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heart disease, and we may begin to see an increase in,say, renal failure. Depending on how the humanlifespan is engineered by evolution, radical

gerontology may, and some would say must,

eventually run into limits of the same kind. As we getthe better of malignant tumolirs we will have toconcentrate on heart and vessels-or in the reverseorder. When we achieve both, we may have to convertoncology and cardiology units to renal units. Andwhen we begin to address lifespan setting mechanismswe may need to shift all these interventions to a higherage range. The pace of such changes is likely to bemanageable, but there is no harm in thinking aboutthem and preparing a range of options.

1. Weindruch R, Walford RL. The retardation of aging and disease bydietary restriction. Springfield, Illinois: CC Thomas, 1988.

2. Olshansky SJ, Cames BA, Cassel C. In search of Methuselah: estimatingthe upper limits of longevity. Science 1990; 250: 634-40.

3. Putter A. Die Ältesten menschen. Naturwissenschaften 1921; 9: 875-80.4. Fries JF. Aging, natural death, and the compression of morbidity. N Engl

J Med 1980; 303: 130-35.

TENS for chronic low-back painThe use of electrical stimulation for pain relief has a

long history, but modem stimulators were developedas a result of the gate control theory and were firststudied in man in 1967.1 For transcutaneous electricalnerve stimulation (TENS),2,3 the patient carries asmall, portable, battery-powered stimulator which isconnected via wires to electrodes. The electrodes areself-adhesive or applied to the skin with adhesive tapewith an intervening layer of conductive gel. Dual-output machines allow two sites to be stimulated

simultaneously. The stimulator produces a pulsatileoutput, which can be of different wave-forms anddelivered either continuously or in bursts. The patientcan alter the output by adjusting the current intensityand pulse frequency and width to produce a tinglingsensation.TENS has been tried in numerous conditions,

although how it works is unclear. The technique hasbeen used most for chronic pain/,3 often for low-backpain. Does it help?

Several trials have indicated benefit from TENS in30-50% patients with chronic low-back pain, benefitoften outlasts the period of stimulation.4-9 However,many of these studies were uncontrolled, the patientshad different underlying conditions, and follow-upwas often short. TENS has also been compared withother forms of treatment. High-intensity TENSseems to be as effective as acupuncture and distantice-massage,l1 and more effective than conventionalmassage.12 Deyo and colleagues13 lately comparedTENS, sham TENS, a programme of exercises andTENS, and exercises and sham TENS in 145

patients. After a month’s treatment, TENS had

produced no clinically or statistically significant effecton pain, function, or back flexion. Exercise was

modestly beneficial for two months. The researchersconcluded that TENS is no more effective than

placebo for patients with chronic back pain and that itadded nothing to exercise alone.Deyo et al acknowledged that their results could

have been due to successful blinding that included acredible placebo and strong suggestion, but theydiscounted several factors that might have contributedto these negative conclusions-eg, inadequatestatistical power to detect important differences;inclusion of patients unlikely to respond to therapy;measures of outcome too unresponsive to detectclinically important differences; and inadequacy in themanner of the intervention to produce efficacy. Otherpossible factors were recruitment of the patients bynewspaper advertisement, which probably attracts anatypical sample of back-pain sufferers; the

requirement that they attend twice weeklyappointments; the short duration of therapy; and thattwo types of TENS were used.

The bewildering variety of TENS proceduresillustrates the general confusion surrounding this

technique. Studies have used stimulation that is

subliminal,14 comfortable,4 mildly uncomfortable,15tolerable,12 intense,16 produces a sensation ofvibration or tingling,17 or is sufficient to producemuscle contractions.18 In many trials, high-frequencylow-intensity stimulation has been used, althoughlow-frequency high-intensity ("acupuncture-like")stimulation may sometimes be more effective.19 Burststimulation does not appear to be any better thanconventional stimulation. 20

Electrodes are usually placed on or around theregion of maximum pain or near the nerves

innervating that area. Trigger points associated withmusculoskeletal pain often correlate with acupuncturepoints,21 and some,22 but not all,1s workers believe thatthese are important for effective TENS, andreduction in trigger-point sensitivity may not berequired.23 In practice, most therapists use trial anderror.

