**6.4 Application of rational mechanical prophylactic measures**

In the absence of contrary evidence, there is a *prima facie* case for using the measures described in section 6.2 throughout the period of bed rest for all acute medical patients and for surgical patients with limited mobility. That would be in line with the recommendations of NICE in the UK, of similar bodies in other European countries, and of the Joint Commission on Accreditation of Healthcare Organizations in the USA (see earlier discussion), except that anticoagulants would be given a less central role in prophylaxis. (There would be little or no need for any such measures in ambulant patients.)

One area that might need more or less radical reconsideration is surgery involving general anaesthesia. However, we emphasise that our comments here are conjectural and would require detailed evaluation before they were considered for practical application. Also, they relate to a potential risk associated only with very prolonged anaesthesia, involving sustained muscle relaxation, not to surgical operations in general.

It is self-evident that relaxant anaesthesia is inherently thrombogenic. For short operations, this is unlikely to matter greatly; but for longer operations (say over 100 minutes) during which the patient is totally motionless, there is a major risk that undetectable prothrombogenic nidi will form in valve pockets. The muscular paralysis induced during anaesthesia will therefore cause thrombosis in every case unless the duration of the resultant streamline (non-pulsatile) blood flow in all veins of the body is constantly kept in mind. If the unconscious patient's veins are not squeezed, e.g. by mechanical movement of limbs, then valve pocket hypoxaemia cannot be avoided. In reality, of course, the risk depends on how quickly the patient recovers from the effects of the muscle relaxant so that the skeletal muscles of the limbs start to contract again. It would seem ideal for the state of total muscle paralysis to last only ½ to ¾ of an hour, after which the anaesthetist would have to administer another dose of curare, but that is speculation; confirmation from practice and experience would be essential before such proposals were applied.

All anaesthetists are acutely aware of the respiratory support (sustained oxygen supply) needed for safe anaesthesia, but they are far less likely to be concerned about the restoration of pulsatile blood flow in the patient's legs/ abdomen. Appropriate practice must relate to the acceptability of temporary recovery of patient motion to the surgeon, and of course the delicacy of the operation and the disturbance likely from any change in anaesthetic practice.

Besides monitoring relaxant anaesthesia, the practice of end-to-end Trendelenburg/anti-Trendenburg tilting discussed in section 6.2 could be re-employed. However, the number of degrees elevation and reduction would be only 2 x 5% to and fro in 'horizontal patient' operations. Operations performed e.g. in sitting positions would require different manoeuvres appropriate to the particular patient posture. The objective in all cases would be to limit drugged-immobility to a specific duration that must be established by experience (after an initial informed guess based on e.g. the evidence from Hamer *et al.*, 1981).
