M.K. Sykes

Closing volume and pregnancy.

Measurements of closing volume have been made in 20 women between the 36th and 40th weeks of pregnancy. The patients were studied in the erect and supine positions and the point of airway closure was related to functional residual capacity. The results show that airway closure occurred during tidal ventilation in 10 patients in the erect position and in six patients in the supine position. These results may explain the variation found in maternal arterial oxygen tension during pregnancy.

pH and blood—gas analysis

During the past ten years there have been considerable advances in the design of apparatus used for pH and blood gas-measurements. Many hospitals have been equipped with suitable instruments and the value of these measurements in clinical practice has become widely recognised. Unfortunately all such instruments are subject to error. The errors vary widely in magnitude; some are obvious, others difficult to detect. During the past eight years one of the authors (MKS) has noted errors of up to 0.3 units in pH, 15mm Hg in Pc02 and 20mm Hg in Po2 during the use of electrode systems which were apparently functioning perfectly when checked with buffer solutions or reference gases. Such gross errors, if unsuspected, may lead to serious mistakes in diagnosis and treatment. In this article the authors discuss thr. main sources of these errors and indicate methods by which they may be prevented, detected and eliminated. These errors are discussed under the headings of blood sampling and storage and analytical errors.

Rebreathing with the Magill attachment

The occurrence of rebreathing in anaesthetic circuits has been studied by a number of authors ; the behaviour of semi-closed circuits during spontaneous ventilation being first subjected to analysis by Wynne in 1941 1. In 1954 Mapleson2 predicted that, for the Magill attachment, rebreathing of alveolar gas would not occur if the fresh gas flow rate equalled or exceeded the alveolar ventilation. He also predicted that there would be no rebreathing of any gas either alheolar or deadspace gas if the fresh gas flow rate equalled or exceeded the minute volume. Since the former prediction was based on the assumption that there would be no mixing of alveolar, deadspace and fresh gas in the apparatus or conducting airways, Mapleson suggested that, in clinical practice, the fresh gas flow rate should always exceed the minute volume. Woolmer & Lind3 and Bracken & Sanderson4 using model systems confirmed that where the fresh gas flow rate exceeded the minute volume there was no rebreathing. Similar results have been found during studies on anaesthetised patientss.6. Kain & Nunn7 have recently determined the lowest fresh gas flow rate that could be used without the occurrence of rebreathing in anaesthetised patients. They detected the occurrence of rebreathing by an increase in alveolar (end-tidal) carbon dioxide concentration and by an increase in the expired minute volume two obvious effects of the re-inhalation of expired carbon dioxide. Using this method of assessment these authors found that rebreathing did not occur until the fresh gas flow rate was less than the minute volume: on occasion the fresh gas flow rate could be reduced to less than half the minute volume before rebreathing occurred. Although the presence of rebreathing is usually revealed by changes produced by the re-inhalation of carbon dioxide it must be remembered that rebreathing also causes a fall in the inspired oxygen concentrations. The fall in the inspired oxygen concentration and the rise in alveolar carbon dioxide concentration will lead to a fall in the alveolar oxygen concentration. However, Kain & Nunn found that as the fresh gas flow rate was

Evaluation of a multigas anaesthetic monitor: the Datex Capnomac

A laboratory investigation was carried out to evaluate the performance of the Datex Capnomac multigas anaesthetic agent analyser, with particular emphasis on accuracy, response and delay times, zero and gain stability and interference from water vapour. The analysis of anaesthetic vapours was less accurate than the analysis of CO2, O2 and N2O, but acceptable for clinical use. The response to square wave changes in gas composition was accurate at frequencies up to 60 per minute for CO2 and 30 per minute for O2, but with N2O and the anaesthetic vapours there was a decrease in accuracy at frequencies above 20 breaths/minute. The instrument appeared to be unaffected by water vapour.

