dfferent manometer systems wth unform response out to 6 cycles per second and above. In ths partcular study we have been concerned wth the systolc and dastolc levels rather than the actual confguraton of the pressure pulse waves. The system was underdamped at the hgher frequences snce t produced a 100 per cent response flat to 17.5 cycles per second followed by overshoot as the frequency ncreased. To determne the accuracy of ths recordng system, a comparatve test was carred out. A second recordng system whch produced a 100 per cent response flat to only 7.5 cycles per second (fg. 2) was arranged by usng a 22.5 cm. length of small bore (0.5 mm. nsde dameter) polyvnyl catheter. Ths catheter was threaded nto the brachal artery through an 18 gage needle. A 15 gage needle was nserted besde t. The orfces of both catheter and needle were at the same level wthn the artery. Smultaneous recordngs were made and analyzed. Fgure 14 llustrates the fndngs. No apprecable dfference exsts n the pressure levels recorded by the two systems. The only notceable alteraton produced by the catheter was a phase lag of 0.03 second. A second factor whch concevably could account for dfferences n drect and ndrect pressure measurements was consdered. The ndrect methods determne the lateral pressure aganst the wall of the artery, whereas the drect method used here determned the end-on thrust of pressure aganst the daphragm of the transducer. In the latter nstance hgher readngs mght result from the addtve effect of knetc energy mparted to the stagnant flud column of the recordng system. To determne the error ntroduced by measurng an end-on systolc pressure, a second comparatve experment was carred out. A smultaneous recordng of end-on. and end-away pressures was obtaned by ntroducng two 15 gage needles n opposte drectons nto the brachal artery. The results are shown n fgure 15. It wll be VAN BERGEN, ET AL I:: :l 1,' :I :! I ": :;: ; l I lli ::l I~l 4!I'~? : ^ :. I: '""e r.ll.[_.1: HL 150 ~~~~~~~~~l:: Itl! **'.':,; ::, '!,!;:! [!!1 ' I~:L:! ~: ^ElE~~~~~~zEEElEEEllzz 200- o------~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ I-- )O : :I::^^^::^ =^-E^? FIG. 14. Smultaneous records of ]:ressure pulses from the brachal artery as recorded wth a 15 gage needle (upper) and a fne bore polyvnyl catheter (lower). Note that the base lne of the upper tracng s shfted upward :4±:::::: 0- -H tci. : 100- r : : 'I:: M:I\'I..'\' N (:::t J!ft,. If\'- \t:i-1 IVU-I -- t! X. \ :ltrlil:f:: %:~::jjf~ I 0- FIG. 15. Smultaneous tracngs of brachal pressure pulses as recorded wth a 15 gage needle drected centrally (upper) and a 15 gage needle drected perpherally (lower). noted that no apprecable dfference exsts between these two recordngs. Oscllometrc Method. In observng oscllometrc systolc pressures t s concevable that the values obtaned may be hgher than the ntra-arteral pressures. Ths may be explaned by the pulsatng column of blood proxmal to the nflated cuff strkng ts upper edge and producng an oscllaton of the mercury column. Ths condton was most apt to occur when measurng the systolc pressure of chldren wth a small szed cuff. Auscultatory Method. Contrary to the fnd- ki; I I 1 1 I._;_ 1 1 I I \ I; I tj

8 488 48MEASUREMENT OF ARTERIAL BLOOD PRESSURE Downloaded from by on November 27, I0C 5C Ilm _ I1r _ T- t,.-f1- + t I,I t.rl;l t/ f 1, ;E!. tl. t+ t L1 I t( 1.t t-lt-f.t IrTtl- +,.l ":: "" Ilr.. Th~~htf~~~I -I (+ rl T-- tl rt t't.flt Irlt + '(.1 L t.fl! [C e l+t+. I+t (,. I; II -.. I. I lt et!)11."' tt tf-,,... (II.!lt r -1. -tt, rf. I " r t Itt ll ; I -tl -t.tc.l l 4 ' r^% U-' ; I' 4 ;L1L4tI FIG. 16. Pressure pulse waves recorded durng cuff pressure of 130 mm. Hg. The arrows ndcate the negatve dp whch precedes the anacrotc rse when Korotkoff sounds are present. ngs of others3 the ndrect auscultatory and palpatory systolc observatons never exceeded the smultaneous drect pressure readngs n our observatons. Although the mechansms effectng the Korotkoff sounds are not wholly understood, the conclusons of Erlangera0 concernng a preanacrotc phenomenon seem tenable. He found that at a compresson pressure just below the proxmal endarteral pressure a small ncrement of blood s forced through the flattened segment of artery. Ths blood s nsuffcent n amount to dstend the compressed segment and the segment remans flattened untl the compresson pressure s reduced further. The pulse then succeeds n penetratng the entre length of the segment and a dastolc resduum results. Not untl ths dastolc resduum s formed s t possble for the preanacrotc phenomenon (fg. 16) to occur wth ts concomtant sounds of Korotkoff. In one phase of our study endarteral and auscultatory readngs were recorded smultaneously from the same arm and dstal to a compresson cuff (fg. 17). The procedure was as follows. The cuff was nflated rapdly to a level well above the endarteral systolc pressure, mantaned there for several seconds and then slowly deflated. Pror to deflaton the endarteral pressure level fell toward the equlbrum pressure of 20 mm. Hg. Durng deflaton of the cuff a slow rse n the level of the drect tracng developed, followed by the appearance of small pressure waves. Shortly,,4 thereafter, sounds of Korotkoff could be obtaned. It was noted that the ntal slow rse n the endarteral pressure level occurred