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Pulmonary Hypertension of the Newborn (PPHN)
What is Persistent Pulmonary Hypertension of the Newborn (PPHN)?
PPHN is also often referred to as Persistent Fetal Circulation, or PFC. PPHN is a very serious problem in the newborn. It remains one of the leading causes of death in term or near-term infants.
In the fetus (the baby when still inside the mother during the pregnancy), unique situations exist.
- To obtain oxygen, the blood must flow through the placenta rather than the lungs
- The fetal lungs do not have air in them, therefore, no oxygen is available
- The placenta supplies oxygen from the mother's blood to the baby's blood
- A special blood vessel (Patent Ductus Arteriosus, or PDA) is open in the fetus
- The PDA diverts or shunts blood away from the lungs and back to the placenta
Blood can be shunted because:
- The placenta acts like a large pool with very low resistance to blood flow
- The fetal lung blood vessels are constricted and restrict blood flow
- After delivery, the lungs must fill with air, and blood must begin to flow through the lung to produce oxygen
- Normally, when the lungs fill with air, the lung blood vessels dilate, resistance to blood flow drops, and it is easier for blood to go through the lungs
- At delivery, the umbilical cord is clamped, which removes the placenta as a pool
- This helps to increase the resistance in the blood vessels outside the lungs
- The end result is blood can easily be pumped through the lungs by the right side of the heart
The primary problem of babies with PPHN is failure of the lung blood vessels to relax and dilate.
- Result is decreased blood flow through the lung
- Decreased lung blood flow equals decreased blood oxygen levels
What causes PPHN?
There is a variety of factors that cause the lung blood vessels to relax (dilate) or to constrict. Factors that relax the lung blood vessels are:
- Most important is adequate lung inflation with air
- Increased oxygen also relaxes these vessels
- Lower levels of acid in the blood also dilate the lung blood vessels
- Effects of lung inflation, oxygen, and blood acid levels are through other chemicals
- Nitric oxide is probably the most important
Factors that cause the lung blood vessels to tighten or constrict:
- Low levels of oxygen
- Collapsed lungs
- Lungs that are over-expanded (have too much air)
- Increased acid levels in the blood
- Very small lungs (or pulmonary hypoplasia)
- Blood that is too thick (polycythemia)
- Severe infection (sepsis) can produce factors that cause constriction of lung vessels
- Any problem that interferes with blood from the lung going into the left heart
- Normally, blood is pumped by the right heart into the lung for oxygen pickup
- Oxygenated (red) blood then returns to the left heart to be pumped to the body
- A wide range of diseases can cause PPHN to persist after birth or recur within the first few days of life
Any disease of the lung can result in PPHN. These may include:
- Respiratory Distress Syndrome (RDS)
- Pneumonia
- Meconium Aspiration Syndrome
- Lung hypoplasia , such as in Congenital Diaphragmatic Hernia
- Severe infection is a common cause of PPHN
- Certain cardiac malformations or diseases - Hypoplastic Left Heart and coarctation of the aorta, Severe deformity of the aortic or mitral heart valves, Abnormal formation of the pulmonary veins
- Problems that cause the heart to work very poorly
- Inadequate oxygen/blood flow to the fetus (Hypoxic-Ischemic Encephalopathy)
- Sometimes, the specific cause is unknown
How do we diagnose PPHN?
Most often, PPHN is suspected because of low blood oxygen levels, or cyanosis. There are many causes of low blood oxygen levels, some of which can also include PPHN as part of the reason (i.e., pneumonia). Additionally, certain infants are known to be at high risk for developing PPHN.
High index of suspicion if the following are present:
- Low APGAR scores
- Thick meconium-stained amniotic fluid
- Suspected pulmonary hypoplasia
- Severe infection
Evaluation for PPHN might include:
- Pulse oximetry - Normal oxygen saturation by pulse oximeter should be greater than 95%. All four extremities should have the same measurement. Frequently, in PPHN, leg measurements will be lower than the right arm value.
- Echocardiography - This is the best test for diagnosing PPHN. Not every nursery has the ability to perform echocardiography on infants, but not always needed for diagnosis
- Cardiac catheterization - Rarely needed with today's echocardiography equipment
How do we treat infants with PPHN?
Although a wide variety of therapies are used in the treatment of infants with PPHN, most of these therapies have not been systematically studied to determine their overall ffectiveness. A step-wise approach is usually undertaken, depending on the severity of PPHN and the response to each therapy.
It is important to make sure the basic needs of the infant are being met, which are:
- Adequate blood sugar levels (glucose)
- Adequate blood cell numbers (hematocrit) - Red blood cell transfusions might be required
- Adequateblood pressure - Medications, such as dopamine, dobutamine, and epinephrine, might be used to increase the body's blood pressure. These treatments may also increase the pulmonary blood pressure.
Othere therapies may be used including:
- Sedation - used in most infants with PPHN. "Outside" stimuli can worsen PPHN. Paralysis (medication so the baby cannot move) might be used at some centers to further minimize the effects of outside stimuli.
- Oxygen -
- This is always the first and most important therapy. Generally, we try to keep arterial blood gas oxygen levels between 80-100. Oxygen levels need to monitored carefully as very high blood oxygen levels may be toxic.
- Mechanical ventilation is almost always required for treating severe PPHN.
- Hyperventilation (breathing at excessive rates or volumes) is a traditional approach.
This can increase secondary injury to the lung. Some centers use High Frequency Ventilation to try to minimize this injury.
- pH - The pH is a measure of the amount of acid in the blood. As acid levels go up, PPHN can get worse. Many centers try to decrease acid levels in an effort to treat PPHN. This may be done by alkaline therapy, basically, giving sodium bicarbonate. An alkaline compound interacts and neutralizes an acid. As another method to lower pH some centers might use alkaline treatment instead of hyperventilation. With any method care must be taken. Severe alkalosis can decrease brain blood flow. Can cause hemoglobin to bind oxygen more tightly and not give it up to the cells. Long-term risk of extreme changes in pH by either approach is unknown.
- IV medications - specific medications such as Dobutamine, Isoproterenol, or Priscoline may be used to attempt to decrease pulmonary blood pressure (vasodilator therapy). These may decrease the body's blood pressure.
- Inhaled Nitric Oxide (iNO) - iNO is currently considered the most effective therapy available for PPHN. It is effective because, at the doses used, iNO only alters lung blood pressure. iNO was approved for treatment of PPHN in term and near-term infants in 1999. iNO is very, expensive.
- ExtraCorporeal Membrane Oxygenation (ECMO) - ECMO might be the last resort for infants with severe PPHN who do not respond to iNOT. ECMO is effective in over 90% of infants with PPHN (except Diaphragmatic Hernia) but has many risks
What are the long-term effects associated with PPHN?
Most infants with PPHN recover without significant problems. Potential complications include the following
- Chronic pulmonary problems; usually, due to the underlying cause of PPHN
- Neurodevelopmenatal problems; especially, hearing abnormalities
- Death can still occur in infants with severe PPHN
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