Are we There Yet??
With just a few miles to go before arriving home, mom and dad noticed that their 4-month old son was cyanotic and unresponsive. They immediately divert to the nearest hospital. The parents report to the emergency department (ED) staff that their child has Hypoplastic Left Heart Syndrome (HLHS) and has received a Norwood procedure. They were discharged home following a preoperative cardiac catheterization for an upcoming scheduled Hemi Fontan Procedure. Pulse oximetry (SpO2) on admission is 60% (the child’s “normal” is in the mid to high 70’s per parent report). The child remains unresponsive and dusky.
The transport team for the children’s hospital that this child has received care from is immediately activated. Upon arrival 30 minutes later, the team finds an intubated infant being manually ventilated with a self-inflating bag at 15 lpm of oxygen. It was suspected that the child had aspirated during the multiple intubation attempts performed. Vital signs include a heart rate of 132 bpm, blood pressure of 83/30, a rectal temperature of 38.8o C and pulse oximetry reading unchanged at 60%. With this clinical picture, a concomitant diagnosis of sepsis and acute lung injury could not be ruled out at this point.
A trans-thoracic echocardiogram revealed unrestricted flow across the atrial septum (an atrial septal defect (ASD) is created or extended during a Norwood procedure), small left atrium and left ventricle (expected finding in HLHS), mitral and aortic valve atresia, moderately dilated right ventricle (again, expected with this diagnosis), moderate tricuspid regurgitation (compromising forward flow), a patent neoaortic valve and neoaortic arch (created during the Norwood), decreased central shunt flow (a key component to this child’s current problem), normal coronary arteries, normal systemic venous drainage and normal pulmonary artery branches. The immediate and fundamental question with all of this assessment data is: How do we stabilize and transport?
Clinical Question
In order to answer this key clinical question, we must have a basic understanding of this child’s current anatomy and how the aforementioned assessment findings play into his current pathophysiology. Once this is taken into consideration, the treatment plan (believe it or not) is fairly intuitive. Again, the first step is understanding how the anatomy (and any other “typical” assessment findings) could be causing the current physiological derangements.
The video below provides a quick review of the Norwood procedure (pioneered in the 1980’s at the Children’s Hospital of Philadelphia).
Since this infant is a patient of the academic center that he is being flown to, the transport team had access to the medical records prior to liftoff. These records revealed that he had received his initial procedure at 4 days of life. He has a 3.5 mm “central” shunt that was eventually clipped (reducing the flow through it) due to pulmonary overcirculation (the differences in location of these shunts are pictured below). Subsequently the distal portion of the shunt was stented open due to excessive narrowing. The pre-operative heart catheterization that the child was returning home from when all of this occurred revealed proximal shunt narrowing to 1.5mm and moderate to severe TR (causing a reduction in forward blood flow).
The Balancing Act Between Systemic and Pulmonary Circulation
While enroute to the referring hospital, the team reviewed the data that they had collected including surgical and procedural history up to this point, current clinical status and expected physiology / hemodynamics. Attention was brought to the central shunt and the increased possibility that the patient’s hypoxemia could be caused by a decrease in blood flow to the lungs due to the known proximal shunt narrowing and moderate tricuspid regurgitation (TR). After the Norwood procedure a primary objective is managing pulmonary and systemic perfusion balance. It is clear upon initial patient presentation to the referring ED that there is a derangement in this. The fundamental management goal in this instance is promoting forward flow through an incompetent tricuspid valve while ensuring that systemic and pulmonary blood flow are appropriately balanced. This was what the crew addressed immediately upon their arrival to the bedside.
The amount of oxygen being provided was immediately addressed and deemed currently appropriate. As oxygen is a potent pulmonary vasodilator and a reduction in pulmonary blood flow is suspected, it would be feasible to increase this in an effort to increase pulse oximetry to 75%. However, it cannot be overemphasized that with an aortopulmonary shunt, the pulmonary and systemic circulations must be balanced. Typically, a shunt must be large enough to allow for adequate pulmonary blood flow (to avoid excessive cyanosis), but restrictive enough so that there is not pulmonary over circulation and decreased systemic blood flow, thus causing acidosis and tissue hypoperfusion. In this specific case, the former needs attention over the latter.
Additionally, the forward flow issue needs to be addressed in order to promote an increase in systemic circulation. This was evidenced by a capillary blood gas revealing a pH of 6.98. A downward trending mean arterial pressure (MAP) and a desire to drive more blood across the compromised pulmonary shunt, the decision was made to initiate and titrate an epinephrine infusion.
In lesions with parallel circulation (such as in this case), the total cardiac output (CO) of the single (right) ventricle is shared between pulmonary and systemic circulations. This child’s balance is somewhere in the area of an oxygen saturation (SpO2) of 75%. A value higher than this can cause pulmonary overcirculation and lower can exacerbate hypoxia. Dehydration is deadly to these infants as the clotting of an aortopulmonary shunt will quickly cause loss of blood flow to the lungs, hypoxia and lactic acidosis quickly leading to multi-system organ failure.
There are a multitude of tools at the clinician’s disposal to manipulate systemic vascular (SVR) and pulmonary vascular (PVR) resistance. These tools are not exclusive to the child with an aortopulmonary shunt. They include judicious use of oxygen that would increase pulmonary blood flow at the expense of systemic blood flow. Adjust ventilatory settings for appropriate waveform capnography readings and avoid overventilation (remember what overventilation does to intrathoracic pressure and venous return). Consider pressors to increase SVR or vasodilators to reduce SVR. Remember that Inhaled nitric oxide (iNO) is administered to reduce PVR.
Take Home Points
- PALS DOES apply: while management of shock shares similarities, the one thing that this transport team did to ensure that their treatment plan was appropriate was that they utilized the data at their disposal. While not everyone will have access to the medical record, this child’s parents are most sincerely experts in his care. LISTEN TO THEM!
- Be cautious of high oxygen levels (FiO2) in a child with single-ventricle physiology. Remember that one ventricle is supplying blood flow to both the lungs and the body.
- Mild illness and dehydration are cause for concern as this increases the chance of increased coagulation and thrombus formation in the shunt. Though seemingly benign, these conditions will prompt care givers to seek immediate medical attention.
- You already have the tools: it is just a matter of appropriately using them and understanding when and how you need to manipulate SVR and PVR.
References
Bacha, E. (2017). Intervention on Surgical Systemic-to-Pulmonary Artery Shunts.
Ramaswamy P et al: “Systemic to Pulmonary Artery Shunting for Palliation.” http://www.emedicine.com/ped/topic3024.htm. Updated December 29, 2016. Accessed April 6, 2019.
Shunt, M. B. T. (2018). Aortopulmonary and Cavopulmonary Shunts. Consults in Obstetric Anesthesiology, 45.
Tabbutt, S., Tweddell, J. S., & Ghanayem, N. (2016). Hypoplastic left heart syndrome and other shunt-dependent single ventricles. Pediatric Critical Care Medicine, 17(8_suppl), S318-S322.