210 Correspondence

Nasal intubation was achieved by guiding a tracheal tube into the larynx over a paediatric fibreoptic bronchoscope.

Since one nostril was obstructed by the broncho- scope and tube, a second Portex tube was placed in the other nostril, so that one-third of the gently inflated cuff remained outside; this was combined with apposition of the lips which enabled ventilation of the lungs to be maintained with ease. Thus laryngoscopy and bronchoscopy wcre performed without haste, the paticnt's gas exchange was maintained at all times and the dangers of removal and re-insertion of the broncho- scope and tube were avoided.

This casc is rcported firstly to emphasise the neces- sity of both keeping accurate records and reading those of others. and secondly. to bring to the attention of other anaesthetists a technique which may permit ventilation to be maintained during fibreoptic laryii- goscopy.

Arterial to end tidal CO, tension differences

The paper by Shankar et nl. (Anursthe.u'u 1986; 41: 698- 702) is a useful contribution to our knowledge of gas exchangc during controlled vcntilation, but may I be permitted to comment on some errors in the discus- sion?

Regarding the third paragraph in the discussion, we showcd that all patients (cxccpt those with pul- monary embolus. carcinoid Hush or hypovolaemia) had a positive slope to phase 111 of the single breath test for C 0 2 (SBT-C02). that is, cnd tidal Pco2 ( k ' r o l ) is also peak Pro,. This is regardless of fre- quency or tidal volume. Many factors influence this slope: amongst these are tidal volume ( VT) and frequency (1 ) . for when V-r is largc and f i s small. the slope is least. Nevertheless. large volume, low fre- quency ventilation gives the greatest chance of PE'co2 exceeding Pam, in a well perfused, healthy lung, because gas distribution and mixing are better.

Alveolar Pro, ( P A ~ o ~ ) varies cyclically. being least at end-inspiration and greatest at end-expira- tion. However. because of mixing in the heart and syringe. Pac.02 sampled a t the radial artery is the spatial and temporal mean of alveolar Proz (Riley's physiological integrator2) and therefore i t is quite possible for PF'co, to excced P a r o 2 thus sampled. This is not temporal deadspace: if i t were. il would be a negative one! Temporal deadspace refers to the effect, during controllcd ventilation. of ventilation ((9 being out ofphase with perfusion (0). As inspiration proceeds, 8 is reduced. Theoretically, tern- p i x i 1 deadspace should be greatest during large Vr. low / ventilation, hut under these conditions the alveolar dcadspace fraction is least; ' therefore we have argued ,3

that temporal deadspace is of less importance than that caused by airway diseaae, that is, spatial, between- and within-units mismatching.

I t is true that the reduced functional residual capa- city (FRC) in pregnant patients should give rise to a greater cyclical variation in P.\co2. which increases the likelihood of sampling a Pr'ro, greater than P a m 2 . The lung model developed for controlled ventilation '

supports this. Ilowever, the authors' statement that there i s a greater incidence of positive phase I l l 4opc during Caesarean section anaesthesia. should be niodilied to read 'it is possible that phase 111 slope is increased . . as a result of the reduced F R C ' Thc normal incidcnce o f positive phasc I l l slope is already I 00 ?'" .

There is a second explanation for the 7ero and negative arterial to cnd tidal P ( w , differences. I n ohcse patients with healthy lungs. there is a biphasic dope. reminiscent of phase IV of the N, clusing \,olume test. The ProL of most of the alveolar g:rs I S less than P;ic .oL, hut in the tcrniinal part of the expirate. Pto, rises rapidly and may exceed Pace?. Becausc pregnant patients have some features in common with obese ones, namely, rcduccd FKC due to a high diaphragm. and low total c ~ m p l i a n c e . ~ they may also show (his phenomenon. a s discussed i n my thesis.'

The important issues raised by the paper are firstl) that gas exchange during Caesarean section with con- trolled ventilation would appear to be good. I t may he comparable to that in children.5 and IS considerably better than i n older patients, many of whom h a w airway disease and large alveolar dcadspaces. Thc second point concerna the non-pregnant contrcil group. These were only 7 years older than the prtyianl patientc, but their ParoL - P F ' C O ~ I S greater than that of our middle-aged patients ' Thib is strange. con- sidering that they had nevci. smoked and that dead- spacc is highly dependent on age. ' The uncxpcctedly large deadspace that must cause this is, perhaps. B

rrsult of fairly vigorous (mean Pace? 3.3 kPa) hypei-- ventilation with 0.5 -0.1 'A halothane. Perhaps this disturbs local pulinonary homeostatic I-ellexes Our patirnts icceivcd nitrous oxide and opiates and were less hyperventilated (mean P a m 2 4.2 kPa).

Finally. the authors state that Raemer ('1 u/.- 'have shown scver:iI factors. nainsly changes i n hlood prea- sure, pulmonary blood flow. variations in compliance. anaesthetic agents, and surgical position lead to alte- rations in v:Q ratios'. This is a misrepresentation.

