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COVERS for anaesthetic machines: an audit and standard

Krimmer, M.; Lake, A. P. J.; Wray, I.

Author Information
European Journal of Anaesthesiology: September 1997 - Volume 14 - Issue 5 - p 505-513



It is not disputed that safety concerns and appropriate anaesthesia apparatus are essential for today's anaesthetist. There is likewise no dispute that the anaesthetist is ultimately responsible for performance, both personal and equipment, both of which the anaesthetist should ensure is properly prepared for the case. To this end the Association of Anaesthetists of Great Britain and Ireland (Association) has produced guidelines [1] for checking procedures but as these do not detect all possible faults [2-5], they are not considered to be adequate by themselves. The use of the Federal Drug Administration anaesthesia apparatus checkout recommendations was shown to detect four pre-arranged faults on only 29.9% of occasions [6].

Previous studies have used a checklist to eliminate faults [7] and assessed checking practice in terms of performance and time expended [8,9]. The use of shortened checklists either for general application [10] or individually drawn up [11] has been suggested, and also the value of visual aids promoted [12] and tested [13]. The possible scale of checking that can be carried out is considerable [14] and we must ask whether it is still necessary to promote a full check, including disconnection and cylinder test by two operators with a check list taking an absolute minimum of 10 min per machine prior to every operating half day, on machines in fixed locations in theatre, or is an alternative approach justified? Current Association guidelines, on which the checking practice at our hospital was previously based, are recommendations only and in reality such extensive checklists may not be regularly carried out because they are perceived as not an overwhelming necessity and too time consuming [15, 16]. Three to five minutes was found to be acceptable in Norway, most anaesthetists would not adopt recommendations requiring more time [17].

We report, first, an initial Audit of all our anaesthesia machines to look for faults. Second the establishment of a standard comprising COVERS (Table 1), a simple 'quick check' to be performed prior to each list by the rostered anaesthetist, together with a system of comprehensive checks. Third a re-audit to measure the effectiveness of the new standard.

Table 1
Table 1:
COVERS: a pre-use equipment quick check


The Checklist for Anaesthetic Machines suggested by the Association of Anaesthetists of Great Britain and Ireland was taken as a base to which we added the following checking procedures to ensure comprehensive coverage:

  1. The test for leakage described by Page [4] for the anaesthetic machine itself. The common gas outlet is connected to a pressure gauge and the oxygen valve slowly opened and adjusted to maintain a pressure of 16 kPa (120 mmHg) at the common gas outlet. The oxygen flow measured in this way is an indication of leakage; more than 200-300 mL min−1 is considered unacceptable.
  2. The test for any obstruction to the expiratory limb and the scavenging system as recommended by James [2] using a dummy lung.
  3. A check of the Bain [5] and Lack co-axial systems.

For the audit, the 30 anaesthetic machines used in the District General Hospital at that time (Table 2) were each checked by the same post-fellowship registrar (MK) according to a checklist (Appendix 1) on three occasions over 8 weeks to ensure that data to support any necessary changes would be unequivocal. Ninety checks were therefore performed, each machine being checked before the start of a list, out of hours and randomly on a third occasion. Machines in group A each have integrated oxygen failure protection and 0.5 L min−1 limited flow for carbon dioxide. The machines in group B (accident and emergency resuscitation room) have integrated oxygen failure protection devices but no cylinders or carbon dioxide facility. The remaining machines in group C were older British Oxygen Company (BOC, now part of Ohmeda Group, Hatfield, Herts) machines of which three used cylinders only and three had cylinders and piped oxygen.

Table 2
Table 2:
Anaesthetic machines tested

We defined the number of cylinders (BOC 'E' - internal volume 5 litres) required as one oxygen and one nitrous oxide when piped gases were available and two each without piped gases and fully correct labelling as 'in use' and 'full labels' on all cylinders. The number of cylinder keys should be two or more [14, 18]. We assessed a reserve cylinder to be full when 120 × 102 kPa (120 bar) or more registered for oxygen and 40 × 102 kPa (40 bar) or more for nitrous oxide. For 'in use' cylinders we were satisfied when the indicator was not in the red area on corresponding gauge which varied with the machine (15-30 × 102 kPa (15-30 bar) = 75-150 litres for oxygen and 5-10 × 102 kPa (5-10bar) =30-65 litres for nitrous oxide) as the machine would always either carry a full cylinder or pipeline connection. For carbon dioxide cylinders we took 30 kPa or more as a satisfactory filling state.