at a compresson pressure approxmately equal to the prevously determned endarteral systolc pressure. Some workers3' 6 have publshed smlar tracngs wheren t was demonstrated that the frst small pressure waves dstal to the compressng cuff could be synchronous wth the frst auscultatory sound. We assume from these studes that these small pressure waves represent ncrements of blood penetratng the artery under the cuff durng systole at a compresson pressure whch completely flattens the compressed segment durng dastole. When the compressng pressure becomes low enough for the endarteral pressure to mantan a dastolc resduum, the prealnacrotc phenomenon develops. And f ths resduum develops wth the frst pulse wave to penetrate the cuff, the auscultatory and drect readngs can be equal. We are thus led to beleve that, usng the proper sze of cuff and slow deflaton, the 200oo I 00--, Ilt'ffett c'~~h- \. r1f~ 1 A -Th I t H!~eH~ H 't31)lf#t.r.flfff 'It#trtlttt 'lti 11 1' m IIn l *: 'I t.essl 1- t! FIG. 17. Smultaneous records of changes n the brachal artery pressure (lower) related to the compresson cuff pressure (upper). A, ntra-arteral pressure rse begnnng at compresson pressure of 165 mm. Hg. B, frst auscultatory sound heard at compresson pressure of 152 mm. Hg. 1t4.S 1- : 200- rr tl; ~t' '_ L lq:!:l~~~~~~~~~~~~~~~~~ (- 'I L -4~.,,, H lff IIfW 7.l. -

9 auscultatory method for measurement of systolc blood pressure call be of varyng degrees less than (or equal to) the true drect systolc pressure readng, but can never exceed ths drect readng. Roberts and assocates' suggest that, as anl ndex of dastolc pressure, the mufflng of Korotkoff sounds s more accurate than the dsappearance of the sounds. The frequency wth whch our dastolc readngs grossly underestmate the ntra-arteral pressure seems to support ths vew. We recorded levels accordng to the dsappearance of sounds as recommended by Bordley and hs colleagues.8 Wave Summaton. Accordng to Wggers" a centrpetal rebound wave of pressure summates upon the prmary centrfugal systolc wave resultng n the peak systolc level. That the magntude of the summaton wave s a functon of perpheral resstance has been demonstrated n a crculaton model." Drect tracngs were recorded n whch the perpheral resstance was ncreased by occluson of the brachal artery dstal to the needle (fg. 18). Invarably ths ncrease n perpheral resstance resulted n an elevaton of the drect systolc pressure of 20 to 30 mm. Hg. It s of nterest to note that the values of auscultatory systolc pressures dd not vary under the two condtons. From ths observaton one mght assume that the dfference between drect systolc and auscultatory systolc pressures could be the result of falure of the auscultatory method to regster the summaton waves. In turn ths mght mean that the appearance of the frst sound of Korotkoff depends prmarly upon the pressure of the centrfugal wave. If ths s true, then the auscultatory systolc level s most lkely an ndex of the central aortc pressure rather than the perpheral pressure n the vessel at the ste of measurement. In the lght of the foregong assumptons an ncrease n general perpheral resstance s prmarly regstered by drect recordng methods, whereas the ndrect auscultatory observatons are affected only secondarly by the ncreased work load on the heart resultng n an ncreased systolc pressure thrlst. Such a relatonshp mght account for some VAN BERGEN, ET AL. w------t m " " K 150g III q-h--~ ~ VLI III! III III -+ -' I ;I:~ I' 7d s &YIThh f-1li 4m' I :9Al' r I 1 I ; I I I 1 1 I I I I : : I I I 1 I 1 I I ll r..rrrri7 77T771 5' B~~ Il!ll - '11- rrrtfl:r::1:tttl I I!IJl.lr Ir7-1.7.T7TS.STT;T IJJ!I: ;-lrl;l.llll: LLLI.1.L rtrr t-. t.:t.t.l. f-, jlt tff...ii I ]11 t.f!l ~.. lllii IIL 11t...;r;t.t.f.;.u.f.: Illll ll 1I[ 11.lll.. liili [[]~ TL L FIG. 18. Pressure pulse waves from the brachal artery. The arrow ndcates the pont at whch the artery was occluded by dgtal compresson dstal to the pont of recordng. of the dscrepances between drect and auscultatory readngs encountered n ths study. We make partcular reference to our fndngs n the young hypertensve patents. In ths group, usually, the arterolar bed mantans a hgh degree of tonus. It seems reasonable to assume that such a state would augment the summaton effect of the reflected wave upon the central wave resultng n a perpheral ntra-arteral pressure hgher than that obtaned wth the auscultatory method. The fndngs n the young hypertensve group pose some nterestng possbltes. If the auscultatory method s frequently n error n a young patent wth moderate systolc elevaton, an ntra-arteral measurement may well be an ndcated procedure n the nvestgaton of such an ndvdual. Early detecton of some cases destned to develop a severe hypertenson syndrome may be facltated. It s also possble that nformatonl ganed from such drect measurements mght be of value n estmatng the degree of hypertenson, as a gude n the evaluaton of therapy and n the formulaton of a prognoss. CONCLUSIONS 1. Drect and ndrect determnatons of blood pressure are c(ompared n 70 patents n the mmedate postoperatve state, and the results of statstcal analyss of these data are presented. 2. These studes demonstrate a frequent dscrepancy between the drect and ndrect readngs. K-r / I/ I/ ll) Ill.II/ 111) I/ I/ II: _JI: t+ct r r-t--t?

