By Karen Allen BBC Health Correspondent

The announcement by the defence Secretary Geoff Hoon that 900 medical reservists could receive call up papers to serve in the Gulf, is bound to send a slight shudder across the shoulders of any NHS Chief Executive.

Although not full time military personnel, many have clinical jobs in the frontline of an already squeezed NHS.

Their deployment could have a significant impact on the government's timetable for meeting hospital performance targets - especially if hostilities persist for several months.

Not all military personnel will be sent abroad in one go, but prospect of losing of 100 reservist doctors (in addition to regular arm staff) is worrying.

Not only that, but predicting where the shortfall will come, will be hard. Reservists have no obligation to inform their employer about what they do in their spare time.

So what's the picture so far?

This week Frimley Park Hospital in Surrey released 90 service men and women to fulfil their military responsibilities.

For managers of hospitals with regular military personnel, staff shortages in times of war is part of the deal.

It is one of six medical defence hospital units which together have a total of a thousand regulars on their books.

In peacetime they look after NHS patients as well as their own - but with the threat of war - just how much the health service depends upon will be revealed.

Ninety posts unfilled may not sound many in a large district general hospital - it represents 5% of the hospital's clinical staff.

But they include orthopaedic surgeons; anaesthetists; burns and chest experts not to mention technicians and nurses.

All are being deployed to serve in 33 Field Hospital - a 200 bedded unit that will receive casualties in the event of war.

Derriford hospital in Plymouth has already shed 40 naval staff.

All are being sent to serve on RFA Argus - part of the Royal Naval taskforce that's being kitted out to act as a hospital ship and is currently on exercise in the Mediterranean.

Staff shortfall

For managers of hospitals with regular military personnel, staff shortages in times of war is part of the deal.

A public hungry to see the NHS turned around may just have to wait.

But in peacetime the NHS gains allowing hospitals like Frimley park to run two more wards.

Frimley Park's Chief Executive Andrew Morris is confident they'll be able to make up the shortfall in staff, by calling on agency doctors and nurses to help fill the gaps.

It has worked in the past when regulars were deployed to Kosovo, but any long term deployment could make this hard at a time when the health service is already strapped for staff.

No one is criticising the fact that military action requires medical backup, but the defence medical services have seen their numbers slashed, making them increasingly dependent upon volunteers.

The landscape of the NHS has changed since the last major deployment to the Gulf.

Hospitals are under pressure to get waiting times down and achieve their performance goals, but as a member of the British Medical Association's defence committee intimated, troop deployments and rigid targets don't match.

A public hungry to see the NHS turned around may just have to wait.