EXPERIENCE WITH THE CEREBRAL FUNCTION MONITOR DURING DELIBERATE HYPOTENSION

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1 Br. J. Anaesth. (98), 53, 639 EXPERIENCE WITH THE CEREBRAL FUNCTION MONITOR DURING DELIBERATE HYPOTENSION H. PATEL SUMMARY Forty-fve healthy patents (30 aged 5-46yr and 5 aged yr) undergong varous types of plastc surgery were studed durng delberate hypotenson produced by a combnaton of ganglon- and betablockade, halothane, head-up tlt and postve end expratory pressure. A Cerebral Functon Montor was used to assess the adequacy of cerebral perfuson. Cerebral actvty dd not undergo any sgnfcant change f the systolc arteral pressure was decreased gradually and mantaned at approxmately 60 mm Hg at heart level. However, a rapd decrease n pressure may depress cerebral actvty n a reversble manner. Snce the ntroducton of delberate hypotenson (Davdson, 950; Enderby, 950; Organe, 950) fears have been expressed about the adequacy of cerebral perfuson durng reduced arteral pressure and head-up tlt, the elderly, hypertensve patent beng partcularly at rsk. Electroencephalography (e.e.g.) has been wdely used to study the effects of decreases n arteral pressure on cortcal actvty n anmals and man (Beecher, McDonough and Forbes, 938; Bromage, 953; Schallek and Walz, 954; Wlson, 957; Spoerel, 960; Backman, Lofstrom and Wden, 964; Rollason, Dundas and Mlne, 964; Rollason, 965; Schettn, Freund and Owre, 967). Although these observatons are useful durng anaesthesa and surgery, the conventonal e.e.g. has lmtatons because of the complexty of equpment and dffculty n nterpretng the record. In an attempt to smplfy both the equpment and the record for clncal use the Cerebral Functon Montor (CFM) was ntroduced by Maynard, Pror and Scott (969). It s a smple and portable machne convenent for use n the operatng theatre, and provdes a contnuous record of the overall concal electrcal actvty. The record s relatvely easy to nterpret and has been of value n montorng cerebral actvty n a varety of clncal condtons (Pror et al., 97; Branthwate, 973; Schwartz et al., 973; Dubos, 975; Dubos et al., 978; Dubos, Scott and Savege, 978; Savege et al., 978; Pror, 979). Comparson of the CFM wth the e.e.g. has shown that t can ndcate sgnfcant alteratons n H. PATEL, F.F.A.R.C.S., Department of Anaesthetcs, Queen Vctora Hosptal, East Grnstead, Sussex RH93DZ /8/ the functonal state of the bran (Pror, 979). The best poston for the placement of electrodes s over the stes of greatest vulnerablty to decreases n blood flow the boundary zones between the terrtory of the anteror, mddle and posteror cerebral arteres (Pror, 979;Brerley etal., 980). The present study was undertaken to determne the usefulness of the CFM as a montor durng delberate hypotenson. PATIENTS AND METHODS Ethcs Commttee approval was obtaned for ths study. Thrty healthy female patents aged 5-46yr (younger group) undergong breast reducton and 5 patents between the ages of yr (older group) undergong varous plastc surgcal procedures were studed (table I). All patents were premedcated wth levorphanol tartrate mg, promethazne mg and atropne 0.6 mg or hyoscne 0.4 mg,.m. h before surgery. Anaesthesa was nduced wth thopentone mg.v. and tracheal ntubaton performed after the admnstraton of decamethonum 4 5mg and suxamethonum 0-20mg.v. Anaesthesa was mantaned wth % halothane n a 50% mxture of ntrous oxde n oxygen. Delberate hypotenson was nduced by the admnstraton of pentolnurn 4-8 mg.v., and practolol 5-0mg.v. where ndcated. Hypotenson was augmented by postural schaema wth a 0-20 head-up tlt, and ntermttent postve pressure ventlaton of the lungs (IPPV) wth the addton of postve endexpratory pressure (PEEP) where necessary to acheve the requred arteral pressure: usually Macmllan Publshers Ltd 98

2 640 BRITISH JOURNAL OF ANAESTHESIA TABLE I. Mean values (+ SEM) of age, arteral pressure, and changes n CFM trace n two groups of patents. The mean CFM levels of the two groups both before hypotenson and at the end of ntal decrease n pressure were sgnfcantly dfferent Age (yr) Systolc arteral pressure (nunhg) Before hypotenson Durng hypotenson Decrease n arteral pressure Intal rate of decrease of arteral pressure (mmhgmn"') Average of upper and lower margns of CFM trace measured (mm) from base (uv n parentheses) Before hypotenson Young group ±2.5 63± ±.37.30± ±0.9 ±.6 (3.98±.3) Older group ±7.27 8± ± ± ±.38 ±2.54 (2.0±2.28) Sgnfcance f><0.00 P< >P>0.00 End of ntal decrease n pressure ±0.86 (2.03 ±0.85) 27.8 ±.45 (6.20±.79) 0.05>P> mmHg (approx.) systolc at heart level. Fentanyl mg Was admnstered.v. A radal artery was cannulated n 28 patents for the contnuous recordng of arteral pressure (Devces two-channel recorder and a Statham P23 Gb transducer). In the remanng