Thorax 1987;42:1 1-18

Spinal cord damage and operations for coarctation ofthe aorta: aetiology, practice, and prospects

ABSTRACT An inquiry was made into the clinical practice and paraplegia rate associated with oper-ations for coarctation of the aorta conducted by surgeons in the United Kingdom and Ireland.Paraplegia occurred in 16 patients in a total of 5492 operations, an incidence of03%, or once in 343operations. The aetiology and measures that may be taken to avoid this complication are dis-cussed.The medicolegal consequences need no emphasis and it is recommended that finger pal-pation alone as a method of assessing the adequacy of distal aortic pressure should be abandonedin favour of continuous electromanometric visual display of the aortic pressure in all patients. In thelong term, serious consideration must be given to developing spinal cord monitoring using somato-sensory evoked potentials.

One of the greatest tragedies that can befall an activechild is the development of severe and disabling neu-rological damage after an apparently straightforwardoperative procedure. The consequent deprivation andanguish to the patient, family, and surgeon, to saynothing of probable litigation, makes this a subjectworthy of serious consideration.The potential for damage to the spinal cord arises

on account of its variable, disputed, and sometimesprecarious blood supply. The spinal cord is at risk notonly during operations on the thoracic aorta but alsoin orthopaedic and neurosurgical operations on thethoracic spine, particularly those for the mechanicalcorrection of scoliosis. Although the risk of paraple-gia in operations for traumatic rupture of the aortaand of acute dissection is high (indeed, in these condi-tions the patient may present with paraplegia), theincidence after operations for coarctation of the aortais fortunately low. Nevertheless, since many oper-ations for these conditions are undertaken worldwide,there is a group of patients suffering consequentialparaplegia; and because of the understandable reluc-tance of surgeons to publish these tragedies, this num-ber is probably considerably underestimated.'The blood supply of the spinal cord is derived from

the anterior spinal artery, which provides a rich anas-tomotic network. This artery is supplied by branchesof the vertebral arteries, radicular branches from eachof the intercostal arteries, and other collaterals.

Address for reprint requests: Mr G Keen, Bristol Royal Infirmary,Bristol BS2 8HW.

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Whereas in most individuals the anterior spinal arteryis a continuous vessel, unhappily it is occasionally dis-continuous. Since much of the blood supply to theanterior spinal artery arises from the cervical vesselsand lower thoracic and upper lumbar vessels (the lat-ter through the great radicular arteries, particularly ofAdamkiewicz2), possibly clamping of the aorta or ofintercostal vessels for operative procedures may inter-fere appreciably with the blood supply to an isolatedsegment of the thoracic spinal cord, with consequentischaemic transection.A recent study of the vascular anatomy of the lower

spinal cord has thrown interesting light on theaetiology of paraplegia in man. Although the use ofdistal shunting after aortic cross clamping almostinvariably protects from paraplegia, some cases ofparaplegia are reported despite these measures. Sven-son and his colleagues in 19863 studied the hae-modynamics, paraplegia rate, and spinal cord bloodflow with radioactive microspheres in adult baboons,with particular reference to the arteria radicularismagna (the main lumbar arterial branch to the lowerspinal cord). There were no significant left ventricularhaemodynamic advantages with shunting, but shunt-ing significantly increased lumbar spinal cord bloodflow, which correlated with the distal aortic meanpressure. Lower thoracic spinal cord blood flow didnot, however, increase during shunting and did notcorrelate with the distal aortic pressure. This is due tothe vascular anatomy of the anterior spinal artery,which, as in man, was much smaller above than belowthe entry of the arteria radicularis magna.4 Resistanceto flow, as calculated by Poiseuille's equation, is 11

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12times greater in man up the anterior spinal artery thandown this artery. These findings show that distal per-fusion of the aorta using shunts after cross clampingmay provide adequate lumbar spinal cord protection,but the arrangement of the vascular anatomy sodescribed leaves the lower thoracic spinal cord at riskfrom ischaemia. They concluded that during thoracicaortic cross clamping without distal perfusion thelumbar spinal cord is the area most sensitive toischaemia. A distal shunt, however, increases lumbarspinal cord blood flow, but does not alter the protec-tion of thoracic spinal cord blood flow during pro-longed thoracic aortic cross clamping, thus explainingsome cases of paraplegia following an apparently wellconducted operation for coarctation of the aorta evenwhen adequate distal shunting is used.Wadouh and his colleagues undertook direct mea-

surements of the oxygen tension on the surface of thespinal cord of pigs after occlusion of the descendingaorta in a serious of elegant experiments, and showedthat oxygen tension decreased dramatically after thisprocedure, and that the decrease was time related.Furthermore, the production of paraplegia in theseanimals could be predicted at certain levels of surfaceoxygen tension, and these changes were directlyrelated to the low blood pressures recorded distal tothe aortic cross clamp.5

