EFFECT OF SURGICAL STIMULATION ON THE AUDITORY EVOKED RESPONSE

Previous studies have shown a dose-related effect of a numberof general anaesthetic agents on the early cortical waves inthe auditory evoked response (AER). In this study the effectof surgical stimulation on these waves was examined in 11 patientsanaesthetized with thiopen-tone, nitrous oxide and halothaneand paralysed with pancuronium. The inspired nitrous oxide concentrationand end-tidal halothane concentration were held constant at70% and 0.3%, respectively, and baseline AER recordings weremade. Following surgical stimulation there was a progressiveand significant increase in the amplitude of waves Nb and Pb/Pc.Unambiguous autonomic responses were seen in three patients,but these were not significantly correlated with changes inthe AER. We conclude from this, and previous studies, that theamplitude of cortical waves in the AER are sensitive not onlyto anaesthetic concentration but also to surgical stimulation.The may, therefore, provide a useful index of depth of anaesthesia,that is the balance between the effects of surgical stimulationand anaesthetic depression on central nervous system activity. Present address: University Department of Anaesthesia, 24 HydeTerrace, Leeds LS2 9LN.     

Haemoptysis in healthy children due to unsuspected foreign body

Abstract: Haemoptysis in otherwise healthy children is an uncommon event. Two cases of massive haemoptysis, subsequently requiring lobectomy, are discussed. In each case, foreign vegetable matter was identified despite previously normal bronchoscopy and minimal changes on chest radiograph.     

EARLY CORTICAL AUDITORY EVOKED RESPONSE IN ANAESTHESIA: COMPARISON OF THE EFFECTS OF NITROUS OXIDE AND ISOFLURANE

Previous studies showing graded changes in the early corticalwaves Pa and Nb of the auditory evoked response (AER) with increasingconcentration of volatile anaesthetic agents demonstrated highamplitudes of these waves in the period immediately followinginduction of anaesthesia and tracheal intubation, when the patientbreathed nitrous oxide alone. These high amplitude waves werenot consistent with extrapolation of the data or observationsof patients under steady-state nitrous oxide anaesthesia. Inorder to discriminate between effects in the period immediatelyfollowing induction of anaesthesia and tracheal intubation,and effects caused by nitrous oxide alone, a randomized cross-overstudy was performed. Eight patients breathed either nitrousoxide or isoflurane at 0.6 MAC for three consecutive 10-minperiods following intubation and before surgery. The amplitudesof Pa and Nb were significantly less for isoflurane with respectto the same MAC fraction of nitrous oxide in all periods, butfor both agents the amplitudes were significantly greater inthe 10 min following intubation than in subsequent periods,presumably as a result of stimulation. ^*Present address: Hospital for Sick Children, Great Ormond Street,London WC1.     

Relationships of haptoglobin level to FEV1, wheezing, bronchial hyper-responsiveness and allergy

The relationships of haptoglobin level to respiratory and allergic parameters have been assessed in an epidemiological study conducted in a working population surveyed twice 5 years apart. At the first survey conducted in 892 working men, haptoglobin level was significantly related to FEV1 (r = -0.18; P less than 0.001) and smoking habits. After adjustment for smoking, a history of wheezing was significantly related to lower haptoglobin level. A second survey conducted in 304 men of the original sample 5 years later confirmed that haptoglobin was related to FEV1 (r = -0.21; P less than 0.001) and that wheezing was significantly related to hypohaptoglobinaemia (lower decile; P = 0.04). Men who exhibited bronchial hyper-responsiveness to methacholine had haptoglobin levels 0.35 g/l higher than those who did not (P = 0.01). Haptoglobin level was unrelated to IgE level and skin prick tests. These results support the hypothesis of the role of inflammation in both lower lung function and bronchial hyper-responsiveness. They suggest that some heterogeneity exists within subjects with a history of wheezing.     

THE AUDITORY EVOKED RESPONSE AS AN INDICATOR OF AWARENESS

The latency of the early cortical wave Nb of the auditory evokedresponse (AER) was compared with responses to Tunstall's isolatedforearm test, while, the concentration of nitrous oxide wasprogressively reduced during light anaesthesia in seven patients.A threshold Nb latency of 44.5 ms was chosen to discriminatebetween an early cortical AER containing three waves and thatwith two waves of longer latency. When Nb latency decreasedbelow this threshold, four of the patients has positive responses,indicating awareness. The addition of a volatile anaestheticabolished any response, and increased Nb latency to more than44.5 ms. The three wave AER pattern, therefore, is associatedwith a depth of anaesthesia at which awareness occurs.     

