TSRA Content:
Author: Aakash M. Shah, MD
This is a revision and update from the previous edition of the TSRA Primer in Cardiothoracic Surgery written by William Stein, MD.
A proper understanding of cardiopulmonary bypass (CPB) is critical to the safe conduct of pediatric cardiac surgery. The goal of this chapter is to familiarize the resident with common techniques and terminology used in congenital CPB.
Exposure
Exposure for most cases is obtained through a median sternotomy. The skin incision begins just below the sternal notch and extends to the tip of the xiphoid. The midline is carefully identified, and the sternal notch and sub-xiphoid space are dissected. The sternotomy is performed with a sternal saw. Hemostasis is achieved, and an appropriately sized sternal retractor is placed. In children, the thymus can be quite large and frequently requires excision of one or both lobes for exposure. In older children, it can be split and preserved. Care must be taken to avoid injury to the innominate vein as well as the phrenic nerve which can be easily injured if the dissection proceeds too far superior or lateral while the thymus is elevated. The pericardium is opened off the midline to allow for creation of a pericardial patch if necessary. Pericardial retraction sutures are placed.
For reoperations, there are two approaches to the median sternotomy. The sub-xiphoid approach involves dissection beginning in the preperitoneal space with retractors lifting either side of the xiphoid. A short segment is cleared with cautery and/or sharp dissection below the xiphoid and sternum. The sternal saw or scissors are used to divide the sternum for that short segment. This pattern continues using a long bovie tip and sometimes bilateral mammary retractors to lift on the cut edges of the sternum until the sternum is completely divided. The standard reoperative sternotomy approach used in adults with an oscillating saw can be used in larger and older children. Hemostasis and dissection of the heart then proceeds in a standard fashion. In the hemodynamically stable patient, dissection of the AP window, main pulmonary arteries, aorta, SVC, and IVC are completed prior to cannulation to help decrease bypass time. If applicable, a patent ductus arteriosus can be dissected at this point as well.
Cannulation
Bypass circuit: The manifold and lines are set up according to institution preferences. The length of the tubing is kept as short as possible to avoid hemodilution. The tubing diameter may be small and vulnerable to kinks. The size of the tubing and cannulas are determined by the surgeon and perfusionist, taking into account the size of the patient and desired flows. Tubing diameters for neonates and infants can range from 3/16" to 3/8" with cannulas ranging from 8 to 12 Fr. Older children may have cannulas that approach the size of adult cannulas.
Arterial cannulation:
Arterial cannulation in the pediatric patient is performed in a similar fashion to the adult although the sutures and purse strings are smaller depending on the cannula size. In small aortas, longitudinal purse strings can minimize the risk of stenosis of the ascending aorta when tying them down. Cannulation is performed distal enough to allow room for the antegrade cardioplegia cannula, aortic cross clamp, and aortotomy, if needed. Heparin (300-400 units/kg) is given to achieve an ACT between 400-600 seconds. A single, superficial purse string is placed, usually with 5-0 Prolene (non-pledgeted) or Ethibond. The adventitia is cleared with tenotomy scissors. The assistant stabilizes and retracts the aorta medially and inferiorly with forceps while the surgeon makes a stab incision with an 11 blade into the aorta. An appropriately sized straight aortic cannula is placed and secured with heavy silk ties. The cannula is placed 5-10 mm into the aorta taking care to prevent direct cannulation of the carotid or subclavian arteries. The circuit is attached, de-aired, and tested to confirm appropriate placement. The cannula is carefully positioned and secured on the operative field.
Venous cannulation:
Venous cannulation for congenital surgery is often bicaval to allow access to a bloodless right atrium and an uncluttered operative field. Right-angle cannulae are often used in the SVC. If not already completed, the posterior SVC is dissected from the right PA. A single 5-0 Prolene oval purse string is placed on the midposition of the SVC. This purse string is quite narrow to avoid stricturing of the SVC. A linear incision is made and dilated with a fine hemostat. The cannula is engaged perpendicular to the SVC and the tip rotated cephalad. Alternatively, a straight venous cannula may be inserted into the atrial appendage and thread into the SVC.
The IVC is then cannulated with a straight or right-angled cannula. The right atrium is retracted cephalad with a pair of forceps, and the diaphragm is retracted caudal. A 5-0 Prolene or Ethibond purse string is placed at the right atrial-IVC junction. A stab incision is made and dilated with a tonsil clamp. There is potential for substantial blood loss and adequate suction should be available using the cardiotomy suction catheters as this blood can be immediately transfused through the aortic cannula. Care must be taken to ensure that the IVC cannula does not obstruct the hepatic veins. Caval snares are placed around the SVC and IVC.
