Respiratory Failure and ARDS in Pregnancy
Respiratory failure in pregnancy poses unique challenges, as you are ventilating two circulatory systems using only one set of diseased lungs. Gas exchange will be complex. It is hard enough ventilating and oxygenating the mother, but then you need to take into account the fact that the foetus is in there, with its own brand of haemoglobin. On top of that, there is the various drug selection and position limitations on the patient (i.e. good luck turning her prone). In short, everything becomes more difficult, includng the social scenario (particularly if the family are then expected to help with decisionmaking regarding ongoing foetal or maternal survival).
This topic has come up a couple of times in the SAQs. Question 26 from the second paper of 2010 presented a (then-topical but 12 months late) scenario of a pregnant patient with H1N1 influenza, and Question 12 from the second paper of 2006 asked more broadly about the possible causes of respiratory failure in of the pregnant woman. Specifically, sudden respiratory failure was being asked about. Clinical features which discriminate between these different causes were sought.
A good thorough resource for this topic can be found in UpToDate. For those cursed with poverty, one can recommend this 2015 review article by Stephen Lapinsky. Management strategies and caveats are well-explored by Jain et al, 2015.
Causes of respiratory failure in pregnancy
This list could be super long, seeing as pregnant human beings remain susceptible to the causes of respiratory failure which affect the normal non-pregnant population. Thus, here is an abbreviated list of respiratory disorders which are somehow (at least loosely) associated with pregnancy, and which have a sudden (or at least subacute onset. As such, this table makes a suitable answer for Question 12 from the second paper of 2006.
|Cause||Cardinal features and brief discussion|
|High epidural/spinal block||
|Amniotic fluid embolism||
|Pre-eclampsia leading to pulmonary oedema||
|Tocolytic-associated pulmonary oedema||
(also known as Hamman's syndrome)
|Accidental magnesium overdose||
|Causes which are not unique to pregnancy, but which commonly co-exist with pregnancy|
|Peripartum opiate use||
Management of ARDS during pregnancy
Major pregnancy-related limiting factors which complicate the management of ARDS are as follows:
- Intubation will be difficult, for well-known reasons.
- Need to maintain normoxia to maintain a satisfactory foetal oxygen transfer gradient
- Cannot tolerate permissive hypercapnea as this runs the risk of foetal acidosis. Foetal hemoglobin will decrease in its affinity for oxygen, thus CO2 should be kept no higher than 45mmHg. The second part of the college answer to Question 26 from the second paper of 2010 mentions this. An article commenting on one author's experience suggests that the intensivist should ignore this, as making attempts to increase minute ventilation will overload the right heart.
- Pressures will be high: the gravid uterus contributes to decreased respiratory compliance
- Prone ventilation will be difficult, because of the belly. However it is not completely off the table: see this case report by Samanta et al (2014) who safely ventilated a third-trimester patient with H1N1.
- Sustained paralysis is out of the question because of the risks to the foetus (arthrogryposis)
- Pulmonary vasodilators are still available, including nitric oxide - though there is no experience in pregnant ARDS, only in pulmonary hypertension as in Robinson et al (1999).
Thus, a management strategy mentioning all the important points would resemble the following list:
- Prepare for intubation with optimal senior ICU/anaesthetic expertise
- Expect a difficult airway and greatly decreased reserve
- Ensure normoxia by using a higher FiO2 target (minimum saturation 95%)
- Aim for normocapnea, CO2 no higher than 45mmHg
- Rely on sedation to relax the respiratory muscles; sustained NMJ blockade is out of the question
- Consider inhaled nitric oxide or prostacycline to improve V/Q matching
- If these measures fail, the next option will be determined by local level of experience in prone ventilation or ECMO. Both will be risky and difficult to manage.
- Ensure good central venous filling; high ventilator pressures may give rise to haemodynamic instability in the volume-depleted woman
- Nurse the patient in alternating left and right recovery position (this may also improve V-Q matching)
- Avoid long-acting opiates and benzodiazepines to protect the foetal respiratoryt drive
- Continuous CTG monitoring
- Early O&G/neonatology involvement
- Preparation of the foetal lung for delivery with steroids
- Maternal in-hospital mortality may be in the realm of 45%. Foetal mortality is more difficult to estimate. Family need to be aware of this.