**17. Conduits in cardiac surgery**

group is limited by the need for miniaturization and ECMO is still the most common form of

ECMO is now accepted modality of treatment in neonates with a variety of parenchymal and vascular lung disease (meconium aspiration, diaphragmatic hernia, persistent hypertension of newborn). The outcome irrespective of the indication depends *on early diagnosis, prompt institution and reversible nature of dysfunction.* The use of ECMO for respiratory indications has progressively decreased (due to increased availability of High frequency oscillatory ventila‐ tion, nitric oxide, and surfactant therapy) and there is a steady increase in its use after con‐

genital cardiac surgery or as a bridge to transplantation to support failing circulation.

The cardiac indications for ECMO are in preoperative resuscitation (critical AS, pulmonary hypertension, obstructed TAPVC, transposition of great arteries), inability to wean from CPB, cardiomyopathy, myocarditis, after in- hospital cardiac arrest and CPR, and bridge to trans‐

Venovenous ECMO is used in patients who need ventilatory support alone. Arteriovenous cannulation is used for cardiac ECMO. In the postoperative period single right atrial and ascending aortic cannula is preferred, internal jugular vein and carotid artery can be used in other settings. ACT is maintained around 180-200 seconds,flow rates between 100-150 ml/kg/ min for children on full ECMO, hematocrit level between 35 -45% and platelet count > 100,000/

The daily management of ECMO needs assessment of cardiorespiratory function, end organ perfusion and evolving complications such as bleeding or sepsis. Assessing adequacy of flow

When instituted in post cardiac surgery setting, myocardial recovery should be anticipated in 2-3 days, failing which listing for transplantation or withdrawal of support must be considered. Patients with cardiomyopathies or with severe bronchiolitis due to viral infection may require

Weaning depends on the underlying indication. Inotropes are recommenced or increased, intravascular status is optimized and ventilator settings are adjusted, flows gradually de‐ creased over a period of time and the circuit is clamped. ABG, serum lactated levels and mixed venous oxygen saturation levels are closely monitored and decannulation is done once patient

IABP (Intra-aortic Balloon Pump) has not been very successful in paediatric population because of a number of reasons, the usual right heart nature of pathology, rapid heart rate in children making timing difficult, distensible nature of aorta and collaterals making coronary flow augmentation and afterload reduction less. The smallest size available is 2.5ml and generally volume of 0.5ml/kg is recommended. Technical improvements including smaller sized consoles, catheters and M-mode echocardiography for timing may enhance its applica‐

Ventricular assist devices require direct cannulation of heart. Reported indications are decompensated cardiomyopathy, ALCAPA (ischemic myocardium as a result of anomalous

has maintained stable circulation and acceptable gas exchange for upto 4hrs.

mechanical circulatory support for pediatric patients.

250 Principles and Practice of Cardiothoracic Surgery

and systemic perfusion is of paramount importance.

longer period of support (1-2 weeks).

bility in the future.

plantation.

mm3.

Allografts – tissues obtained from human body which are cryopreserved are used in repair of variety of congenital cardiac defects. Due to availability and preservation problems it fell into disrepute in 1980's only to re-emerge in 1990's mainly due to late complications of porcine valved Dacron conduits which became popular in 1980's and availability of better cryopre‐ servation technology.

Allografts are currently available standards against which any conduit is compared. Factors which can reduce the longevity of allografts are – use of excessively large conduits (> +2 z score), aortic allografts which tends to calcify earlier probably because of higher elastin content (controversial) and immunogenicity due to blood and tissue incompatibility (controversial). No particular method is proven to enhance the longevity of allograft. Decellularisation process using anionic detergents and nucleases have been used to provide acellular matrix for the native tissue to repopulate and grow, but the long term structural impact of the decellulari‐ sation process is yet to be seen. Treating the decellularised tissue in a bioreactor using the recipient cells so as to repopulate the matrix is now in experimental stages.

Contegra which is made from Bovine jugular vein is one of the popular valved conduits in pediatric cardiac surgery; This conduit shows predictable function with survival approaching homografts over mid-term. The advantages are easy availability (12- 22mm sizes) and predictable quality. It is important to keep the conduit short and straight to prevent 'telescop‐ ing' and distal stenosis.

Alternative conduits have been used made of Polytetrafluoroethylene (PTFE) wall and leaflets, which has minimal tissue reactivity. Long term results are still awaited.

Conduits fashioned out of autologous pericardium with PTFE (0.1mm) leaflets are also increasingly being used, the disadvantages being non-availability in reoperations and limitation with respect to the length and diameter.

While a variety of choices are available for older children and adults, an ideal conduit for neonates and small children is still elusive. The availability of 'conduits which grow' using tissue engineering technology can potentially provide curative surgery for a number of conditions which require multiple reoperations for conduit replacements. [28]
