D.W. Bethune

A comparison of five heat and moisture exchangers

Five heat and moisture exchangers were investigated to compare their efficiency of humidification, their ability to filter bacterial spores and their various physical properties. The results are presented and the various mechanisms of heat and moisture exchange are reviewed. The Pall Ultipor BB50. because of its hydrophobic properties, has a slightly different action from heat and moisture exchangers already in use. The place of the Pall Ultipor BB50 in clinical practice is discussed.

Test of delayed memory recall suitable for assessing postoperative amnesia

The application of a test of delayed memory recall to determine the extent of postoperative (anterograde) amnesia is described. Results from studies following the application of different anaesthetic techniques to patients undergoing minor surgery suggests that a period of anterograde amnesia is inevitable following general anaesthesia.

Management of donors for heart and heart–lung transplantation

The quality of donor organs will determine the quality of life for the recipient and the importance of optimal management of the multi‐organ donor is that the organs may benefit up to five, critically ill, patients. The basic principle is to maintain sufficient preload to minimise the need for inotropic support and it is recommended that all multiple organ donors should have central venous and arterial pressure monitoring in addition to adequate venous access. The importance of the choice of fluid for volume expansion and the management of the hormonal disturbances which follow brain death are considered.

Removal of anaesthetic waste gases

operatively, her haemoglobin concentration was 6.9 g/dl. Before induction of anaesthesia, the probe of a Drager transcutaneous oxymeter (cat. no. I 1 1 40605) was applied to the skin in the intraclavicular region, permitting continuous intra-operative monitoring of the transcutaneous PO, by means of both a digital display and paper chart trend recorder. The relative heating power required to keep the skin under the detector at 45°C was also displayed; this permitted early changes in peripheral circulation to be detected and enabled optimal peripheral perfusion to be maintained with fluids and mild a-blocker therapy, thus preventing capillary sludging and possible sickling. Intra-operative measurements made on blood taken from a radial artery cannula showed that the transcapillary PO, rapidly reflected changes in arterial PO,, and the values obtained indirectly were consistently 1.2 kPa less than those obtained directly. Following surgery the child was transferred to the intensive care unit and oxygen was administered for 24 hours postoperatively. During this period the transcutaneous PO, was continuously monitored and observed by the nursing staff. The ability to measure the transcutaneous PO, represents a significant advance in the anaesthetic management of patients with sickle cell states and this facility should be available in centres dealing with large numbers of these cases.

Air embolism during anaesthesia

Earlier investigations hoped to show that fazadinium had two advantages, namely speed of onset and secondly a short duration of action. A competitive neuro-muscular blocking drug with both these properties would be ideal. Clinical trials using fazadinium demonstrated rapid onset but a duration of action not far removed from other known drugs. The question asked was whether or not the rapid speed of onset would have advantages if shown to be similar to suxamethonium and this can only be decided by individual opinion. We therefore attempted to measure the speed of onset on a comparative basis with suxamethonium and alcuronium which, too, has a fairly rapid speed of onset and equally important to assess the comparative conditions for intubation. The implications from our findings must be left to the individual reader. Dr D.J.Hill rightly points out that the complications of suxamethonium are not in dispute but he answers his own question referring to prior injection of the non-depolarising relaxant, namely the possibility of sudden failure of access of a vein and delay of injection of the induction agent, but he does not mention how he gauges his dose of the induction agent to avoid possible awareness a t the time of intubation. Dr D.M. Jackson is surely reiterating the need for a quick acting non-depolarising relaxant with short duration which at present we do not have. There is no doubt that suzamethonium for the time being provides optimum conditions for intubation and, in that sense, whether a difficult intubation might be foreseen or not, in a known situation where aspiration risk is high, suxamethonium must be the choice of relaxant, and this applies with equal importance to obstetric anaesthesia. There are situations in anaesthesia where speed of onset and rapidity of intubation are desirable but in which it would be better to avoid suxamethonium and, in many elective cases, many anaesthetists nowadays tend to reject suxamethonium in favour of a rapidly acting non-depolarising relaxant with fewer side effects. Fazadinium appears to have marginal advantages over alcuronium from the results of work so far undertaken. The longer duration of action of fazadinium overrides any significant advantage in cost when comparing this drug with alcuronium although I agree that a drug doing an equal job of work and costing less would be appropriate. May we also point out to Dr Hill that hundredths of a minute may be converted to seconds simply by multiplying by a factor of 60. The Omega stop watch used measured time in hundredths of a minute and certainly made for simpler mathematics.

Mesenteric ischaemia and endotoxaemia during cpb may cause significant splanchnic injury in transplant patients

Under normal conditions endotoxins are retained within the lumen of the qut and are prevented from leaking into the portal circulation by the integrity of the qut mucosa.If small quantities permeate through the barrier and reach the liver they are filtered out by the Kupfer cells. Eowever it has been shown that during cardiopulmonary bypass (CPB) there is a significant level of endotoxin passing through the liver and reachinq the systemic circulation (1) , The mechanism for this is complex. The vasopressor response to CPB is well recognized and includes the combined effects of catecholamine release, activation of the renin anqiotensin system, increased secretion of vasopressin and local tissue agents such as thromboxane A2. The resulting low flow in the splanchnic circulation accompanied by reperfusion injury causes mucosal disruption and leakaqe of endotoxins (2). This low flow in the portal system also adversely effects the liver and the ability of the Kupfer cells to effectively deal with the endotoxin challenge, and so they respond by producing cytokines such as tumour necrosis factor.