Not all breathing systems available for use were checked on each occasion. If not already attached to the common gas outlet of the anaesthetic machine one was taken from the selection available. If the circle system was attached to the common gas outlet another breathing system was chosen and checked in addition to extend the checking process.

Checking for leaks in the circle proved difficult as we have different machines in use and the distensible reservoir bag cannot be excluded in all of them. Attached to a ventilator we took full bellows after 3 min of ventilation 10 breaths min−1 with tidal volume 1000 mL and a basal fresh gas flow of 300 mL min−1 (P max 4 kPa) as an absence of significant leak.

Three different types of oxygen analysers were present: Ohmeda 5120 (Ohmeda, USA), Datex Capnomac Ultima and Datex Cardiocap (Datex, Finland). The East Apollo machines also have an indicator that shows green for oxygen on line and red no oxygen available.

Only when ventilators were available for use with a machine was the presence of a disconnection and high airway pressure alarm and a tidal/minute volume monitor considered to be necessary.

Adequate suction power is described as minus 600 mmHg (80 kPa, 0.8 bar) [14] or minus 500 mmHg (67 kPa, 0.67 bar) within 10 s [19] [BS (British Standard) 4199] which we adopted.

Following the initial audit, a standard for anaesthetic machine checking comprising COVERS (Table 1): a simple 'quick check' to be performed prior to each list by the anaesthetist, and a system of comprehensive checks using a fixed schedule was established by the department of anaesthesia and put into effect.

After 1 year in operation a re-audit of all machines using the same checklist by the same consultant anaesthetist (APJL) and/or senior operating department assistant (IW) was carried out.



The time required for checking varied widely with the complexity of the machine and decreased progressively on each of the three occasions, first mean 18 min (range 3-30), second mean 13 min (range 5-27) and third mean 10 min (range 4-15). The 3-5 min checks were for the East Apollo Induction machines, which are simple, with piped gases only. For the full series of three checks per machine, 24 of the tests performed revealed faults (Table 3) with a range from 1.1-64%. For the remaining tests in the schedule (Appendix) all the equipment functioned correctly.

Table 3
Table 3:
Faults: tests from comprehensive checklist revealing faults. Audit (90 check lists) and re-audit (102 check lists)

On 16 occasions an oxygen analyser was not present. Those present were faulty (usually the fuel cell) on 34 out of 74 occasions (46%). Inadequate cylinder labelling (16.6%) and cylinder key provision (64%) were common faults. Where O2 cylinder contents were found to be inadequate reserve cylinders were always adequately filled. Four Lack co-axial Mapleson A rebreathing circuits failed the test having a communication between the inner and outer tube, but all Bain co-axial Mapleson D breathing circuits were satisfactory. Circle systems with parallel breathing circuits examined on 25 occasions leaked on 10 (attached to a ventilator with descending bellows at basal fresh gas flow), which in two cases could be abolished by switching off the carbon dioxide absorber.

On five occasions when ventilators were in use there was neither a disconnection nor a high airway pressure alarm, and on nine occasions no functioning volume monitor was present. On 14 occasions scavenging was absent, but on the remaining 76 it was correctly configured, although not always connected to the breathing system. On three occasions there was no valve and on five the scavenging hose had been taped to the wall to secure it at the outlet.

An inadequate negative pressure [<500 mmHg (67 kPa, 0.67 bar)] was generated by the suction unit on 15 of the 90 tests. On five occasions faults arose from connections to the vacuum pipeline.

Of the faults detected in the 30 machines undergoing the full series of three tests each, 171 (72.5%) were considered non-recurring and would be detected and eliminated by the scheduled check and maintenance. The remaining 65 (27.5%) would require to be identified and rectified prior to each use (Table 3).


The following was agreed and established after the initial audit:

  1. COVERS - a pre-use checklist. Prior to each operating list the anaesthetist must complete the checklist to detect possible faults with the equipment and readiness for endotracheal/airway management (Table 1) and sign the audit diary in which any problems must also be recorded.
  2. Comprehensive checklist. Modern machines need servicing 6 monthly and older 3 monthly. The schedule requires each machine to be comprehensively tested against a checklist (Appendix) following return from service or breakdown and midway between routine services. The tests are carried out by an anaesthetist and trained operating department assistant (ODA) together and include an examination of the COVERS diary.


The equipment replacement programme for anaesthetic machines changed during the year (Table 2). Six MIE Cavendish 460 (group A) became available for use in main theatre anaesthetic rooms.