SOMATOSENSORY AND AUDITORY EVOKED RESPONSES RECORDED SIMULTANEOUSLY: DIFFERENTIAL EFFECTS OF NITROUS OXIDE AND TSOFLURANE

Auditory (AER) and somatosensory evoked responses (SSER) wererecorded simultaneously in eight patients under anaesthesiabefore surgery. We studied the effects of equi-MAC end-expiratoryconcentrations of isoflurane (0.65–0.75%) and nitrousoxide (60–65%). The anaesthetics were changed at randomin three consecutive 10-min periods so that each patient receivedboth drugs. From the AER recorded from the vertex and inion.Pa and Nb latency and amplitude were measured. N13, P20 latencyand N13 amplitude were measured from SSER recordings from theneck and P15, N20. P25, N35, P45 latency and P15-N20, N20-P25,P25-N35 and N35-P45 amplitude from the scalp over the hand areaof the sensory cortex. Compared with nitrous oxide, isofluranesignificantly increased the latencies of the AER waves Pa (P= 0.02) andNb (P = 0.02), and the SSER waves N20 (P = 0.001)and P25 (P = 0.04). We were unable to demonstrate significantdifferences in Pa and Nb amplitude between isoflurane and nitrousoxide that we had seen previusly. However, the amplitude ofthe SSER wave N20 was reduced significantly by nitrous oxidecompared with isoflurane (P = 0.0004). This wave (N20) is thoughtto emanate from the thalamo-cortical radiations, and our findingsmay be explained by an analgesic effect of nitrous oxide mediatedby endogenous opioids. ^*Present address: Department of Anaesthesia, Royal Free Hospital,Pond Street, London NW3.     

CHRONIC EXPOSURE OF RATS TO ENFLURANE 200P.P.M.: NO EVIDENCE OF TOXICITY OR TERATOGENICITY

Adult male or female rats were exposed either to enflurane 200p.p.m. or to air for up to 63 days before mating and subsequentlythroughout the pregnancy of the females. The treated animalswere exposed for 8 h per day, 5 days per week for a maximumtotal of 100 days. The parents and progeny were studied. Additionalpositive controls with vitamin A palmitate demonstrated thatthe strain of animals was susceptible to a known teratogen.No abortifaaent effect was observed with these conditions ofexposure to enflurane. Skeletal examination of the fetuses failedto reveal any major teratogenic effect. The parents remainedhealthy clinically, whilst autopsy and histopathological examinationfailed to reveal any consistent organ injuries which could beblamed on exposure to enflurane. In rats at least, environmentalpollution with enflurane appears not to be associated with significanttoxicity or teratogenidty.     

Modification of the iso-shunt lines for low inspired oxygen concentrations

Arterial Po₂ (Pa_o2) has been related to inspired oxygen concentration(Fl_o2 range 0.21–0.60) in 20 patients, breathing spontaneously,while recovering from a range of conditions treated in an intensivetherapy unit. There was a highly significant linear trend (P< 0.001) for the mean venous admixture ("virtual shunt"),calculated from the iso-shunt diagram (model A), to increasefrom 0.11 to 0.19 as Fl_o2 decreased from 0.60 to 0.21. As thisappeared to be caused by relative ventilation/perfusion (V/Q.)mismatch, we prepared a second model (model B) with a variabletwo-compartment relative V/Q mismatch in addition to a shunt.Regression of the degree of mismatch on the shunt gave goodagreement between predicted and observed Pa_o2 values (mean bias–0.17 kPa, pooled within patient SD 1.22). Model B isnow proposed to extend the applicability of the iso-shunt diagramfor Fl_o2 values less than 0.35. Published postoperative Pa_o2data from other workers showed good agreement with our finalmodel (mean bias 0.10 kPa, pooled SD 0.41). The new diagrammay therefore have wide applicability in the field of anaesthesiaand intensive care. (Br. J. Anaesth. 1994; 72: 515–522)