patents arteral pressure was montored contnuously usng a Boultte Oscllometer (Enderby, 962; 974). The changes n systolc arteral pressure and the tmes of ther occurrence were recorded and marked on the CFM trace to enable calculaton of the rate of decrease n pressure. End-expred carbon doxde concentraton was measured contnuously from an endotracheal samplng catheter usng an nfra-red carbon doxde analyser and recorded on the second channel. A contnuous CFM trace was obtaned as descrbed by Pror and colleagues (97) usng bparetal needle electrodes. A guard electrode was nserted to decrease the effects of hgh level electrcal nterference. The rate of decrease n systolc pressure was calculated at the tme of ts greatest change after the nducton of hypotenson. Durng the same perod changes n the average of upper and lower margns of the CFM trace were measured n mm from the baselne and the percentage change from the prehypotensve value calculated. The deflectons were measured n mm rather than uv because the dstrbuton of ampltudes n uv s not normal; the semlogarthmc CFM scale s desgned to gve a close approxmaton to a normal dstrbuton (Maynard, 979). RESULTS Young group In the majorty of the young patents studed, systolc arteral pressure decreased gradually to about 60mmHg. Overall, there were no gross changes n the CFM traces (table I). However, n ndvdual patents there were sgnfcant changes related to the followng: Rate of decrease n arteral pressure. Fgure shows the relatonshp between the rate of decrease n arteral pressure and % change of CFM level durng the same perod. A decrease n arteral pressure of less than 0 mmhgmn"' was seen n 26 patents. In nne of these there was no change n cerebral actvty, whle an ncrease of 5-20% was seen n another four patents. In the remanng 3 patents an average decrease of 5 5% n cerebral actvty occurred n 2 patents and a more marked decrease of 25% n one. In three patents n whom the rate of decrease n arteral pressure was greater, 4.5mmHgmn"', ^.SmmHgmn" and 20 mm Hg mn ~' depresson of the CFM trace averaged 7%, 38% and 4 % respectvely. In one young patent (aged 5yr) there was only a small (0%) change n cerebral actvty n spte of a decrease of 80mmHgmn"' n arteral pressure. There was a good correlaton between rate of decrease n pressure and the percentage change of cerebral actvty (r = 0.694). Fgure 2 shows an example of a marked and rapd decrease n arteral pressure followng a sudden change n posture, together wth the accompanyng decrease n cerebral actvty. PEEP. Eleven patents requred mld to moderate PEEP to nduce the requred degree of hypotenson and showed no undue depresson of the CFM trace. There were four epsodes of short perods of ntense PEEP n three patents, wth no effects on the CFM trace. There were eght epsodes of prolonged perods of ntense PEEP n four patents and all resulted n depresson of

3 CEREBRAL FUNCTION MONITOR AND HYPOTENSION E I I SO ] change of CFM level FIG.. Relatonshp between rate of decrease n aneral pressure (logarthmc scale) and percentage change of CFM (average of upper and lower margns (mm) from base-lne) from pre-hypotens ve value n both groups of patents. = young group; O = older group. r = 0.694; r = cerebral actvty. Fgure 3 depcts events n one such patent and shows the effects of short and long duratons of PEEP. A "crtcal arteral pressure". In two patents a "crtcal arteral pressure" was observed whch appeared to correlate wth cerebral actvty. A systolc pressure of less than 60 mm Hg n these patents was assocated wth depresson of cerebral actvty whch returned to ts prevous value when the arteral pressure had returned to baselne. End-expred carbon doxde concentraton was unchanged durng ths tme. Older group The lower half of table I shows means of the measured varables n 5 patents aged yr. Systolc arteral pressure, before and durng hypotenson, was dfferent from that observed n the prevous group. There were no gross changes n CFM traces n the majorty of patents studed. However, n ndvdual patents there were changes related to the followng: Rate of decrease n arteral pressure. Fgure shows the relatonshp between the rate of decrease n arteral pressure and the percentage change n cerebral actvty n 2 patents (r = 0.79). As n the young group, the rate of decrease jn pressure was less than lommhgmn" n all but one patent; n 0 cerebral actvty decreased by 5-20% whle an ncrease of 0% was seen n one patent. However, n one patent n whom pressure decreased at a rate of 5mmHgmn~ l, cerebral actvty was depressed by about 30%. PEEP. Sx patents had mld to moderate PEEP durng hypotenson and showed no apprecable depresson of the CFM trace except for one n whom pressure decreased rapdly. No patent n ths group had perods of ntense or prolonged PEEP.

