536

Correspondence

Table 3. OMV + bypass: pressure decreases at 40 I.min

OMV Full on Full on Full on

Occluded

~

I.

Bypass

Pressure (cmH,O)

Absent Clamped Patent Patent

-3 -3 -1.8 -3

otherwise excessive isoflurane drawover output of the OMV, but was unsatisfactory for plenum OMV use. Before any ‘home-made’ external bypass for the OMV is employed clinically, it should be tested by vapour analysis (and checked by the pressure drop tecchnique), at the temperatures, minute volumes and plenum/drawover flow patterns in which it is to be used. Launceston General Hospital, Launcest on, Tasmaniu 7250 Australia

C.G. MERRIDEW

~

6 1.min-I Table 3 shows pressure decreases associated with drawover air flow of 40 1.min-I (1.2 I in 1.8 s) at 22°C through the OMV, with or without the bypass. Pressures were measured with a water manometer (clear plastic tubing of 5.5 mm internal diameter) attached to a T-piece at the ventilator intake. Similarity of the Table 3 values was predicted by the 55°C outputs in Tables 1 and 2, and by the drawover calibration data, but not by plenum data. In conclusion, at temperatures from about 30 to 55°C. the specified external bypass reduced desirably the

References [ I ] RESTALLJ. JOHNSTON IG. Anaesthesia in the field. Anarsthesiu 1991: 46:320-1. [2] MERRIDEWC G , FRASERSJ. Field anaesthesia at extreme temperatures. Anaesthesia 1991; 46: 1000. [3] OHMEDA. Operators’ Manual, Tec 3 Continuous Flow Vuporixr. BOC Health Care, 1987. [4] HOUGHTON.IT. The Triservice Apparatus. Anuesthesiu 198 I: 36: 1094-108.

Secondary transfer of patients We are grateful for the reply of Vyvyan et al. (Anaesthesia 1992; 47: 73-4) to our letter concerning the ‘secondary transfer of patients’. We suggested that helicopters equipped to the minimum standards, as recommended by the report of the working party on such matters [I], possess insufficient monitoring to undertake secondary transfers. In reply, Vyvyan and his colleagues correctly comment that the standards document covered both primary and secondary transfers of patients; however, it did not distinguish between the equipment deemed necessary for each type of transfer. Indeed, it did not mention that a capnometer and facilities for invasive pressure monitoring should be included in the minimum standards required for secondary transportation of patients. The standards document will inevitably be used by the directors of helicopter systems as guidance for equipping such craft, and as such, should be more specific in its advice. On the rare occasions that our hospital has been required to transfer patients by helicopter we have been offered a variety of craft, most of which were unacceptable in one way or another. In the incident referred to in our first letter, recognising the need to provide standards of care and monitoring equal to that in our own intensive therapy unit, we took considerable care to ensure that the helicopter in question would reach our own standards. Complete details of the patient’s condition were discussed with the duty

flight registrar and he was made fully aware of our monitoring, ventilation and other equipment requirements. The transfer was accepted in the full knowledge of these. Furthermore, the medical director of the helicopter scheme in question is a member of Dr Bristow’s working party and would therefore be fully aware of the standards recommended for the secondary transfer of an intensive care patient. We agree with Dr Vyvyan and his colleagues that the craft used for secondary transfer must be appropriate for its intended role. However, it cannot reasonably be expected that a referring hospital, faced with an urgent need to transfer a critically ill patient and the offer of a helicopter service purporting to be adequately staffed and equipped, should be responsible for verification of the expertise of that system. Queen Alexandra Hospital, Portsmouth PO6 3LY

G.B. SMITH A.S.J. PROSSER B.L. TAYLOR

Reference [ I ] BRISTOW A. Medical helicopter systems-recommended minimum standards for patient management. Journal (!/’ the Royal Society of’ Medicine I99 I ; 84: 242-4.

