Download Update 11 - Update in Anaesthesia - WFSA

34Update in Anaesthesiamore vigorously. At this point, let go of the lever, and theneedle will display systolic pressure. Pull the lever forwardagain. As the pressure is reduced, the needle jumps morevigorously. If the lever is released at the point of maximumneedle oscillations, the dial will read the mean arterialpressure. If it is released at the point when the needlejumps get suddenly smaller, the dial reads diastolicpressure.Automatic non-invasive blood pressure measurementAutomatic devices which essentially apply the sameprinciple as the oscillotonometer have been produced (e.g.the ‘Dinamap’ made by Critikon). They require a supplyof electricity. A single cuff is applied to the patients arm,and the machine inflates it to a level assumed to be greaterthan systolic pressure. The cuff is deflated gradually. Asensor then measures the tiny oscillations in the pressureof the cuff caused by the pulse. Systolic is taken to bewhen the pulsations start, mean pressure is when they aremaximal, and diastolic is when they disappear. They canproduce fairly accurate readings and free the hands of theanaesthetist for other tasks. There are important sourcesof inaccuracy, however. Such devices tend to over-readat low blood pressure, and under-read very high bloodpressure. The cuff should be an appropriate size. The patientshould be still during measurement. The technique reliesheavily on a constant pulse volume, so in a patient with anirregular heart beat (especially atrial fibrillation) readingscan be inaccurate. Sometimes an automatic blood pressuremeasuring device inflates and deflates repeatedly “hunting”without displaying the blood pressure successfully. If thepulse is palpated as the cuff is being inflated and deflatedthe blood pressure may be estimated by palpation andreading the cuff pressure on the display.Invasive arterial pressure measurementThis technique involves direct measurement of arterialpressure by placing a cannula in an artery (usually radial,femoral, dorsalis pedis or brachial). The cannula must beconnected to a sterile, fluid-filled system, which isconnected to an electronic monitor. The advantage of thissystem is that pressure is constantly monitored beat-bybeat,and a waveform (a graph of pressure against time)can be displayed. Patients with invasive arterial monitoringrequire very close supervision, as there is a danger ofsevere bleeding if the line becomes disconnected. It isgenerally reserved for critically ill patients where rapidvariations in blood pressure are anticipated.RESPIRATORY GAS ANALYSIS IN THEATREDr J.G.McFadyen, Royal Devon and Exeter Hospital , Exeter, UK. Previously: Edendale Hospital,Kwazulu-Natal, South Africa.CAPNOGRAPHYCapnography is the measurement of carbon dioxide (CO 2 )in each breath of the respiratory cycle. The capnographdisplays a waveform of CO 2 (measured in kiloPascals ormillimetres of mercury) and it displays the value of theCO 2 at the end of exhalation, which is known as the endtidalCO 2 .It is useful to measure CO 2 to assess the adequacy ofventilation, to detect oesophageal intubation, to indicatedisconnection of the breathing system or ventilator, and todiagnose circulatory problems and malignant hyperthermia.Applications of CapnographyProvided the patient has a stable cardiac status, stablebody temperature, absence of lung disease and a normalcapnograph trace, end-tidal carbon dioxide (ETCO 2 )approximates to the partial pressure of CO 2 in arterialblood (PaCO 2 .) Normal PaCO 2 is 5.3kPa (40mmHg).In these patients, ETCO 2 can be used to assess adequacyof ventilation i.e. hypo-, normo-, or hyperventilation.ETCO 2 is not as reliable in patients who have respiratoryfailure. The increased V/Q mismatch is associated with awidened P(a-ET) gradient, and can lead to erroneousETCO 2 values.The capnograph is the gold standard for detectingoesophageal intubation. No or very little CO 2 is detectedif the oesophagus has been intubated.The capnograph is also useful in the followingcircumstances: As a disconnection alarm for a ventilator or abreathing system. There is sudden absence of thecapnograph trace.May detect air embolism as a suddendecrease in ETCO 2 , assuming that the arterial bloodpressure remains stable.