Correspondence

1 103

Fig. 1.

sequence induction using thiopentone 5 mg.kgi' and suxamethonium I mg.kg-'. The trachea was intubated and the lungs ventilated with I % isoflurane in 70% nitrous oxide and oxygen using a Nuffield Penlon 200 ventilator and a Bain coaxial system. Fentanyl 100 pg was given after delivery of a healthy male infant. Fifteen minutes later the endtidal carbon dioxide was noted to be 2.9 kPa. As all other vital signs including pulse and blood pressure were entirely stable. hyperventilation was diagnosed and ventilation was adjusted accordingly. However, the end-tidal carbon dioxide continued to decrease and a further cause was sought. Disconnecting the Bain system from the machine revealed a faint hiss from the swivel connector. The oxygen flush button was found to be partially depressed and the problem was rectified. The inspired isoflurane concentration was increased and intravenous hyoscine was given in an attempt to increase sedation and amnesia. The patient was not sweating or lacrimating unduly. Chest movements and inflation pressures were normal and there was no tachycardia or change in blood pressure. At the end of the operation the oxygen flush button was examined and it was found that the return spring (arrowed. Fig. 1) was missing from the assembly. The machine had recently returned from a service and we presume that the return spring had been omitted during re-assembly. The patient experienced some awareness of events after induction but did not feel any pain. The second case involved a similar method of anaesthesia. However, soon after tracheal intubation and connection to the Bain system and Nuffield Penlon ventilator, the combination of high airway pressures and low end-tidal carbon dioxide ( I .6 kPa) was noted. At the same time chest movements were seen to be large. Institution of manual ventilation confirmed a high fresh gas flow and inspection of the Boyles machine rapidly revealed that in this case the oxygen flush button was locked in the 'on' position; no awareness occurred. Cases of awareness during general anaesthesia as a result of the emergency oxygen flush button remaining locked in the 'on' position are well reported [ 1-31. A typical scenario is that at some stage the button is leant against and an

Fig. 2.

additional rotational movement allows it to become locked, as probably happened in the second case described. Using a ventilated Bain system, the high fresh gas flow usually produces exaggerated chest movements and abnormally high inflation pressures which help to identify the problem. as in case two. In our first case the fault was much more subtle, and in the absence of an inspired oxygen analyser, only capnography gave a clue to the problem. To our knowledge only one author [3] has previously drawn attention to the usefulness of capnography in these circumstances. Cooper cites the overlapping absorption bands for nitrous oxide and carbon dioxide as the cause for an elevation of end-tidal carbon dioxide readings obtained during inhalational anaesthesia [3]. Once the nitrous oxide has been largely displaced with high flow oxygen, it is suggested that the resultant readings will be relatively low, hence indicating a problem. We believe, in our case at least, that the explanation is more simple and results from the oxygen flush button causing continuous hyperventilation. In an effort to prevent this type of incident occurring again we have fitted a protection bar in front of the oxygen flush button (Fig. 2). Bristol Maternity Hospital, Bristol BS2 8EG

A. MCCRIRRICK J.P. WARWICK T.A. THOMAS

References BRAHAMS D. Anaesthesia and the law. Awareness and pain during anaesthesia. Anuesrhesiu 1989; 44.352. [2] PUTTICK N. Hazard from the oxygen flush control. Anueslhesiu

[I]

1986; 41: 222-4. [3]

COOPER CMS. Capnography. Anoesthesiu

1987; 4 2 1238-9.

No need to modify the Sprotte needle Aglan and Stansby (Anuesthesia 1992; 47: 506-7) propose a modification to the Sprotte needle. The excellent clinical characteristics would not be affected if the lateral eye of the Sprotte needle were to be reduced to the size of the internal diameter. The authors maintain that this modification results in greater stability to the tip of the needle as well as a reduced risk for inadequate spinal anaesthesia. However, my modification to the Whitacre needle was carried out quite deliberately, with the lateral eye to considerably exceed the physically requisite size for laminar flow. There were two reasons behind this. Firstly, even with

correct positioning in the subarachnoid space, the cauda equina and arachnoid may well obstruct, either completely or partly, the eye of the needle. 'Dry' taps and uncertainty as to whether the needle has entered the subarachnoid space have their own risks and tend to increase the failure rate of spinal anaesthesia. Together with the extra thin wall, the apparently large lateral eye of the Sprotte needle makes for rapid flow of cerebrospinal fluid (CSF), even if the eye is partly obstructed. This characteristic of the needle enables diagnostic lumbar puncture to be performed more frequently at the first attempt, with an incidence of

