First Stage of Univentricular Hearts
Blalock-Taussig-Thomas Shunt (BTTS)
BTTS represents the first stage of univentricular palliation surgery in
patients with restricted pulmonary blood flow. This procedure allows the
growth of pulmonary arteries, where it regulates pulmonary blood flow to
the lungs until the size of the pulmonary artery is suitable for the
second stage of univentricular palliation surgery. The classic BTTS is a
direct anastomosis between the transected subclavian artery (or the
innominate artery) and the pulmonary artery (Figure 1 ). It
requires extensive surgical dissection where it sacrifices the
subclavian artery.6 The main disadvantages of classic
BTTS include long operative dissection time, phrenic nerve injury,
technical difficulties during the takedown, as well as possible arm
ischemia. In 1975, this technique was modified by Marc R. de Leval and
his colleagues, where a polytetrafluoroethylene (PTFE) graft was used to
interpose between the subclavian artery and the ipsilateral pulmonary
artery (Figure 2 ). Since this modification, the procedure is
now more popularly known as the modified BTTS. PTFE conduits are
considered ideal compared to Dacron because they have smaller pore sizes
that limit ingrowth of tissue, yet allowing for fibroblastic
incorporation to bind the conduit to its surrounding
structures.7
To maximize postoperative outcomes of the modified BBTS, a few things
regarding perioperative management such as surgical consideration as
well as strategies to deal with over-shunting and under-shunting must be
considered. Surgical considerations include the approach to apply the
modified BTTS. An approach through median sternotomy gives the surgeon
exposure to the right subclavian artery. It will also expand the
surgical field and allows the surgeon to initiate cardiopulmonary bypass
if needed. The approach through sternotomy is correlated to the extended
use of mechanical ventilatory support, the length of stay in the
intensive care unit and hospital, as well as a higher mortality rate.
Meanwhile, the thoracotomy approach provides ease in creating a proximal
anastomosis beyond the bifurcation of the innominate artery. The
thoracotomy approach will lead to the need for a more extended graft.
The diameter of the graft becomes less critical in estimating shunt
resistance so that the flow can be a problem. A longitudinal
arteriotomy incision in the subclavian and pulmonary arteries provides a
wider circumferential area of the anastomosis for a larger shunt size.
This technique proves to be more beneficial for hypoplastic pulmonary
arteries. Pulmonary artery distortion at the anastomotic site is also
less common with the longitudinal incision technique. Using a smaller
needle size of 8/0 polypropylene will help provided better anastomosis
and less bleeding from the needle hole.8
The basic principle to deal with over-shunting and under-shunting
include balancing Qp:Qs by decreasing systemic vascular resistance (SVR)
and increasing pulmonary vascular resistance (PVR) in modified BTTS to
achieve an oxygen saturation of 70-85%. Strategies to increase the PVR
include reducing the fraction of inspired oxygen (FiO2) to 0.21,
avoidance of hyperventilation with targeted permissive hypercapnia,
administration of high PEEP (PEEP 6-8), and maintaining blood pH
7.35-7.40. Evaluation of lactic acid and arterial blood gas may be done
every 4-6 hours. Some inodilator are practical and can be used to
decrease SVR. Dobutamine, milrinone or levosimendan may be considered
while maintaining a diastolic blood pressure of over 25 mmHg.
Administration of nitroglycerin can be considered if blood pressure is
still high after using the inodilator.
After establishing the signs of under-shunting (oxygen saturation
<70%, despite being given FiO2 > 0.60), we must
first exclude the cause of under-shunting, especially shunt thrombosis
where urgent echocardiography is needed to evaluate shunt patency.
Respiratory system failure, such as displaced endotracheal tube,
obstructed endotracheal tube, pneumothorax, and failure of hemodynamic
support and equipment may also cause under-shunting. If those mentioned
above are already excluded, strategic management include increasing FiO2
to achieve an oxygen saturation of 70-85%, avoid acidosis, and keeping
blood pH around 7.40 to 7.45. Adequate volume status must be ensured by
titrating fluid slowly according to the patient’s response with 5 ml 5%
albumin. If the blood pressure is low, consider starting
vasoconstrictors, such as norepinephrine or epinephrine. In addition, a
pulmonary vasodilator (sildenafil and inhaled nitric oxide) may be
needed to decrease PVR.8
Pulmonary Artery Banding (PAB)
PAB represents the first stage of univentricular palliation surgery in
patients with unrestricted pulmonary blood flow. PAB aims to decrease
pulmonary artery pressure and pulmonary vascular resistance to levels
suitable for future univentricular palliation surgery. The golden period
for performing PAB is considered to be 2 to 4 weeks of age. By this
time, the neonatal pulmonary vascular resistance decreases, allowing for
a tighter band.9
A median sternotomy approach was used for PAB. We use Mersilene tape
with 5 mm width (Ethicon Inc., Somerville, NJ, USA) to band the
pulmonary artery (Figure 3). To prevent migration, the tape was
fixated with two 6/0 Polypropylene stitches. As a guide, the Trusler
formulae (for univentricle hearts: 22 mm + weight in kg) are used as the
first steps, then finer adjustments being made by systemic O2 saturation
(75 - 85%, with inspired oxygen fraction 0.50) and distal PAP (Target
distal PAP 50% or the optimal mean PAP < 15
mmHg).9