The most effective durations for each session andfor the course of TENS treatment have not beenestablished. In the study of Wynn Parry and Girgis,9TENS was given by a trained physiotherapist for aminimum of eight hours daily for two weeks; many oftheir 101 inpatients with chronic back pain, 72% ofwhom had not improved with surgery, were able toreturn to work, and TENS proved to be the singlemost useful treatment.

TENS is a very safe procedure, with fewcontraindications (eg, patients with pacemakers,pregnancy, use over the carotid sinus) and withside-effects largely limited to skin irritation under theelectrodes, which can be diminished by use ofnon-irritant gels. The patient needs to attend hospitalfor instruction in the technique. Despite the expense([80-[120 for the unit alone), if a small percentage ofpatients can lead a useful life, return to work, control

463

their own therapy at home, and avoid further

expensive treatment, this represents excellent valuefor money.TENS has a placebo effect as powerful as other

pain-relieving methods, especially at the start of

treatment/4 about 30% patients being helped bysham stimulation. This figure is probably less thanthat for patients helped by true TENS,2s and truestimulation mayb or may not provide longer benefitthan placebo. The impossibility of achieving goodcontrolled trials has been recognised ;26 apart fromsubliminal stimulation, which is as successful as shamstimulation,14 devices that induce tingling sensationscannot be compared with devices giving no electricaloutput. What is important is that TENS can producesome relief in over 30 % patients with intractable backpain. This is an impressive therapeutic result,achieved at little risk. It therefore seems sensible tooffer an adequate, supervised course of TENS topaients with chronic low back pain. Moreover, it

may well be irrelevant to subject TENS to furthertrials.

1. Wall PD, Sweet WH. Temporary abolition of pain in man. Science 1967;155: 108-09.

2. Mannheimer JS, Lampe GN. Clinical transcutaneous electrical nervestimulation. Philadelphia: FA Davis, 1984.

3. Gersh MR, Wolf SL. Application of transcutaneous electrical nervestimulation in the management of patients with pain: state-of-the-artupdate. Phys Ther 1985; 65: 314-22.

4. Indeck W, Printy A. Skin application of electrical impulses for relief ofpain in chronic orthopaedic conditions. Minn Med 1975; 58: 305-09.

5. Ersek RA. Low-back pain: prompt relief with transcutaneous neuro-stimulation. Orthop Rev 1976; 5: 27-31.

6. Long DM, Campbell JN, Gucer G. Transcutaneous electrical

stimulation for relief of chronic pain. Adv Pain Res Ther 1979; 3:593-99.

7. Bates JAV, Nathan PW. Transcutaneous electrical nerve stimulation forchronic pain. Anaesthesia 1988; 35: 817-22.

8. Fried T, Johnson R, McCracken W. Transcutaneous electrical nervestimulation: its role in the control of chronic pain. Arch Phys MedRehabil 1984; 65: 228-31.

9. Wynn Parry CB, Girgis F. The assessment and management of the failedback, Part II. Int Disabil Stud 1988; 10: 25-28.

10. Fox EJ, Melzack R. Transcutaneous electrical stimulation and

acupuncture: comparison of treatment for low-back pain. Pain 1976; 2:141-48.

11. Melzack R, Jeans ME, Stratford JG, Monks RC. Ice massage andtranscutaneous electrical stimulation: comparison of treatment forlow-back pain. Pain 1980; 9: 209-17.

12. Melzack R, Vetere P, Finch L. Transcutaneous electrical nerve

stimulation for low back pain. A comparison of TENS and massage forpain and range of motion. Phys Ther 1983; 63: 489-93.

13. Deyo RA, Walsh NE, Martin DC, Schoenfeld LS, Ramamurthy S. Acontrolled trial of transcutaneous electrical nerve stimulation (TENS)and exercise for chronic low back pain. N Engl J Med 1990; 322:1627-34.