Operations on the descending thoracic aorta areindicated for acute traumatic rupture, for some acutedissections, and for chronic aneurysms, but a com-mon indication is for the resection and anastomosis ofcoarctation of the aorta, a congenital abnormality.

These operations are complicated by the need tocross clamp the aorta high up and at the same timeprevent left ventricular strain and to protect the kid-neys, spinal cord, and abdominal viscera from theeffects of ischaemia.

Paraplegia and surgery for coarctation

The adequacy of blood supply to the lower spinalcord after cross clamping should depend logically onthe number of intercostal vessels that are sacrificedand on the duration of aortic cross clamping. At firstsight it Would appear that the more intercostal arte-ries are divided or the longer the period of aortic crossclamping the greater is the risk to the patient of devel-oping paraplegia. This, however, is not necessarily thecase and the conclusions of Brewer are important.This review was summarised as follows:I An incidence of 0-41% of spinal cord compli-cations in 12 532 patients who had surgical correctionof coarctation of the aorta was found by the author ina survey completed by leading thoracic surgeons (onecase in 250 operations).2 Twenty three additional cases with and without

Keensurgery were collected from published reports.3 There were eight cases of spontaneous spinal cordinjury without operation, six collected from publishedreports and two from his own series.4 The concept of the segmental blood supply to thecord by multiple consistent radicular branches fromthe intercostal vessels is erroneous.5 Neither sacrifice of intercostal arteries norduration of occlusion of the aorta was related to spi-nal cord injuries in all cases reported in this series.6 Although the extent of the collateral circulationmay be estimated preoperatively, the sufficiency ofthis circulation must be verified at operation. Themost reliable test is the accurate measurement of theaortic pressure above and below the coarctationbefore and after occlusion of the coarctation. Thistest should be added to the techniques that allsurgeons use in repairing coarctation of the aorta.7 The surgeon must also be prepared at every oper-ation for coarctation of the aorta to provide addi-tional protection of the spinal cord in patients withproved poor collateral circulation by one of threemeans: hypothermia, left heart bypass, or jumpgrafts.8 The variation of the blood supply to the anteriorspinal artery may not permit even the briefest crossclamping, despite the use of the above mentioned pro-tective measures.9 Since this may occur in any case and is beyond thecontrol of the surgeon, it is essential that this is madeknown to the patient before the operation.

Detennination of adequacy of distal aortic perfusionduring operations for coarctation of aorta

After aortic cross clamping it is necessary to have anaccurate assessment of the adequacy of blood supplyto the lower limbs, abdominal viscera, kidneys, andespecially the spinal cord. In coarctation of the aorta,when there is a completely obstructing diaphragm,cross clamping of the aorta above and below thisdiaphragn should cause no change in distal aorticperfusion, for adequate collateral circulation to thelower half of the body is provided through the hyper-trophied internal mammary, parascapular, and otherarteries, which return blood in a retrograde mannerinto the lower aorta via the intercostal arteries. Inmost cases, however, the coarctation is not a completediaphragm and there may be appreciable blood flowthrough the narrowed segment. In these cases aorticcross clamping will deprive the lower aortic segmentof a varying amount of blood, depending on thedegree of narrowing produced by the coarctation. Inthose patients with a less than tight coarctation and

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Spinal cord damage and operations for coarctation of the aortawith appreciable blood flow through the coarctationthere will be other indications that this is so. In thesecases the femoral pulses may be palpable, althoughdelayed, and there may be less than the usually readilypalpable collaterals over the posterior chest wall.Further, upper limb hypertension may be modest orabsent. In this group of patients special care isrequired to monitor the lower aortic segment aftercross clamping.