4 642 BRITISH JOURNAL OF ANAESTHESIA Events FIG. 2. Effect on cerebral actvty of rapd and severe decrease n arteral pressure as a result of sudden head-up tlt (). Tme markers at 0-mn ntervals for CFM (vertcal lnes at 2-mn ntervals) and at -mn ntervals for arteral pressure. Recovery after surgery Postoperatve recovery was uneventful n all patents. Although a detaled neurologcal examnaton was not performed, there was no obvous clncal ndcaton of any neurologcal change n any patent n the perod mmedately after operaton. DISCUSSION Beecher, McDonough and Forbes (938) observed that decreases n arteral pressure produced reversble depresson of cortcal actvty and concluded that the results of such decreases, although not dentcal, were smlar to those occasoned by an ncrease n the depth of anaesthesa. In three conscous subjects, submtted to nduced vascular hypotenson by a combnaton of head-up tlt and ganglon blockade, Bromage (953) found that a rapd ancj uncontrolled decrease n pressure resulted n complete abolton of alpha rhythm and loss of conscousness. When the decrease n arteral pressure was gradual, to not less than 55mmHg systolc, the change n alpha rhythm (40% reducton n ampltude), was ndcatve of mpendng nadequacy of the cerebral crculaton. Schettn, Freund and Owre (967) produced delberate hypotenson n elderly patents (50-78 yr) wth halothane, ncreased arway pressure and head-up tlt and found only mnmal depresson of cortcal functon when mean arteral pressure was greater than 60mmHg. The CFM traces n the present seres confrm that there s no gross alteraton n cerebral actvty, n ether young or elderly patents, provded the systolc arteral pressure s decreased gradually to approxmately 60mmHg. A rapd decrease n systemc pressure may cause marked mparment of cerebral actvty (fg. 2). Schneder (963) 00 -] S S 5- Tme (mn) ' M 3 00 ". I «H I 20- < I! :! L t! r U 9 IP m t 3 f - m l I M I - H mm Events FIG. 3. EflFects of ntense PEEP ( and 2) on cerebral actvty durng delberate hypotenson. Tme n mnutes. Arteral pressure n mm Hg. CFrom Pror (979); reproduced wth permsson of the author and publshers.) ^ } m.an " l W' I! - ; > tt ' I I,) f

5 CEREBRAL FUNCTION MONITOR AND HYPOTENSION 643 quotes Hrsch as demonstratng n anmals that a sudden decrease n systemc arteral pressure from 200 to loommhg caused a marked decrease n cerebral blood flow (c.b.f.), but 4mn later the c.b.f. had almost returned to control values. In 954, Schallek and Walz studed the effects of drug-nduced hypotenson on the e.e.g. of the dog and suggested that changes observed depended not only on the value to whch arteral pressure was decreased, but also on the rate at whch t was decreased. In one group arteral pressure decreased at 45-90mmHgmn~ and there was transent flattenng of the e.e.g. In another smlar group the rate of decrease was 6-60 mm Hg mn ~' and only transent depresson was noted. At rates of l-0mmhgmn~ l there was no change n the e.e.g. and Schallek and Walz (954) suggested that arteral pressure should not be decreased at a rate greater than 0 mm Hg mn ~'. They concluded that the cerebrovascular resstance was unable to accommodate quckly to rapd changes n arteral pressure and, consequently, a decrease n cerebral blood flow and depresson of e.e.g. actvty could occur. When the arteral pressure decreases slowly, autoregulaton mantans a normal cerebral blood flow and the e.e.g. s unaffected. In the present study there was a good correlaton between the rate of decrease of systolc pressure and the decrease n cerebral actvty (fg. ). In the majorty of patents n both groups