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Congenital Heart Disease

General Reference: 

Geggel's Congenital Heart Disease Library (OpenPediatrics/Boston Children's Hospital)


Intraoperative Considerations

Terms:

  • Cardiopulmonary Bypass Time: Time spent "on pump," generally with cannulae draining the venous system (i.e. in SVC or RA) and one providing systemic/cerebral blood flow (i.e. Aorta). Cardiopulmonary bypass, due to factors such as blood/tubing interfaces, generally leads to a systemic inflammatory response which may contribute to low cardiac output syndrome and/or immune dysfunction. In addition, it can lead to complications such as clotting, air embolism, and hemolysis. 
  • Aortic Cross Clamp Time: This is the amount of time spent where the aorta is cross clamped and since the heart must be stopped during this time, also reflects the cardiac arrest time, or time for cardioplegia. Importantly, there is no coronary blood flow during this period of time and myocyte survival and health depends on drastically reducing the myocardial oxygen demand through cardioplegia.
  • Cardioplegia: Solution (typically with high potassium) injected into the coronary arteries (or retrograde through the coronary sinus) in order to induce cardiac arrest and thereby minimize myocardial oxygen demand during aortic cross clamp (allows the surgeon to operate on a non-beating heart and limits the amount of blood in the surgical field)
  • Deep Hypothermic Circulatory Arrest (DHCA): Process in which the patient is cooled to approximately 18C (64F) and the blood drained from the body. This is used to allow for aortic arch repairs (i.e. Norwood procedure) where an aortic cross clamp and cardioplegia would not be sufficient. It also provides for a bloodless field. While the patient is in DHCA, their blood circulates extracorporeally through the bypass circuit. Periods of DHCA greater than 30 minutes are generally associated with increasing rates of neurological complications
  • Modified Ultrafiltration (MUF): ultrafiltration of the patient and circuit volume immediately after cardiopulmonary bypass which mitigates some of the fluid retention induced by the inflammatory response to cardiopulmonary bypass. It also serves to hemoconcentrate the available blood, thereby increasing the patient's ultimate hematocrit (and oxygen carrying capacity), and also allows for return of the blood present in the circuit (to which the patient has already been exposed) to be given back to the patient and not wasted

Factors to Consider

  • In addition to DHCA,cross clamp, and bypass time, other important intraoperative factors relevant to postoperative care include:
    • Significant arrhythmias noted intraoperatively
    • Residual lesions (whether pressure gradients, valvular regurgitation, or shunts)
    • New lesions (ie significant atrio-ventricular valve regurgitation)
    • Issues with airway or oxygenation/ventilation

Single Ventricle Physiology

  • Hypoplastic Left Heart Syndrome (HLHS) can be viewed as the prototypical lesion resulting in single ventricle physiology
Figure 1: HLHS

    • Other single ventricle lesions include: Tricuspid atresia, double outlet left ventricle, some heterotaxy defects, and hypoplastic right heart syndrome
  • Essentially, one ventricle becomes responsible for both flow to the pulmonary and systemic circulations. This also requires mixing (i.e. via a patent ductus arteriosus) to sustain life- hence, these are generally ductal dependent lesions
  • The single ventricle is significantly volume loaded
  • Relative pulmonary blood flow (Qp) and systemic blood flow dependent upon resistances of the respective vascular beds
  • Major preoperative consideration is balancing the circulations to maintain adequate end organ oxygen delivery (you need pulmonary blood flow to saturate hemoglobin with oxygen but you also need adequate blood flow to perfuse your brain and vital organs- more of one by definition leads to less of the other)
  • Norwood procedure is the first step in palliating HLHS. The 3 major goals of the Norwood procedure are:
    1. Unobstructed Systemic Blood Flow (basically, you need to augment the hypoplastic aorta by creating a neoaorta utilizing the pulmonary artery and graft material)
    2. Unobstructed pulmonary venous return (you need complete mixing into your "common" atrium- this generally consists of an atrial septectomy)
    3. Restricted pulmonary blood flow (you have divided the main pulmonary artery so you need to create a shunt between the systemic circulation and pulmonary artery to provide pulmonary blood flow. However, any blood that flows to the pulmonary circulation means less blood flowing to the systemic circulation and so this pulmonary blood flow has to be restricted- PVR is lower than SVR in general). 
      • The two shunts utilized include:
        • Modified Blalock Taussig shunt (BT shunt) connecting the subclavian artery and pulmonary artery. The size of the shunt is critical for determining relative Qp and Qs
        • Sano shunt (or RV-PA conduit)
      • The SVR trial looked at outcomes following these two types of shunts and found increased transplant free survival in the RVPA conduit group vs. the mBT group at 12 months but more unintended interventions and complications. However, the transplant free survival at 32 months was not different. Many conclude that centers and surgeons that are experienced with mBT shunts should continue doing those while those who are experienced doing RVPA conduits should continue doing those.

Figure 2: Norwood Procedure with modified BT shunt (left) and Sano RV PA conduit (right)

  • Hybrid Procedure: Variation of the Norwood procedure but avoids cardiopulmonary bypass and sometimes performed on higher risk patients. Combines procedures via cardiac catheterization as well as sternotomy/OR
    • Unobstructed Systemic Blood Flow Instead of creating a neoarorta, you stent the PDA to provide systemic blood flow
    • Unobstructed pulmonary venous return (you need complete mixing into your "common" atrium- this can be performed via catheterization and ballooon atrial septostomy (BAS/Rashkind Procedure))
    • Restricted pulmonary blood flow PA bands are placed to provide restricted pulmonary blood flow. This is done surgically but does not require cardiopulmonary bypass
Figure 3: Hybrid Procedure


Postoperative Considerations

    Low Cardiac Output Syndrome
  • Inadequate oxygen delivery to tissues generally due to depressed cardiac output
  • Develops ~6-12 hours post cardiopulmonary bypass as a result of systemic inflammation from blood interfacing with foreign material. Myocardial ischemia from aortic cross clamping, hypothermia, reperfusion injury to myocardium, ventriculotomy, and inadequate cardioplegia can all contribute as well.
From: Wernosky et al, Circulation 1995

  • Milrinone often used to prevent LCOS after CHD repair based on PRIMACORP study (Hoffman et al, Circulation 2003). Multicenter RCT of 238 children undergoing biventricular repair comparing placebo vs, milrinone at 0.25 mcg/kg/min, and milrinone at 0.75 mcg/kg/min demonstrating less development of LCOS/Death (primary outcome)
From: Hoffman et al, Circulation 2003 (PRIMACORP)



    Postoperative Tachyarrhythmias:
  • Most commonly atrial or junctional tachycardias in pediatrics
  • Can obtain 12 lead EKG with atrial wire to clearly identify p-wave and timing of atrial conduction
  • common postoperative tachyarrythmias include SVT, JET, and a flutter/fibrillation (less common)

Stage 1 Norwood Procedure
    • Balancing Circulations 
    Stage 2 Hemifontan/Bidrectional Glenn Procedure

    • Cerebral perfusion pressure
    • Passive pulmonary blood flow
    Stage 3 Fontan Procedure

    • Passive pulmonary blood flow
    
    Tetralogy of Fallot

    • Right ventricular compliance
    • Junctional Ectopic tachycardia (JET)


References