Correspondence 21 1

Raemer et 01. speculuted on the effects of anaesthctic agents, surgical positioning, changes in temperature and pulmonary blood flow. Compliance was not men- tioned. What they dei~ionstrufrd was that for each of 1.1 variables. in about three of 15 patients (different patients for each variable), there was a significant cor- relation with PacoL - F%'CO,. This result is scarcely more significant than that which would be expected due to chance. and underlines the potential dangers of doing enormous batteries of linear regressions, in thts casc IhS of them. Raemer's data and conclusions do not support the statements made in the present paper.

Lusaretti~t I Lund, 221 85 Lund. Sweden

R. FLFTC'HFR

Refbrenccs I . Fl.t.TCHER R, JONSON R. Dcddspace and the single breath test

for uarhnn dinxidc during anaesthesia and artificial venti- lation. Effects of tidal voluinc and frequency of respiration. Brtrisli J i~urnul of Anoc.de.riu 1984: 5 6 109-19.

On the dcterminatinii of the physiologically effective piebsures of oxygen and carbon dioxide in alveolar air.

2 . RILIY RL, LILICNTHAL JL. PROEMMIX DD. FRANKE RE.

AnlPrri Ut2 Jourl ld Of P / l L ' S f / l / O j i l 1946: 147: 191-8 3. FI ET('H1.R R. JON SO^ B, C C M M I N t i G. BK1.W J . The concept

of deadspace with special refermce t u the single breath test for carbon dioxide. Brirts/i Journul of Anap~thrsin 1981: 53: 77 4 8 .

4. FLI:IUII:R R A method for producing nnrmocarhi;~ during general anaesthesia for Caesarean section. A n a f s i h c ~ r i ~ 1985: 40: 378

5. FLF~C.HI:R R The single breath test tor carbon dioxide. Thesis, Lund, 1980 (obtainable from author)

6. F L ~ . I C I I I ~ R. N I K L A S ~ N L, J N ~ r ~ i n r R. Gas exchange during controlled ventilation in children with normal and ahnormal pulmonary cir~ulatiim: a study using th r wngle breath test for ciirbon dioxide. A n r . ~ r h r \ i o nnd A n o l ~ r ~ i u 1986: 65: 645-52.

7 . RAEMER DB, FRANCIS D. PHILIP JH. GAIEL RA. Variation in Pro2 hetween arterial blond and peak expired gas during anrsthesia. Anurhesiu und Andgmu 1983; 63: 1065-9.

A reply

We thank you for allowing us to reply to the above correspondence. We appreciate Dr Fletcher's keen interest in our study and thank him for his expert comments on the various mechanisms operative during pregnancy to cxplain the negative or zero arterial to end tidal CO, tension difference. However, we would like to make the following points.

We agree with Dr Fletcher that Raemar et a/ . did spcculate that a rcdistribution of the ratio of ventila- tion to perfusion throughout the lung would be expected owing to anaesthetic agents, surgical posi- tioning, changes in temperature, changes in pulmonary blood flow, mechanical ventilation and cardiopul- monary bypass. The redistribution of ventilation relative to perfusion can produce changes in physiological dcadspace that are reflected by changes in arterial to end tidal C 0 2 tension. During Caesarean section anaesthesia, as was stated in our paper, one could expect thcre to be changes in pulmonary vcnti- lation, pulmonary blood flow. blood pressure and pulmonary compliance. Therefore these changes are likely to influence the ratio of ventilation to perfusion and thereby arterial to cnd tidal CO, values. Hence we suggest that there is a wider scatter between P a m , and PE'CO, during anaesthesia for Caesarean section.

Queen Elizubptlr Hospital, K.B. SHANKAR University of the West hidies, H. MOSELEY 3~1rhudo.s Y . KLMAR

V. VEMULA

Nitrous oxide in early pregnancy

I t is encouraging to note that for the fetus, general anaesthesia is a s safe as regional analgesia when the mother undergoes cervical cerclage between weeks 8 and 28 of gestation. This observation still shed5 no light on thc pcriod betwcen 4 and 8 wecks of gestation during which the three germ layers give rise to tissues and organ systems in the human fetus. Prior to 4 weeks. abnormality of the blastocyst will probably cause no sign of pregnancy;' and after X weeks, changes occur mainly in fetal length, weight and cellular maturation. It is this period of between 4 and 8 weeks gestation, during which pregnancy is often only a n unconfirtncd suspicion in the mother's mind, that has been highlighted in animal studies.' -4 These studies demonstrate fetal malformation in response to nitrous oxide exposure.

Extrapolation o f animal studies to man is always contentious. but drug teratogenicity studies must

always he performed in an animal and if a drug is found to have teratogenic properties in this model, it is doubtful whether i t would gain a product licence for use in pregnancy. One therefore questions whether any stress and anxiety caused to the anaesthetist by administration of nitrous oxide during the fourth to eighth weeks of gestation would be wholly unjustified, but at the same time would not advocate administra- tion of folinic acid to all women of child-bearing age!

The difficulty i n performing any critique of the administration of nitrous oxide during this period is appreciated. and I look forward to the results of Drs Crawford and Lewis's yecond paper on the subject.

Queen hlury'.s Hospituljbr Children, ( 'urshalton, Surrej, S M 5 4NR

P. KEELING