Experienced cardiothoracic surgeons can assessfairly adequately the distal aortic pressure after aorticcross clamping by digital palpation of the distal aorta.The educated finger can certainly distinguish betweena soft and a tense aorta, but many surgeons claim(without much evidence) that digital palpation helpsthem to assess this pressure to within 10 or 15 mm Hg.Undoubtedly the pressure in the distal aorta is mea-sured more accurately by an indwelling needle in thedistal aorta or by means of a femoral artery catheterattached to an electromanometric device giving con-tinuous visual display of the arterial pressure.6

Past and present surgical practice

In March 1985 I corresponded with those 90 membersof the Society of Thoracic and CardiovascularSurgeons of Great Britain and Ireland whom I knewto have personal experience of operations forcoarctation of the aorta. My interest arose from beingconcerned with the legal actions of several patientswho had suffered paraplegia after coarctation oper-ations by other surgeons. In these cases inquiry hadbeen made by the plaintiffs about the method ofmonitoring distal aortic perfusion after aortic crossclamping.

In my questionnaire I asked which method theyroutinely used-whether it was digital palpation orwhether it was by electromanometric measurements. Iwas interested to know what their practice was in1985 and furthermore what had been their practice in1980 as I was attempting to determine what mightreasonably be considered to be usual and thereforeprudent practice at that time. I also inquired into theincidence of paraplegia after the operation.

I received 74 completed questionnaires from the 90surgeons, and an analysis of these has proved inter-esting, although inconclusive. Forty two surgeonsreplied that in 1980 they did not use electro-manometric measurements in the distal aorta afteraortic cross clamping, but that they relied upon digi-tal palpation of the lower aortic segment to give infor-mation on the adequacy or otherwise of distalperfusion.Twenty seven surgeons routinely measured distal

aortic pressures after aortic cross clamping; a furtherfive did not do so regularly, but did use monitoring

when digital palpation of the distal aorta suggested aninadequate blood supply. Since therefore the majorityclearly did not use electromanometric measurementin 1980, failure to do so could not be regarded asunusual or unacceptable practice at that time.

In 1985 35 of this same group of surgeons did notmeasure distal aortic pressure electromanometricallyand continued to rely on digital palpation, but 39reported that they now invariably measured distalaortic pressures. Thus 12 surgeons who did not rou-tinely measure distal aortic pressures in 1980 weredoing so in 1985.

Neither method of distal aortic pressure assessmentwas especially preferred by any particular age groupof surgeons, although the digital method of palpationof aortic pressure was used frequently by thosesurgeons with very great experience. No surgeon mea-sured distal aortic pressure while operating oncoarctation of the aorta in the first few weeks of life,as this is not practical and cross clamp times are veryshort.

Incidence of spinal cord damage

In this inquiry 74 surgeons reported a total of 5492operations for coarctation of the aorta. Among these,there were 16 patients with consequent paraplegia, anincidence of 0 3% or one in 343 operations. Fourteenof these followed primary operations, one occurringsuddenly and permanently one month after oper-ation. The remaining two followed second or thirdoperations for coarctation, and in neither case wasany form of bypass used.A further three patients were reported to have had

postoperative temporary paraplegia, all of whomrecovered completely within 10 days of operation. Afew surgeons separately reported that several of theirpatients had developed an unusual gait after oper-ation, which may have been due to a mild caudaequina lesion. Therefore in addition to 16 patientswith permanent paraplegia three sustained temporaryparaplegia and an unknown number of other patientsmay have developed less obvious or transient lesionsof the spinal cord or cauda equina.

Somatosensory evoked potentials

A most attractive method of assessing spinal cordfunction during these operations is by the continuousmonitoring of spinal cord function by means ofsomatosensory evoked potentials. The stimulation ofvarious sensory systems will produce a signalidentifiable by an electroencephalogram on the rele-vant part of the surface of the brain provided that thetransmitting nerve is intact. This has been understoodfor many years in ophthalmic surgery, where the

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14flashing of a light before the eye is picked up as asignal at the occipital cortex provided that the opticnerve and other tracts are intact. Similarly, stimu-lation of peroneal or tibial nerves will produce a con-tinuous signal on the cerebral cortex provided thatthe spinal cord is intact. This simple concept isunfortunately much more difficult to transfer toclinical practice than superficial consideration mightsuggest. The equipment used is specialised and inter-pretation of the signals produced are best supervisedby neurophysiologists and trained technicians in theoperating theatre. Experience has shown that oncethe technique has been learned a trained member ofthe team, usually the anaesthetist, will be able tointerpret normal and abnormal cerebral responses toperipheral stimulation during surgery.

