A three-lead ECG displays surface electrical activity of the heart; this is useful in the assessment of rhythm and rate, particularly in lead II. A CM5 configuration (negative lead, right sternal border; positive lead, same as V5) is the most sensitive in detecting intraoperative ischaemic events.
The cuff must be appropriate for the arm size. If the cuff is too small, it may give falsely high readings. All cuffs tend to be more inaccurate the lower the systolic pressure. Obviously arrhythmias and sudden changes in blood pressure may affect cuff accuracy.
It is easy to measure nasopharyngeal (core) and skin temperature. The gradient between the two provides a marker of peripheral perfusion and, indirectly and together with many other clinical signs, of vascular resistance and cardiac output.
This provides accurate beat-to-beat measurement of blood pressure. It also provides useful information on rhythm and a gauge of the efficiency of individual beats, for example, in extrasystoles. In addition, it enables blood sampling. Careful cannulation of limb arteries is associated with a low incidence of side-effects. Before cannulating the radial artery, some anaesthetists routinely check for collateral flow from the ulnar artery.
Measurement of central venous pressure (CVP), pulmonary artery occlusion or wedge pressure (PAOP), cardiac output,
TOE, etc., are undertaken during complex surgery and/or in ill patients but they are outside the scope of this book.
Monitoring of the equipment
There are a multitude of oxygen analysers available based on several different electrophysical principles. They may be affected by water vapour and nitrous oxide and they must be calibrated regularly.
Monitors normally operate by infrared gas analysis. There can be a considerable difference between the vaporiser setting and the delivered concentration. Volatile agent concentration monitors should help to avoid excessively light or heavy anaesthetics.
There are many different kinds and all are designed to detect an absence in cyclical pressure fluctuations within the breathing system rapidly. The two most important causes of this are disconnection and ventilator failure.
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