Tet Spell in the OR #2: Rapid CPB Initiation

A refractory hypercyanotic spell during induction or early operative preparation for tetralogy of Fallot is a time-critical physiologic emergency. The fundamental mechanism is an acute increase in right-to-left shunting across the ventricular septal defect, usually caused by worsening dynamic right ventricular outflow tract obstruction, decreased systemic vascular resistance, or both [1].

Initial medical stabilization should be immediate and simultaneous with preparation for cannulation. Oxygenation, deepening of anesthesia, volume administration, alpha-adrenergic vasoconstriction, beta-blockade, and correction of acidosis may restore a more favorable balance between systemic and pulmonary blood flow. However, when severe desaturation or circulatory instability persists, the operative priority should shift rapidly from pharmacologic stabilization to cardiopulmonary bypass as the definitive bridge to repair.

1. Physiologic Basis of a Refractory Tet Spell

In tetralogy of Fallot, pulmonary blood flow is highly dependent on the degree of right ventricular outflow tract obstruction and the balance between systemic and pulmonary vascular resistance. During a hypercyanotic spell, right-to-left shunting increases because blood preferentially exits the right ventricle through the malalignment VSD into the overriding aorta rather than through the obstructed RVOT.

The spell may be triggered or amplified by:

1. Dynamic infundibular contraction
2. Hypovolemia or reduced preload
3. Decreased systemic vascular resistance
4. Increased sympathetic tone
5. Acidosis and hypoxemia
6. Inadequate anesthetic depth or painful stimulation

Intraoperative echocardiographic observations have demonstrated that acute changes in RVOT obstruction and systemic vascular resistance can directly alter shunt direction and systemic oxygen saturation [1]. This explains why phenylephrine and beta-blockade are physiologically rational: phenylephrine increases systemic vascular resistance and reduces right-to-left shunt fraction, whereas beta-blockade reduces dynamic infundibular obstruction.

Disopyramide has also been reported to improve oxygenation through negative inotropic effects on the right ventricle, but its role is limited and should not distract from standard acute management or timely surgical rescue [2].

2. When CPB Becomes the Definitive Rescue Strategy

Medical therapy is appropriate only if it rapidly reverses the spell. If severe hypoxemia persists, ongoing delay may worsen acidosis, myocardial dysfunction, bradycardia, and circulatory collapse. At that point, cardiopulmonary bypass becomes the most reliable method to restore systemic oxygen delivery.

CPB rescues the physiology by:

1. Providing systemic oxygen delivery independent of native pulmonary blood flow
2. Restoring controlled systemic perfusion
3. Interrupting the cycle of hypoxemia, acidosis, and increasing RVOT obstruction
4. Allowing transition from emergency stabilization to definitive intracardiac repair

The decision should not wait until cardiac arrest. In a prepared operating room, persistent desaturation despite appropriate medical maneuvers should trigger the command:

“Prepare for rapid bypass.”

3. Aortic Cannulation in TOF

In most patients with tetralogy of Fallot, the ascending aorta is relatively generous because it receives the dominant systemic outflow through the overriding aorta. This anatomy often makes ascending aortic cannulation straightforward, even in infants.

Important technical principles include:

1. Select a safe cannulation site
• Usually on the anterior ascending aorta.
• Avoid the planned cross-clamp site, cardioplegia site, and sinotubular junction.
• Maintain sufficient distance from the innominate artery, especially in small infants.
2. Secure arterial access efficiently
• Place the purse-string sutures rapidly but precisely.
• Avoid unnecessary traction or manipulation that may worsen instability.
3. Coordinate with perfusion
• The arterial line should be de-aired and ready.
• The perfusionist should be prepared for immediate initiation once venous drainage is established.

The immediate goal is not the final operative configuration. The immediate goal is safe and rapid establishment of extracorporeal support.

4. Rapid Right Atrial Venous Cannulation

For emergency initiation of CPB during a refractory spell, the fastest venous strategy is often single right atrial cannulation. This provides rapid venous drainage and allows bypass to begin before completing the more controlled bicaval setup required for intracardiac repair.

Practical sequence

1. Place a purse-string suture on the right atrium or right atrial appendage.
2. Insert a venous cannula into the right atrium.
3. Connect to the venous limb of the circuit.
4. Initiate CPB once venous drainage and arterial inflow are adequate.
5. Stabilize oxygenation, systemic perfusion pressure, temperature, and acid-base status.
6. Convert to bicaval drainage after physiologic control is restored.

This strategy prioritizes time to bypass. In the setting of profound hypoxemia, single RA cannulation is often the most practical bridge from crisis to control.

5. Conversion to Bicaval Venous Drainage

Definitive TOF repair usually requires right atriotomy, exposure through the tricuspid valve, VSD closure, and relief of RVOT obstruction. Therefore, after the patient is stabilized on CPB, venous drainage should be converted to a bicaval configuration.

