Finely Balanced: Preoperative Congenital Heart Disease

Author: Beth Osmond, Consultant Neonatologist

At St Mike’s, we provide care for newborn infants with an antenatal diagnosis of congenital heart disease. For newcomers to the medical team in the unit, attending these deliveries can be an anxious prospect. This blog post is written to give some general pointers about the clinical problems you could encounter in the infant with preoperative congenital heart disease.

Preparation

Birth: Placenta-ectomy

When you think about it, the fetus is on antenatal “life support”: the placenta providing both ECMO and dialysis- so the moment of “placenta-ectomy” is the first moment of challenge for the baby’s cardiovascular system to function independently.

In reality, most infants with congenital heart disease transition well from intrauterine life to air-breathing. When we run into early problems in the delivery room, this is usually because there is an additional problem with the respiratory system. The respiratory system blood circuit has hither-to been collapsed and insignificant, but must now expand to become a major circulatory route- imagine suddenly turning a Cornish country lane into a motorway. Our focus in the delivery room is providing good basic life support- warmth, airway position and chest expansion- as per NLS principles. Once the lungs are expanded, blood will be drawn into the pulmonary circulation as evidenced by an increase in heart rate. Target saturations are less important here than chest expansion, heart rate and other immediate clinical measures of adequate end-organ perfusion: tone & reactivity.

Neonatal Unit Care

Congenital heart lesions which are dependent on a patent ductus to achieve blood flow are the babies who are interesting and challenging to look after in the newborn period. We put these infants on a continuous infusion of prostaglandin E1- this must be given by a secure route – with an alternative source of venous access swiftly available in case there is an unforeseen line-patency problem. The prostin line is the patient’s lifeline.

Regardless of the specific cardiac lesion, the key aim is to achieve a balance of circulations. This simply means ensuring there is both adequate blood flow to the pulmonary vascular bed and the systemic vascular bed.

Thinking again about the traffic analogy, blood flowing through the circuit has the option of following 2 major motorways – if more goes one way or the other we will run into trouble.

Ventilation/ perfusion matching

As well as achieving adequate blood flow to both circulations, once the blood reaches the pulmonary vascular bed, it needs to be matched with adequate ventilation so oxygen can be taken up and carbon dioxide disposed of. When there is a mismatch in a large area of lung, this will cause significant issues in an already compromised system. Perhaps the motorway to Cornwall is open but none of the shops or beaches are accessible (maybe they are in lockdown) and thus the journey is rendered pointless.

Hypoxia causes pulmonary vascular constriction (unlike in the systemic ciruclation, where it causes vasodilatation). Hypoxic pulmonary vasocontriction is a built- in mechanism to reduce areas of VQ mismatch- diverting blood away from areas where there is poor oxygenation/ atelectasis.

In preoperative duct-dependent congenital heart disease, we also have the added component of shunting of blood away from the pulmonary vascular bed. Increasing the amount of inspired oxygen will not improve the situation and could infact make matters worse.

Unbalanced

When making your bedside assessments of a baby with a prostin-dependant circulation, consider both circulations and whether you have adequate flow to each vascular bed or if the system is unbalanced. You do not need an echo to tell you this.

Think about the vulnerable post-ductal large vascular beds and you will probably be able to infer what we need to measure and track in NICU for the babies with a duct dependent circulation.

Cyanosis

Cyanosis to some degree is expected with many lesions. Target oxygen saturations will be given by the cardiologists depending on the expected degree of pulmonary blood flow. If the saturations are falling, especially if the baby is becoming compromised, they need an urgent cardiology review and may require in interventional procedure.

High Saturations

Do not be falsely reassured by monitors reading sats in the high 90s-100% in a patient with a duct-dependant circulation- be worried and assess the baby. Whilst in the delivery room we sometimes have problems encouraging blood into the lungs, after the pulmonary vascular resistance falls, we can run into the opposite scenario- too much blood going to the lungs and therefore, inadequate perfusion of the systemic system.

Think what the expected saturations are for this patient (and check the bedside card from the cardiologists with target levels). If the sats are high and the baby looks unwell, too much blood is flowing into the lungs.

