A.P. Adams

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.

Inadvertent suppression of a fixed rate ventricular pacemaker using a peripheral nerve stimulator.

Electromagnetic interference usually produces only minor effects in patients with pacemakers. Nevertheless, the possibilities of serious and even fatal consequences of this complication must be recognised. This case reports an unusual anaesthetic source of interference, caused by activation of a popular nerve stimulator, resulting in cardiac arrest in a patient with a fixed‐rate ventricular pacemaker.

The effect of mechanical ventilation after open-heart surgery.

The cardiorespiratory effects of mechanical ventilation in patients with normal lungs and circulation have been extensively studied. However, relatively few investigations have been reported on patients with cardiac or respiratory disease. This paper details a number of observations made both during and after operation on patients who underwent open-heart surgery under total cardiopulmonary bypass.

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

The end-tidal carbon dioxide detector : assessment of a new method to distinguish oesophageal from tracheal intubation

A new method to distinguish oesophageal from tracheal intubation using an end‐tidal carbon dioxide detector was evaluated. In a prospective study on 50 healthy adult patients, the end‐tidal carbon dioxide detector was reliably used to detect initial oesophageal intubation in 22 cases, and then to confirm tracheal intubation in all 50 patients. We conclude from this study that the end‐tidal carbon dioxide detector is a reliable, rapid and easy method for the detection of oesophageal intubation.

Endotracheal intubation skills of medical students.

The ability and confidence of clinical medical students to insert endotracheal tubes correctly and quickly and to recognize oesophageal misplacement was evaluated. Ten (33%) of the medical students intubated the trachea correctly at their first attempt but 14 (47%) incorrectly identified the position of the endotracheal tube. However, recognition improved by their second and third attempts (70% and 80% respectively). Ninety-three percent of students intubated correctly on their third attempt. Although medical students can obtain better results at correct tube placement with repeated attempts under optimum conditions--a practice effect--and do better at recognizing correct tube placement there is still a persistent failure to recognize endotracheal tube misplacement, ie oesophageal intubation. It is the ability to recognize oesophageal intubation promptly that is a life-saving skill. This essential skill should be taught during the introductory anaesthesia programme through the use of clinical patients.

OXYGEN DELIVERY SYSTEMS : A COMPARISON OF TWO DEVICES

Two low‐volume, variable performance oxygen delivery Systems were compared in conscious spontaneously breathing volunteers. Oropharyngeal oxygen concentrations were measured during periods of nose and mouth breathing. The Systems were studied at oxygen flow rates of 2 or 4 litres/minute. The performance of both Systems was similar under the test conditions but the nasal catheter is preferable in terms of cost.

Checking anaesthetic machines--checklists or visual aids?

