This chapter is relevant to Section G1(iv) of the 2017 CICM Primary Syllabus, which asks the exam candidate to "describe the fetal circulation". By tracking this occurence of an Americanised spelling in an Australian professional document, one can suppose that, like many syllabus items, this was cut-and-pasted from the ANZCA Syllabus for the Basic Sciences. Incidentally, as a member of a Commonwealth country where etymological fallacies are viewed as a marker of cultured literary prestige, the author has chosen to keep his foetuses full of Oe.
This has never appeared in the First Part papers, which is somewhat weird. The topic of foetal circulation and the circulatory changes which occur at birth has appeared multiple times in the Fellowship exam (Question 10 from the second paper of 2007, Question 11 from the first paper of 2005, Question 6 from the first paper of 2001), but vanished completely with the establishment of the CICM Primary Exam, which did not exist pre-2007.
The key to learning this subject would be to find an explanation which focuses on the reasons behind the structural and functional differences between adult and in-utero circulations. Without that rationale to stimulate their faculties, the trainee turns to mindlessly memorising the list of differences. This produces suboptimal trainees, but also answers the SAQs, i.e. either approach has merit. The author himself has understood and re-forgotten this topic on at least eight separate occasions, in spite of epic efforts to self-educate.
- The umbilical vein carries blood from the placenta to the IVC via the ductus venosus
- From the IVC, blood splits into two parallel circuits:
- via the foramen ovale into the LA
- via the RV into the pulmonary circulation
- From the pulmonary trunk to the aorta via the ductus arteriosus
- From the aorta, some of the blood flow returns to the placenta via the umbilical artery
Systemic oxygen saturation 65% (in the left atrium) 97-100% (in the aorta) Venous oxygen saturation 25-40% (IVC) 65-70% (pulmonary artery) Arrangement of circulation Parallel circuit with shunts Serial circuit Pulmonary blood flow 8% of the cardiac output 100% of the cardiac output Pulmonary vascular resistance High Low Pressures in the ventricles Equal Left ventricle has a much higher pressure than right Mean arterial pressure
15-40 mmHg 65-90 mmHg Compensation Minimal capacity to increase stroke volume; must increase heart rate to increase cardiac output Can increase stroke volume (by Starling mechanism) as well as heart rate
As far as peer-reviewed articles go, the best one would probably have to be the free BJA article by Murphy (2013), because it's short and to the point. Kiserud (2005) or Kiserud & Acharya (2004) might also be useful, depending on which links die first (in fact, anything by Torvid Kiserud seems to be gold). All of these authors have one flaw, which is the insistence on trying to represent the circulatory system with some semblance of anatomical accuracy. One could, however, make the arguments that CICM trainees are a) never going to operate on the unborn, and b) have a very function-oriented focus in the primary exams, which makes anatomy completely superfluous in this specific topic area. Therefore, a purely functional flowchart diagram is offered, to liberate the trainees from confusing tangles of poorly drawn greater vessels.
This diagram of the foetal circulation is an uncharacteristically austere version which should be reasonably easy to reproduce in an exam setting.
In summary, the path of blood flow is,
- Oxygenated blood from the placenta returns via the umbilical vein
- The umbilical vein distributes
- 40% of its flow to the liver
- 60% of its flow to the inferior vena cava, via the ductus venosus
- The inferior vena cava drains into the right atrium
- From the right atrium, the flow splits into parallel circuits:
- Into the right ventricle, and then
- Into pulmonary circulation, then back to the left atrium and left ventricle
- Into the systemic circulation via the ductus arteriosus, which connects the pulmonary trunk to the proximal descending aorta
- Into the left atrium via the foramen ovale, and then to the left ventricle (this is mainly IVC blood, directed there by the Eustachian valve)
- From the left ventricle, into the aorta
- From the aorta, upper body blood flow (brain and arms) is purely from the left ventricle, whereas lower body blood flow is the combined output of the LV and RV (via the ductus arteriosus)
- From the systemic circulation, venous blood returns:
- From the common iliac arteries, via the umbilical arteries, to the placenta
- From the upper body, to the right atrium
- From the lower body, to the right atrium
In case this information is ever of interest, MRI imaging has been able to measure the following flow rates in these vessels, (indexed to kilograms of body mass in the 37 week foetus):
There are a few structures which the trainee should probably know a little bit about, because they are unique to prenatal life. These will be discussed in order of circulatory appearance:
Apart from being structurally quite unlike the adult system, the foetal circulation has enough functional weirdness to fill an extensive list:
Though these differences have never been a topic of an SAQ (none of this has), they are a ripe low-hanging fruit for an easily graded question, where a tabulated answer is expected. Where possible, the oxygenation values and other numbers came from the Murphy article.
|Source of oxygenated blood||Placenta||Lungs|
|Systemic oxygen saturation||65% (in the left atrium)||97-100% (in the aorta)|
|Venous oxygen saturation||25-40% (IVC)||65-70% (pulmonary artery)|
|Arrangement of circulation||Parallel circuit with shunts||Serial circuit|
|Pulmonary blood flow||8% of the cardiac output||100% of the cardiac output|
|Pulmonary vascular resistance||High||Low|
|Pressures in the ventricles||Equal||Left ventricle has a much higher pressure than right|
|Mean arterial pressure
|15-40 mmHg||65-90 mmHg|
|Compensation||Minimal capacity to increase stroke volume; must increase heart rate to increase cardiac output||Can increase stroke volume (by Starling mechanism) as well as heart rate|