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.

Acute Respiratory Failure in Pregnancy
CauseCardinal features and brief discussion
High epidural/spinal block
  • Abnormal (diaphragmatic) respiratory movements
  • Flaccid paralysis of the extremities
  • Otherwise preserved consciousness
  • Predominantly hypercapneic respiratory failure
Amniotic fluid embolism
  • Clinical evidence of right heart failure
  • Petechial rash
  • DIC
  • Seizures
  • Haemodynamic compromise
Pre-eclampsia leading to pulmonary oedema
  • Severe hypertension
  • Clinical features of pre-eclampsia, eg. proteinuria
Tocolytic-associated pulmonary oedema
  • Improves with the withdrawal of tocolytics
  • Associated with tachycardia
Peripartum cardiomyopathy
  • TTE evidence of LV dilatation and decreased LVEF (by definition, under 45%)
  • Displaced apex beat
  • Audible S3
Air embolism
Pneumomediastinum
(also known as Hamman's syndrome)
  • Follows a Valsalva manoeuvre during the second stage of labour
  • Typically, associated with pneumothorax;
    more rarely associated with an oesophageal rupture
  • Neck and facial crepitus (surgical emphysema)
  • Impressive CXR and CT features
  • A "mediastinal crunch" may be heard on auscultation, also known as "Hamman's Sign" and more familiar from the examination of cardiothoracic ICU patients who have just returned from a CABG. It is a fine crackle heard in synchrony with the heart beat.
Accidental magnesium overdose
  • Depressed or absent reflexes, flaccid paralysis
  • Clinical features of hypermagnesaemia
  • Historically, pre-eclampsia or eclampsia
  • Usually, associated with the use of premixed bags of MgSO4
Causes which are not unique to pregnancy, but which commonly co-exist with pregnancy
Sepsis
Peripartum opiate use
  • Predominantly hypercapneic respiratory failure
  • Clinical features of opiate toxidrome (pinpoint pupils, etc)
PE
  • Clinical features of right heart failure
  • Hypoxia, tachypnoea, tachycardia
  • Characteristic ECG changes (S1 Q3 T3, right heart strain)
Fluid overload
  • Historically, vigorous fluid resuscitation
  • Clinically, evidence of peripheral oedema
Aspiration
  • Auscultation findings consistent with aspiration
  • Characteristic CXR appearance
  • A history of vomiting or decreased level of consciousness (eg. seizures)
Transfusion reaction
  • History of recent large volume blood transfusion 
  • Usually, this is a TACO as opposed to TRALI
  • Haemolysis and haemoglobinuria may be present if this was really incorrectly crossmatched blood

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:

Airway:

  • Prepare for intubation with optimal senior ICU/anaesthetic expertise 
  • Expect a difficult airway and greatly decreased reserve

Ventilation:

  • 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.

Circulation:

  • 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)

Sedation

  • Avoid long-acting opiates and benzodiazepines to protect the foetal respiratoryt drive

Foetal wellbeing:

  • Continuous CTG monitoring
  • Early O&G/neonatology involvement
  • Preparation of the foetal lung for delivery with steroids

Social issues:

  • 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.

 

References

 

Oh's Intensive Care manual:

Chapter 64   (pp. 684) General  obstetric  emergencies by Winnie  TP  Wan  and  Tony  Gin

Chapter 65   (pp. 692) Severe  pre-existing  disease  in  pregnancy by Jeremy  P  Campbell  and  Steve  M  Yentis

Lapinsky, Stephen E. "Acute respiratory failure in pregnancy." Obstetric Medicine: The Medicine of Pregnancy 8.3 (2015): 126-132.

Samanta, Sukhen, J. Wig, and A. K. Baronia. "How safe is the prone position in acute respiratory distress syndrome at late pregnancy?." (2014).

Rubal, Bernard J., et al. "The'mill-wheel'murmur and computed tomography of intracardiac air emboli." Journal of the American Association for Laboratory Animal Science 48.3 (2009): 300-302.

Lifschultz, Barry D., and Edmund R. Donoghue. "Air embolism during intercourse in pregnancy." Journal of Forensic Science 28.4 (1983): 1021-1022.

Balkan, M. Erkan, and Göknur Alver. "Spontaneous pneumomediastinum in 3rd trimester of pregnancy." Annals of thoracic and cardiovascular surgery 12.5 (2006): 362.

Jain, Vikyath. "Acute respiratory distress syndrome, Respiratory failure, Pregnancy." ACUTE RESPIRATORY DISTRESS SYNDROME IN PREGNANCY 7540 (2015).

Robinson, Julian N., et al. "Inhaled nitric oxide therapy in pregnancy complicated by pulmonary hypertension." American journal of obstetrics and gynecology 180.4 (1999): 1045-1046.