Explanation:

A "Tet spell" (hypercyanotic spell) is a sudden, severe decrease in pulmonary blood flow, typically triggered by a decrease in systemic vascular resistance (SVR), an increase in right ventricular outflow tract (RVOT) obstruction, or an increase in sympathetic tone.
Pathophysiology
The "spell" creates a vicious cycle:
1. Trigger: Increased contractility (e.g., pain, crying, induction) or decreased SVR leads to increased right-to-left shunting across the ventricular septal defect (VSD).
2. Hypoxia: Deoxygenated blood enters systemic circulation, causing profound arterial desaturation.
3. Acidosis: Resulting hypoxemia and metabolic acidosis stimulate the respiratory center, increasing sympathetic tone and heart rate, which further worsens RVOT obstruction and exacerbates the shunt.
Management Strategy: "Break the Cycle"
The goal is to increase SVR and decrease RVOT obstruction/infundibular spasm.

1. Knee-chest position- Increases SVR by kinking the femoral arteries and compressing the abdominal aorta, redirecting blood toward the lungs.
2. Oxygenation- Oxygen is a potent pulmonary vasodilator and systemic vasoconstrictor (mild), aiding pulmonary flow.
3. Volume resuscitation- Improves right ventricular preload, helping distend the RVOT.
4. Morphine- Reduces sympathetic tone and alleviates agitation, which reduces infundibular contractility.
5. Phenylephrine- Alpha-agonist to rapidly increase SVR, reducing the right-to-left shunt.
6. Beta-Blockers (Esmolol/Propranolol)- Reduces infundibular spasm and heart rate (allowing for longer diastolic filling).
7. Sodium Bicarbonate- Corrects metabolic acidosis, which is a potent pulmonary vasoconstrictor and stimulant for tachycardia.
References:
1. Miller’s Anesthesia (9th Ed); Smith’s Anesthesia for Infants and Children (9th Ed).
2. Kaplan's Essentials of Cardiac Anesthesia for Noncardiac Surgery.