Each of the 34 machines were comprehensively checked according to the schedules (Appendices 1, 2 and 3) on three separate occasions. Twenty of the tests performed revealed faults (Table 3) with a range from 1.0-11.6%. For the remaining tests all the equipment functioned correctly.

Four new oxygen analysers were supplied during the re-audit and only one was still absent from a machine at the end; faults with them have been largely eliminated as have those in respect of medical gas supplies. Re-breathing circuit faults continue to feature in particular with respect to reservoir bag holes and connection between inspiratory and expiratory limbs of Lack co-axial circuits. We are transferring to parallel systems. Circle system leaks and suction problems have also been essentially eliminated. Volume monitor provision has improved but remains unsatisfactory. Scavenging extraction faults persist despite a new system having been installed in the main theatre suite. Three machines have no access to ducted scavenging and are being provided with anaesthetic vapour adsorbers (Cardiff Aldasorber™).


The first part of the study showed that faults arose with a frequency that had not changed significantly since 1992 when Barthram and McClymont [8] found faults in 60% of their machines.

It is recommended that anaesthetists perform preuse checks according to the Association procedure, but in practice this is often not the case. An extended complete checking procedure involving pipeline disconnection is not only time and cost consuming but also can damage terminal outlets. Many senior anaesthetists do not see the value or necessity of such a practice.

Unfortunately by insisting that the recommended procedure is the only approved method of checking results on many occasions in no checking whatsoever.

A picture guide of the full checklist has been suggested to improve compliance [13] but as Petty comments [10] 'there must be a balance between superficial observation of the machine and a three page checkout list devised from the procedures recommended by certain anaesthesia machine manufacturers'. An acceptable checkout routine is a compromise between comprehensiveness and speed.

The initial audit of our machines showed problems that could have lead to critical incidents, and we are sure that our findings are typical. They suggest the need to establish a checking system that is not only practical and acceptable but also robust and effective.

The modern anaesthetic machine is not only complex but also difficult to test by standard procedures as the range of inbuilt alarms and safety features decrease the ability of the anaesthetist to perform a thorough check. Furthermore, it is now a recommendation [20] that the anaesthetist 'must' be present throughout the conduct of general anaesthesia and this highly trained person is available to detect faults as they arise in both patient and equipment. The pilot analogy is commonly advanced as support for a comprehensive immediate pre-use check but in reality the pilot accepts that extensive checking procedures have already been carried out by others to whom this function is delegated. Modern anaesthetic practice generally revolves around machines in fixed locations with pipelines connected, and under these circumstances repeated daily comprehensive checks to exclude many non-recurring faults are surely excessive.

COVERS takes but a few minutes to perform and, although not comprehensive, encompasses those aspects required to make sure that the anaesthetic machine is safe to use. The removal of the most commonly detected faults at the initial audit (72.5%) would be the task for those undertaking the scheduled comprehensive check because they are in general non-recurring and inherent in the type of equipment used. The delegated anaesthetist and ODA are responsible for correcting the faults or removing the machine from service and implementing corrective maintenance.

The re-audit has demonstrated the effectiveness of our new procedures. Continuing improvement in equipment provision (oxygen analysers, volume monitors and scavenging) is a matter being addressed by the department of anaesthesia. A beneficial closer liaison with the EBME (Electro Bio-Medical Engineering) department who maintain the equipment has developed.

We believe that a COVERS check as the mandated requirement before every list together with the formal comprehensive check according to a fixed schedule are appropriate for the delivery of safe anaesthesia.