Health Equipment(Dis)Information: HEI 166 Evaluation of heat and moisture exchangers

Health Equipment Information Bulletins (HEI) are produced to help and guide purchasers to make rational choices between various items of equipment. They are intended to offer an efficient and accurate method to allow comparisons of equipment which have been tested and evaluated with due regard to its use in the NHS. The value of the reports will depend on the evaluators using relevant tests to compare the devices. A unique test method is used to evaluate heat and moisture exchangers (HMEs) 1 , 2 in HE1 166; unfortunately, the chosen test method is not relevant when the clinical situation is considered. Eight groups at least have published methods over the last 30 years for testing HMEs experimentally with lung analogues which deliver air saturated with water vapour to the devices (a complete list of references is available from the author). It is possible to determine the ‘efficiency’ of the device by determining the reduction in water loss from the analogue lung. In addition to the credibility that methods which involve the use of a saturated test gas have acquired from previous workers, the committee responsible for the current draft I S 0 standard have specified saturated gas in that document. The evaluators who produced HE1 166 decided, for reasons which are not tenable, that when an HME is used clinically the expired air must become progressively desaturated. They then devised an analogue lung which fits in with their hypothesis and which is unable to produce saturated air at clinically relevant minute volumes. The concept of significant desaturation of expired air is not tenable. Alveolar air, in the living organism, must always be fully saturated and forms the greater part of the expired air. The alveolar air cools in its passage through both natural and artificial airways, losing water vapour as it cools, but remains fully saturated. Published experimental data obtained when the original HME was used on a patient demonstrated that the expired air was fully saturated. Another study of intubated patients without a heat and moisture exchanger showed again that the expired air was fully saturated4 at all points from 10 cm below the carina to the expiratory limb of the system. We have confirmed, using a commercially available Dewpoint Hygrometer * with a heated sampling probe, that the expired air was fully saturated in every patient we tested with a hydrophobic HME (Pall BB50) in use for 24-48 hours and, in one patient, have found that the expired air was still fully saturated after the use of such hydrophobic HMEs for over 400 days. There can thus be no justification on clinical grounds for the use of an analogue lung model, as found in HE1 166, which does not provide fully saturated expired air. The reply to a request to the DHSS for clarification on this point merely reiterated the points made in Appendix 2 of HE1 166, to the effect that the evaluators believe that expired gas is not necessarily saturated in the respiratory tract; no clinical or experimental evidence is advanced to support this proposition or to refute the findings of other ~ o r k e r s . ~ , ~ However, is this experimental aberration important? The authors of HE1 say that their results will be suitable for comparisons between devices. They refer in the article to acceptable levels of humidification which cannot possibly be achieved with their test rig since no HME can add heat and moisture to a system and the ‘expired air’ they provided from their test rig did not even reach the levels which they considered acceptable for ‘inspired air’. The effect of an increase in minute and tidal volume is

Respiratory obstruction following vocal cord injection. A complication on induction of anaesthesia

Two patients who each had a paralysed left vocal cord are discussed. Both had poor coughing ability preoperatively. The affected cords had been injected with tetrafluoroethylene (Teflon) paste 3 days before operation and this had given immediate improvement in coughing and speech. During induction of anaesthesia, respiratory obstruction occurred that was related to the injected cord. This cause of obstruction has not been reported previously. The potential benefits of cord injection for voice, cough and laryngeal competence are reviewed.

A safety block for scavenging systems.

A safety block is described which protects the patient from expiratory obstruction when using scavenging systems, and provides the anaesthetist with a visual and auditory check on the functioning of the system.

Alarm systems for infusion pumps

There is an increasing use of calibrated infusion pumps in anaesthesia and intensive care. These pumps provide an efficient means of delivering fluid or drugs to patients over predetermined intervals. The use of any automatic system is potentially dangerous and such life support systems must have comprehensive visual and audio alarms. It is our practice to infuse vasodilator and inotropic support drugs through Imed 927 and 928 infusion pumps. We have become concerned that the audio alarm has been cancelled in many instances; no doubt due to the irritating noise which emanates during priming. This has led to dangerous situations when alarm conditions have resulted in the pumps stopping and the unobtrusive visual indicators have not been noticed for some time. We have subsequently modified the audio alarm cancel switch by fitting a ‘hold-off switch which allows the alarm to be held off during priming but as a result of the automatic resetting, subsequent dysfunctions of the infusion system are audibly communicated to the operator. (The switch (RS317162) is available from R.S. Components Limited and costs f1.24.) It is noteworthy that the newer Imed 960 possesses an audio alarm.delay rather than cancel switch.