Spinal cord monitoring

Laschinger and his colleagures7 8 undertook aninvestigation of the experimental and clinical assess-ment of the adequacy of partial bypass in the mainte-nance of spinal cord blood flow during operations onthe thoracic aorta using spinal cord impulse conduc-tion (somatosensory evoked potentials). They con-cluded that maintenance of a distal aortic pressuregreater than 60-80mmHg will uniformly preservespinal blood flow in the absence of critical intercostalexclusion. Should distal aortic pressure be inade-quate, early reversible changes in the somatosensoryevoked potentials will alert the surgeon; and failure toinstitute measures to reverse these changes may resultin paraplegia.With the availability of these methods many cen-

tres (commonly in the United States and two or threein the United Kingdom) are modifying their tech-niques and use somatosensory evoked potentials.Inquiry at these centres shows that a prolonged learn-ing process is required before the interpretation ofdata is of sufficient accuracy for this to become a clin-ically reliable technique, and consequently it must beregarded as an experimental technique in the hands oflearner groups. Although this appears to be a for-midable problem, it is clear that we have a duty todevelop these techniques and to transmit them to theoperating theatre, for surgery of the thoracic aortaand for scoliosis. The initial and revenue costs of sucha system are high, and so is the cost of training appro-priate personnel; but the equipment and personnel inany one city could be made available to help thosesurgeons or neurologists who might from time to timeneed this assistance. It has, of course, yet to be provedthat in the present state of the art the use of somato-sensory evoked potentials offers greater reliabilitythan the monitoring of distal aortic pressure. Pollockand his colleagues in Glasgow, who are evaluating

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somatosensory evoked potential monitoring, believethat their initial experience is promising and that itsintraoperative use can identify quickly the patient atrisk of ischaemic cord damage and allow alternativerepair methods, avoiding cross clamping, to be used.Although they believe that the technique holds greatpromise they are of the opinion that much workremains to be done before an appropriate degree ofrefinement is obtained.9A complicating factor is that cortical somato-

sensory evoked potentials are subject to attenuationby anaesthesia'0" and by hypotension. Thisincreases the difficulty of obtaining good qualityrecordings in the operating theatre, which is never anideal electrophysiological environment.Although somatosensory evoked potentials are

used to measure the function of the entire spinal cord,and some contribution to the potentials may be trans-mitted by the anterior part of the spinal cord, themain pathway is via the dorsal columns. So far, inacute conditions-for example, in trauma, spinaloperations, or vascular insults-the somatosensoryevoked potential does seem to be a sensitive indicatorof global spinal cord function.7 This is not necessarilythe case in chronic paraplegia.

Clearly, we do not yet have sufficient informationto know whether, in man, paraplegia, or at least para-paresis, may occasionally result when somatosensoryevoked potentials have not been abolished. This willbe difficult to discover with the given incidence ofneurological sequelae. The relationship of intra-operative somatosensory evoked potential changes tothe occurrence and severity of postoperative neu-rological sequelae has so far not been established.'2

In my inquiry I asked the 90 surgeons specificallyabout spinal cord monitoring using somatosensoryevoked potentials. Of the 74 who replied, threesurgeons only in the United Kingdom and Irelandwere using this method and a surprising number hadnot heard of its existence.

Need for bypass

When digital palpation is used the blood supply willbe considered inadequate when the distal aortabecomes slack after cross clamping; when electro-manometric methods are used a mean pressure below60mmHg is usually considered an unsafe pressurewith which to proceed with the operation withoutsome form of bypass. Again, when somatosensoryevoked potentials are used the disappearance of thesepotentials will indicate the need for some form ofbypass.

Should distal aortic perfusion be inadequate, distalbypass is required. The forms in use include left atrio-femoral bypass, heparin bonded shunts, femoral vein

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Spinal cord damage and operations for coarctation of the aorta

;--

I.M.A.

Fig 1 The usual type ofcoarctation, which is seen in theinset illustration to produce an almost total obstruction toflow. During the operation it is unnecessary to cross clampthe left subclavian artery and therefore bilateral collateralflow is ensured via the internal mammary and other branchesofboth subelavian arteries. IMA internal mammary artery.

to femoral artery bypass, or surface cooling to 30°Cwith any of these forms of bypass. The exponents ofvery rapid aortic surgery13 14 seem to manage withnone of these aids (although their patients do notappear to be immune from paraplegia), but thesurgeon of average ability is advised to err on the sideof safety and use a bypass technique.'5 We need topay particular attention to those patients in whom

I.M.A. I

there is an appreciable blood flow through thecoarctation, which will be lost to the lower aorta dur-ing aortic cross clamping.