1 The Association of Anaesthetists of Great Britain and Ireland. Checklist for Anaesthetic Machines, a Recommended Procedure Based on the Use of an Oxygen Analyser. London: 1990.
2 James RH. Checking the expiratory limb of the breathing system. Anaesthesia 1994; 49: 646-647.
3 Jackson IJB, Wilson RJT. Association of Anaesthetists checklist for anaesthetic machines. Anaesthesia 1993; 48: 152-153.
4 Page J. Testing for leaks. Anaesthesia 1977; 32: 673.
5 Foëx P, Crampton Smith A. A test for coaxial circuits. Anaesthesia 1977; 32: 294.
6 March MG, Crowley JJ. An evaluation of anesthesiologists' present checkout methods and the validity of the FDA checklist. Anesthesiology 1991; 75: 724-729.
7 Cundy J, Baldock GJ. Safety check procedures to eliminate faults in anaesthetic machines. Anaesthesia 1982; 37: 161-169.
8 Barthram C, McClymont W. The use of a check list for anaesthetic machines. Anaesthesia 1992; 47: 1066-1069.
9 Mayor AH, Eaton JM. Anaesthetic machine checking practices. Anaesthesia 1992; 47: 866-868.
10 Petty C. The Anaesthesia Machine. Edinburgh: Churchill Livingstone 1987: 217-218.
11 Zorab JSM. Anaesthetic machine checking practices. Anaesthesia 1993; 48: 267.
12 Adams AP, Morgan M. Checking anaesthetic machines - checklist or visual aids? Anaesthesia 1993; 48: 183-186.
13 Groves J, Edwards N, Carr B. The use of a visual aid to check anaesthetic machines. Anaesthesia 1994; 49: 122-125.
14 The North American Dräger Company. Anesthesia Apparatus Checkout Recommendations: Pre-use Checkout and Inspection Procedures (based on North American Dräger Safety guidelines). Pennsylvania: 1989.
15 Higham H, Beck GN. Checking anaesthetic machines. Anaesthesia 1993; 48: 536.
16 Lake APJ, Webb TB. Checking anaesthetic machines. Anaesthesia 1993; 48: 536.
17 Berge JA, Gramstad L, Grimnes S. An evaluation of a time-saving anaesthetic machine checkout procedure. Eur J Anaesthesiol 1994; 11: 493-498.
18 Chapman JM. Checking equipment before anaesthesia. Br J Hosp Med 1987; 38: 470-472.
19 Davey A, Moyle JTB, Ward CS. Ward's Anaesthetic Equipment, 3rd Edn. London: W.B. Saunders, 1992: 48.
20 Association of Anaesthetists of Great Britain and Ireland. Recommendations for Standards of Monitoring During Anaesthesia and Recovery, Revised Edn. London: 1994.

Appendix 1

Pipeline machines

A O2 analyser

1 present on common gas outlet (CGO)

2 calibrated 21% set lower alarm limit (20%) set upper alarm limit (100%)

B Medical gas supplies disconnect all supplies, turn all vaporizers off, turn electricity and gas supply switch on (if appropriate)

1 cylinders required present

2 cylinders labelled correctly

3 cylinders securely seated

4 blanking plugs on empty yokes

5 correct number of cylinder keys turn off all cylinders open all flowmeter control valves turn on O2 reserve cylinder

6 O2 reserve cylinder opens easily

7 O2 reserve cylinder contents adequate (≥120 × 102 kPa)

8 O2 flowing

9 flowmeter fully adjustable where second O2cylinder is available, turn off reserve and turn on in-use cylinder

10 O2 in-use cylinder opens easily

11 O2 in-use cylinder contents adequate (≥15-30 × 102 kPa=75-150 litres)

12 O2 flowing

13 flowmeter fully adjustable set O2flow at 5 L min−1turn on reserve N2O cylinder

14 N2O reserve cylinder opens easily

15 N2O reserve cylinder contents adequate (≥40 × 102 kPa)

16 N2O flowing

17 flowmeter fully adjustable where second N2O cylinder is available, turn off reserve and turn on in-use cylinder

18 N2O in-use cylinder opens easily

19 N2O in-use cylinder contents adequate (≥5-10 × 102 kPa=30-65litres)

20 N2O flowing

21 flowmeter fully adjustable set N2O flow at 5 L min−1, turn off O2cylinder, press O2flush (emergency O2) until system is empty

22 cylinder gauge now zero

23 audible O2 failure alarm working

24 O2 failure protection working (cut off N2O) connect O2pipeline

25 O2 flow restored

26 O2 failure alarm discontinued perform tug test

27 O2 pipeline secure

28 terminal outlet leaks

29 O2 pipeline pressure reading 400 kPa turn off N2O cylinder and allow to empty, connect N2O pipeline

30 N2O flow restored perform tug test

31 N2O pipeline secure

32 terminal outlet leaks

33 N2O pipeline pressure reading 400 kPa turn on CO2cylinder where available

34 CO2 cylinder opens easily

35 CO2 cylinder contents adequate (≥30 × 102 kPa=40 litres)

36 CO2 flows

37 flow fully adjustable connect air pipeline

38 air flow restored perform tug test

39 air pipeline secure

40 terminal outlet leaks

41 air pipeline pressure reading 400 kPa turn off N2O, CO2, air flowmeters, attach sphygmomanometer to CGO; FGF O2300 mL min−1, ≥ 120 mmHg pressure

42 check for gas leaks vaporizer(s) 'on'

43 check for gas leaks vaporizer(s) 'off'

44 pressure relief valve operates (back bar) - audible switch off all flowmeter control valves operate O2flush

