Why is pregnancy ASA 2?

The short answer

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ASA 2 reflects altered physiological status rather than the presence of disease.

It's also not a particularly useful risk stratification score, and doesn't take into account the nature of the surgery or anaesthetic complexity.

I thought pregnancy was 'natural'?

You might have noticed that as soon as an ASA 1 woman becomes pregnant, her ASA grade jumps up to 2.

This puts her in the same category as someone with well controlled hypertension, asthma or a smoker.

Pregnancy absolutely is one of the most natural processes in mammalian life, and it certainly isn't a 'disease state', but that doesn't mean it doesn't cause the anaesthetist any issues.

Pregnancy is an entirely physiological, non-pathological process
However it has profound physiological and pharmacological impacts on the body
These affect the perioperative risk, and your anaesthetic plan

So from a 'how do I manage this patient's anaesthetic' point of view, it's similar in nature to them having well-controlled asthma or diabetes.

Sometimes.

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ASA 2 is a patient with mild systemic disease, without substantial functional limitation.

Remind me of the ASA classification?

Certainly.

Devised in 1941, the ASA classification categorises patients based on their baseline physiological status, without any further information in the way of test results or recent scans.

Its purpose is to assess and communicate a patient’s operative medical comorbidities.

ASA 1 - normal healthy patient - non smoker with no disease
ASA 2 - mild systemic disease - well controlled HTN, DM, smoker
ASA 3 - severe systemic disease - poorly controlled HTN or DM, renal disease
ASA 4 - severe systemic disease that is a constant threat to life - active cardiac ischaemia, sepsis with organ dysfunction
ASA 5 - moribund, not expected to survive without the operation - ruptured AAA, massive trauma
ASA 6 - brain-dead patient for organ donation

BMI 30-39 is ASA 2, BMI 40 and over is ASA 3.

As you can see, the jump from ASA 2 to 4 is enormous, meaning a large proportion of patients sit firmly in ASA 3 territory.

And one would argue that 'normal' is edging ever closer to ASA 2 in modern society, but that's a rant for another day.

Things to know about the ASA grading

It describes preoperative physical and physiological status
It does not describe procedural or anaesthetic risk
It is associated with perioperative mortality and morbidity on a population scale, but is not a risk prediction tool
It does not take the complexity of surgery or anaesthesia into account

It's a bit like GCS, in that everyone uses it, but not for the reasons it was intended.

It's still useful, mind - it's fast and doesn't need any investigation results - and if someone hands over an 'ASA 1 appendix' or an 'ASA 4 laparotomy', you immediately know exactly what they mean and how worried you need to be.

What's the E modifier?

It makes sense that an emergency procedure is always going to be at least slightly higher risk than a planned elective case, even if it's for the same patient.

E is added for emergency procedures where delaying treatment would increase the threat to life or limb

For example:

ASA 2E - emergency C section for healthy pregnant woman
ASA 4E - emergency ruptured triple A repair

How often this nomenclature is actually observed is another question.

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Pregnancy is not an illness, but it changes how the body responds to drugs and anaesthesia.

What if they also have a medical problem?

As soon as you add a further complicating disease process into the mix, they quickly tip into ASA 3 territory, depending on whether this is causing them 'substantial functional limitation'.

Severe pre-eclampsia
Poorly controlled gestational diabetes
Cardiac disease
Obesity class III (BMI >40)
Poorly controlled HTN
Symptomatic anaemia

You get the idea.

Which brings us nicely onto a classic FRCA exam question.

What changes occur in pregnancy?

This is a standard FRCA question at both Primary and Final levels, and you should be able to reel off the following answers fairly quickly on exam day.

As always, categorise your response by system rather than scattergunning individual facts.

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Most of the changes are due to the effects of progesterone and increased cardiovascular demand.

Respiratory

Progesterone-induced airway mucosal oedema and vascular engorgement
Reduced functional residual capacity
Increased oxygen consumption
Increased minute ventilation (lower CO2 and physiological alkalosis)

This means a heavily pregnant patient will desaturate faster and have a more difficult, smaller airway.

Cardiovascular

Increased cardiac output (by up to 50% by 20 weeks)
Increased blood volume
Reduced systemic vascular resistance
Increased heart rate
Aortocaval compression when supine after 20 weeks

This means a heavily pregnant patient will be uncomfortable lying flat on their back and may have profound haemodynamic instability under anaesthesia.

Gastrointestinal

Reduced lower oesophageal sphincter tone
Increased intra-abdominal pressure
Delayed gastric emptying if in labour or taking opioids

This means a heavily pregnant patient will have increased risk of regurgitation and aspiration under general anaesthesia.

Haematological

Dilutional anaemia of pregnancy
Physiological thrombocytopenia of pregnancy
Increased risk of venous thromboembolism as pregnancy is a hypercoagulable state

Patients may be on low molecular weight heparin during their pregnancy, which can influence your anaesthetic plan.

Pharmacological

Increased cardiac output and blood volume alter the volume of distribution
Increased plasma volume affects protein binding
Reduced anaesthetic requirement
Increased sensitivity to neuraxial local anaesthetics

You also need to think about placental drug transfer and foetal effects, you know, just in case you weren't already stressed enough.