Anatomical variation and risk of paraplegia

There are two main surgical varieties of coarctation.In the usual type the narrowing at the level ofinsertion of the ligamentum arteriosum into the aortais an extremely tight and sometimes totally occludingdiaphragm (fig 1). This is by far the most commonvariety and collaterals are obtained almost exclusivelyfrom the large branches of the subclavian arteries,which are the internal mammary and parascapularvessels. These patients present with classical signs,such as severe upper limb hypertension, absence offemoral pulsation, easily palpable collaterals over theshoulders and back of the chest wall, and well markedrib notching seen on the plain chest radiograph. In thesecond and rather less common variety the aorta hasa longer and less severe stricture and total occlusion israre. In this group of patients, apart from there beinga variable and possibly inadequate collateral circu-lation provided by the branches of the subclavianarteries, very often a considerable amount of bloodpasses through the coarctation (fig 2a)

In the first variety cross clamping of the aortaabove and below the coarctation does not interfereappreciably with distal blood flow. Furthermore, inthese patients it is usually unnecessary to cross clampthe left subclavian to gain access, and the only inter-ference with the lower anastomotic channels is bytemporary clamping or ligation of one or two pairs offeeding intercostal arteries just proximal to the lower

Fig 2 The less common type of coarctation. This produces less extreme narrowing and allows appreciable bloodflowthrough the stenosis. When patch aortoplasty is undertaken the left subclavian artery has to be cross clamped. When this isdone, the distal circulation is deprived not only of the significant flow through the stenosis but also of the blood supplied bythe large branches of the left subclavian artery. IMA -internal mammary artery.

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clamp. On the other hand, in the second variety,where there is a long stricture, the operation of choiceis often a patch annuloplasty: the stricture is incisedlongitudinally and the aorta widened by inserting awide gusset of either prosthetic or biological material.To enable as large a gusset as possible to be inserted,the aorta usually has to be incised well proximally andup into the root of the left subclavian artery, whichrequires the left subclavian artery to be clampedthroughout the procedure (fig 2b). In this group ofpatients therefore aortic cross clamping not only cutsoff a perhaps appreciable blood flow that previouslypassed through the coarctation but also causes theloss of a most important contribution from thebranches of the left subclavian artery during the pro-cedure. Not surprisingly therefore, 10 of the 16 casesof paraplegia reported in this series occurred inpatients with a long, narrow segment who underwentpatch aortoplasty rather than resection and an endto end anastomosis. Given that this variety ofcoarctation is less common than is the complete oralmost complete diaphragm and that aortoplasty isless common than resection with end to end anas-tomosis, these findings may well be significant.

In their review' Brewer et al make no mention ofthe operation of patch aortoplasty. Nearly all theoperations reported in his paper were undertaken inthe earlier years of surgical correction and before theintroduction of the operation of aortoplasty. Patchaortoplasty was introduced by Vosschulte in 195716but was not popularised until Cooley and his col-leagues reported a substantial series of patients oper-ated on from 1967 onwards.'7 18 In the report ofBrewer et al great emphasis is placed on the numberof intercostal arteries occluded and the duration ofaortic occlusion, and their data support the widelyheld view that neither of these factors in itself isdirectly related to the development or otherwise ofpostoperative paraplegia. He did, however, makepassing reference to the collateral circulation pro-vided by the left subclavian artery, but stated thatonly five of more than 100 surgeons consideredclamping of the left subclavian artery to be animportant factor in the aetiology of paraplegia.Our own findings therefore expose a further

important incremental risk factor that has a bearingon this operation. During the first two decades ofcoarctation surgery the almost universal operationwas resection and end to end anastomosis betweenoccluding aortic clamps. When a long, narrow seg-ment was present, this was excised; if it was then notpossible to approximate the divided aortic ends atubular graft had to be inserted. It was usually possi-ble to place a Potts's spoon shaped clamp above thecoarction in such a way as to enable good flow to becontinued up the left subclavian artery. Although the

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tight, short coarctation still remains the most com-mon form, patch annuloplasty is an increasingly fre-quent operation nowadays. It is in many waystechnically attractive as it does not require resectionof the aorta and, in children, it avoids the problem offailure of growth of an end to end anastomosis. Thegreater frequency of this operation may, however,have introduced an increased risk of paraplegia.