14. Lehmann TR, Russell DW, Spratt KF, Colby H, Liu YK, FairchildML, Christensen S. Efficacy of electroacupuncture and TENS in therehabilitation of chronic low back pain patients. Pain 1986; 26: 277-90.

15. Wolf SL, Gersh MR, Rao VR. Examination of electrode placements andstimulating parameters in treating chronic pain with conventionaltranscutaneous electrical nerve stimulation (TENS). Pain 1981; 11:37-47.

16. Melzack R. Prolonged relief of pain by brief, intense transcutanoussomatic stimulation. Pain 1975; 1: 357-73.

17. Linzer M, Long DM. Transcutaneous neural stimulation for relief ofpain IEEE Trans Biomed Eng 1976; 23: 341-45.

18. Santiesteban AJ. The role of physical agents in the treatment of spinepain. Clin Orthop 1983; 179: 24-30.

19 Enksson MBE, Sjolund BH, Nielzén S. Long term results of penpheralconditioning stimulation as an analgesic measure in chronic pain. Pain1979; 6: 335-47.

20. Field GM. Spanswick CC, Hunter ME, Main CJ. A comparison of twomodes of transcutaneous electrical nerve stimulation in chronic back

pain. Pain 1990; 5 (suppl): S231.21. Melzack R, Stillwell DM, Fox EJ. Trigger points and acupuncture points

for pain: correlations and implications. Pain 1977; 3: 3-23.22. Berlant SR. Method of determining optimal stimulation sites for

transcutaneous electrical nerve stimulation. Phys Ther 1984; 64:924-28.

23. Graff-Radford SB, Reeves JL, Baker RL, Chiu D. Effects oftranscutaneous electrical nerve stimulation on myofascial pain andtrigger point sensitivity. Pain 1989; 37: 1-5.

24. Evans FJ. The placebo response in pain reduction. Adv Neurol 1974; 4:289-96.

25. Thorsteinsson G, Stonnington HH, Stillwell GK, Elveback LR.Transcutaneous electrical stimulation: a double-blind trial of its

efficacy for pain. Arch Phys Med Rehabil 1977; 58: 8-13.26. Deyo RA, Walsh NE, Schoenfeld LS, Ramamurthy S. Can trials of

physical treatments be blinded? The example of transcutaneouselectrical nerve stimulation for chronic pain. Am J Phys Med Rehabil1990; 69: 6-10.

To ventilate or not

Should ventilators be used to treat acute

exacerbations of chronic lung disease? The difficultiesof weaning such patients off ventilation are widelyrecognised. Intensive care beds may be occupied formany weeks, and patients may even require a

long-term tracheostomy, with or without mechanicalventilation via this route, when they return horned Todeny this therapy often condemns the patient to apremature death during the acute illness. Althoughknowledge of the previous forced expiratory volumein 1 second (FEV1), arterial blood gases, exercisecapacity, and quality of life may be helpful, this is notalways the case2and such data are often unavailable atthe time hard decisions have to be made. The

approach in the UK tends to be more conservativethan that in the USA, for example, where many ofthese patients are intubated and ventilated.The work of Brochard et aP suggests that this

dilemma may be receding. These researchers describea method of inspiratory pressure support ventilationwith a face mask that they have used during acuteexacerbations of chronic airflow obstruction. The

system provides a constant but adjustable positivepressure during inspiration; it is triggered by thepatient’s respiratory effort and stops when the

inspiratory flow rate falls below a threshold value.This treatment improved the p02, pCOz, and pH andreduced diaphragmatic activity. Intubation was

avoided in most patients. By comparison with amatched group of control patients who had beenmanaged conventionally in the intensive care unitduring the preceding two years, duration of stay in theunit was reduced and the survival rate was the same.

Although inspiratory pressure support ventilationhas become popular in intensive care units as a methodof weaning patients from conventional ventilation,4 4this method cannot be recommended unless facilitiesfor intubation are immediately available should thetechnique not prove successful. The inspiratory phasecan be terminated only when the flow rate through theventilator falls. An air leak around the face or nasalmask, for example, may prevent the ventilator from