Conduct of Cardiopulmonary Bypass
Initiation of CPB:
Arterial flow is gradually initiated followed by either gravity or vacuum-assisted venous drainage, depending on institutional preferences. Vacuum-assisted drainage may provide better drainage and prevent the creation of airlocks in the circuit while gravity drainage may prevent undue trauma and hemolysis of the red blood cells. The desired temperature is specified to achieve mild (32-34 degrees C), moderate (28-32 degrees C), or deep (18-20 degrees C) hypothermia. The atrium should be completely decompressed. Otherwise, adjust the IVC cannula until adequate drainage is achieved. Inadequate emptying may impair visualization and lead to distention-induced myocardial dysfunction and decreased myocardial protection. In cases where an aorto-pulmonary connection exists, the pulmonary arteries must be controlled immediately after the initiation of CPB. This includes ligation of any PDA present.
Left Ventricular/Left Atrial Vent:
To aid with exposure and prevent left ventricular distention, a vent can be placed through the left atrial appendage or the right superior pulmonary vein into the LA or LV. Some surgeons favor venting the left heart directly through an incision in the right atrium and atrial septum immediately after aortic cross clamp in order to avoid possible pulmonary vein stenosis from the vent purse string. Collateral blood flow to the lungs can flow retrograde through the pulmonary artery and compromise exposure. A vent or "sucker" in the main PA can resolve this problem.
Cardioplegia:
Antegrade cardioplegia is given via a catheter placed in the ascending aorta. A 5-0 non-pledgeted Prolene horizontal mattress suture is placed and the catheter is positioned. Care must be taken that the catheter is not placed through the back wall of the aorta. This cardioplegia line can also serve as the "root vent" for decompression during the case and de-airing. The aorta is cross clamped and a specified amount of cardioplegia is administered. Maintenance doses are given throughout the case as needed.
On Pump:
CPB is conducted with close monitoring of hemodynamics. Blood pressure and perfusion flows are followed and maintained throughout CPB. Target perfusion flows depend on institutional policy and vary between 75-150 mL/kg/min. Vasodilating agents can be used to allow vasodilation of vascular beds, allow high perfusion flows, maintain a uniform temperature while cooling, and allow adequate perfusion of tissues. Vasoconstricting agents may be needed in case of excessive vasodilation leading to hypotension. Arterial blood gases and lactate levels are followed throughout CPB. Adjunct monitoring devices such as brain near-infrared spectroscopy (NIRS) and transcranial Doppler may be used to monitor brain and tissue perfusion.
Deep hypothermic circulatory arrest (DHCA) and low-flow cerebral perfusion:
While performing aortic arch reconstruction (i.e. hypoplastic left heart syndrome (HLHS), interrupted aortic arch) or repair of total anomalous pulmonary veins, the need for either low flow cerebral perfusion or DHCA will arise. For DHCA, the patient is cannulated as above, cooled to 18 degrees C, and pump flow is arrested to allow a blood-free operative field. A dose of cardioplegia is given prior to arrest.
In low-flow or antegrade cerebral perfusion, a Gore-Tex graft (typically 3.5 mm) is anastomosed to the innominate artery with 7-0 Prolene (in a similar fashion to a Blalock-Taussig (BT) shunt). Once the anastomosis is completed, the arterial cannula is inserted into the graft and de-aired. Initially, standard flows are used. When needed, the head vessels are snared, and pump flow is decreased to 1 mL/kg/min while distal aortic work is performed. Regional blood pressure can be monitored through a right radial arterial line since flow is directed to the right carotid and subclavian from the innominate artery graft. If necessary, the graft can be subsequently used for creation of a BT shunt.
Weaning Cardiopulmonary Bypass
Rewarming is initiated, and the heart is de-aired. The cross clamp is removed with the root vent on. Weaning then proceeds in a manner similar to the adult. Suture lines are inspected, adequate rhythm is ensured, ventilation is resumed, and the heart is allowed to fill and eject. The echo is carefully inspected to rule out any residual defects or air in the chambers. Flow is gradually decreased while the heart takes over. Contractility is constantly assessed during this process by echo, visual inspection, and hemodynamics. Once weaned, venous cannulae are removed with tourniquets left in place in case of re-entry. Test-dose protamine is administered, and the aortic cannula is removed. The rest of the protamine dose is completed, and hemostasis is achieved. Heparin is reversed as in adults with 3-4 mg/kg of Protamine (or as a ratio of 1.3:1 mg of protamine to mg of heparin).
Post-bypass modified ultrafiltration and continuous bypass ultrafiltration:
Despite miniaturization of CPB circuits, the priming volume can be many times the blood volume of the patient. This leads to an increase in total body water and subsequent peripheral, pulmonary, visceral, and cerebral edema. Aggressive diuresis, ultrafiltration on CPB, and modified ultrafiltration (MUF) have been used to manage the increase in total body water. In addition to removal of excess water, ultrafiltration potentially decreases the circulating levels of inflammatory mediators.
Continuous ultrafiltration can be achieved through the pump while on CPB. MUF is performed after weaning from CPB but prior to removal of the cannulas. A right atrial cannula (typically the previously removed vent) is placed. Blood is removed from the venous cannula, circulated through the ultrafiltration filter (where it is hemo-concentrated), and returned to the patient through the right atrial cannula.