Timing of surgery in congenital diaphragmatic hernia We read with interest the article regarding the timing of surgery in congenital diaphragmatic hernia (CDH) by Charlton et al. (Anaesthesia 1991; 4 6 820-3), reporting their experience with 86 neonates with C D H presenting within 6 h of life, and demonstrating significant stabilisation of 57 of the infants associated with delay in surgery and an overall mortality of 29%. While these results are certainly very encouraging, the authors should refrain from comparing their results to those of extracorporeal membrane oxygenator (ECMO) centres who use rigid criteria for the initiation of ECMO. Current ECMO criteria usually involve the use of the oxygen index (01) (mean airway pressure x 100 + Pao,) with an 0 1 of greater

than 40 on three occasions as the most common criteria for the initiation of ECMO[I]. In the absence of ECMO availability, Bohn el al. have demonstrated, using the ventilatory index (ventilator rate x mean airway pressure) and the Pace?, either before or, more reliably, 2 h after surgery [2. 31 that such indicators are very useful in selecting a population of infants with C D H having 90% or greater mortality. Others have defined infants with C D H as responders or nonresponders on the basis of the ability to achieve a postductal Pao, of greater than 13.3 kPa or a Paco, approaching normal [4, 51. Subsequent reports have concluded that these indicators are inaccurate when ECMO is available, and that no infant should be denied support

Correspondence using ECMO on the basis of any of these criteria [6-lo], although one recent report describes blood gas and ventilation criteria which the authors consider to be predictive of death even with the use of ECMO [ l I]. If Charlton et ul. wish to conclude that ECMO is of no benefit or would have only benefited a small number of their patients, then they should attempt to demonstrate that their infants with CDH were at least as ill and satisfied the criteria which have been demonstrated in the past to be associated with a high mortality. While the authors have referenced the use of the alveolar oxygen gradients and the use of the ventilatory index (their references 7 and 8), they have not provided any information in their paper that would allow the reader to compare the severity of the infants described using such indicators. The authors should be encouraged t o provide this information if they wish to conclude that their results represent a significant improvement from other current experiences. Charlton et ul. also consider that a lung weight less than 35% of predicted excludes the possibility of ECMO, presumably because this is believed to indicate lethal pulmonary hypoplasia. We do not know how to diagnose lethal pulmonary hypoplasia, other than in an infant who dies despite the use of ECMO. and we would consider those six infants with < 35% predicted lung weight as ECMO candidates. We agree with the conclusion of Charlton e t u l . and many other groups that pre-operative stabilisation in congenital diaphragmatic hernia is beneficial, although a recent report from Liverpool found no difference in outcome between early and delayed surgery [ 121. We wish to emphasize that these discussions and comparisons concerning the merits of the timing of surgery and use of ECMO will only be useful if authors provide blood gas and ventilation data so that the severity of the cases described can be compared to the published indicators. Ro!d AIe.rutidru tlospitul. Edtnon ton Alhrrtu, Cunudu TSH 3VG

N.N. FINER P.C. ETCHES

Rqf>rencr.r [I] BARTLETT RH. CHAPMAN RA. SNEDECOR SM. HULTQUIST K. E.rtrucorporeu1 Life Support Orgunhution Registry. University of Michigan Medical Center Publication. (21 BOHN DJ. JAMESI, FILLERRM, EIN SH, WESSONDE, SHANDLING B, STEPHENSC, BARKERGA. The relationship between Paco, and ventilation parameters in predicting survival in congenital diaphragmatic hernia. Journal of Pediutric Surgery 1984; 1 9 666-71. [3] BOHND. TAMURA M, PERRIN D, BARKER G , RABINOVTCH M. Ventilatory predictors of pulmonary hypoplasia in congenital diaphragmatic hernia. confirmed by morphologic assessment. Journul of Pediatrics 1987; 11 1: 423-3 I . [4] VACANTIJP. O'ROURKE P. LILLEHEICR, r i a l . The cardiopulmonary consequences of high-risk congenital diaphragmatic hernia. Pediurric Surgery Iniernaiionul 1988; 3: 1-5. [S] WEBERTR. CONNORS RH. TRACY T F . BAILEYPV. STEPHENS C. KEENANW. Prognostic determinants in extracorporeal membrane oxygenation for respiratory failure in newborns. Annuls of Thorucic Surgery 1990; 50: 720-3. [6] BAILEYPV, CONNORS RH, TRACY TF, STEPHENSC, PENNINGTON DG, WEBER TR. A critical analysis of extracorporeal membrane oxygenation for congenital diaphragmatic hernia. Surgery 1989; 106: 61 1-6. [7] NEWMAN KD, ANDERSON KD, VANMEURSK, PARSONS, LOE W, SHORTB. Extracorporeal membrane oxygenation and congenital diaphragmatic hernia: Should any infant be excluded. Journal of Pediatric Surgery 1990; 2 5 1048-53. (81 HOWELL CG, HATLEYRM, BOEDYRF, ROGERSDM, KANTO WP, PARRISHRA. Recent experience with diaphragmatic hernia and ECMO. Annuls of Surgery 1990; 211: 793-8.