arteral pressure was decreased at less than lommhgmn"' and the respectve depresson of CFM was 0-20%. A more marked depresson of the CFM trace was notced when the rate of decrease was greater than lommhgmn". In fve patents there was an ncrease n the CFM trace; all had a slow rate of decrease n arteral pressure (3 mm Hg mn" or less). The mean CFM values before hypotenson were sgnfcantly dfferent between the two groups young group: ; older group: 30.75±.38 mm of heght (0.0 >P>0.00). The mean CFM values were also sgnfcantly dfferent n the two groups at the end of the ntal decrease n arteral pressure ( and 27.8 ±.45mm of heght (0.05>P>0.02)). However, there was no statstcal dfference between the two groups n respect of the percentage change n these two values (young: ± 2.46%; older: 9.456±2.99% (0.5>P>0.)). Smth and Wollman (972) pont out that the drugs commonly used to nduce hypotenson durng anaesthesa have no drect effects on c.b.f. or cerebral metabolc rate for oxygen (Sokoloff, 959) and that the cerebral vasculature autoregulates to mantan c.b.f. constant over a wde range of systemc pressures. Autoregulaton s sad to be ntact durng general anaesthesa (Smth et al., 970) although decreased by deep anaesthesa (Schneder, 963) and mpared at hgh Pa^ (Harper, 965). In two patents a "crtcal arteral pressure" seemed to correlate wth depresson of cerebral actvty. Schneder (963) denned crtcal pressure as the level at whch oxygen uptake decreased and mental dsturbances occurred n awake subjects. Ths pont s usually marked by a transton of the conventonal e.e.g. from a desynchronzed to a slow-wave state. In anaesthetzed patents the CFM trace n ts dfferent "ntegrated" form seems to reflect such a pont by showng depresson of cerebral actvty when arteral pressure reached the crtcal value for these patents. Depresson of the CFM trace can occur as a result of varous factors: ncrease n depth of anaesthesa, hypoxa, hypotherma, schaema. It does not necessarly mply rreversble damage to the bran, but suggests mparment of neuronal functon and ndcates that crtcal lmts are reached. Durng stable, delberate hypotenson and n the absence of change n other factors, such as the concentraton of anaesthetc agents, a decrease n the level of the CFM trace would suggest a decrease n cerebral perfuson and cortcal depresson. At ths stage there s tme to take acton to prevent permanent damage (Astrup et al., 977; Morawetz et al., 979; Brerley et al., 980). The sudden applcaton of prolonged, ntense PEEP durng delberate hypotenson s undesrable as shown by the depresson of cerebral actvty (fg. 3). Sustaned PEEP by nducng an acute decrease n arteral pressure and by ncreasng venous pressure decreases cerebral perfuson markedly. Delberate hypotenson has been practsed for more than 30 years. Among the factors whch make t as safe as normotensve anaesthesa are the correct selecton of patents, proper tranng n the use of hypotensve drugs and technques, accurate and contnuous montorng of arteral pressure, and an apprecaton by both surgeon and anaesthetst of the problems nvolved. In conjuncton wth these, CFM can gve useful nformaton. It can detect changes n cortcal actvty whch are a warnng of

