Hypotheses

Adjustable Systemic–Pulmonary Arterial Shunts

The idea of an adjustable systemic to pulmonary artery(S–P) shunt has been pondered since the introduction ofthe Blalock–Taussig Shunt [1] in 1945. If such a shuntcould be developed and manufactured, somatic growthissues and physiologic parameters could be addressedwith more certainty, leading to better outcomes. As amatter of information, I could not find any scientificreference, based on a computer search, to the “adjustablesystemic to pulmonary artery shunt” in humans or ani-mals. It seems that adjustable devices and techniques incongenital heart surgery have been confined to adjust-able interatrial communications [2], and adjustable pul-monary artery bands [3]. The following reports by Drs.Douglas and Abdulla concerning the adjustable S–Pshunt represent excellent conceptual forays into this veryinteresting and important idea.

The pertinent anatomical and physiologic issues re-garding an effective S–P shunt center around: the size ofthe proximal and distal anastomoses, the cross-sectionalarea of the conduit, the length of the conduit, the fluidviscosity, the inlet pressure, and the outlet resistance.Conceptually then, the adjustable S–P shunt would haveto be constructed “too big” with appropriate downsizingadjustments if the goal is to gradually make the shuntlarger with somatic growth. For this reason, Douglas’sconcept seems more appealing since the proximal anddistal anastomoses are performed at the maximal sizewhich is dependent on the prosthesis size. The downsiz-ing takes place within the center of the prosthesis whichcan be adjusted. The confounding isuses then are clottingpossibilities, problems with the regulatory mechanism(indwelling computer) and the compliance of the con-stricting bellows. The design by Abdulla takes advantageof the extant balloon dilatation technology to enlarge thepreformed restrictive diaphragm which has been manu-factured into the center of the prosthetic graft. The dif-ficulty with this design is that abrupt constrictions, suchas the diaphragm that has been described for the center ofthe tube graft, tend to remodel with fibrous tissue, form-ing a gradual upstream and downstream constriction with

the diaphragm at the center. A ballon catheter would nowhave to rupture the diaphragm and constrict the accumu-lated fibrous tissue. The dangers of unwanted intralumi-nal clotting caused by thrombogenic material and lowflow states are present in both designs. The fact that nodevices, such as the adjustable S–P shunt, have beendeveloped speaks to the difficulties that have to be over-come for an effective and safe product. Nevertheless,these ideas are clearly important and warrant further in-vestigation.

Constantine MavroudisChildren’s Memorial HospitalNorthwest UniversityChicago, Illinois 60614

References

1. Blalock A, Taussig HB (1945) The surgical treatment of malforma-tions of the heart in which there is pulmonary stenosis or pulmonaryatresia.JAMA 128:189

2. Laks H, ear JM, Drinkwater DC, Jarmakani J, Isabel-Jones J,George BL, Williams RG (1992) Partial biventricular repair of pul-monary atresia with intact ventricular septum. Use of an adjustableatrial septal defect.Circulation 86(5 Suppl):II159–66

3. Ahmadi A, Rein J, Hellberg K, Bastanier C (1995) Percutaneouslyadjustable pulmonary artery band.Ann Thorac Surg 60(6 Suppl):S520–2

Expandable Systemic to Pulmonary Arterial Shunt

In many instances the pulmonary blood flow providedthrough the modified Blalock–Taussig (B–T) shunt (sideto side shunt using synthetic vascular graft) becomesinadequate as the child grows due to limited blood vol-ume allowed to cross the fixed cross-sectional area of theB–T shunt. This may necessitate the placement of a sec-ond B–T shunt on the opposite side to provide morepulmonary blood flow until the child reaches a bodyweight deemed important for good outcome of a subse-quent surgical procedure. The necessity for a second pro-cedure may be eliminated if the shunt could be dilatedto accommodate the increasing amount of pulmonaryblood flow required for a growing child. This need forms

Correspondence to:R. Abdulla at Rush Children’s Hospital, 1653West Congress Parkway, Chicago, Illinois 60612, USA

Pediatr Cardiol 20:445–447, 1999

PediatricCardiology© Springer-Verlag New York Inc. 1999

the basis of this modified “Expandable or flow controlB–T shunt.”

The proposed shunt is a tubular structure made froma synthetic or natural material. The lumenal diameter ofthe tube would accommodate enough blood flow to thelungs for a toddler (5–6 mm); however, the blood flowwould be restricted by a diaphragm situated in the cross-section of the lumen (Fig. 1A). The diaphragm has acentral orifice (3–5 mm in diameter) which would re-strict the blood flow to the lungs, and therefore accom-modate a neonate’s or a young infant’s pulmonary bloodflow. The restricting diaphragm has weak diagonal areaswhich facilitate rupture of the diaphragm when dilatedby a balloon valvuloplasty catheter to enlarge the effec-tive orifice of the shunt to that of the tube, thereby in-creasing the blood flow to the lungs.