Anaesthetists have always been in the forefront of medical audit and there have been a number of investigations over the years into anaesthetic accidents. Obtaining accurate figures is difficult as it is impossible to ensure that all incidents are reported. The most reliable pertain to mortality and are epitomised by the triennial Confidential Enquiries into Maternal Deaths and the National Confidential Enquiry into Pen-operative Deaths. Information also comes from isolated reports from single hospitals or hospital groups, but the findings bear a remarkable similarity from institutes throughout the world. Human error is the most common causative factor resulting in a critical incident. In 1984, Cooper et al. [I] reported on 1089 preventable critical incidents and found that 4% of those associated with a ‘substantive negative outcome’ involved equipment failure. There were 115 incidents of the latter, 14% being due to the anaesthetic machine and 15% due to ventilators. The commonest associated factor cited in a critical incident was failure to check and the authors stressed the importance of apparatus and equipmenk checking in the prevention of critical incidents. A much smaller study in the UK [2] listed 81 misadventures in 83 12 anaesthetics and reported that human error was more frequently responsible than equipment failure, and failure to perform a normal check was the factor most frequently associated (27/81 incidents). In New Zealand, 100 events were reported from 3546 operations. The most frequently reported incidents were related to gas delivery, the airway, ventilation and drug administration [3]. Again, human factors were most frequently implicated as the primary cause, the anaesthetist being the most common individual involved. Failure to check equipment resulted in 22% of critical incidents in a series of 549 reported over an 18-month period [4]. A more recent study recorded 125 critical incidents in 16 379 anaesthetics; the most frequent associated factor was inadequate checking of equipment [5]. These authors therefore emphasised adequate checking of equipment at all their departmental meetings throughout the year and yet incidents continued to occur. It is difficult to understand why anaesthetists fail to check equipment, especially as they are highly trained, skilled and motivated. A common feature of many mortality and morbidity reports is that mishaps occur not from lack of knowledge, but from failure to apply such knowledge, from inattention or even carelessness [6]. Senior anaesthetists are involved as frequently as trainees. Perhaps the most insidious hazard in anaesthesia is its relative safety [I]. It therefore requires constant effort to avoid the feeling of complacency. Failure to check equipment is a major contributory factor to disasters or potential disasters in anaesthetic practice and this must be rectified if improvements in patient care are to be made. The similarity between anaesthesia and flying is often made and anaesthetists in the Netherlands incorporated the help of the chief pilot of the Royal Dutch Airlines (KLM) to develop a checklist for the Drager NS650 anaesthetic machine [7]. As with pilots, an anaesthetist calls out items and an assistant checks that that item is working correctly. This is probably the best method of checking, but not feasible if someone is working alone. And there is another corollary with civil aviation. When we sit in an aircraft, we do so confident in the knowledge that it has been checked and found to be working satisfactorily by the ground crew and the pilots before taking off. We know that the pilots have not just kicked the tyres a couple of times and had a quick look round to see that there is nothing obvious hanging off. We also know that twice each year, by law, pilots have to practice for all known problems that might occur in flight, even though they will probably meet none of them in their entire careers. Will we be going this way? Like the passengers, should our patients expect anything less? Although operating department assistants frequently check machines, it is as well to remember that final responsibility rests with the anaesthetist administering the anaesthetic. In 1990, the Association of Anaesthetists of Great Britain and Ireland recommended a procedure for the checking of anaesthetic machines incorporating an oxygen analyser. This checklist is also available on laminated plastic sheets which can be attached to the machines. The checklist documents a number of procedures which should be performed at the start of an operating session. The oxygen analyser is checked and then there is a list of I 1 items concerning the gas supply. This is followed by four items each, to check vaporizers, breathing systems and the ventilator. Checking suction equipment and checks for leaks are included. That this document is read by anaesthetists is attested by the number of references to it in articles and the correspondence columns of this journal. But it is also clear that pursuance of these guidelines is by no means universal. Mayor and Eaton [8] found that up to 41 % of anaesthetists perform inadequate tests, or none at all, while of those that do, few follow the Association’s guidelines. It is extraordinary that although 90% of the anaesthetists questioned had read the guidelines and believed that safety would be improved, they did not follow the recommendations. Using the checklist, Barthram and McClymont [9] reported that faults were found in 60% of machines checked and that 18% of these were deemed to be serious. They found that the mean time to check one machine was 8-9 min (range 5-19 min) and for two consecutive machines was 18.25 min (range 10-30 min). These times are very reasonable considering the possible implications of failing to carry out a proper check, and should be considered a normal part of an anaesthetist’s working schedule. Detection of

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

Normocapnic anaesthesia with enflurane for intraocular surgery

The intraocular pressure was measured by applanation tonometry in 13 patients undergoing lens extraction with normocapnic anaesthesia with 1 % enflurane and controlled ventilation of the lungs with large tidal volumes (13 ml/kg). The intraocular pressure was consistently reduced by about 35% and was associated with a similar reduction in systolic arterial pressure. Provided care is taken with those patients with hypertension or heart disease who might become hypotensive, the technique is suitable for lens extraction surgery especially where it is desirable to avoid the administration of halothane.