Occlusion of the left subclavian artery alone canpredispose to paraplegia, as one case reported to mein this series illustrates. This was a case of coarctationwhere the narrow segment was several centimetreslong, and it was decided to insert a bypass Dacrongraft from the left subclavian artery to the descendingaorta without excising the coarctation or undertakingan aortoplasty. During this operation the left sub-clavian artery was cross clamped throughout but thedistal aorta was at no time occluded, access for thelower anastomosis being provided by a side clamp.Nevertheless, this patient develped paraplegia.

Future attitudes and prudent practice

In the recent past, digital assessment of distal aorticpressure could be accepted as providing an adequatestandard of care during operations for coarctation ofthe aorta. With the development of accurate mano-metric methods, an increasing number of surgeonsnow use this technique-in 1985 more than half of thesurgeons questioned did so. The conscientious use ofeither of these methods could in the past be regardedas adequate evidence of good practice. Eventually,however, the undertaking of aortic surgery withoutcareful monitoring of spinal cord function might con-ceivably be regarded as negligent, especially if such anunmonitored procedure should be followed by neu-rological damage. There is certainly no guarantee thata distal pressure determined as adequate either by dig-ital palpation or by electromanometric recordingafter cross clamping will prevent paraplegia. Even-tually a more certain way of monitoring spinal cordfunction, such as the evoking of somatosensorypotentials, is likely to be available to all surgeons; butas yet these techniques are imperfect. My own experi-ence indicates that whereas the majority of surgeonspractising these operations are unaware of somato-sensory evoked potential as a method of spinal cordmonitoring, lawyers dealing with medical negligenceclaims are well aware of these developments and haveasked whether spinal cord monitoring was used dur-ing an operation on their paraplegic client. There is,however, no firm evidence to sustain the view that theabsence of somatosensory evoked potential moni-toring could have influenced the outcome of a par-ticular operation and thus label such practise asnegligent. The lawyers will invariably ask whether or

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Spinal cord damage and operations for coarctation of the aorta

not electromanometric methods rather than digitalpalpation were used to assess the adequacy of distalaortic pressure after cross clamping, and here theymay be on firmer ground. Whereas in the past wecould claim bad luck when patients developed para-plegia after an operation for coarctation, this may notbe so in the future. It is therefore predicted that, if thereliability of the technique can be established, the useof spinal cord monitoring by the evoking of somato-sensory potentials will be universally adopted. Whatthen should be our attitude towards the monitoring ofoperations on the descending thoracic aorta?

I believe that the monitoring of spinal cord func-tion by somatosensory evoked potentials may even-tually be shown to be accurate and reliable, but thishas not yet happened in the United Kingdom. On theother hand, the monitoring of distal aortic pressuresby electromanometric methods is clearly more sensi-tive and accurate than is the assessment of digitalpalpation alone. Although many experienced and dis-tinguished surgeons would disagree with this last sen-timent and claim that in their hands digital aorticpalpation has never been followed by paraplegia, itis likely that an uncomfortably large number ofsurgeons will have an operation complicated by para-plegia should they operate on a sufficient number ofpatients, whichever method of monitoring is used.

Cases are reported of paraplegia following shortperiods (under 20 minutes) of aortic cross clamping,confirming the view that in a very small number ofpatients even short periods of aortic cross clampingwill not be tolerated-regardless of the skill andexperience of the surgeon. It is advocated that solereliance on estimation of aortic pressures by digitalpalpation should be abandoned and replaced in allcases by electromanometric measurement, to give ourpatients the most accurate spinal cord monitoring ser-vice universally available today, and to establish anew yardstick of acceptable care. Although the use ofsomatosensory evoked potential monitoring of thespinal cord is largely undeveloped in the United King-dom, the work of Laschinger in the United States andPollock in Glasgow shows clearly that this methodhas great potential.7-9

It is recommended that interested users, includingthoracic surgeons, orthopaedic surgeons, and neuro-physiologists collaborate and develop somatosensoryevoked potential monitoring in their owndepartments, in the expectation that they will even-tually have at their disposal a comprehensive andreliable system of spinal cord monitoring.

I am indebted to Dr Hilary Morgan of the BurdenNeurological Institute, Bristol, for her review of this

paper and to my many colleagues in cardiothoracicsurgery who assisted with this survey.

G KEENBristol Royal Infirmary

Bristol BS2 8HW

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