Trends in anaesthesia: reading the course behind the current value

When I am called to help during an anaesthetic, it is often because of a value that already worries the team. It might be a low heart rate, reduced blood pressure or a high end-tidal carbon dioxide (EtCO₂).

At that point we have to assess the patient and deal with the problem in front of us. But I also ask another question, and to me it carries a lot of weight: how did we get here?

The current value shows one moment. The readings before it may show which way the change is heading, whether it persists and how it relates to the procedure. Reading the monitor over time is what I teach, and what I do in clinical practice.

A call for help with an EtCO₂ of 75 mmHg

In one episode from my practice, I was called to an anaesthetic with an EtCO₂ of 75 mmHg on the monitor. The team included a colleague experienced in anaesthetic monitoring.

That was the value I found when I arrived, and there was no documented sequence. So I cannot say when the change began, how quickly it developed or whether the rise was continuous. A gradual rise is possible, but it is only a hypothesis: no data reconstruct that episode that way.

It leaves me with a question: do we recognise changes as they develop, or only once the current value is worrying enough for someone to ask for help?

In my experience, that request may come after several attempts to resolve the situation. I want to understand what information was available and how it was interpreted, without judging the person who asked.

An illustrative example

Consider the sequence below, put together only to explain the reasoning. The numbers are illustrative and are not measurements from the episode I described.

Time since the first observation	EtCO₂
0 minutes	40 mmHg
20 minutes	44 mmHg
25 minutes	46 mmHg
30 minutes	50 mmHg

If we look only at the last number, we lose some of the information. Each value is higher than the one before, but the intervals between measurements are unequal. The table does not show everything that happened between readings, nor does it allow a future rise to be extrapolated.

On its own, the sequence does not define a complication or an indication for treatment. Its meaning depends on the patient, ventilation, anaesthetic technique and measurement reliability. The example makes visible a direction that deserves assessment.

From trend to assessment

A persistent change should prompt questions. Is the measurement credible? Is the change expected in this context? What has changed in the meantime?

AAHA recommends interpreting monitor information alongside clinical assessment of the patient ¹. For rising EtCO₂, its guidance includes considering anaesthetic depth and possible rebreathing or breathing-circuit problems ².

Spotting a rise does not, by itself, identify the cause. And bringing a number down without understanding the mechanism may leave the problem unresolved.

The idea is to start assessing the context before we rely only on an extreme value. Whether to observe, check, seek help or treat still calls for clinical judgement.

It helps to place the trend among the events of the case. These include drug administration, changes to maintenance anaesthesia or ventilation, changes in position and stages of the procedure.

Closeness in time may guide the questions, but it does not establish causation. A change after an intervention may be related to it or have another explanation.

So a useful record holds values, events and responses. Without times, we may remember giving a drug and seeing the blood pressure change, yet lose the order of the two or the interval between them.

EtCO₂ is interpreted with the capnogram, respiratory rate and pattern, the breathing circuit and the other available clinical data. AAHA distinguishes assessment of ventilation, oxygenation and cardiovascular function ¹.

Reading these together does not require every variable to move in the same direction, and which variable changes first varies between patients and situations.

The aim is to gather coherent information to understand the case. When the data contradict each other, the discrepancy deserves checking too. An isolated change may be real or come from a measurement problem, and the sequence alone does not settle which.

Keeping the sequence in view

Without records or trend displays, comparing the present with the past depends heavily on memory. In a long procedure, we have to remember earlier values, interventions and responses while we keep monitoring the patient.

A chart filled in with times lets us recover part of that sequence. Monitor trend screens, where available, may add information. Either way, you need to know the time scale and see where the gaps are.

Two values far apart in time do not show that the patient was stable in between. A break in the record is a gap, so resist filling it in mentally as if nothing had happened.

Reading over time carries on after we intervene. We should record what was done, when, and how the patient progressed. The trend may help show whether the change persisted, stabilised or reversed. That alone does not prove the intervention caused the response, but it keeps the information the assessment needs.

An improvement at a single moment tells only part of the story, so we keep observing the patient and interpreting what happens next.

The difficulty of following this evolution was one of the reasons I created VetNoa. We aim to support data capture from existing monitors and the organisation of information over the course of a case, so the team has the sequence closer to hand.

None of this demonstrates that VetNoa can predict deterioration. Data quality, gaps and clinical interpretation still count. We are trying to help people understand the patient's course, and responsibility for decisions stays with the team.

Questions for when something changes

These are educational prompts: they do not replace assessment of the patient and are not a treatment protocol.

• Is the value credible and compatible with your assessment of the patient?
• What earlier measurements do we have, and when were they taken?
• Is the change isolated, repeated or persistent?
• What events or interventions happened in that interval?
• What do the waveform and the other available parameters show?
• Are there gaps that prevent the course from being reconstructed?
• Does the current situation call for help or intervention?
• After acting, what changed, and was it documented?