Lactate Levels:

We monitor these regularly and would be concerned that a rising lactate could represent inadequate end organ perfusion.

Femoral Pulses:

These are a good bedside marker for blood flow to the “south”- at the start of your shift it’s a good idea to palpate them and then you will have a reference point against which to check every few hours.

Urine Output:

Since placenta-ectomy (birth) the kidneys have been functioning independently. They are essentially an elaborate seive and are dependent on adequate blood flow to work efficiently. A reduction in urine output is therefore a worrying early sign that they are inadequately perfused- and will alert staff on NICU in advance of any lab tests of renal function, that something isn’t right.

Gut blood flow:

This is an area which can cause us significant anxiety, particularly if the baby is concurrently preterm and has a duct-dependent circulation. Gut perfusion can be estimated on echo by looking at flow in the mesenteric artery. A good beside test is monitoring for bilious gastric aspirates and regularly palpating the abdomen for distension, discolouration or tenderness. If there are concerns of poor flow to the gut, parenteral feeding is given, although we would always try to offer non-nutritive expressed breast milk to promote healthy development of the microbiome.

Other important clinical signs:

Monitor heart rate and BP (invasive if possible).Examine and touch the patient- feel their skin temperature with the back of your hand centrally and peripherally. Check their central capillary refil time. Observe their colour- a baby with a duct dependent lesion who is pale but 100% saturated should raise alarm bells. A baby who is cyanosed with a good urine output, normal femorals and lactate is less worrying.

Manipulating the Pulmonary Circulation

Lung Expansion

As we know from NLS principles, lungs need to be optimally expanded in order to draw blood into them. You might think that deflating the lung could therefore reduce excessive pulmonary blood flow. Unfortunatley, since  placenta-ectomy (birth) we no longer have an alternative vascular-bed to use for oxygenation. Lung atelectasis is therefore of no benefit and will cause lung injury.

What we need to achieve is matched ventilation/perfusion without atelectasis.

Does PEEP/CPAP increase or decrease pulmonary blood flow?

This depends if there is atelectasis.

In a lung which is unhealthy or under-expanded (eg RDS) PEEP will reduce atelectasis and draw blood into the pulmonary circulation, reducing VQ mismatch.

In a healhy, expanded lung, PEEP might actually reduce blood flow, by a simple pressure mechanism of expanded air sacs on the pulmonary vascular bed. Our cardiologists will sometimes therefore ask us to give CPAP to some babies who are pulmonary over-circulating, to see if this will reduce pulmonary blood flow.

In a ventilated baby with a duct-dependent circulation, manipulation of intra-thoracic PEEP becomes a little easier and more controlled, so we can reduce or increase the PEEP depending upon the status of balance of the circulations. High mean airway pressures/ high PEEP can be used to reduce pulmonary blood flow.

pH

Acidosis (both metabolic and respiratory) constricts the pulmonary vasculature and increases pulmonary vascular resistance, whereas alkalosis selectively decreases PVR.

In a ventilated patient, we can therefore quickly manipulate the CO2, depending upon which direction of blood flow we wish to achieve. We can also manipate the pH by controling any metabolic acidosis, provided we can adequately clear any CO2 which accumulates.

CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3

Promotion of homeostasis: trying to be a placenta

We are trying to do the same job that the placenta did so well- promoting normal physiology will help the baby. We therefore aim for normal electroytes, calcium levels and normothermia. Keeping the baby settled is important- if they are aggitated, they can quickly decompensate. Choice of sedatives is important and should be discussed with a consultant as some agents will cause hypotension. If a baby with a duct-dependent circulation requires induction agents for intubation, this must be done with caution and with guidance from a senior clinician- they can rapidly decompensate in these circumstances.

Controling the size of the Duct

Ductal blood flow is regulary assessed by echo.

The size of the duct and it’s contribution to shunting blood towards or away from the lungs can be manipulated by changing the rate of the prostin delivery.

If you are currently working with us, or are coming to work in our unit or another cardiac unit in the future I hope this post has helped to explain some management points in this fascinating and vulnerable group of patients.

.