In pregnancy, the asthmatic has several added features to bewilder and confound the intensivist. It is actually the most common condition to complicate pregnancy. The main problem is that ventilation is going to be impaired by the gravid uterus as well as by the high airway resistance (i.e. there will be a horrible combination of poor compliance and high airway resistance). Many of the drugs used to combat asthma may be either contraindicated (eg. sustained neuromuscular blockade) or counterproductive (β-agonists). Hypercapnea, tolerated as a necessary evil in the non-pregnant population, can have a disastrous effect on the acid-base balance of the foetus. In short, everything is more complicated.
This topic has come up before, in Question 18 from the first paper of 2009. The college wanted us to "outline the challenges specifically associated with the management of a pregnant patient with status asthmaticus". An ideal single best reference for this topic would have to be either the 2005 article by Hanania et al or the more recent (2014) paper by Chan et al. The trainee in advanced stages of last-minute revision is also directed to the LITFL article on this topic.
The main issues are:
- Intubating them is a high-risk deathsport
- Ventilating them deals with both poor compliance and high airway resistance
- There are restrictions on the drugs you can use (steroids and sustained paralysis are out)
- On should be more concerned with hypercapnia then one ordinarily would be.
The Pregnant Asthmatic
- More reflux, thus more exacerbations
- More allergic exacerbations, particularly in the last trimester
- Less capacity to compensate, thus more frequent presentations
- They are already tachypnoeic, with a mild respiratory alkalosis, which can confuse their presentation.
- The preload is already decreased by uterine compression of the vena cava, let alone the intrinsic PEEP.
- Prolonged maternal hypoxia is associated with intrauterine growth retardation.
- There is a greatly increased risk of pre-term delivery with poorly controlled asthma
Physiological features of pregnancy which complicate ventilation
- Non-invasive ventilation may be more hazardous, due to decreased gastric emptying rate and increased risk of aspiration.
- But, on the other hand, the pregnant airway is already difficult - if to this you add critical hypoxia and bronchospasm, the airway becomes the domain of a true Jedi.
- Respiratory compliance will be poor; not just because of the bronchospasm, but because the diaphragm has risen by 4cm, and the ventilator has to fight the gravid uterus for space.
Strategies which we know are harmful
- Don't use adrenaline! It will cause placental vasoconstriction, or so they say.
- Avoid permissive hypercapnea; foetal hemoglobin will have poor affinity to oxygen in the presence of a respiratory acidosis. Of course, one can up-titrate one's oxygen supply instead, making this somewhat safer. Most authors will mention this piece of physiological trivia, and then happily report successful ventilation with permissive hypercapnea.
- Avoid prolonged neuromuscular blockade - it can cause arthrogryposis, nasty congenital joint contractures.
Strategies which we must use with caution
- Corticosteroids in the first trimester - they may cause a cleft palate.
- of course, in later stages of pregnancy, you can really hit the steroids
- Beta-agonists in labour - they have a tocolytic effect, slowing everything down.
On top of that (possibly undesirable) effec, there is also the known potential to precipitate diastolic heart failure and pulmonary oedema
- Routine ICU sedatives - if it crosses the blood-brain barrier, it probably also crosses the placenta. If the plan is proceed with caesarian, a floppy infant may be produced.
Strategies which we suspect are safe
Usually, there is no reason to perform a caesarian. However, strangely, delivery seems to improve the status asthmaticus - at least in case reports. certainly, one would not think twice about it if CPR is in progress.