Transitional circulation fetal heart simulator

At birth, the fetal circulation, where the left and right sides of the heart work in parallel, transitions into the adult pattern of circulation with the two sides of the heart work in series. In the fetal parallel pattern the two sides of the heart pump out different volumes of blood and this does not cause any problem because of the connections between the left and rignt sides, - at the foramen ovale and the ductus arteriosus. Any excessive return to the right side of the heart can be shunted through the foramen ovale. And excess output from one ventricle can be diverted through the ductus arteriosus. After birth with a full functional pulmonary circulation and closure of the shunt connections, the two sides of the heart, working in series, must pump out the same volume of blood if the blood is not to accumulate in the lungs or in the systemic circulation.

This transition needs to be achieved quickly after birth but not instantaneously. The shunts can remain a little open until both sides of the heart are able to perform equally. In the fetus the output from the right side of the heart is about 250mls/kg/min while the output from the left is only about 200mls/kg/min. The ductus arteriosis allows the excess output from the right side of the heart to supplement the output from the left side which, after birth, needs to increase in response to demand from the muscles and gut circulation. At a physiological and healthy birth the baby's first steps towards transition occurs with inspiration. This first breath leads to a fall in the resitance of the pulmonary circulation and, as breathing becomes established, the majority of the output from the right ventricle goes through the pulmonary circulation. This has a number of effects.
(1) The distended capillaries around the alveoli helps to keep the alveoli open.
(2) The volume of blood returning through the pulmonary vein is greatly increased and the pressure in the left atrium tends to close the valve of the foramen ovale.
(3) The oxygenated blood pumped out by the left ventricle is no longer diluted by blood from the ductus arteriosus, and oxygenated blood arrives at the umbilical arteries. The high oxygen tension together with agents from the newly distended lungs constrict the umbilical ateries.
(4) Less blood is pumped out into the placental circulation but pressure within the uterus maintains a venous return.
(5) As the circulation of the lungs fills, the circulation of the placenta collapses.
(6) Reduced and finally no return of blood from the placenta leads to lower pressure in the right atrium and the valve of the foramen ovale is kept closed by the higher pressure on the left.
(7) High oxygen tension of the blood leads to constriction of the ductus arteriosus.

In the next 24 to 48 hours the left ventricle hypertophies, allowing the ductus arteriosis to close completely by which time the output from the left and right have become equal. The process is slow and controlled. It needs to be. Minor changes in pressure or flow could precipitate a cascade of events. Unfortunately birth is often far from easy for the baby or mother. The baby may already be hypoxic and acidotic. When there is sufficient concern an operative delivery will be carried out. Let's consider what this involves. If it is possible, and considered safe, a vaginal delivery by forceps or ventouse will be carried out, otherwise a caesarean section is necessary. At delivery the umbilical circulation is immediately clamped and the baby handed to the paediatrician for resuscitation. Sometimes the baby may have already taken its first breaths but the pulmonary circulation will not have become established, nor the umbilical circulation closed. Clamping the functioning placental circulation results in a dramatic increase in the systemic resistance and an equivalent increase in the afterload of the heart. ( The placental circulation forms 40% of the combined [left + right] cardiac output). Even a healthy heart may have difficulty in recovering from this sudden load. It is uncertain what effect this will have on the homeostatic and regulatory systems of the circulation of the baby. A further effect is hypovolaemia. The pulmonary circulation has to open up at the expense of the rest of the circulation since the blood in the placenta has been blocked off. The hypovolaemia may lead to hypotension and hypoperfusion of vital organs. This heart simulator demonstrates what must happen to the circulation when the cord is clamped suddenly at birth before the pulmonary circulation is established. Like any simulation it is not perfect. With any medical intervention it is obligatory to establish that the intervention is benficial. All the evidence suggests that the intervention is harmful.