45 basal oxygen 2-300 mL min−1 flowing

46 no significant drop in pipeline/cylinder pressure

47 O2 analyser reading 99%/100%

C Vaporizers

1 fitted correctly on anaesthetic machine

2 locking mechanisms fully engaged ('O' rings in place)

3 control knobs rotate fully through range turn vaporizer off

4 filled to correct level (over/under)

5 filling ports tightly closed O2flow 6-8 L min−1, temporarily occlude CGO turn on vaporizers in turn

6 no leak from vaporizer fitment

7 no leak from filling port

D Circle system If no pressure gauge in circuit move sphygmomanometer to patient end

1 configuration correct

2 soda lime canister filled, colour correct

3 APL or other expiratory valve fully mobile close APL valve, attach dummy lung (1.0 litre reservoir bag), reduce O2flow to 200 mL min−1or machine minimum, manually ventilate

4 unidirectional flow valves OK

5 no leaks (including canister)

6 CO2 absorption on/off switch operational

7 pressure increasing in system (circle pressure gauge or sphygmomanometer) open APL valve

8 pressure falls to atmospheric (no PEEP)

E Breathing systems, type: move sphygmomanometer to patient end of each circuit in turn, O2flow 5 L min−1

1 configuration (bag, valves, etc.) correct

2 APL valve fully mobile

3 connections tight occlude patient end, close APL valve

4 bag fills up (visual check for leaks) Bain: occlude inner tube, flush

5 bag does not fill, pressure relief valve (back bar) opens Lack: apply positive pressure to inner tube only

6 bag does not fill attach dummy lung (1.0 litre reservoir bag)

7 pressure increasing in circuit open APL valve

8 pressure falls to atmospheric (no PEEP)

F Ventilator, type: attach/turn on ventilator

1 all ventilator controls work (e.g. VT, F, I:E)

2 disconnection alarm present and working

3 high airway pressure alarm present and working

4 volume monitor present

G Scavenging

1 configuration correct switch on

2 extraction indicator present and working

3 unobstructed (test flow down conducting tube)

H Suction connect vacuum pipeline, perform tug test

1 vacuum pipeline secure

2 terminal outlet leaks turn on

3 adequate negative pressure generated rapidly (500 mmHg in 10 s)

4 suction tubing/catheter present

Appendix 2

Cylinder only machines

As appendix 1 except

B Medical gas supplies turn on CO2cylinder where available

22 CO2 cylinder opens easily

23 CO2 cylinder contents adequate (≥30 × 102 kPa=40 litres)

24 CO2 flows

25 flow fully adjustable set N2O flow at 5 L min−1, turn off CO2flowmeter, turn off O2cylinder, press O2flush (emergency O2) until system is empty

26 cylinder gauge now zero

27 audible O2 failure alarm working

28 O2 failure protection working (cut off N2O) turn off N2O flowmeter, turn on in-use O2cylinder, attach sphygmomanometer to CGO;FGF O2300 mL min−1, 120 mmHg pressure

29 check for gas leaks vaporizer on

30 check for gas leaks vaporizer off

31 pressure relief valve operates (back bar) - audible switch off all flowmeter control valves operate O2flush

32 no significant drop in cylinder pressure

33 O2 analyser reading 99%/100%

Appendix 3

East Apollo machines

As appendix 1 except

B Medical gas supplies Disconnect all supplies, turn vaporizer off open all flowmeter valves connect O2pipeline

1 O2 flows perform tug test

2 O2 pipeline secure

3 terminal outlet leaks

4 O2 pipeline pressure reading 400 kPa

5 flow fully adjustable set O2flow 5 L min−1connect N2O pipeline

6 N2O flows perform tug test

7 N2O pipeline secure

8 terminal outlet leaks

9 N2O pipeline pressure reading 400 kPa

10 flow fully adjustable set N2O flow 5 L min−1disconnect O2pipeline

11 N2O flow cut off, O2 failure alarm sounds red warning indicator reconnect O2pipeline

12 O2, N2O flows restored, green indicator turn off N2O flowmeter, attach sphygmomanometer to CGO; FGF O2300 mL min−1, 120 mmHg pressure

13 check for gas leaks vaporizer on

14 check for gas leaks vaporizer off

15 pressure relief valve operates (back bar) - audible switch off O2flowmeter operate O2flush

16 no significant drop in pipeline pressure

17 O2 analyser reading 99%/100%

D Circle system - omitted


EQUIPMENT, anaesthetic machine, checklist; SAFETY, checklist; AUDIT

© 1997 European Academy of Anaesthesiology