10 490 MEASUREMENT OF ARTERIAL BLOOD PRESSURE 3. The greatest dscrepancy s found n the young hypertensve group. The possble clncal value of ths fndng s consdered. 4. The decreasng order of accuracy for ndrect methods of measurement s shown to be: oscllometrc, auscultatory and palpatory. 5. The drft of ndrect readngs s to fall ncreasngly below the drect measurement as blood pressure rses. 6. The usual practce of employng one ndrect technc only s not subject to crtcsm f the observer regards such a pressure estmaton as an ndex to the true ntra-arteral pressure. Changes n blood pressure can be detected by such a method but no concluson values. should be drawn concernng absolute 7. An attempt s made to explan some of the factors contrbutng to the varable error by whch auscultatory readngs underestmate the true ntra-arteral pressure. SUMARIO ESPANOL 1. Determnacones de pres6n arteral drectas e ndrectas son comparadas en 70 pacentes en el estado postoperatoro nmedato, y los resultados de un analss estadstco de estos datos se presentan. 2. Estos estudos demuestran una frecuente dscrepanca entre las lecturas drectas e ndrectas. 3. La mayor dscrepanca se encuentra en el grupo hpertenso joven. El posble valor clnco de este hallazgo se consdera. 4. El orden descendente de precs6n para los metodos ndrectos de medr la pres6n ha mostrado ser; osclom~trco, auscultatoro y palpatoro. 5. El rumbo de las lecturas ndrectas es el de caer progresvamente por debajo de las lecturas drectas a medda que la pres6n arteral aumenta. 6. La prgctca usual de emplear una t6cnca ndrecta solamente no esta sujeta a la crtca s el observador consdera tal estmado de la pres6n como un ndce de la pres6n ntraarteral verdadera. Cambos en pres6n arteral se pueden determnar por este m~todo pero una conclus6n sobre los valores absolutes no se debe aceptar. 7. Un atentado se hace para explcar algunos de los factores contrbuyentes al error varable por el cual lecturas auscultatoras ndrectas resultan mas bajas que la pres6n verdadera ntraarteral. ACKNOWLEDGMENT The authors are ndebted to Mss Nancy Thel, R.N., and Mss Marlyn Johnson, R.N., for ther panstakng assstance n recordng of ndrect blood pressures, and to Loren R. Sheldahl, B.A., Research Assstant n the Dvson of Bostatstcs, for hs complaton and statstcal analyss of the data. REFERENCES 1 ROBERTS, L. N., SMILEY, J. R., AND MANNING, G. W.: A comparson of drect and ndrect blood pressure determnatons. Crculaton 8: 232, BAZETT, H. C., AND LAPLACE, L. B.: Studes on the ndrect measurement of blood pressure. I. Sources of error n the Rva-Rocca method. Am. J. Physol. 103: 48, RAGAN, C., AND BORDLEY, J.: MIeasurements of blood pressure. Bull. Johns Hopkns Hosp. 69: 526, EATHER, K. F., PETERSON, L. H., AND DRIPPS, R. D.: Studes of the crculaton of anesthetzed patents by a new method for recordng arteral pressure and pressure pulse contours. Anesthesology 10: 125, HAMILTON, W. F., WOODBURY, R. A., AND HARPER H. T.: Physologc relatons between ntrathoracc, ntraspnal and arteral pressures. J. A. M. A. 107: 853, STEELE, J. M.: Measurements of arteral pressure n man. J. Mt. Sna Hosp. 8: 1049, MALTBY, A. B., AND WIGGERS, C..J.: The effects of partal and complete occluson on actual pressures n compressed arteres. Am. J. Physol. 100: 604, BORDLEY, J., CONNOR, C. A. R., HAMILTON, W. F., KEN, W. J., AND WIGGERS, C. J.: Recommendatons for human blood pressure determnatons by sphygmomanometers. Crculaton 4: 503, WOOD, E. H.: Study of mnmal dynamc response characterstcs of manometer systems requred for adequate recordng of perpheral arteral pressure pulses n man. Am. J. Physol. 163: 762, '0ERLANGER, J.: Movements n an artery under pneumatc compresson. Am. J. Physol. 56: 84, WIGGERS, C. J.: Crculatory Dynamcs. New York, Grune and Stratton Pp. 9, 10.