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36Update in AnaesthesiaAbnormal Traces1. Rebreathing3. Cardiac oscillationsA waveform that does not return to the baseline duringinspiration indicates that rebreathing of exhaled gas isoccurring.Causes: Fresh gas flow too low in non-rebreathingsystem. Soda lime depleted in circle system.Cause: Cardiac impulses transmitted to capnograph.Explanation The oscillations reflected on the capnographtrace result from transmission of cardiac impulses to theairways.4. “Curare cleft”2. Sloping PlateauCause: Obstructive airways disease, because of impairmentof V/Q ratio.Explanation In patients with obstructive airways disease,the lungs are perfused with blood as normal, but the alveoliare unevenly ventilated. CO 2 that is transferred to thealveoli from the bloodstream may take longer to exhalebecause of the narrowed bronchi. This delayed emptyingof the alveoli varies in different parts of the lungs. Thisresults in the sloping plateau on the capnograph trace, asthe CO 2 from those parts of the lungs with more severebronchial narrowing is exhaled later than those parts withless severe narrowing.Cause: Reversal of neuromuscular blockade in a ventilatedpatient.Explanation When a paralysed patient starts taking smallbreaths as the neuromuscular blocking agent reverses, deep“clefts” are seen on the capnograph trace.OXYGEN CONCENTRATION ANALYSERSIt is important to measure the oxygen concentration in thegas mixture delivered to the patient during anaesthesia.There are three techniques of measuring the inspiredoxygen concentration (FiO 2 ): galvanic, polarographic, andparamagnetic techniques. The paramagnetic method ismost widely used in anaesthesia. These analysers measurethe oxygen partial pressure in a gas sample but they display
Update in Anaesthesia 37a percentage. Regular calibration of oxygen analysersis vital.Paramagnetic oxygen analysers Oxygen possesses theproperty of paramagnetism, which means that it is attractedto a magnetic field. This is because it has two electrons inunpaired orbits. Most of the gases used in anaesthesia arerepelled by a magnetic field (diamagnetism).The sample gas is delivered to the analyser via a samplingtube, which should be placed as close as possible to thepatient’s trachea. The analyser has two chambers separatedby a sensitive pressure transducer. The sample gas isdelivered to one chamber. Room air is delivered to thereference chamber. An electromagnet is rapidly switchedon and off creating a changing magnetic field to which thesample gas is subjected. The magnetic field causes theoxygen molecules to be attracted and agitated. This resultsin changes in pressure on either side of the pressuretransducer. The pressure difference across the transduceris proportional to the oxygen partial pressure differencebetween the sample gas and the reference gas (room air).Paramagnetic oxygen analysers are very accurate andhighly sensitive. The analysers should function continuouslywithout any service breaks. They have a rapid responseallowing measurement of inspiratory and expiratory oxygenon a breath-to-breath basis. They are affected by watervapour and have a water trap incorporated into theirdesign.The Galvanic Oxygen Analyser (Fuel Cell) Thegalvanic analyser is placed on the inspiratory limb of thebreathing system. Oxygen molecules diffuse across amembrane and an electrolyte solution to a silvercathode, which is connected through an electrolytesolution to a lead anode. An electrical current isgenerated which is proportional to the partial pressureof oxygen in the inspired gas.The galvanic analyser has a response time of approximately20 seconds. It is accurate to within 3%. Calibration isachieved using 100% oxygen and room air (21% O 2).Water vapour does not affect its performance. It isdepleted by continuous exposure to oxygen due toexhaustion of the cell, so limiting its life span to about oneyear.The Polarographic Oxygen Analyser (ClarkElectrode) The polarographic analyser has similarprinciples to the galvanic analyser. Oxygen moleculesdiffuse across a Teflon membrane. Current flows betweena silver cathode and a platinum anode, which isproportional to the partial pressure of oxygen in theinspiratory gas. A battery powers it. Its life expectancy islimited to about three years because of deterioration ofthe Teflon membrane.Further reading1. Williamson JA, Webb RK, Cockings J, Morgan C. The Australiaincident monitoring study. The capnograph: applications andlimitations - an analysis of 2000 incident reports. Anaesthesia &Analgesia 1993; 21: 551-72. Schmitz BD, Shapiro BA. Capnography. Respiratory Care Clinicsof North America. 1995; 1: 107-173. Al-Shaikh B, Essentials of Anaesthetic Equipment. ChurchillLivingstone 1995.MONITORING DURING CAESAREAN SECTIONDr James Eldridge, Consultant Anaesthetist, Queen Alexandra Hospital, Portsmouth, UKRecommendations for monitoring during Caesareansection (CS) have been developed by the American Boardof Anesthesiologists and the Obstetric AnaesthetistsAssociation (OAA) in the UK. The OAA’srecommendations are reproduced in full in Box 1. Not allanaesthetists have access to complex equipment, but everyanaesthetist should be aware of the potential problemsthat may be encountered and make appropriate use of themonitors they do have. The requirements for regional andgeneral anaesthetics are different and so consideredseparately. All obstetric patients undergoing CS shouldbe positioned with left lateral tilt to avoid aorto-cavalcompression. (See Update in Anaesthesia 1999).Regional anaesthesiaMost of the monitoring is clinical since awake mothers areexcellent monitors of their own physiology. The anaesthetistshould be continuously present from the start of anaesthesiato the completion of surgery.Assessment of analgesiaA major cause of maternal complaint is pain during CSunder regional anaesthesia. For CS, a block should extendfrom S4 to the upper thoracic dermatomes. One commonreason for inadequate pain relief is a failure of the block tospread to the sacral dermatomes. Although this happensmore frequently with epidural than spinal anaesthesia,
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