I am grateful to Falstad for providing this computer analogue circuit simulator. Copy the program below first then load the circuit simulator from

http://www.falstad.com/circuit/
On the circuit simulator, goto File Import and paste in the program below. Complete the installation by clicking the import button. The cicuit is fairly self explanatory.
$ 1 5.0E-6 22.331051418620845 43 5.0 50
r 160 208 208 192 0 578.0
d 208 192 272 208 0
w 272 112 272 48 0
w 272 48 336 48 0
w 336 48 336 256 0
w 272 48 272 16 0
r 272 16 32 16 0 44.96
w 32 16 32 208 0
w 32 208 160 208 0
r 400 64 400 160 0 999.0
w 400 64 448 64 0
w 400 160 448 160 0
s 448 64 448 96 0 true false
r 448 160 448 96 0 40.0
w 272 208 400 160 0
w 368 304 464 304 0
w 352 320 464 320 0
s 464 320 496 320 0 false false
r 544 272 544 368 0 12.99
w 464 304 544 272 0
w 496 320 544 368 0
w 304 112 400 64 0
r 336 48 304 80 0 0.21
s 304 80 304 112 0 false false
w 336 256 304 320 0
w 304 320 320 352 0
w 304 320 272 352 0
w 320 352 352 320 0
w 272 352 160 400 0
x 372 13 388 13 0 24 Fetal to adult sim
x 68 44 84 44 0 24 Brain and upper body
x 127 366 143 366 0 24 Lower body
x 390 319 406 319 0 24 Cord
x 562 244 578 244 0 24 P
x 563 264 579 264 0 24 l
x 564 286 580 286 0 24 a
x 567 309 583 309 0 24 c
x 570 334 586 334 0 24 e
x 568 358 584 358 0 24 n
x 569 383 585 383 0 24 t
x 566 406 582 406 0 24 a
x 484 70 500 70 0 24 L
x 485 93 501 93 0 24 u
x 486 116 502 116 0 24 n
x 486 141 502 141 0 24 g
x 486 168 502 168 0 24 s
x 277 80 293 80 0 24 DA
w 288 272 368 304 0
w 256 256 272 224 0
w 304 240 288 272 0
x 242 295 258 295 0 24 DV
x 280 226 296 226 0 5 Liver
w 160 112 304 112 0
i 160 208 160 144 0 0.25
i 272 208 272 160 0 0.2
v 160 144 160 112 0 0 40.0 5.0 0.0
v 272 160 272 112 0 0 40.0 5.0 0.0
x 137 175 153 175 0 72 R
x 254 175 270 175 0 78 L
x 188 206 204 206 0 31 F
r 272 224 304 240 0 28.27
r 160 400 160 272 0 150.0
w 160 272 160 256 0
w 160 256 160 240 0
w 256 256 160 240 0
w 160 240 160 208 0
r 256 256 240 288 0 40.76
s 240 288 288 272 0 false false
All the flows approximate to those thought to occur in the fetus. The equivalent of current is blood flow, the equivalent of blood pressure is voltage. There is a switch on the umbilical cord for clamping and a switch on the lung circuit to open up the full pulmonary circulation. You can experiment with different values, increase the placental resistance as in placental insufficiency. Right clicking on a component allows you to alter the value. One of the most interesting ( and worrying) effects that I noted form the simulator was the reverse flow in the aortic isthmus when the cord is clamped before the pulmonary circulation is opened as would happen with a baby requiring resuscitaion. The heart is both volume and pressure driven. Until the volumes are equal the circuit will not function with the switch on the ductus arteriosus open. (ie the ductus closed).
Please read my review article in The Obstetrician and Gynaecologist Hutchon D. A view on why immediate cord clamping must cease immediately. The Obstetrician & Gynaecologist 2008;10:2:112-116
This link for a summary Summary of TOG article at - onlinetog.org/cgi/content/abstract/10/2/112

Author
David J R Hutchon BSc, MB, ChB, FRCOG
Consultant Gynaecologist, Memorial Hospital, Darlington, England.
Please get in touch if you have any difficulty in getting the simulator to function. I would appreciate an E-mail if you find it interesting whether or not you agree with my view. E-mail to me at DJRHutchon@hotmail.co.uk
31/8/2008