This can be achieved by:

1. Adding a separate SVC cannula
2. Advancing the initial RA cannula into the SVC, depending on the cannula design and anatomy
3. Adding an IVC cannula when complete bicaval isolation is required

Bicaval drainage provides:

• A controlled right atrial operative field
• Reduced venous return into the atriotomy
• Improved exposure of the tricuspid valve and VSD
• Stable conditions for RVOT assessment and repair
• Standard conduct of complete TOF repair

The key distinction is practical:

Single RA cannulation is the rescue setup; bicaval drainage is the repair setup.

6. Oxygen Management on CPB in Cyanotic Patients

Once CPB is initiated, restoration of oxygen delivery must be balanced against the risk of abrupt reoxygenation injury in chronically cyanotic myocardium and end organs. Randomized data in cyanotic pediatric patients suggest that controlled normoxic CPB may reduce oxidative stress and inflammatory injury compared with hyperoxic CPB [3]. Additional transcriptomic data in patients undergoing TOF repair support the concept that controlled reoxygenation may attenuate molecular injury responses during bypass [4].

In an emergency Tet spell, the first priority remains survival and restoration of systemic oxygen delivery. However, once bypass is established, the perfusion strategy should avoid unnecessary hyperoxia when feasible, particularly in chronically cyanotic infants.

7. Team-Based Algorithm

A severe intraoperative Tet spell should trigger a shared mental model among surgery, anesthesia, perfusion, nursing, and ICU teams.

Suggested operative sequence

1. Recognize the spell
• Abrupt desaturation
• Hypotension or hemodynamic instability
• Worsening acidosis
• Bradycardia or poor perfusion
2. Start immediate medical stabilization
• Increase inspired oxygen
• Deepen anesthesia
• Administer volume
• Increase systemic vascular resistance
• Reduce dynamic RVOT obstruction
• Correct acidosis
3. Prepare cannulation simultaneously
• Expose ascending aorta
• Prepare arterial cannula
• Prepare RA venous cannulation
• Alert perfusion to prime and stand ready
4. Initiate CPB if instability persists
• Ascending aortic cannulation
• Rapid RA venous cannulation
• Start bypass
5. Stabilize on CPB
• Restore systemic oxygen delivery
• Correct acidosis
• Optimize perfusion pressure and temperature
• Avoid unnecessary hyperoxic overshoot when feasible
6. Convert to bicaval drainage
• Add SVC and IVC cannulation, or advance the RA cannula into the SVC as appropriate
• Establish a controlled field for intracardiac repair
7. Proceed to definitive TOF repair
• VSD closure
• RVOT muscle resection
• Pulmonary valve and annular assessment
• Transatrial, transpulmonary, or transannular strategy as dictated by anatomy

8. Surgical Principle

The core operative principle is:

If a Tet spell persists in the operating room despite immediate medical stabilization, rapid CPB initiation is the definitive bridge to repair.

In tetralogy of Fallot, the ascending aorta is usually favorable for arterial cannulation. A single right atrial venous cannula can be used to initiate bypass quickly, after which the setup can be converted to bicaval drainage for controlled intracardiac repair. This sequence minimizes the duration of severe hypoxemia and allows the team to regain control of systemic oxygen delivery, perfusion, and operative exposure.

The available evidence is stronger for the physiologic mechanisms of Tet spells than for standardized emergency CPB protocols. Therefore, the decision to initiate rapid CPB is best understood as a surgically grounded rescue strategy derived from the pathophysiology of refractory right-to-left shunting, the predictable anatomy of TOF, and the practical realities of intraoperative crisis management.

References

[1] Greeley WJ, Stanley TE 3rd, Ungerleider RM, Kisslo JA. Intraoperative hypoxemic spells in tetralogy of Fallot. An echocardiographic analysis of diagnosis and treatment. Anesth Analg. 1989;68(6):815-819.

[2] Tomita H, Fuse S, Hatakeyama K, Suzuki M, Chiba S. Disopyramide: a promising new approach to the medical treatment of the hypercyanotic spell complicating tetralogy of Fallot. Jpn Circ J. 1998;62(11):807-810.

[3] Caputo M, Mokhtari A, Rogers CA, Panayiotou N, Chen Q, Ghorbel MT, Angelini GD, Parry AJ. The effects of normoxic versus hyperoxic cardiopulmonary bypass on oxidative stress and inflammatory response in cyanotic pediatric patients undergoing open cardiac surgery: a randomized controlled trial. J Thorac Cardiovasc Surg. 2009;138(1):206-214.

[4] Ghorbel MT, Mokhtari A, Sheikh M, Angelini GD, Caputo M. Controlled reoxygenation cardiopulmonary bypass is associated with reduced transcriptomic changes in cyanotic tetralogy of Fallot patients undergoing surgery. Physiol Genomics. 2012;44(22):1098-1106.