The best monitoring is provided by a trained anaesthetist applying continuous clinical observation skills throughout the entire anaesthetic. A patient with pink, warm and moist mucocutaneous membranes, a capillary refill of about 1 s, good strong regular peripheral pulses, adequate respiratory excursion of both hemithoraces and appropriate urine volumes is almost certainly in a stable condition and in no immediate danger.
In addition to good clinical judgement, there are technical devices which may provide early warning of problems; they constitute a much welcome aid to modern anaesthesia and have been associated with a reduction in legal claims and an increase in safety . They help the anaesthetist to maintain the wellbeing of patients and reduce fatigue and stress. As a general principle, alarms must not be turned off out of habit, as this defeats their purpose. Rather, the abnormality setting the alarm off (e.g. tachycardia) should be addressed or the alarm limit adjusted accordingly, for example, in paedi-atric practice. Some of the more important instruments are discussed below.
Monitoring of the patient
This is based on the physical principle of different but characteristic absorptions of light by oxygenated and deoxygenated Hb. Software developments have enabled separation of light absorption by blood (Hb) from that of skin and tissues.
Pulse oximetry measures blood oxygenation non-invasively. Indirectly it provides an indication of ventilationperfusion match, heart rate and rhythm and peripheral perfusion. It warns of deterioration in oxygenation but does not indicate its aetiology. Abnormal Hb (carboxyhaemoglo-bin and methaemoglobin) can give false readings, as can severe hypoxaemia .
CO2 analysers normally work by infrared gas analysis. This is based on the principle that gases with more than one atom in their molecule absorb radiation at a characteristic wavelength. It is most useful in children where the anatomical dead space, is small. In adults, because of the larger dead space, dilutional effects may make interpretation more difficult. The graph of concentration versus time can reflect the adequacy of ventilation. It is possible to detect rebreathing (high CO2 during inspiration) and obstructive airways disease (delayed rise towards the expiratory plateau).
Expired CO2 analysis is valuable in the detection of the following potentially lethal conditions: • Oesophageal intubation: Even experienced anaesthetists can occasionally find oesophageal intubation very difficult to detect. Absence of CO2 production after five consecutive 'breaths' is nearly pathonogmonic of intubation of the oesophagus. It is an indication to remove the tube and oxygenate the patient by other means, that is, a face mask.
• Deficiencies in ventilation: Although airway obstruction or gas trapping have characteristic expiratory patterns, interpretation can sometimes be difficult and, if in doubt, it is best to obtain a sample of arterial blood for analysis of blood gases.
• Detection of air or pulmonary embolism: The increased deadspace created by ventilated but underperfused alveoli, causes a sharp decrease (by dilution) in expired CO2 and in the ratio of expired/arterial CO2. This is particularly useful in surgery where the position of the head is above that of the heart (head and neck surgery, neurosurgery, etc.). In general, a low cardiac output (impending cardiac arrest or cardiogenic shock, in addition to the causes discussed above) features an abnormally low perfusion, a high ventilation-perfusion ratio and, consequently, a low expired CO2 (increased deadspace).
• Malignant hyperpyrexia: A frank increase in CO2 often precedes other signs.
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