How much blood do we want to pass through the lung?

As one of the world’s leading fully integrated air ambulance providers, Unicair was tasked to transport a two-and-a-half-year-old girl from Lahore in Pakistan to Bristol in the UK for cardiac surgery. The child suffered from a large ventricular septal defect (VSD) and pulmonary artery hypertension (PAH). She was a refugee in Pakistan and seen by a paediatric cardiologist in an outpatient clinic while in the country. She was treated with Captopril, her oxygen saturation (SaO2) on ground in room air was around 88–90%. She presented with a marked failure to thrive, a body weight of around 7kg and no ability to walk or crawl.

Unicair, with a fleet of 12 dedicated aeromedical jets, dispatched one of its three long-range, multi-patient configured Challenger 604 aircraft with a paediatric cardiologist and a PICU nurse for this transport. The Unicair operations team, highly experienced in coordinating complex missions around the world, dealt with the necessity to circumnavigate Afghan airspace.

The patient and parents arrived from their residence at the airport without a medical transport team. Prior to take-off, the aforementioned diagnosis was confirmed with Point-of-Care (PoC) ultrasound. The scan showed a very large inlet VSD with bi-directional shunting. After take-off, SaO2 dropped to 70% and a repeat scan showed a now predominantly right-to-left shunt. With oxygen at 2l/min, SaO2 rapidly increased to 100%. To prevent pulmonary over-circulation, oxygen was titrated to 0.5 l/min which achieved SaO2 around 90% on cruise level altitude. In-flight echocardiography confirmed continued bi-directional shunting over the VSD.

Further interventions did not become necessary, blood gas analysis remained normal and urine output (as a surrogate for systemic perfusion) was sufficient during transport. Upon arrival in the UK, oxygen supplementation could be ceased and the child was accompanied to the admitting hospital by the Unicair Medical Team.

In children with large VSDs, shunting normally occurs along the pressure gradient from the left ventricle (systemic blood pressure) into the right ventricle (pulmonary artery pressure) – i.e. a left-to-right shunt with clinical signs of heart failure, resulting in pulmonary over-circulation and reduced systemic perfusion.

PAH can mitigate that shunt by reducing the pressure gradient or even reversing it (bidirectional or right-to-left shunting with resulting cyanosis). While hypoxia is a pulmonary vasoconstrictor, oxygen results in a pulmonary vasodilatation. Other pharmacologic interventions to reduce pulmonary vascular resistance include sildenafil (available as oral suspension and as intravenous medication (Revatio) on board of Unicair aircraft) and inhaled nitric oxide in ventilated patients (iNO, available on Unicair aircraft on request).

In conclusion, a good understanding of the physiology and hemodynamics is of critical importance when transporting children with congenital heart disease in an air ambulance. PoC ultrasound/echocardiography and blood gas analysis does enable the team to tailor therapy as needed by the individual patient.