PDA

By Nicole Stevens

Human fetal circulation works differently to extrauterinecirculation, and there are 3 key differences:

The presence of a placenta in the “circuit”

Blood flowing through the foramen ovale between theright atrium and the left atrium

Blood bypassing through the ductus arteriosus from thepulmonary artery to the aorta

There are also many other embryonic circulatory vessels;these are slowly obliterated and remain in the adult only asfibrous remants

ROLES OF THE PLACENTA:

Secretes endocrine hormones which increase throughthe pregnancy, causing progressive changes inmaternal metabolism.; this creates an increase in thematernal glucose and amino acids available to thefoetus

Facilitates the transfer of these (and other) essentialnutrients from the maternal to the fetal circulation, andconversely, of metabolic wastes from the fetal to thematernal system

Adequate maternal and fetal blood flow through theplacenta is vital through the pregnancy to maintainhealthy growth and development of the fetus

ROLES OF THE PLACENTA cont..:

Exchange of CO2 and O2 also occurs in the placenta

The maternal and fetal circulations are not joined,instead, the respiratory gases, carbohydrates, lipids,vitamins, minerals and amino acids diffuse across theplacental membrane

The placenta is a low resistant component of fetalcirculation

The fetal heart generates the force to pump bloodthrough the umbilical arteries (2) to the placenta

Blood returns from the placenta to the fetus via theumbilical vein

FORAMEN OVALE (FO):

Connects the right atrium to the left atrium; it permitsoxygenated blood to bypass the right ventricle andpulmonary circuit and go directly to the left atrium

A “flap” known as the septum primum is located onthe left side of the atrial septum, it is pushed open byblood flow going from right to left, but any flow goingfrom left to right will push it closed (this preventsblood flowing in the wrong direction)

DUCTUS ARTERIOSUS:

Connects the pulmonary trunk to the descending aorta

Allows bypassing of the non-functioning lungs

Returns blood to the placenta for oxygenation

Systemic circulation returning to the heart goes into theright atrium then the right ventricle, this is pumpedinto the pulmonary artery; blood wants to travel on thepath of least resistence, there is high resistence in thelungs of the fetus and low resistence in systemiccirculation, the blood will prefer to flow across theductus arteriosus into the aorta, rather than down theright and left pulmonary arteries to the lungs.

THE PATH: (may help to track it on your heart drawings)

Deoxygenated fetal blood is carried to the placenta bythe two umbilical arteries; these arise from the internaliliac arteries

Gas exchange occurs in the placenta

Oxygenated blood travels from the placenta via theumbilical vein and enters the IVC

About 50% of this passes through the liver and the restbypasses the liver via the ductus venosus; the IVC alsodrains blood returning from the lower part of the body

THE PATH cont..

On reaching the heart at the right atrium the blood isdivided into 2 streams (of unequal sizes); the largerstream is shunted to the left atrium through the FO,

the smaller stream joins the flow of blood returningfrom the SVC (returning from the myocardium andupper body)

The blood that goes through the FO ends up in the leftventricle and is ejected into the aorta

The blood that goes into the right ventricle is pumpedinto the pulmonary artery but most will go across theDA and end up in the aorta as well

Most of the relatively O2 rich blood that comes up theascending aorta from the left ventricle will end up atthe head and upper extremeties (the vessels feedingthese areas come off the aorta prior to the DA unionpoint)

The patency of the DA is maintained by the relativelylow O2 tension and the vasodilating effects ofprostaglandin E2 (maternal hormone coming throughthe placenta)

FETAL

CIRCULATION

At birth, the circulation of fetal blood through theplacenta ceases; the umbilical vessels are obliteratedwhen the cord is clamped externally

There is a reduction in blood flow through the IVC andthe ductus venosus (this will close within 3 – 10 days)

There is a dramatic fall in PVR with lung expansion(opening up of pulmonary vessels)

A reduction in hypoxic pulmonary vaso-

constriction and stimulation of pulmonary stretchreceptors contribute to this process

The rise in pulmonary blood flow leads to an increasein blood returning to the left atrium; the left atrialpressure therefore exceeds the right atrial pressure; thisreversal of pressure causes the flap of the FO to bepushed closed (the initial closure occurs withinminutes, anatomical closure takes several days

The ductus arteriosus consticts due to the high partialpressure of oxygen; this process is usually complete 2days after birth (we will discuss the failure of this tooccur later)

NEONATAL

CIRCULATION

In some circumstances a neonates circulation mayrevert back to be fetal-like; this state is known aspersistent foetal circulation (PFC)

A neonates pulmonary arterioles remain very reactiveand will constrict in response to: hypoxia, hypercarbia,acidosis and cold; this can lead to an increase in PVR,which favours right to left shunting through the FOand ductus arteriosus

The result of this is the neonate reverting to PFC, withone major difference – there is no placenta to provideoxygenation; a vicious cycle of worsening hypoxia andacidosis is then set in motion.