1 I04

Correspondence

postlumbar puncture headache comparable to that of spinal anaesthesia [ I , 21. Secondly, the elongated lateral eye of the Sprotte needle is more suitable for preventing inadvertent intraneural injection in neural blockade procedures. These definite advantages of the large lateral eye have to be taken into account when considering the theoretical drawbacks referred to by Aglan and Stansby. With regard to the alleged greater failure rate, this was not apparent during spinal anaesthesia in the 6-year testing phase preceding initial publication [3]. A prospective random double-blind study revealed a significantly lower number of incomplete blocks for the 24-gauge Sprotte needle compared to a 25-gauge Quincke needle (1.6% vs 7.8%) 141. Furthermore, routine use of the 24-gauge Sprotte cannula in a number of neuroradiological departments over a period of several years indicated that there were no problems with double epidural-spinal injection of contrast media during myelography, i.e. with free CSF aspiration, the entire contents of the syringe are distributed solely in the subarachnoid space. Naturally, the stability of the needle tip is greater if the lateral eye is smaller. Therefore a special quality steel is employed for the production of these needles to compensate for the lateral eye and reduced wall thickness 141. For use in the USA the FDA require comprehensive physical testing. To avoid any misunderstandings it should be pointed out at this juncture that for comparative purposes we were forced to refer to 22 and 25-gauge Whitacre needles from Becton and Dickinson, which

despite their smaller lateral eye proved to be more unstable than the 22 and 24-gauge Sprotte cannula (Test Certificate of TUV Product Service Abt. Chemie, Medicalprodukte Munich 18.06.91). I should like to express my thanks to Drs Aglan and Stansby for their constructive contribution, which discusses one critical aspect of the Sprotte needle and at the same time offers me the opportunity to comment on this publicly. Institut fur Anaesthesiologie, Universitat Wurzburg, Germany

G. SPROTTE

References [I] JAGER H, SCHIMRIGK K, HAAS A. Das postpunktionelle Syndrom selten bei der Punktionshadel nach Sprotte (Eng. Abstr.) Postlumbar puncture headache prevention by means of an “atraumatic” needle Akt. Neurology 1991; 18: 6 1 4 . 121 ENGELHARD A, OHEIMS, NEUNDORFER B. Lumbar puncture with a new atraumatic needle. Journal of Neurology 1990; 237: 54. [3] SPROTTEG, SCHEDEL R, PAJUNKH, PAJUNKH. Eine atraumatische Universalkaniile fur einzeitige Regionalanaesthesien (Eng Abstr.) Regional-Anaesthesie 1987; 1 0 104-8. [4] BOTTNERJ, WRFSCH KP, KLOSE R. Bietet eine konisch geformte Kaniilenspitze Vorteile bei der Spinalanesthesie?(Eng Abstr.) Regional-Anaesthesie 1990; 13 124-8.

Intravenous magnesium therapy in critically ill patients Magnesium homeostasis 11-31, the use of magnesium in the control of tachyarrhythmias in critically ill patients [4, 51, and the effect of magnesium on mortality following acute myocardial infarction (LIMIT-2 study) [6], have been the subject of several recent reports. A number of different dosing schedules have been documented. Some regimens suggest fairly rapid infusion of magnesium sulphate (20 mmol (5 G) over 1 min [4], 8 mmol (2 G) over 5 min [6]), while others are more circumspect: 40 mmol (10 G) over 24 h [3]. Using a ’rapid’ regimen (8 mmol.5 min-I), we administered magnesium sulphate to three patients suffering from Gram-negative septicaemia, who were subject to a variety of tachyarrhythmias (atrial fibrillation, nodal/supraventricular tachycardia, recurrent ventricular ectopy). In two patients, hypotension (due to a fall in systemic vascular resistance (SVR) and requiring increased inotropic support) persisted for over 4 h. A third patient again showed a fall in SVR but unaccompanied by hypotension because of a significant increase in cardiac output. The hypotensive response was not unexpected although we were surprised by its duration, particularly as hypotension had not been a significant problem in the LIMIT-2 study [6] whose dosage schedule we were using. Despite this, in all three patients, the arrhythmias were significantly blunted. Since these experiences, we have altered our regimen, so that magnesium (48 mmol) is infused over 24 h.

Hypotension has not been noticeable. We suggest that in septicaemic patients (who are subject to global myocardial depression, unlike most patients suffering from acute myocardial infarction), magnesium should be infused slowly, unless there is clear evidence of clinical hypomagnesaemia (e.g. fasciculation, hypotonia etc). South Cleveland Hospital, Middlesbrough Cleveland TS4 3B W References [I] GAMBLING DR, BIRMINGHAM CL, JENKINS LC. Magnesium and the anaesthetist. Canadian Journal of Anaesthesia 1988; 35: 64454. [2] CHERNOW B, BAMBERGER S, STOIKO M, VADNAISM. MILLSS , HOELLERICH V, WARSHAW AL. Hypomagnesemia in patients in postoperative intensive care. Chest 1989; 9 5 391-7. [3] JAMESMFM. Magnesium in critical care medicine. Care

No need to modify the Sprotte needle.

Correspondence 1 103 Fig. 1. sequence induction using thiopentone 5 mg.kgi' and suxamethonium I mg.kg-'. The trachea was intubated and the lungs ve...
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