Equipment - Introduction to Monitoring

Question 1

What does the AAGBI consider essential for a GA?

Answer 1

Monitoring equipment supplements clinical assessment, rather than replacing it, and provides a warning of potential deterioration, as well as reminding the user of the most important parameters to assess.

For any procedure
Anaesthetist present and maintaining a record
Monitors appropriately set up with audible alarms & visible parameters as necessary (ECG, NIBP, SpO₂, measured electroically minimum every 5 minutes)

For General Anaesthesia
FiO₂
EtCO₂ until artificial airway removed and able to talk
Age adjusted MAC
Airway and cuff pressure
Temperature if > 30 minute GA
Nerve stimulator
DOA monitoring if using TIVA

For Regional Anaesthesia
ECG and NIBP
SpO₂
To begin before the procedure and at least 30 minutes or more after

For Procedural Sedation
ECG and NIBP
SpO₂
Capnography if there is loss of response to voice

Blood sugar monitoring
Capillary blood glucose and ketone monitoring should be immediately available if required, and done minimum hourly for diabetic patients

Question 2

Explain calibration and drift

Answer 2

Calibration
The process of testing a known input value (the standard) to see what value is given by the process being calibrated, enabling the measuring instrument to be adjusted to ensure the process is accurate

1-point calibration compares a single measurement to the standard
2-point calibrateion compares with 2 standards
3-point calibration compares with 3, and confirms a linear relationship

Drift
When the measured value systematically differs from the true value across all ranges of inputes, and often an error which can be zeroed

Linear/Offset drift means that the values differ by the same amount at all points on the scale

Gradient drift means that the difference increases disproportionately as values increase

If only one of these drifts are present, then 1-point calibration is sufficient, otherwise if both are present, 2-point calibration is required

Question 3

Explain sensitivity and linearity

Answer 3

Sensitivity
A measure of how big the output is for a given input (or how small an input the instrument can detect)

A more sensitive system is better for small input values, but for values over a wider range, then a less sensitive system may be more useful

Linearity
A linear relationship means that output increases in direct proportion to the input, giving a straight line graph, with the gradient providing the sensitivity of the instrument

A non-linear relationship means the output changes disproportionately as input changes - such as with a rotameter

Question 4

Explain dynamic and step response

Answer 4

Dynamic response
The ability of a measuring system to react to changes in an input variable

Zero-order means the displayed output directly tracks with the true output value
First-order means the displayed output gets exponentially closer to the true value over time
Second-order means the displayed output value tends to oscillate around the true value (such as invasive BP)

Step response
A measure of how quickly a system can react to a sudden change in input variable, with two main components

Response time: How long it takes from the change in input variable occuring, to 90% of the output being generated

Rise time: How long it takes for the output to rise from 10% to 90% of the end value