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Dr. Alessandro Giardini
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The period between discharge after the Norwood procedure and the Glenn operation, typically spanning three to five months, is one of the most delicate periods for children treated for HLHS. The baby is at home. The surgery is behind them. The next operation is months away. And yet the circulation remains profoundly abnormal, vulnerable to rapid deterioration, and dependent on a level of vigilance that few families anticipated when they first heard the word parenthood.
Understanding what the interstage period involves, what monitoring is required, and what warning signs mean is extremely important.
After the Norwood procedure, the baby's circulation is maintained by the single right ventricle pumping blood both to the body and to the lungs via a shunt. The shunt is a small synthetic tube. It can narrow. It can clot. The balance between pulmonary and systemic blood flow remains delicate, and a shift in either direction can produce rapid deterioration. Pulmonary vascular resistance continues to fall during the first months of life, which means the proportion of cardiac output going to the lungs tends to increase over time, gradually reducing systemic perfusion.
Published data consistently show that interstage mortality, deaths occurring between hospital discharge after stage one and the Glenn procedure, represents a significant and disproportionate portion of overall HLHS mortality. Historical interstage mortality rates of 10 to 15% have been substantially reduced at centres with formal interstage surveillance programmes, in some series to below 5%. The difference between those figures is largely parental education, home monitoring, and a responsive clinical team.
This is worth stating plainly: what parents do during the interstage period matters to survival outcomes.
Every centre with an active interstage programme uses some version of the same three core components.
Weight measurement is the most sensitive early indicator of deteriorating haemodynamics. A baby who is retaining fluid because the heart is struggling to maintain output will gain weight before any other sign appears. Families are given specific thresholds: an unexplained weight gain of more than thirty grams per day, or a trend of unexplained weight gain over several consecutive days, warrants a call to the cardiac team the same day.
Pulse oximetry monitoring involves measuring the baby's oxygen saturation daily using a home pulse oximeter. In a baby with a well-functioning Norwood circulation, saturations typically run between 75 and 85%. A reading consistently below 75%, a sudden significant drop, or any reading below a centre-specific threshold warrants immediate contact with the team. Families are taught how to obtain a reliable reading, how to troubleshoot the oximeter, and critically, what to do with an abnormal result.
Feeding monitoring tracks the volume of feeds and the time taken to complete them. A baby who was previously finishing feeds in fifteen to twenty minutes and is now taking forty minutes, or who is consistently taking significantly less volume than before, may be showing early signs of cardiovascular decompensation. Feeding is effortful work for small babies, and a deteriorating circulation often manifests as feeding difficulty before more dramatic signs appear. The interpretation is more difficult for children who require NG feeding as a baseline modality of feeding. In this children vomiting or worsening of reflux is a potential sign of clinical deterioration.
Before discharge from the surgical centre after the Norwood procedure, families receive structured teaching from specialist nurses. This is not a brief handout. It is a programme of education that covers how to use the equipment, how to interpret the readings, how to recognise concerning trends, and what action to take at every level of finding.
Families are given direct telephone access to a cardiac nurse and the cardiac medical team. The threshold for calling is deliberately low. A family who calls and everything is fine has done the right thing. A family who waits because they do not want to bother anyone is the clinical risk.
Adequate nutrition during the interstage period is not just about growth. It is about keeping a baby healthy enough for the Glenn procedure. Babies with a Norwood circulation have significantly higher caloric requirements than healthy infants because the extra workload on the right ventricle increases metabolic demand. Many interstage babies receive calorie-fortified feeds, nasogastric tube supplementation, or both, to meet these requirements without exhausting the baby through feeding effort.
Feeding difficulties are common and distressing for families. They are also clinically important. A baby who is not gaining weight adequately before the Glenn procedure is at higher surgical risk. The feeding plan is managed by the cardiac team, often with input from specialist cardiac dietitians and speech and language therapists with experience in congenital heart disease.
Families are given clear written guidance on this before discharge. The general framework used at most UK centres has three tiers.
Call the cardiac team the same day for: unexplained weight gain above the threshold, saturation readings below the lower threshold, feeds taking more than twice the usual duration or completing less than half the usual volume, increased work of breathing that is new or worsening.
Go to the nearest emergency department immediately and call the cardiac team for: oxygen saturations that will not recover above a very low threshold despite repositioning, grey or mottled skin colour, a baby who is not rousing normally, or any sudden dramatic change in the baby's appearance or behaviour.
Call emergency services for: a baby who is unresponsive, not breathing, or whose colour is rapidly deteriorating despite immediate repositioning.
The clinical team provides each family with a written card summarising these thresholds individually, because the target numbers vary slightly between babies depending on their specific anatomy and what their typical baseline readings are.
Beyond the monitoring, the interstage period is simply also early parenthood, conducted under conditions of exceptional stress. Families describe this period as simultaneously filled with the joy of finally being home and the constant anxiety of knowing how fragile the situation is. Many parents sleep poorly. Some develop anxiety and depression. The psychological burden on families during the interstage period is well documented in the literature, and it is not something families should manage alone.
Peer support from other HLHS families is enormously valuable. Charities including Little Hearts Matter and the Children's Heart Federation provide connection, information, and emotional support specifically for families of children with univentricular heart conditions. The cardiac social worker at the surgical centre is another important resource, and families should not hesitate to access this support from the beginning of the interstage period rather than waiting until a crisis occurs.
For most babies, the interstage period lasts approximately four to six months, from discharge after the Norwood procedure to the Glenn operation. The timing of the Glenn is determined by the fall in pulmonary vascular resistance, which is assessed at a pre-Glenn cardiac catheterisation. Some babies are ready slightly earlier, others slightly later, depending on how their pulmonary circulation matures.
Short travel within reasonable distance of a hospital with paediatric intensive care capability is generally manageable, but families should discuss specific plans with the cardiac team before any trip. Air travel, travel to countries with limited paediatric cardiac facilities, and holidays in isolated locations are strongly discouraged during the interstage period.
Interstage babies are not immunocompromised in the way that transplant patients are, but intercurrent illness, particularly respiratory viral infections, can seriously destabilise a baby with a Norwood circulation. Sensible precautions include hand washing before contact with the baby, keeping the baby away from people with active respiratory illness, and following the advice of the cardiac team about specific infections including RSV prophylaxis during winter months.
This is a very common feeling and not a sign of inadequacy. Families caring for interstage babies are managing an extraordinary situation. The cardiac team will not think less of any parent who calls frequently, who expresses anxiety, or who needs reassurance. If the anxiety is significantly affecting daily functioning or mental health, speaking to the GP or the cardiac social worker about support is strongly encouraged. You do not have to manage this alone.
Author: Dr. Alessandro Giardini, MD, PhD, Consultant Paediatric Cardiologist
Written 22/06/2026