6 644 BRITISH JOURNAL OF ANAESTHESIA mpendng danger and thus allow tme for ts safe correcton. ACKNOWLEDGEMENTS The author wshes to thank hs surgcal colleagues for allowng hm to study ther patents, Professor Leo Strunn, Dr Hale Enderby, Dr Pamela Pror and Dr D. E. Maynard for ther advce and help, Mrs Tutcher and Mrs Audy for ther secretaral help, and the Department of Medcal Illustraton, The Queen Vctora Hosptal for photographs. It s a pleasure to acknowledge the fnancal help gven by The Murhead Trust and the South East Thames Regonal Health Authorty. REFERENCES Astrup, J., Symon, L., Branston, N. M., and Lassen, N. A. (977). Cortcal evoked potental and extracellular K + and H + at crtcal levels of bran schaema. Stroke, 8, 5. Backman, L. E., Lofstrom, B., and Wden, L. (964). Electroencephalography n halothane anaesthesa. Ada Anaesthenol. Scand., 8, 5. Beecher, H. K., McDonough, F. K., and Forbes, A. (938). Effects of blood pressure changes on cortcal potentals durng anaesthesa. J. Neurpkysol.,, 324. Branthwate, M. A. (973). Factors affectng cerebral actvty durng open-heart surgery. Anaesthesa, 28, 69. Brerley, J. B., Pror, P. F., Calverley, J., Jackson, S. J., and Brown, A. W. (980). The pathogeness of schaemc neuronal damage along the cerebral arteral boundary zones n Papo anurs. Bran, 03, 929. Bromage, P. R. (953). Some electroencephalographc changes assocated wth nduced vascular hypotenson. Proc. R. Soc. Med., 46, 99. Davson, M. H. A. (950). Pentamethonum odde n anaesthesa. Lancet,, 252. Dubos, M. (975). Presentaton d'apparel: lc monteur de foncron cerebrale. Ann. Antslh. Franf., 6, 59. Savege, T. M., O'Carroll, T. M., and Frank, M. (978). General anaesthesa and changes on the cerebral functon montor. Anaesthesa, 33, 57. Scon, D. F., and Savege, T. M. (978). Assessment of recovery from short anaesthesa usng the cerebral functon montor. Br. J. Anaesth., 50, 825. Enderby, G. E. H. (950). Controlled crculaton wth hypotensve drugs and posture to reduce bleedng n surgery. Prelmnary results wth pentamethonum odde. Lancet,, 45. (962). Controlled Hypotenson: Frst European Congress of Anaesthesology. Post-graduate Courses, 7,. (974). The practce of Delberate Hypotenson n a Plastc Surgcal Unt. Plastc Surgery of the Head and Neck and the Female Breast: 5th Meetng of the Assocaton of German Plastc Surgeons, 327. Harper, A. M. (965). Physology of cerebral bloodflow. Br. J. Anaesth., 37, 225. Maynard, D. E. (979). EEG processng by the Cerebral Functon Montor (CFM) Ann. Anaesth. Franc., 20, 70. Pror, P. F., and Scott, D. F. (969). Devce for contnuous montorng of cerebral actvty n resusctated patents. Br. Med. J., 4, 545. Morawetz, R. B., Crowell, R. H., dc Grolam, U., Marcoux, F. W., Jones, J. H., and Halsey, J. H. (979). Regonal cerebral blood flow thresholds durng cerebral schaema. Fed. Proc., 38, Organe, G. (950). Change and progress n anaesthesa. Proc. R. Soc. Med., 43, 8. Pror, P. F. (979). Montorng Cerebral Functon, st edn. Amsterdam: ElseveT/North Holland Bomedcal Press. Maynard, D. E., Sheaff, P. C, Smpson, B. R., Strunn, L., Weaver, E. J. M., and Scott, D. F. (97). Montorng cerebral functon: clncal experence wth new devce for the contnuous recordng of electrcal actvty of bran. Br. Med. J., 2, 736. Rollason, W. N. (965). The montorng of hypotensve anaesthesa. Anaesthesa, 20, 479. Dundas, C. R., and Mlne, R. G. (964). ECG and EEG changes durng hypotensve anaesthesa for "no catheter" prostatectomy. Proc. Ill Cong. Mund. Anaeslhol., Sao Paulo, Brasl,, 06. Savege, T. M., Dubos, M., Frank, M., and Holly, J. M. P. (978). Prelmnary nvestgaton nto a new method of assessng the qualty of anaesthesa: the cardovascular response to a measured noxous stmulus. Br.J. Anaesth., SO, 48. SchaUek, W., and Walz, D. (954). Effects of drug-nduced hypotenson on the electro-encephalogram of the dog. Anesthesology, 5, 673. Schettn, A., Freund, H. R., and Owre, E. S. (967). Delberate hypotenson wth halothane/oxygen anesthesa n head and neck