Airway closure and pregnancy

During pregnancy there is a fall in functional residual capacity and expiratory reserve volume. The point of airway closure in relation to vital capacity and the expiratory reserve volume was determined in 11 post-partum patients who had previously been studied during pregnancy. The lung volume at which airway closure occurred in pregnancy and the post-partum period did not change in relation to the vital capacity whereas the expiratory reserve volume did, so that airway closure became imminent in the pregnant women during tidal ventilation, especially whilst in the suping position. The significance of this alteration in pulmonary function to obstetric anaesthesia is discussed.

An evaluation of the Brüel and Kjær monitor 1304

A laboratory evaluation was performed on the Brüel and Kjœr multigas monitor 1304, incorporating a pulse oximeter. The instrument was tested for accuracy, stability, response and delay times, frequency response and the effects of water vapour, alcohol, cyclopropane and sevoflurane. The instruments performance was found to be within or very close to the manufacturers specifications for accuracy, stability and response and delay times. It was unaffected by water vapour and alcohol and the effect of cyclopropane on the vapour channel was lower than has been reported for other analysers. The response to sevoflurane was of the same order as that of the other vapours. A 90% response to square wave changes of gas composition was maintained up to 60 breaths.min−1 for CO2, O2, and N2O and up to 40 breaths.min−1 for the vapours when the nafion sampling tube was used.

Cardio-pulmonary resuscitation. A report on two years' experience.

A resuscitation service was initiated at Hammersmith Hospital in September 1959; the object was to treat patients who developed respiratory or cardiac arrest as a result of any medical or surgical procedure and patients who collapsed unexpectedly in out-patients, casualty or the wards. It was also recognised that patients with renal, cardiac or respiratory failure would be a special risk and it was decided that attempts should be made to resuscitate these only if there was a good prospect of them returning to a reasonably normal existence after the period of acute disability. Provision was made to designate patients with rapidly progressive, incurable disease as being ‘not on cardiac call’. During the first three years, the service dealt with 185 episodes of cardiac arrest occuring in 161 patients. Twenty-one patients (13.1 %) were ultimately discharged from hospital 1. Since the autumn of 1963, detailed records of the treatment of each cardiac call, other than those occurring during the course of cardiac surgery, have been kept by members of the Anzsthetic Department. Specially prepared forms have been used for this purpose: the information recorded provides the basis for the present review.

Clinical measurement and clinical practice

During the last 60 years we have witnessed the transition of anaesthesia from a n art to a science. A science may be defined as a body of systematic and formalised knowledge. In anaesthesia we encounter a wide range of scientific disciplines, each of which has a body of knowledge which can be described with a variable degree of precision. However, the one common requirement of all these sciences, whether it be physics or psychology, is that there is a need to measure the input and output functions of the system under study. In the past, such measurements were usually made in a laboratory, since this could be designed to provide the most sympathetic environment for the purpose. But, even in that environment, many measurements proved difficult. The reason was nearly always the same-the problem of the biological interface, where sensor meets tissue and complex reactions occur. It is, therefore, not surprising that many years of development were often required before such measurements could be successfully transferred to the clinical environment. It is, of course, the development of the microprocessor which has revolutionised the practice of clinical measurement and monitoring over the past two decades. Microprocessors have made a major impact on the processing of data, the control of apparatus function and the display of results in an easily read format. However, the major benefit resulting from their use has been the development of sophisticated artefact rejection algorithms which have dramatically increased the signal-to-noise ratio of many measurement systems. This remarkable advance has not only increased the range of measurements which can be made in the clinical environment, but has also enabled the results to be presented on-line, so that measuring instruments can now be used for the continuous monitoring of the patient’s condition. Many anaesthetists believe that the widespread use of clinical measurement has revolutionised anaesthetic practice and resulted in a reduction in morbidity and mortality. For example, Keenan and Boyan [ l ] have reported that, in their institution, the incidence of preventable cardiac arrest during anaesthesia in the period 1979-1988 was half that observed during the previous decade, and that this decrease in mortality was due to a decrease in preventable respiratory causes. This change might well be attributed to the increased use of monitoring devices such as end-tidal CO, analysers and pulse oximeters during the second decade. The decrease in mortality has also been accompanied by a reduction in the number of major accidents reported to one of the major medicolegal insurance groups, who were able to reduce their premiums as a result [2]. However, during this period there was a similar reduction in the frequency of claims in a number of other high risk specialties [3]. A number of other factors could well have accounted for the decrease in complications. For example, in 1976 only 52% of anaesthesiology residents in American hospitals were trained in U.S. Medical Schools, whilst in 1988 the figure had risen to over 90% [4]. The staff/patient ratio and average duration of residency training had also increased and it is reasonable to assume that this must have resulted in an improvement in the standard of training. The Harvard Insurance figures suggest that the decrease in complications coincided with the introduction of the Harvard Standards of Monitoring in 1985 [S], but it must be remembered that the first clause of this document stipulated that the anaesthetist should remain with the patient throughout the anaesthetic, and it is quite possible that this injunction did more to reduce the incidence of complications than the provision of extra monitoring apparatus. Indeed, the real cynics question whether we need any monitoring equipment in the operating theatre a t all. They point out that no one has actually proved that any of this expensive equipment produces a higher quality of outcome for the patient, and they suggest that, in many circumstances, its presence may prove counterproductive, in that it may distract the anaesthetist from direct observation of the patient. I think that many of us who have struggled to come to terms with the latest piece of high technology equipment sympathise with this view, for we have all experienced periods when we have become so absorbed in a problem with our equipment that we have not noticed an obvious change in the clinical state of our patient. It thus seems to me that this is an appropriate time to re-examine the question: ‘Has clinical measurement really improved clinical practice during this last 60 years?’ In the hope of throwing some light on the problem, I