The management of this requires treatment of theunderlying causes:

Such as, escalating respiratory support to providebetter ventilation

Use of nitric oxide if there is pulmonary hypertension,to aid in dilating the pulmonary vessels

Management of infection with antibiotics

Management of thermoregulation

BY: NICOLE STEVENS

Patent Ductus Arteriosus is a functional connectionbetween the pulmonary artery and the descendingaorta.

In the fetus the DA allows blood to be diverted awayfrom the pulmonary artery (the high resistancepulmonary circulation) to the descending aorta (thelow resistance systemic circulation)

The duct remains open due to the prostaglandinsupply coming from the placenta.

The DA protects the lungs from circulatory overload.

After the birth the baby’s lungs expand as they begin totake breaths.

Pulmonary resistance decreases and the oxygen levels inthe blood rise causing the DA to constrict.

Sometimes the DA doesn’t constrict and the baby is leftwith a PDA.

This can be due to lack of oxygen or an increase in theamounts of prostaglandin.

Sometimes the PDA will functionally close but will reopenin response to certain stimuli such as hypercarbia,acidosis, infection, hypoxia and cold

PATENT DUCTUS ARTERIOSUS

It is approximately 10mm in diameter

Functional closure occurs between 12 hours and 4

days of age in healthy term babies

Structural closure takes between 2 to 6 weeks

It is common in premature infants

Serious concern as the pulmonary artery is not gettingenough blood for the lungs to function effectively

It can cause worsening RDS and an escalation in therespiratory support that is required

Diagnosed on ECHO

Treated with Indomethacin and sometimes may requiresurgical closure (called a PDA ligation)

PDA in the preterm infant:

There is nothing wrong with the heart, the PDA justdoesn’t close and usually causes a left to right shunt

The tissue is immature; the fetal haemoglobin has ahigher affinity for oxygen (doesn’t release it as easily)

The lower the gestational age the greater the risk ofPDA

VLBW infants will have up to 50 – 60% chance of aPDA

The immature ductal tissue is much less reactive tooxygen, therefore less likely to readily close in responseto the increase in partial pressure of oxygen that occursafter birth

PDA in the term infant:

If there is pulmonary hypertension (numerous causes forthis, eg. meconium aspiration, infection) the blood shuntsright to left through the PDA

Deoxygenated blood that should be going to the lungsgoes back to the body, this causes hypoxia and acidosis,which causes vascular smooth muscle to constrict, andthe end result can be persistent fetal circulation (withoutthe presence of placenta for oxygenation)

When trying to determine if shunting of blood is occuringacross a PDA you may be asked to measure pre and postductal oxygen saturations

Pre ductal SaO2 are measured on the right hand/wrist,and post ductal SaO2 can be measured on any of theother limbs, usually a difference of 10% or more isconsidered significant and may be caused by shunting ofblood across a PDA

The arteries that supply part of the brain, and right arm,branch off the aorta prior to where the duct joins it, andthe arteries that supply the other limbs branch off afterthe point where the PDA joins the aorta – hence, thereason we can measure pre and post ductal SaO2 at thesepoints.

Pulmonary oedema

Poor perfusion

Bounding pulses

Swinging saturations

Desaturations

Renal compromise

Heart murmur

Congestive cardiac failure

Mottled or pale

If a neonate is symptomatic of an existing PDA theymay require treatment

Chemical treatment: Indomethacin (usually given IVbut oral route can be used, although thought to be notas effective)

(Note: there is currently a world-wide shortage ofindomethacin as its production has stopped, in thenear future an alternate drug may be used)

If chemical treatment fails surgical treatment may beused: PDA ligation

First line treatment

Urea & Creatinine are checked beforecommencing Indomethacin to ensure normalrenal function before commencement of thedrug.

Platelets are also checked to ensure normal range

3 daily doses of 200mcg/kg at 2200hrs

Side Effects include impairment of renalfunction, reduce platelet aggregation, GI tractbleeding, hypoglycaemia, hyponatreamia,hyperkaleamia and can mask infection.

NURSING RESPONSIBILITIES FOR INDOMETHACIN

Monitor urine output

Daily Urinalysis

BP monitoring

Observe for bleeding and or NEC

Contraindicated in infants with recent IVH andNEC

Some cardiac abnormalities are considered “duct-dependent”; these can be cyanotic or acyanotic

Duct-dependent defects can be related to pulmonary flow(for example pulmonary stenosis or pulmonary atresia);or related to systemic flow (for example hyperplastic leftheart or transposition of the greater arteries)

If there is a definite or suspected diagnosis of a cardiacabnormality, that is duct dependent, a prostaglandininfusion is the treatment of choice (a synthetic version ofthe hormone that keeps it open inutero); the use ofprostaglandin before further investigations and/orsurgery has improved the mortality and morbidityoutcomes for these neonates

$C:\Users\Nicole\AppData\Local\Microsoft\Windows\Temporary Internet Files\Content.IE5\0QJEC6QS\MPj04447520000[1].jpg$