In a second version, the tubular graft has a dual wall(Fig. 1B). The outer layer is stiff and forms the cylindri-cal outer shape of the tubular graft. The inside layer ismade of a stretchable material which bulges inwardforming an “hourglass” shape, the middle portion re-stricts the lumen to a desired dimension. The middleportion of the hourglass-shaped inner tube could be sta-bilized by placing a stent on the outside (i.e., in between

the two tubes). Balloon dilation of the inner layer wouldresult in enlarging the lumen, which would accommodatemore pulmonary blood flow (U.S. patent #5,445,600).

Ra-id AbdullaRush Children’s Hospital1653 West Congress ParkwayChicago, Illinois

The Adjustable Systemic–Pulmonary Artery Shunt

Systemic-pulmonary artery shunts are commonly used inpediatric cardiac surgery. They may be used to palliateright-sided obstructive lesions in children who will even-tually undergo a two-ventricle repair (e.g., tetralogy ofFallot), or they may be used to palliate children withsingle ventricle physiology until such time that they mayundergo a cavopulmonary connection. A variety of tech-niques have been used to make a systemic-pulmonaryartery shunt, including classic and modified Blalock–Taussig shunts, direct aortopulmonary artery shunts(e.g., Waterston shunt), and aortopulmonary connectionsusing a prosthetic conduit (central shunts). None of thecurrently available methods allow the direct control ofshunt flow (e.g., pulmonary blood flow) once the shunt isin place.

The adjustable systemic-pulmonary artery shunt(Fig. 2) is designed to control the amount of pulmonaryblood flow supplied by a systemic to pulmonary arteryconnection. The device involves a simple prosthetic graftwith a resistor, which is envisioned as being a fluid-filledcompartment covering 60% of the inner circumferenceof the graft and being 6–8 mm in length. A fluid-filledresistor would be connected by a catheter to a smallpump/reservoir. The device also contains an oximeterbuilt into the graft. The oximeter circuit and the reser-voir/pump are both connected to a computer controller.The resistor is inflated or deflated by the controller basedon oximeter readings. The target saturation can be at-tached by communicating with the controller through atelemetry device. Earlier generations may have the ox-imeter omitted.

The physiologic advantages of an adjustable sys-temic-pulmonary shunt will vary with the underlyingcardiac defect; the most important application of the ad-justable shunt will likely be as part of the Norwood(stage I) procedure for the hypoplastic left heart syn-drome. The adjustable shunt will offer differing advan-tages in the early postoperative period and the interme-diate-term before a cavopulmonary connection is com-pleted. The early postoperative period after the Norwoodprocedure is characterized by episodes of hemodynamicinstability and high mortality rates. Although the highmortality rates are certainly multifactorial, the lack of

Fig. 1. (A) A 6-mm diameter shunt with a centrally positioned dia-phragm with a 4-mm diameter fenestration. The diaphragm has diago-nal lines, which would rupture when balloon dilated.(B) A 6-mmdiameter shunt with an inner second layer creating an hour-glass lumenwhich is 4 mm in diameter at its narrowest point. The inner layer ismade of material which could be balloon dilated, thus changing thelumen to 6 mm in diameter.

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ability to control the balance between systemic and pul-monary blood flow is likely a major contributing factor.Patients who have shunts which permit too much or toolittle pulmonary blood flow are currently treated with avariety of indirect measures, including adjusting sys-temic blood pressures and allowing carbon dioxide in theventilator circuit. These indirect maneuvers may be inconflict with other hemodynamic goals of the patient;stability of the post-operative Norwood patient willlikely improve if shunt flow could instead be mechani-cally adjusted. In the intermediate-term period (fromhospital discharge until the next surgical stage), an ad-justable shunt would benefit the patient by limiting thevolume overload placed on the ventricle. The extra vol-ume load on the ventricle lead to ventricular dysfunctionand increased ventricular filling pressures. Minimizingthe volume overload will optimize the patient’s suitabil-ity for a future Fontan procedure.

The ideal systemic-pulmonary artery shunt is not yet

a reality, it would be automatically adjustable withinphysiologic ranges, would be free of mechanical break-down and thrombosis, and would be easy to place andremove. The automatic systemic-pulmonary artery shuntas described above would represent a significant advancefrom currently available shunts; the manufacture of theshunt is feasible given current technology. It is my opin-ion that an automatic adjustable systemic-pulmonary ar-tery shunt would be of significant benefit to those pa-tients with single ventricle physiology, especially thosewho have a systemic-pulmonary artery shunt placed aspart of a Norwood procedure. This device has been pat-ented (U.S. patent #5,662,711).

William I. DouglasDivision of Pediatric Cardiovascular SurgeryC.S. Mott Children’s HospitalUniversity of Michigan Medical CenterAnn Arbor, Michigan

Fig. 2. An illustration of the adjustable systemic–pulmonary shunt.

447Mavroudis et al.: Adjustable Systemic–Pulmonary Arterial Shunts