537

[9] VAN MEURSKP, NEWMAN KD, ANDERSON KD, SHORTBL. Effect of extracorporeal membrane oxygenation on survival of infants with congenital diaphragmatic hernia. Journal qf Pediutrics 1990; 117: 954-60. [lo] ATKINSON JB, FORDEG, HUMPHRIES B. KlTACAWA H, LEW C. GARCM, BUI K. The impact of extracorporeal membrane support in the treatment of congenital diaphragmatic hernia. Journul of Pediuiric Surgery I99 I ; 2 6 79 1-3. [ I l l WILSON JM, LUND DP, LILLEHEICW, VACANTIJP. Congenital diaphragmatic hernia: predictors of severity in the ECMO era. Journal of Pediarric Surgery 1991; 2 6 1028-34. [I21 SHANBHWUELKR, TAM PKH. NINANG. LLOYD DA. Preoperative stabilisation in congenital diaphragmatic hernia. Archives of Diseases in Childhood 1990; 6 5 1043-4. A reply

There appears to be no measure of severity of congenital diaphragmatic hernia (CDH) which has so far met with widespread acceptance and it remains to be seen whether any of the various predictive indices will permit a genuine comparison between centres with both markedly different approaches to treatment and differing populations. We arrange immediate transfer of all C D H babies in our region, undertake prolonged pre-operative medical treatment and aim to deliver all babies with an antenatal diagnosis in our own hospital. In contrast, some ECMO centres appear to take predominantly patients who are selected by survival of delivery, transfer to a paediatric surgery facility, early surgery and later removal to the ECMO centre. These contrasting systems present different problems in the application and timing of severity assessments. Lack of complete data from a referring unit may lead to a patient's 'best effort' being missed. Predictive indices have no place in the actual management of our patients (since we have decided not to offer ECMO). However, they are of interest and their relationship to the outcome of delayed surgery is under investigation. There seems to us little doubt that some C D H patients have insufficient lung to survive on (as low as 15% expected weight in our series). ECMO is not a cure for pulmonary hypoplasia. We have no means of knowing exactly the minimal lung weight compatible with survival, but since Drs Finer and Etches appear to be advocating ECMO for every baby dying from C D H (as now d o other groups [1-3] since the predictive indices are considered unreliable) they should be able to tell us whether the lung weights of babies dying in the 'ECMO era' are now lower than those upon which our estimate off 35% was based [4]. We also await with interest a demonstration that the availability of ECMO has indeed altered the overall mortality from CDH in the communities served by the ECMO centres. Munchester R o y a l I n j r m a r y , Munchester M I 3 9WL

A.J. CHARLTON

References

[I] NEWMAN KD, ANDERSONKD. VAN MEURSK, PARSONS, LOE W. SHORT B. Extracorporeal membrane oxygenation and congenital diaphragmatic hernia: should any infant be excluded? Journal of Pediatric Surgery 1990; 2 5 1048-53. [2] VAN MEURSKP, NEWMANKD, ANDERSON KD, SHORT BL. Effect of extracorporeal membrane oxygenation on survival of infants with congenital diaphragmatic hernia. Journal of Pediatrics 1990; 117: 954-60. [3] HOWELLCG, HATLEYRM, BOEDYRF, ROGERSDM, KANTO WP, PARRISHRA. Recent experience with diaphragmatic hernia and ECMO. Annals ofSurgery 1990; 211: 793-8. [4] NGUYENL, GUTTMAN FM, DE CHADEREVIAN JP, BEARDMORE HE, KARNG M , OWENHF, MURPHYD R . The mortality of congenital diaphragmatic hernia. Is total pulmonary mass inadequate, no matter what? Annals of Surgery 1983; 198: 766-70.

Timing of surgery in congenital diaphragmatic hernia.

536 Correspondence Table 3. OMV + bypass: pressure decreases at 40 I.min OMV Full on Full on Full on Occluded ~ I. Bypass Pressure (cmH,O) Ab...
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