surgery. Am. J. Surg., 4, 543. Schneder, M. (963). Crtcal blood pressure n the cerebral crculaton; n Selectve Vulnerablty of the Bran n Hypoxaema, st edn (eds J. P. Schade and W. M. McMenemey), p. 7. Oxford: Blackwell Scentfc Publcatons. Schwartz, M. S., Colvn, M. P., Pror, P. F., Strunn, L., Smpson, B. R., Weaver, E. J. M., and Scott, D. F. (973). The cerebral functon montor. Its value n predctng the neurologcal outcome n patents undergong cardopulmonary by-pass. Anaesthesa, 28, 6. Smth, A. L., Negh, J. L., Hoffman, J. C, and Wollman, H. (970). Effects of general anaesthesa on autoregularon of cerebral blood flow n man. J. Appl. Physol., 29, 665. Wollman, H. (972). Cerebral blood flow and metabolsm: effects of anesthetc drugs and technques. Anesthesology, 36, 378. Sokoloff, L. (959). The acton of drugs on the cerebral crculaton. Pharmacol. Rev.,,. Spoerel, W. E. (960). Electroencephalographc montorng durng cardac surgery: observatons on the EEG response to severe hypotenson. Can. Anaesth. Soc. J., 7, 2. Wlson, S. M. (957). Electroencephalography n relaton to anaesthesa. Proc. R. Soc. Med., SO, 05. ESSAI AVEC APPAREIL DE CONTROLE DE FONCTION CEREBRALE AU COURS D'HYPOTENSION PROVOQUEE RESUME On etuda 45 patents sans (30 ages de 5 a 46 ans et 5 ages de 54 a 82 ans) qu etaent soums a des operatons de chrurge plastque de types dvers, chez lesquels on avat delbcrcment provoquc un etat d "hypotenson au moyen de blocages beta et ganglonnare, d'halothnne, de basculage de la tete et de

7 CEREBRAL FUNCTION MONITOR AND HYPOTENSION 645 presson expratore tennnale postve combnes. On utlsa un apparel de controle de foncton cerebrale afn d'evaluer s la perfuson cerebrale etat suffsante. On n'enregstra pas de changement sgnfcatf de l'actvte ccrebrale lorsque Ton abassat graduellement la presson arterelle systolque et qu'on la mantenat au nveau du coeur a envron 60mmHg. Toutefos, une basse rapde de la presson peut provoquer une depresson de l'actvte cerebrale de caractere reversble. ERFAHRUNGEN MIT DEM HIRNTATIGKEIT- OBERWACHUNGSGERAT WAHREND EINES ABSICHTLICH HERBEIGEFttHRTEN UNTERDRUCKS ZUSAMMENFASSUNG Es wurden 45 gesunde Patenten (30 zwschen 5 und 46 Jahren und 5 zwschen 54 und 82 Jahren), de sch verschedenen Arten von plastscher Chrurge unterzogen, studert und zwar wahrcnd enes abschdch durch ene Kombnaton von Ganglen- und Beta-Blockade, Halothan, Kopf-Negung nach oben und exspratorschen Druck am postven Ende herbegefuhrtcn Unterdrucks. Bn Hrntatgket-tTberwachungsgerat wurde engesetzt, urn de Suffzenz der Hrndurchstromung zu messen. An der Hrntatgket wurden kene bedeutende Anderungen festgestellt, wenn der systolscher Arteren- Druck allmhlch reduzert und be ca. 60mmHg auf Herzebene gehalten wurde. Ene rasche Abnahme des Drucks kann jcdoch zu ener umkehrbaren Verrngerung der Hrntatgket fuhren. EXPERIMENTO CON APARATO DE CONTROL DE FUNCION CEREBRAL DURANTE HYPOTENSION PROVOCADA Se Ilev6 a cabo un estudo sobre 45 pacentes sanos (30 de ellos de 5 a 46 afos de edad y 5 de 54 a 82 afos de edad) sometdos a cruja plastca en los cuales se provoco delberadamente una hypotenson por medo de bloqueos beta y ganglonaros, halotano, cabeza volteada y preson expratora termnal postva, combnados. Se uso un aparato de control de funcon cerebral para evaluar la sufcenca de la perfuson cerebral. No se produjo nngun cambo sgnfcatvo de la actvdad cerebral cuando se reduda progresvamente la preson arteral sstolca y se la mantena en cerca de 60mmHg a nvel del corazon. Sn embargo, un descenso rapdo de la pres6n puede deprmr la actvdad cerebral de manera reversble.