Methods of measurement and sources of error using electrode systems

In this technique22966~93.94 the pH of the blood is measured before and after equilibration with two gas mixtures of different but known Pc02. The pH of the two equilibrated blood samples is plotted against the log Pcoz of the equilibrating gases and the points joined by a straight line. This is the buffer line of the blood. The pH of the blood as drawn from the patient is then interpolated in this line and the Pcoz read from the ordinate. Since the equilibration technique ultimately depends on only three blood pH determinations it is important to make these measurements as precisely as possible (see section on pH measurement). Astrup et a166 and Siggaard-Andersen et a122 have described a micro-equilibration unit,* comprising two H-shaped thermostatted glass chambers, which allows two samples of the blood to be equilibrated with each gas simultaneously. The whole unit is shaken mechanically at about 2,500 reciprocations per minute. The gases used for equilibration of the blood are usually 4 % and 8 % C02 in oxygent, but unless a certificate of analysis has been supplied by the manufacturers the user must check the composition by analysis. For this purpose the Haldane apparatus is commonly used; for accurate work the calibration of the analysis burette should be checked by mercury displacement. For clinical work the Campbell-Haldane analyser 123 is adequate. When the Haldane type of apparatus i s used the first few analyses will yield a

Dead space during anaesthesia. Effect of added oxygen.

It is known that physiological dead space ( V D ~ ~ ~ ) and dead space/tidal volume ratio (VDIVT) increase when anaesthesia is induced1* 2 and that a further increase occurs when ventilation is controlled 1’3. A short inspiratory period has been shown to increase dead space4-9 whilst VD/VT is also increased by an end-expiratory pressurelo. It has been suggested that the increase in dead space during anaesthesia is due to maldistribution of inspired gas1 or to a fall in pulmonary artery pressure11.12. It is also possible that there might be an increase in stratified ventilation/perfusion inequality at alveolar levell3. Such inequality might be produced by abolition of the homoeostatic mechanisms, which normally tend to minimise ventilation/perfusion inequalities, either by the direct action of the anaesthetic agent on the pulmonary vasculature or by an increase in alveolar oxygen tension. The action of anaesthetic agents is being studied experimentally. The present paper reports the effects of an increase in inspired oxygen concentration on physiological dead space during anaesthesia with controlled ventilation.